Astrobiology Workshop
Final Report
NASA Ames Research Center
December 1996
Edited by:
D. DeVincenzi
Prepared by:
G. Briggs, M. Cohen, J. Cuzzi, D. DesMarais, L. Harper, D. Morrison, A. Pohorille
Table of Contents
Introduction	1
Current State of Knowledge in Key Areas of Astrobiology	2
New Cross-Disciplinary Research Programs	7
Additional Activities	13
Appendices
	Workshop Program	A1
	Abstracts	A4
	Attendees	A61
Introduction
Astrobiology is defined in the 1996 NASA Strategic Plan as
"The study of the living universe." At NASA's Ames Research
Center, this endeavor encompasses the use of space to understand
life's origin, evolution, and destiny in the universe. Life's origin
refers to understanding the origin of life in the context of the origin
and diversity of planetary systems. Life's evolution refers to
understanding how living systems have adapted to Earth's changing
environment, to the all-pervasive force of gravity, and how they may
adapt to environments beyond Earth. Life's destiny refers to making
long-term human presence in space a reality, and laying the foundation
for understanding and managing changes in Earth's environment.
The first Astrobiology Workshop was held at Ames on September 9-11,
1996, bringing together a diverse group of researchers to discuss the
following general questions:
		Where and how are other habitable worlds formed?
		How does life originate?
		How have the Earth and its biosphere influenced each other
		over time?
		Can terrestrial life be sustained beyond our planet?
		How can we expand the human presence to Mars?
The objectives of the Workshop included:  discussing the scope of
astrobiology, strengthening existing efforts for the study of life in
the universe, identifying new cross-disciplinary programs with the
greatest potential for scientific return, and suggesting steps needed
to bring this program to reality.
Ames has been assigned the lead role for astrobiology by NASA in recognition
of its strong history of leadership in multidisciplinary research in
the space, Earth, and life sciences and its pioneering work in studies
of the living universe. This initial science workshop was established
to lay the foundation for what is to become a national effort in
astrobiology, with anticipated participation by the university community,
other NASA centers, and other agencies.
This workshop (the first meeting of its kind ever held) involved life,
Earth, and space scientists in a truly interdisciplinary sharing of
ideas related to life in the universe, and by all accounts was a
resounding success. It was broadly interdisciplinary in attendance, with
the following breakdown of the invited participants:  23 astronomers and
physicists, 37 Earth and planetary scientists, and 38 life
scientists. Attendance was 250 on the first day. The smaller workshop
held on the next two days was nominally restricted to about 100 invitees,
but in fact it attracted an overflow crowd. Peak attendance was actually
reached during the final afternoon. Numerous phone calls were received
from the public wanting access to additional information. The news
media called several times after the workshop to request updates on
and access to the latest thinking, discussion, and speculation.
This report is a summary of the highlights of the workshop. The first
section deals with the current state of knowledge in the fields that
comprise astrobiology as presented by the invited speakers. This was
widely considered to be one of the most significant aspects of the
workshop, as participants were appraised of the latest thinking in
fields outside their own. The next section identifies new cross-disciplinary
research topics which resulted from new information exchanged among all
of the relevant fields. These topics were developed during small group
discussions organized around the 5 key questions noted above and occurred
during two "working lunches." They were summarized and discussed during
the final afternoon plenary session. The last section contains
suggestions for follow-on activities which were proposed by workshop
participants during the final afternoon plenary session. The report
concludes with appendices containing the workshop program, abstracts,
and participant list.
There was no attempt made at the workshop to reach consensus on research
priorities, recommendations, or funding requirements. Rather this
workshop was intended to stimulate cross-discipline thinking and new
ideas for productive research.
Current State of Knowledge
in Key Areas of Astrobiology
Formation and Diversity of Planetary Systems
Fundamental to understanding the distribution of life in the cosmos is
understanding the formation and diversity of planetary systems, which are
the retinues of planets and satellites of different mass and composition
orbiting stars of different luminosities. The conditions under which
these systems form and evolve will determine the diversity of habitable
environments in space and in time. Understanding planetary phenomena
will rely on three key approaches:  direct, multi-wavelength observations
of planetary systems across the entire range of formative and mature
stages; theoretical studies of the behavior of multiple, complex, and
interacting processes under diverse conditions; and laboratory and
astronomical measurements of primitive materials preserved since the
formative stages of our own system.
A consensus theory of planetary formation is generally in hand:  gradual
accumulation of solids within a primarily gaseous, flattened circumstellar
accretion disk, which itself is a byproduct of the formation of its parent
star from a dense, rotating interstellar cloud of gas and dust.
However, this theory has been studied in only a very narrow range of
initial conditions, possibly important physics has been neglected, and
it has little or no predictive capability. For example, recent discoveries
of giant planets in circular orbits very close to solar-type stars were
unexpected and are still not completely understood. There are, as of
this writing, eight new giant planets known to orbit solar-like stars;
at least one of these orbits within the "habitable zone" of its parent
star. These new data provide not only a challenge to the current
theoretical paradigms, but clear direction as to parts of parameter
space in which both theoretical models and observations of extrasolar
systems need more exercise. Furthermore, given the wide range of
conceivable environments, we might ask "what makes a planet habitable?"
(an associated question is "habitable for what kind of organism?").
Advances in technology are enabling not only new observations of these
mature (if unanticipated) extrasolar planetary systems, but also of
"protoplanetary nebulae" within which the planetary formation process
is still ongoing. These observations are capable of telling us the
extent, mass, gas and solid content, and thermal structure of the material
from which planets form. In order to comprehend the new, surprising
diversity of planetary systems, we must continue to study the early
stages of planetary formation under a range of conditions, as well as
to establish the full range of ultimate outcomes of the process.
In addition to observations of remote extrasolar planetary systems from
ground and space, we are fortunate to have in hand, or accessible by
spacecraft, actual material which survives from the days of the early
accumulation of our own planets. So-called "primitive material" preserves
clues as to the materials from which, and the processes by which, planets
formed. To be found in these primitive materials are presolar grains which
carry clues as to the variety and number of stellar precursors of our own
system, complex organic material which might preserve the signature of
interstellar chemistry, once-molten silicate "chondrules" with
composition, size, and mineralogy diagnostic of the pre-accretionary
environment, and, in one recent case, suggestive evidence for past life
on another planet.
Origin of Life
The occurrence of organic compounds in interstellar clouds, planets of
the outer solar system, comets and meteorites suggests a chain of
astrophysical processes which link the chemistry of interstellar
clouds with the prebiotic evolution of organic matter in the solar
system and on the early Earth. Although there is no record of the
evolutionary pathway from this simple organic matter to present-day
life on Earth, the main steps along this pathway can be deduced from
basic physical and chemical principles, environmental conditions on
the early Earth, and the cellular biology and phylogeny of contemporary
organisms.
There is compelling evidence that cellular life existed on Earth 3.56
billion years ago. Recently, a persuasive argument was made that
terrestrial life was already present toward the end of the period of
heavy bombardment of the early Earth by asteroids and comets from 4.0
to 3.9 billion years ago. This implies that ancestors of contemporary
life emerged rather quickly, on a geological time scale, and perhaps
also survived the effects of large impacts. Such catastrophic events
would have strongly favored survival of thermophilic organisms which
thrive at high temperatures. This scenario is consistent with the
phylogenetic record, which indicates that the last common ancestor
was thermophilic. This record also supports the view that life might
have arisen first near marine hydrothermal vents. The possibility
remains, however, that the first common ancestor lived at moderate
temperatures and only later adapted to thermophilic conditions, in
which case ocean surfaces and near-shore shallow environments might
have spawned life.
All present-day forms of life are cellular, with lipid bilayer membranes
forming the primary barrier that separates the interior of a cell from
the external environment. It has been proposed that similar,
encapsulating structures (vesicles) made of simple membrane-forming
material could have self-assembled in the protobiological
environment. The presence of such membrane-forming material in
carbonaceous meteorites is consistent with this idea. Furthermore,
recent experiments showed that vesicular lipid bilayer structures
can grow by spontaneous addition of membrane-forming material from
the surrounding medium, and can encapsulate both ions and
macromolecules. Besides separating intracellular components from the
diluting effect of the environment, cell membranes also provide a
barrier for separating charges, a fundamental process in bioenergetics.
From phylogenetic data we infer that the earliest cells probably used
chemical rather than photochemical energy sources. It has also been
proposed that membranes helped stabilize the secondary structure of
peptides (protein precursors) having appropriate sequences of polar
and nonpolar amino acids. Some of these peptides may have been capable
of performing basic protocellular functions, such as catalysis,
signaling, and energy transduction, without requiring the existence
of separate molecules capable of storing and transmitting genetic
information (i.e., nucleic acids).
Alternatively, it has been postulated that there was a time in
protobiological evolution when RNA played a dual role as both genetic
material and a catalytic molecule ("the RNA world"). However, this
appealing concept encounters significant difficulties. RNA is
chemically fragile and difficult to synthesize abiotically. The known
range of its catalytic activities is rather narrow, and the origin of
an RNA synthetic apparatus is unclear. Therefore, it may be more likely
that RNA and proteins co-evolved in protocells, rather than evolving
independently. The co-evolutionary process leading to division of
cellular functions between these molecules, however, is not at all clear.
Understanding the emergence of life requires studies that extend
beyond the origin of biopolymers and cellular structures. All these
components necessarily assembled into auto-catalytic, self-reproducing
systems capable of evolution and selection. Based on theoretical
arguments, it has been suggested that sets of mutually catalytic
molecules can reproduce and evolve without templating, resulting in
a primitive metabolism without a genome. However, only a limited
number of experimental studies have been performed in this area.
The recent discovery of organic, possibly even biogenic, material in
a martian meteorite (ALH84001) opens the exciting possibility of
extending the search for the origin of life to places beyond the Earth.
Although current findings on ALH84001 are inconclusive regarding
possible life on Mars, future exploration might lead to fundamentally
new insights into prebiotic chemistry and protobiological evolution,
the record of which is lost on the Earth.
Interactions Between Earth and Its Biosphere
The history of life on Earth was directed, at least in part, by changes
in the surface environment. Today we are experiencing rapid environmental
changes of our own making, and our biosphere must adapt and, perhaps
eventually, evolve to a different state. Environmental change surely
has occurred in the past, but can studies of our past help to predict
our future? Also, to the extent that rocky planets have followed
similar evolutionary paths, at least during the early chapters of
their history, can studies of our own biosphere assist us in our
search for extraterrestrial life, past or present?
The processes which modified the environment vary widely both in their
magnitude and time scales. For example, the increase in solar luminosity,
the declining rates of comet and meteorite impacts, the exchange of
volatile materials between Earth's mantle and crustal reservoirs, and
the stabilization of continents have all exerted dominant controls on
the surface environment. However, because these processes themselves
evolved very slowly, they required 108 to 109 year time scales to cause
global changes. The effects of plate tectonics, erosion, sedimentation,
and glaciation acted more quickly, causing changes over 104 to 108 year
time scales. Faster still have been the effects of ocean and climate
dynamics and ocean-atmosphere-biosphere interactions, which can vary on
1 to 104 year time scales. Already, human activity has dramatically
altered patterns of erosion, sedimentation, climate patterns, species
biodiversity, primary productivity and ocean-atmosphere-biosphere
exchange. These changes are happening over a few decades. In the earlier
"natural" world, such changes would have required typically thousands to
millions of years to occur. How will plants, animals and the microbial
world respond to such rapid change?
Microorganisms are supremely adapted for coping with change. Should
global conditions deteriorate, the small size of microbes allows them
to "hide" in niches. Small cell size imparts a high surface/volume ratio,
which allows rapid rates of chemical exchange with the cell's
surroundings. Thus microbes can rapidly exploit favorable conditions. The
diverse biochemistry of microbes permits them not only to survive, but
even to prosper under environmental extremes. Already by 3.5 billion
years ago, widespread microbial communities accommodated large meteorite
impacts, UV irradiation, desiccation, wide excursions in temperature and
salinity, and a long menu of chemical substrates as sources of energy
and organic matter. For example, our early biosphere adapted to major
changes in volcanism, coastal environments, atmospheric composition, and
the oxidation state of the oceans and atmosphere. On the other hand,
microorganisms can themselves contribute to environmental change by,
for example, affecting rates of erosion and sedimentation or by
influencing the atmosphere's inventory of reactive gases. Microbes
responsible for infectious diseases evolve to circumvent medical
treatments, thereby continually challenging human populations.
In contrast with the bacteria, plants and animals are much larger, more
complex and highly specialized. They typically depend upon a more
limited suite of nutrients and a relatively narrow range of conditions
for their survival. Accordingly, environmental change, human-induced
or otherwise, can more easily trigger catastrophe within ecosystems
which sustain these complex eukaryotic organisms. Modern challenges
to the biosphere include rising atmospheric levels of CO2, SO2, CH4,
CO, and N2O due to fossil fuel burning and agriculture (causing
greenhouse climate effects as well as direct biospheric effects),
declining ozone levels (leading to increased ultraviolet radiation),
invasions of foreign species, and land use changes whose effects
include the following:  soil salinization, overgrazing, increased
soil erosion, altered energy balance, loss of biodiversity, species
extinctions, declines in food and fisheries, and chemical pollution.
While large meteorite impacts, such as the one which marks the
Cretaceous/Tertiary boundary, were perhaps more severe than modern
human-induced changes, impacts still serve as useful models for the
effects of catastrophic change on the biosphere. For example, the
severe "winter" which had been predicted to follow a large impact
alerted us to the "nuclear winter" which might follow thermonuclear
war. Also, impacts remind us that catastrophism probably does play
at least a limited, but still important, role in the long-term
evolution of our biosphere. The role of impacts in evolution was
perhaps most pronounced during the earliest stages of Earth's history,
when impact rates were much higher.
Sustaining Life in Space
Because life evolved and developed on the Earth, it is uniquely adapted
to function on this planet. To sustain life beyond the Earth's biosphere
for prolonged periods of time will require a better understanding of the
processes underlying biological adaptation and the interactions among
organisms and their environments. The relationships among the
behavioral, structural, and genetic bases of survival remain to be
elucidated. Adaptability in biological systems is a given, but the
limits of adaptability and the issue of irreversibility of adaptive
changes are major concerns. A concerted effort in enhancing our
knowledge of biological adaptation, and developmental and evolutionary
biology, will be needed if we are to sustain terrestrial life beyond
the Earth's biosphere.
Electromagnetic radiation and gravity are two fundamental environmental
variables that dramatically affect biological systems. On Earth, gravity
is effectively constant in magnitude and direction, and the natural
radiation environment has modest variability. These physical variables
are difficult to control in space, and consequently can severely limit
our ability to sustain life beyond the surface of the Earth.
How the radiation environment beyond the Earth affects biological
systems is only partially understood. In space, galactic cosmic rays
and particles from solar events can be lethal to terrestrial life
forms. We have a very limited ability to predict solar events, and
our understanding of shielding techniques to manage radiation risks
is poor. Further, our ability to characterize the radio-biological
effectiveness of various ionized and non-ionized particles, is limited.
Space travelers beyond low Earth orbit must, therefore, monitor the Sun
for solar storms as a matter of life or death.
Clearly, the effects of various forms of radiation on RNA and DNA are
issues of major concern. Currently we are ignorant of the relationships
among chromosomal damage, chromosomal aberrations, and carcinogenesis.
The direct effects of high energy particles on the nervous system are
also poorly understood, as are biological mechanisms for the repair of
radiation damage.
Gravity profoundly affects many biological systems, both directly and
indirectly. The cardiovascular, musculoskeletal, and neurovestibular
systems all undergo dramatic changes in space, where organisms are
deprived of terrestrial gravity. For example, fluids shift from the
lower limbs and lower torso to the upper torso and the head; blood
volume is reduced; anti-gravity muscles in the lower limbs and torso
tend to atrophy; bones that formerly supported the organism against
gravity become less dense and more fragile; vestibular-ocular reflexes
are altered, and the nervous system re-calibrates itself to function
in the absence of gravity. Although these changes are generally benign
for functioning in space, they can seriously compromise an organism's
ability to function in a new gravitational environment and upon return
to the Earth.
Humans currently use multiple countermeasures to minimize the effects
of non-terrestrial environments on physiological systems for periods
of more than one year. These countermeasures, which include training
procedures, protective garments, physical exercise, conditioning
devices, and various pharmacological agents, may be of only limited
value to sustain life beyond the Earth's biosphere for prolonged
periods of time that ultimately will include multiple generations.
Artificial gravity, provided by continuous or intermittent
centrifugation, lower-body negative pressure exercise chambers, or
other techniques, may be necessary. Our experience with artificial
gravity for humans in space is limited to a single, brief, Gemini
flight experiment, and our current knowledge base is inadequate to
assess the need for artificial gravity to sustain life beyond the
Earth's biosphere.
Critical psychological variables in small group interactions during
prolonged isolation in a perpetually hostile environment away from the
home society are not well understood. The interactions of gravity,
radiation, and isolation in non-terrestrial environments have never
been studied systematically. Thus, many fundamental questions in the
life sciences will need to be answered before we can assure that
terrestrial life forms can be sustained beyond the Earth's biosphere
for prolonged periods.
With current technology, we are able to maintain terrestrial life
beyond the Earth for periods in excess of one year. To sustain
terrestrial life beyond the Earth for longer periods, it is necessary
to create a micro-environment that is similar to that on Earth, at
least initially. This environment must provide an atmosphere with a
ppropriate partial pressures of O2 and allow for gas exchanges to
support metabolism; it must provide adequate liquid water, appropriate
microorganisms, adequate gravity, food, thermal protection, and
radiation protection; it must allow for the partial recycling of
nutrients and waste-products; finally, it must be stable and reliably
sustainable for an indefinite period of time.
Human Exploration of Mars
As described in the section above, we still lack much of the fundamental
knowledge necessary to send humans on extended space journeys beyond the
protection of the Earth's biosphere (including its magnetic field). Only
modest progress is being made towards actually carrying out the life
science experiments and technology tests needed to ensure that a crew
arriving at Mars will be at a sufficient fitness level (albeit that
fitness level needs definition) to assure their well being and the
success of their mission. Thus, fully effective countermeasures to
deal with long duration exposure to microgravity have not yet been
demonstrated, and the appropriate shielding requirements to deal
with extended exposure to heavy galactic cosmic rays have not been
fully defined. However, these issues appear tractable if appropriate
experiments are conducted on the International Space Station and if
appropriate particle accelerator experiments are carried out.
A program to extend human presence to Mars will inevitably have both
exploration and what we may term habitability goals. If evidence that
life once evolved on Mars is discovered, human explorers will provide
much of the scientific capability needed (beyond robotic capabilities
projected for the next several decades) to investigate how the
pre-biotic seeds of microbial life evolved and subsequently prospered
or perished.
Theory, laboratory experimentation, subterranean terrestrial sampling
and meteoritic evidence suggest that microbial life could have evolved
on early Mars. Our present lack of direct knowledge about subterranean
martian environments should make us cautious, therefore, about
concluding (as seems common) that any such early life would inevitably
have become extinct on a planet where present surface conditions are
indeed extremely hostile. To answer questions about possible extant
life we need to explore the subsurface below the cryosphere, which
extends to kilometer depths, and into the warmer martian hydrosphere.
Although a thorough exploration of the martian subsurface by robots
alone is feasible in principle, the combined effects of great
communication distances and intrinsically limited machine intelligence
might well require postponement of such exploration for many
generations. Therefore, some astrobiologists are considering whether
human exploration of Mars may be legitimately identified as a real
scientific priority as the only efficient and timely way in which we
will be able to study, at first hand, a second sample of life (all
terrestrial life being linked to a common ancestor).
The consequences of the discovery of life, past or present, on Mars
in the coming decades will have profound implications beyond just
the intense interest of molecular biologists. (Likewise, although
it will be much harder to disprove the case, the determination that
Mars never evolved life would also have profound implications.)
Scientists and non-scientists alike will immediately appreciate the
improbability that humans are "alone" in our galaxy. The discovery of
life on Mars will surely add priority to the search for life elsewhere
in our solar system (e.g. in the subterranean oceans of Europa), to the
search for Earth-like planets orbiting other stars in our galaxy, and to
the search for extraterrestrial intelligence. More generally, the
stimulation of such a discovery of martian life is also likely to
lead us to a recognition that, having the technological means at hand,
we can be on the verge of becoming a multi-planet civilization, with
Mars as our second abode.
New Cross-Disciplinary Research Programs
Formation and Diversity of Planetary Systems
It was recommended that part of NASA's vision should be to understand
the planetary formation process in enough depth to be able to predict,
or at least constrain, the diversity of habitable planetary systems.
Within this vision, primary science goals might include:  What are the
fundamental processes and conditions that lead to planetary formation?
What kinds of planets form, around what kinds of stars, and at what
distances from their parent stars? What defines and determines
"habitability"?
There are several well-posed problems that are ready for accelerated
study by astronomical observations. Recent indirect radial velocity
observations of extrasolar planets, and millimeter-wave, infrared,
and HST observations of circumstellar gas and particle disks, are
outstanding examples of the tip of this iceberg. Certainly, observations
of mature planetary systems around a large variety of stellar types
of various ages will be needed to determine what kinds of planets
form around what kinds of stars. Direct or indirect detection of planets
in "mature" systems is the focus of NASA's planning efforts to date.
However, there remain critical gaps in our understanding of the
earlier "protoplanetary" stage; these include the actual absolute
(not assumed relative) abundances of gas and solids, the nebula
radial extent under different initial conditions, its radial and
vertical temperature structure, the particle-to-planet accumulation
time scale, the role and distribution of angular momentum of
in-falling material, the properties of stellar winds, and the role
of magnetic fields. We are completely ignorant of whether any of
these properties vary with stellar type and/or with star formation
environment (i.e., solitary or densely clustered), and there are
hints in current data that protoplanetary systems in the two different
star forming regions in Taurus and Ophiuchus have rather different
properties.
The observations need to be conducted over a wide range of wavelengths
between the short microwave (millimeter) and near-infrared spectral
regions to penetrate the thick nebular dust envelopes and sample the
mid-plane where planet formation is occurring. Infrared (Keck and
follow-on) and millimeter-wave interferometers are needed to resolve
protoplanetary disk structure at 1 AU resolution. NASA's planned
"Origins" program proposes a series of space-based infrared telescopes
and interferometers; other approaches were mentioned which are less
connected technologically but are also worthy of consideration
(photometric detection, balloon missions, HST upgrades, far-IR
[100 micron] interferometer, microlensing, etc.). Several key
theoretical questions are ripe for interdisciplinary attack. These
include the properties of a densely clustered protostellar environment,
the role of ionization, grain charging, and electromagnetic forces,
the role of global wave modes and/or energetic infall itself in
nebula evolution, the presence, extent, duration, and energetics
of nebula turbulence, the possibly wide-ranging migration of
protoplanets (and solid material in general) within the nebula
and even relative to the nebula gas. It was noted that the Sun-Earth
Connections theme of the Space Science Enterprise might be tapped to
a larger extent for its expertise in electrodynamics and stability or
instability of weakly ionized media, properties of stellar and solar
winds, and of dusty plasmas.
It was generally felt that augmentation of the numbers of primitive
meteorites returned, catalogued, and analyzed from the Antarctic
would yield commensurate rewards. Much of the nation's analytic
resources are Apollo-era and worthy of considerable upgrading. Of
particular interest might be the sort of ultra-high resolution
analytic equipment capable of studying the internal structure of
putative nano-fossils (such as found in ALH84001) or of obtaining
accurate age dates and/or isotope data on small mineral samples.
Exploration of this sort of "inner space" is probably just as demanding
of technology and expertise, and as rewarding in terms of understanding
of the planetary formation process, as comparable efforts devoted to
exploring "outer space."
Concerning the issue of habitability, there are many unknowns (even
in the case of our own planet). For instance, what stellar and/or
planetary conditions are most important for the origin of life, or
for its evolution and increasing complexity? Are stellar photons of
extreme energies and charged particle fluxes positive or negative
factors? What is the role of internal planetary activity (tectonics)
in truncating or prolonging the habitable era? Water is generally
agreed to be the sine qua non of life, but how is water distributed
across growing planets or reintroduced on mature planets that lost
it or never had it at all? Does chaotic planetesimal dynamics spray
icy objects from the outer solar system onto mostly formed inner
planets? What is the composition of the "primitive" objects that
even today impact the terrestrial planets? What is the mass and
extent of the Kuiper belt of primitive planetesimals? Are
terrestrial-sized satellites of close-in giant planets orbiting M
dwarf stars habitable? Are they dynamically stable? Studies of the
"habitable zone" using planetary scale climate evolution models might
profit from more association with the sophisticated modeling supported
in the Earth science community (such as to treat cloud feedback effects).
Origin of Life
The origin of protobiological self-organization and complexity.
The main thrust in this area should be to establish the principles of
organization and complexity that led a collection of organic molecules
to assemble into the earliest ancestors of contemporary cells. With
these principles as a basis, attempts should be made to create
laboratory versions of cellular, self-reproducing and evolving
systems starting from material that might have existed under prebiotic
conditions. Even though enough knowledge appears to exist to make
fundamental progress in this direction, this research area remains
severely under-represented in the current exobiology program. The
expertise of cellular and molecular/structural biologists and bio-organic
and physical chemists will be required, guided by collaboration between
experimentalists and theorists. The goals of this interdisciplinary
effort are to establish protobiological versions of basic cellular
functions such as energy capture, chemical catalysis, and transport
of solutes across membrane boundaries. These processes must be
accomplished by simple molecules that self-assemble to form
auto-catalytic, self-reproducing systems that evolve in response to
environmental pressures. The laboratory experiments must be guided
by theoretical work aimed at discovering general principles of
organization and complexity from simulations that include realistic
descriptions of intermolecular interactions, energetics and chemical
kinetics.
Conditions on the Earth between 4.6 and 3.8 billion years ago.
Based on current evidence that life on Earth originated earlier than
3.8 Gyrs ago, it becomes especially important to establish plausible
conditions on the Earth during the prebiotic period. This would be a
truly interdisciplinary effort involving astronomers, geologists,
chemists, and planetary and atmospheric scientists. Among the main
unanswered questions are:
(a) How did the temperature and the chemical composition of the
atmosphere, crust and the oceans evolve over this time? (b) What were
the global environmental effects of impacts of varying degrees of
severity on the origin and survival of life? (c) What was the inventory
of organic material on the prebiotic Earth, and what were the relative
contributions of terrestrial and extraterrestrial sources? (d) Could
life have been transported between Earth and Mars? From the biological
side, it would also be important to determine what can be learned about
the early evolution of life and its environment by phylogenetic
characterization of the last common ancestor.
Origin of life elsewhere in the solar system.
An unambiguous answer to the question about extant or extinct life on
Mars will likely come only from direct exploration. Nevertheless, there
is a considerable body of research that should be done prior to or in
parallel with missions to Mars in order to interpret results of analyses
of martian samples. Perhaps the most urgent task is to broaden and
extend the study of the ALH84001 meteorite and Earth analogs with the
goal of better understanding the morphological, chemical and isotopic
characteristics attributable to micro-organisms. Collecting and
analyzing more Mars meteorites is also a key task. Developing reliable
criteria for distinguishing between biogenic and abiotic structures
in the Mars meteorite is critical. Besides the rock record, information
about prebiotic chemistry and possible life on Mars may be gained by
comparing conditions on the early Mars and early Earth.
Since liquid water is required to support life, other, possibly
transient, sub-surface micro-environments in the outer solar system
and large asteroids could have been conducive to the origins of life,
a prime example being Europa. These environments should be considered
in future missions and their prebiotic and biological potential should
be assessed by model studies.
Organic chemistry in astrophysical and planetary environments.
The occurrence of organic matter in interstellar clouds, star-forming
regions, comets and carbonaceous meteorites point to a chain of processes
linking interstellar material to solar system formation and perhaps even
to prebiotic evolution on Earth and Mars. The complexity of molecular
structure that can be achieved in astrophysical environments remains,
however, to be fully explored. In particular, little is known about
what survives molecular cloud collapse to become incorporated in
planetary materials. Are there amino acids, nucleic acid bases or
sugars in interstellar dust and comets? Now that optically active
amino acids have been found in Murchison meteorite, what evidence
can be found for astrophysical mechanisms capable of such
stereoselectivity? These questions can be addressed by astronomical
observations and further laboratory and theoretical studies of
primitive materials.
Interactions between Earth and its Biosphere
The concept that the evolution of the biosphere and its environment are
inextricably related should be investigated in detail. Far beyond simply
demonstrating that such a relationship exists, such an investigation
offers many conceptual and practical benefits. Understanding the extent
to which biological evolution has been a product of environmental change
will reveal the mechanisms of the evolutionary process. The consequences
of human-induced environmental change will therefore be easier to
forecast. The search for life beyond the earth will be improved by a
firmer understanding of how planetary environments influence the
survival of biospheres.
Studies of ancient ecosystems could explore the relationship between
the microenvironment and the diversity of microbiota and how these
changed over time. Comparative studies of modern and ancient ecosystems
could identify those aspects of the microenvironment which are crucial to
microbial diversity and evolution and how they changed over geologic
time. Microbial communities in hydrothermal systems (including hot
spring deposits) and in groundwater are especially important analogs
both for understanding the very early fossil record on Earth and for
guiding the search for evidence of past life on Mars. Also, the
changes in morphology and chemistry which accompany fossilization
should be examined.
Regarding Earth's "macroenvironment," we should identify those
mechanisms which directed the long-term increase in atmospheric O2
and the decline in atmospheric CO2 levels. To accommodate these
changes, microbes modified their pathways of CO2 uptake, invented
protocols for detoxifying oxidants, devised new O2-requiring
biosynthetic pathways, and so forth. What were the nature and timing
of these innovations? What were the composition and abundance of trace
biogenic gases in the ancient atmosphere, particularly before
significant levels of O2 were attained? What were/are the significant
feedback effects involving biota, trace gases and climate? How did
oxygen-utilizing eukaryotes evolve in response to these changes? Given
anticipated land-use changes today, what role will trace gases play in
future climate change?
How does an entire ecosystem, including its microbes, respond to
abrupt environmental perturbations? The natural microbial world is
a rich source of information about the mechanisms which could
permanently change those ecosystems which sustain plants and
animals. Can these microbial effects be detected before permanent
change occurs? Ecosystem-level studies could monitor the effects of
change. For example, such studies could explore the following:  (a)
the ecology of microbial communities which are still relatively
unaltered by human activity, (b) the sensitivity of ecosystems to
changes in specific parameters, singly or in combination (e.g.,
CO2 levels, UV irradiation, soil acidity, reductions in
biodiversity, etc.), and (c) the relationships between the biota,
climate, geography and hydrology.
Studies also could be pursued for plants and animals, specifically in
the following areas:  a) the influence of extraterrestrial phenomena
such as impacts upon evolution, b) the physical and biological drivers
of mass extinctions, and (c) ecosystem, hydrologic and climate changes
which impact the natural ecosystem and also public health.
Extrasolar planets will eventually be examined to search for other
biospheres. Life should ultimately be detectable through spectroscopic
analyses of a planet's atmospheric composition. Under what conditions
does the presence of abundant atmospheric O2 definitely indicate life?
Aside from abundant O2 levels, what other atmospheric compositions are
definitive indicators of a biosphere? Is the early history of our own
atmosphere actually representative of other evolving, habitable planets?
Obviously an effective research and exploration program requires that
new cross-disciplinary technologies be developed to exploit novel
approaches for getting answers. These involve, for example, the
development of new microsensors for probing the dynamics of microbial
ecosystems, field sensors to monitor gas exchange between ecosystems
and the atmosphere, new approaches in remote sensing and so forth. An
effective technology program is one which is closely integrated with
the research program and responds effectively to new needs as they arise.
Integrated quantitative models should be constructed which help to
develop a deeper understanding between physiology, ecology, and the
environment. Such models could account for changes over various time
scales, and ultimately they should be able to predict community
responses to perturbations. We also should model other planets which
might still be habitable but which are different from Earth. How would
different planetary sizes, solar insolation or volatile inventories
affect the evolution of the planet and its biosphere? Such insights
would greatly enrich our understanding of Mars as well as those extrasolar
rocky planets which we are destined to discover.
Sustaining Life in Space
Perspectives from the early evolution and development of life on Earth can
provide perspectives for developmental biology in space. The same
mechanisms that allowed terrestrial life forms to adapt to the earthly
environment will probably be at work in allowing terrestrial life forms
to adapt to non-terrestrial environments. Similarly, experimental
studies investigating gravitational and radio-biological influences
on genetic material and developmental processes can provide perspectives
for an understanding of the evolution of life on Earth. The new field of
astrobiology provides a framework in which this integration can take place.
To sustain terrestrial life beyond the Earth's biosphere for prolonged
periods of time will require new fundamental knowledge and an
integration of that knowledge in many disciplines. Further, we need a
more profound understanding of closed or semi-closed ecological
systems. Interdisciplinary studies involving radiation physics,
gravitational biology, genetics, neurobiology, and developmental
biology are required to provide the critical understanding.
The International Space Station is an essential evolutionary test
bed for research on the effects of the space environment in biological
development and evolution, as well as the only place where the effects
of gravity on living systems can be investigated systematically. For
example, we do not fully understand the role of gravity in
development. Are there thresholds and critical periods for the effects
of gravity; how are phenotypes and genotypes affected by gravity at the
cellular, system, and organism level?
Improved models for definition and prediction of solar events, the
development of advanced radiation shielding techniques, and enhanced
understanding of genetic biological radiation repair mechanisms are of
particular importance. Other interdisciplinary studies involving
gravitational physics and life sciences are also needed. Just as
importantly, we need a new interdisciplinary perspective to integrate
the information that will assure the long term survival of terrestrial
species beyond the Earth's biosphere.
A regenerative life support system (i.e. one which can be fully
restored/replenished), will ultimately be needed to sustain terrestrial
life beyond the Earth. Thus, a biodome will be required. Unfortunately,
we cannot fully specify all the necessary characteristics of the
required micro-environment at this time because we do not understand
all the control mechanisms that function to maintain closed or
nearly-closed ecological systems. A research biodome is an important
tool that will be needed to help us examine these relationships. The
transition from constant re-supplying to the use of in situ resources
will eventually be necessary to sustain terrestrial life beyond the
Earth's biosphere.
Cross-disciplinary studies continue to provide insights into the Earth's
complex ecosystem, and these can ultimately be applied to developing
artificial life support systems. This remains a promising area for
future cross-disciplinary efforts, for the better we can characterize
those events that alter the Earth's biosphere, the more adequately we
will be able to specify what is needed to provide stable and
sustainable life support systems in space.
Human Exploration of Mars
Progress in understanding the origin and evolution of life includes
further efforts to explore the subterranean portion of the Earth's
biosphere. The development of improved technologies to do so should
include equipment that could later be used on Mars; e.g., light-weight,
semi-automated drill rigs. Moreover, techniques should be evolved from
current procedures to ensure that as samples are acquired from new
subsurface environments, those samples are protected from
contamination -- all the way from their source to their detailed
examination within a well-equipped Mars base laboratory. Such aseptic
protection of the samples must be carried out in a way to ensure that
the samples are effectively quarantined until adequately determined to
be free of pathogenic properties. If and when samples of extant
martian life are indeed discovered, we must be prepared to proceed
with their fundamental characterization and to have clear procedures
established to determine if and when such samples should be returned
to Earth. The importance of this issue will surely influence astronaut
selection and training requirements which will likely include
participation in the exploration of the extremes of our own planet's
biosphere, e.g. desert (including Antarctica) and subterranean environments.
If Mars does have an extant subterranean biosphere, then our exploration
of that biosphere raises the serious environmental and ethical questions
that we face on our own world where species are endangered and lost
sometimes even before we have discovered and characterized them. The
need to avoid contaminating and changing a presently unknown biologic
environment was raised at the workshop and acknowledged to be not only
a potential scientific catastrophe but also a real policy issue, as yet
without an assigned advocate.
Further, even if it should turn out that Mars is now a sterile planet,
environmental issues will confront us -- issues relating to the control
of the pollution and waste associated with an expanding human base. In
the much longer term our technology may provide us with the ability to
seriously consider "terraforming" regions of Mars (or even the entire
planet). Today terraforming another planet amounts to little more than
a thought experiment, but human history demonstrates that such conjectures
can indeed become reality, usually with severe unintended consequences.
In the absence of any other organization likely to grapple with the
ethical dilemmas involved in the future expansion of humans beyond the
Earth, the astrobiology component of NASA's space research program
appears to be the natural home for analysis of exploration ethics.
Additional Activities
1.	New Programs
	a. A number of the promising new research directions fit within
	existing NASA programs. In these cases, it is recommended that
	solicitations be included within existing NRAs to recruit research
	offerings in these areas. Because of the multidisciplinary nature of
	these proposals, it is also recommended that existing peer review
	panels be supplemented with reviewers possessing relevant skills.
	High quality research would be funded by the sponsoring program.
	b. Several important new research directions have been proposed
	which cross traditional NASA discipline and programmatic boundaries.
	For these, a funding commitment to support new research is recommended.
	This program should be designed to support an average grant interval of
	approximately three years (the length of time usually needed for a
	graduate thesis). It is recommended that astrobiology research
	proposals be solicited using a NASA Research Announcement and
	peer reviewed by a multidisciplinary panel organized by the Chief
	Scientist's office at NASA Headquarters (HQ) or "hosted" by each of
	the participating HQ Offices in turn:  Space Science, Life and
	Microgravity Science, and Mission to Planet Earth.
	c. Based on the high level of interest expressed
	by the Workshop participants, a funding commitment from
	Ames Research Center is recommended to sponsor one
	focused conference (nano-fossils was proposed as a
	topic to follow-on to the ALH84001 results) per year
	to follow up on the high priority research topics
	that require further refinement and one general
	conference every 3 years to present the latest
	results of research important to astrobiology.
	The results will be made available to HQ and its
	advisory committees for consideration and
	programmatic action as appropriate.
2.	Cross-agency and Cross-disciplinary Collaboration.
	a. In many areas of research summarized
	earlier, it was felt that an increased level of
	NASA-NSF collaboration would yield great
	rewards. Several powerful observational tools
	currently on the drawing boards are ground-based
	facilities (Keck interferometer, mm Array, etc.)
	and lie in a grey area between NASA (which
	traditionally emphasizes flight missions) and
	NSF (which emphasizes ground-based facilities).
	In the area of primitive materials, NSF and NASA
	already share the load in collecting and
	analyzing meteorites. Much theoretical
	modeling is highly computer intensive, yet
	both NASA and NSF are rethinking and/or
	downsizing their dedicated computational
	facilities. An increased level of
	interagency cooperation in all of these
	areas, and perhaps others, might be both
	appropriate and desirable.
	b. While fostering cross-agency cooperation,
	and with the likely support of Congress and the public,
	the workshop participants recommended that NASA
	also rededicate an appropriate fraction of its
	R&A to "interdisciplinary" research. Teaming
	across discipline boundaries is not always
	efficient at the beginning, but can be a
	critical step towards creative insights as
	each team member assimilates the knowledge
	of other disciplines while remaining an
	expert in their own right. Providing
	tangible incentives for working scientists
	to move in this direction is perhaps the
	most obvious tack. Policy-level adjustments
	to the R&A program priorities or practices
	might be appropriate; short "learning"
	sabbaticals might be encouraged to a
	greater extent under the grants programs;
	NAS-NRC associateships in interdisciplinary
	"new direction" research might be endowed.
3.	Outreach and Communication Activities.
	There was widespread public as well as
	scientific interest in the Astrobiology Workshop
	and its results. Beyond its deep fundamental
	scientific value, it was felt that a driving
	force for astrobiology is unquestionably the
	enormous public interest it excites. Hopefully,
	this interest will not flag at any point along
	the long path which now lies ahead. Opportunities
	should be built in from the start to engage the
	public and to provide them with new results and
	information appropriate to their level of public
	investment. A more problem- or theme-focused
	approach perhaps provides more appeal to the public
	and to Congress than a traditional method- or
	discipline-oriented approach.
	Workshop participants urged more multidisciplinary
	discussions and interactions. Some Workshop participants are
	interested in teaching courses in astrobiology. Workshop
	participants also urged that a special effort be made to
	ensure that scientists interested in astrobiology have
	easy access to the latest findings, including access to the
	results of new research efforts described above. Four
	mechanisms are proposed to respond to this interest.
	a. In addition to future astrobiology workshops
	and conferences, astrobiology may be a topic or
	session at other scientific conferences such
	as the American Society of Gravitational and
	Space Biology, COSPAR, the International
	Society for the Study of the Origin of Life,
	the Gordon Conference, and others.
	b. A Web page will be established that
	will not only present formal work but will also
	allow discussion and interaction via chat
	rooms and dialog groups. Part of this Web
	page may be an "electronic textbook" accessible
	to the public to support education efforts in
	this area. Interdisciplinary communication
	can be furthered by using web-based
	connections to allow motivated discipline
	experts better insight into other communities.
	c. A lay person's summary of the
	results of the current Astrobiology Workshop
	will be developed for presentation in a
	publicly accessible medium such as The
	 Planetary Report, Discover Magazine,
	Scientific American, etc.
	d. Special workshops will be convened for educators and students.
Appendices
Astrobiology Workshop
September 9-11, 1996
Program
Astrobiology is defined in the 1996 NASA Strategic Plan as
"The study of the living universe. This field provides a
scientific foundation for a multidisciplinary study of (1)
the origin and distribution of life in the universe, (2)
an understanding of the role of gravity in living systems, and (3)
the study of the Earth's atmosphere and ecosystems." Ames
Research Center has been assigned the lead role for astrobiology
within the agency.
In a response to the challenge from the NASA Administrator to
develop new cross-disciplinary programs and strengthen existing
efforts for the study of life in the universe, Ames will host a
scientific workshop, organized around several major questions in astrobiology:
	 How does life originate?
	 Where and how are other habitable worlds formed?
	 How have the Earth and its biosphere influenced each other over time?
	 Can terrestrial life be sustained beyond our planet?
	 How can we expand the human presence to Mars?
Session 1 (Monday morning, Conference Center Ballroom)
0800	Registration
0900	Harry McDonald 	Welcome and introductions
0930	Carl Pilcher	Astrobiology and space science
0955	Arnauld Nicogossian	Astrobiology and life science
1020	William Townsend  	Astrobiology and earth science
1045	Frank Martin	Astrobiology and exploration
1110	David Morrison	Key questions in astrobiology
1130	Lunch break
Session 2 (Monday afternoon, Conference Center Ballroom)
Co-chairs:  MRC Greenwood & David Morrison
1300	Stuart Kauffman 	Chemical and physical pathways to complexity
1340	Geoff Marcy  	Detection of planets orbiting Sun-like stars
1420	James Kasting 	Habitability of planets
1500	Chris McKay 	The search for life on Mars
1540	William Sprigg 	Near-term evolution of Earth's climate
1620	Emily Holton  	Gravity and biology
1730	Reception
Session 3 (Tuesday morning, Space Science Auditorium)
Co-chairs:  Warren Gore & Kevin Zahnle
0800	Scott Sandford 	Complex molecules in the interstellar medium
0830	John Cronin 	Organic chemistry in the early solar system
0900	David Des Marais	Evolution of the early Earth and its biosphere
0930	Brian Toon  	Extinctions due to impacts, past and future
1015	David Peterson 	Response of Earth's ecosystem to global change
1045	Ken Nealson 	Response of microbial ecosystems to global change
1115	Anne Erlich 	The current great extinction
1200	Working lunch
Session 4 (Tuesday afternoon, Space Science Auditorium)
Co-chairs:  Nancy Daunton & Charles Fuller
1330	Lewis Feldman 	Evolution of light- and gravity-sensing genes in plants
1400	Debra Wolgemuth	Vertebrate development in space:  clues and complications
1430	Muriel Ross  	Gravity sensor plasticity in the space environment
1500	Ben Levine  	Human cardiovascular adaptation to altered environments
1530	Amy Kronenberg 	Biological responses to exposure to the space radiation
			environment
Co-chairs:  Alan Hargens & Sam Pool
1615	Mike Duke  	Science and habitability goals for Mars exploration
1645	Larry Young 	Artificial gravity for human missions
1715	Scott Parazynski  	Destination Mars:  An astronaut's perspective
Session 5 (Wednesday morning, Space Science Auditorium)
Co-chairs:  Jeffrey Bada, Robert Pepin, Frank Shu, & Don DeVincenzi
0800	Jack Welch  	Observations of planetary system formation
0830	Pat Cassen  	Theory of planetary system formation
0900	Don Brownlee  	Primitive materials and planetary formation
0945	Norman Sleep  	Planetary perspective on life on early Mars and the
			early Earth
1015	David Deamer  	Origin of protocells
1045	Norman Pace 	Biological perspective on the Earth and the chemistry
			that spawned life
1115	David McKay 	Evidence for past life on Mars
1130	Jack Farmer 	Exploring Mars for evidence of past or present life
1200	Working lunch
Session 6 (Wednesday afternoon, Space Science Auditorium)
1330	Frank Drake	Intelligent life in the universe
1415	Final panel and discussion session; formulation of recommendations
1730	Adjourn
Astrobiology Workshop
ABSTRACTS
Understanding the Role of Biology in the Global Environment:
NASA's Mission to Planet Earth
William F. Townsend
Headquarters, National Aeronautics and Space Administration
Understanding our Changing Planet
NASA has long used the unique perspective of space as a means of
expanding our understanding of how the Earth's environment functions.
In particular, the linkages between land, air, water, and life-the
elements of the Earth system-are a focus for NASA's Mission to Planet
Earth. This approach, called Earth system science, blends together
fields like meteorology, biology, oceanography, and atmospheric science.
Mission to Planet Earth uses observations from satellites, aircraft,
balloons, and ground researchers as the basis for analysis of the
elements of the Earth system, the interactions between those elements,
and possible changes over the coming years and decades. This
information is helping scientists improve our understanding of how
natural processes affect us and how we might be affecting them. Such
studies will yield improved weather forecasts, tools for managing
agriculture and forests, information for fishermen and local planners,
and, eventually, an enhanced ability to predict how the climate will
change in the future. NASA has designed Mission to Planet Earth to focus
on five primary themes:
	Land Cover and Land Use Change
	Seasonal to Interannual Climate Prediction
	Natural Hazards
	Long-Term Climate Variability
	Atmospheric Ozone
Mission to Planet Earth is far more than a NASA endeavor; indeed, it is
NASA's contribution to the multi-agency U. S. Global Change Research
Program. Because the challenge of understanding the global environment
is so broad, Mission to Planet Earth is a program with extensive
contributions and collaborations with many other organizations. In
addition to providing a broader and more reliable quantity of data,
this cooperation fosters broader confidence in the results of such
research and the decisions that may be considered as a consequence of
the findings. The program benefits from active partnerships with other
nations (including Japan, ESA, Russia, Canada, Brazil, and many others),
Federal agencies (such as NOAA, EPA, USGS, and NSF) and the broad
community of researchers. NASA is also seeking to dramatically expand
its partnerships with the commercial community, recognizing the inherent
overlap between commercial and research needs as well as emerging
commercial capabilities for provision of data and application of results.
Biological Research as Part of Mission to Planet Earth
Broad-scale biological studies have long been an important part of
NASA's Earth science research. Using space as a platform from which to
observe changes in ground cover and ocean productivity enables us to
examine areas of the Earth that would be impractical (or very difficult)
to study globally from the ground. Moreover, the broader perspective
available from satellites and planes is critical to establishing how
biological changes occur over time and aid in explaining why those
changes are happening.
The last few years offer some notable examples of how biological
research from Mission to Planet Earth has significantly advanced our
understanding of the nature of biological activities and changes.
Examples include:
	Biology and Weather: Satellite, aircraft, and ground observations
	by the U. S. and Canada have revealed that the air over the boreal
	forests in northern Canada is actually much drier than previously
	expected, due to much slower release of water by the trees. These
	findings are already being used to substantially improve weather
	models (and thus forecasts) and will also help scientists better
	understand this critical ecosystem. The experiment also enabled
	accurate measurement of the rate of growth of trees over a large
	area for the first time.
	Biology and Land Use: Working closely with their Brazilian
	counterparts, U. S. researchers used satellite and
	ground observations to accurately estimate the rate of
	deforestation in the Amazon. Their studies also revealed
	surprising findings regarding the effect of such
	deforestation on biodiversity and viability of
	individual ecosystems within the rain forest. Using
	these new techniques and findings, scientists now have
	a powerful tool in better understanding forests across
	the globe.
	Biology and the Carbon Cycle: As an example of the linkage
	between biological and physical processes, NASA is
	looking at changes in forest acreage and density as a
	key source or sink in the global distribution of
	carbon, which also involves the oceans and human activity.
	Biology and Agriculture: A combination of satellite,
	aircraft, and ground data has been used to help California
	wine growers mitigate the effects of an infestation of
	phylloxera in the vineyards, thus helping to protect the
	multi-billion dollar wine industry in the state. NASA research
	has also examined the health and productivity of Chesapeake Bay
	marshes, growth rates of phytoplankton in the world's oceans,
	and the patterns of crop use and growth in America's breadbasket.
	Biology and the Oceans: Working with data from satellites
	and surface observations, scientists will be able
	to precisely track changes in phytoplankton productivity
	in the oceans; such measurements will enable researchers
	to better understand the behavior of fish, to assess
	changes in ocean chemistry, and to track water-borne
	pollution and river discharge.
Of course, research in Mission to Planet Earth is closely tied to
that of other NASA science efforts. In turn, these activities
build heavily on biological studies by other Federal agencies and
especially by the nation's universities and research centers. NASA's
primary role in this area is to explore the connections between
biological processes and other facets of the Earth system, primarily
through the collection and analysis of remote sensing data.
A New Era of Earth Observations
Having built a solid record of scientific research, Mission to Planet
Earth has entered an unprecedented period of observational capability
and international coordination. Over the coming years, NASA and its
international partners will launch dozens of spacecraft to observe
the Earth. In 1998, NASA will begin launches of the Earth Observing
System, a multi-spacecraft system which will offer the first
integrated measurements of the Earth's process. EOS will generate a
15-year database focusing on the complex issues associated with
climate change. Many of these new capabilities and research will
observe the biological components of the dynamic Earth system. Notable
among these will be instruments on the EOS AM-1 spacecraft (including
global vegetation dynamics, land cover changes, and ocean
productivity), Landsat-7 (continuing the long-term global land cover
change record), and SeaStar (examining ocean productivity for both
research and commercial customers), as well as a varied set of
international field and research efforts. In the NASA tradition,
Mission to Planet Earth is a constantly evolving program, investing
in new technology today in an effort to both increase program
capabilities and reduce program costs. It is a program that seeks
to understand the global and regional, so as to help answer questions
that are regional and local. The natural link between biological
research and Mission to Planet Earth is strong, and will only grow
stronger in the coming years.
Key Questions in Astrobiology
David Morrison
NASA-Ames Research Center
Astrobiology is a new name for a range of interdisciplinary studies
related to life in the universe. The term is defined in the 1996 NASA
Strategic Plan as "The Study of the living universe. This field
provides a scientific foundation for a multidisciplinary study of
(1) the origin and distribution of life in the universe,
(2) an understanding of the role of gravity in living systems, and
(3) the study of the Earth's atmosphere and ecosystems."
Concepts associated with life in the universe have a long and
checkered history in science. Giordano Bruno was burned at the
stake in part for his speculation concerning other inhabited worlds,
a fate unlikely to be associated today with astrobiology. By the late
nineteenth century the pendulum of public thinking had swung to the
opposite extreme, with a widespread belief (supported by Percival
Lowell and other astronomers) in the presence of intelligent life
on Mars. If we can judge by the science fiction of the time, our
great-grandparents also believed that humans could survive travel
in space and explore other worlds like the Moon with relative
impunity. By the mid-twentieth century, however, these optimistic
ideas had been largely abandoned. Scientists may have been disappointed
by the negative results of the Viking life-detection experiments on
Mars, but they were not really surprised. And when NASA first began
to consider human space flight, it was felt necessary to send
chimpanzees into space first to verify survivability in the space
environment, even for flights of only 15-minute duration.
Today the pendulum is swinging back. We have analyzed complex organic
chemistry in interstellar clouds of gas and dust and have discovered
other planets circling distant stars. On Earth, life has been found
at environmental extremes from the Antarctic ice to boiling hot
springs, and from thermal vents in the deep ocean to aquifers buried
kilometers below the land surface. We know that liquid water, the one
essential ingredient for life as we know it, once flowed on the
surface of Mars and probably exists today below the icy crust of
Europa. Life on Earth has been traced back 3.8 billion years to
the period of heavy cometary bombardment, an era that simultaneously
brought life-giving water and organic compounds to the terrestrial
planets while battering them with lethal quantities of impact
energy. We are discovering both the fragility and the robustness
of life, as we investigate the history of mass extinctions on our
planet (including the extinction taking place today), the subtle
alterations in climate triggered by both volcanic eruptions and
human industry, and the destruction of our protective shield of
ozone by non-toxic, chemically inert, and cheap industrial chemicals.
As more and more humans venture into space, we celebrate their
ability to live and work and achieve wonderful feats of engineering
in this hostile environment, at the same time facing baffling
physiological and chemical changes experienced by astronauts on
long-duration missions. Space and life interact on many levels
and time scales, in ways that we are only beginning to explore.
Astrobiology is playing an increasing role in a wide variety of
NASA programs. The primary objectives of the Earth Observing System
are related to the effects of human activity on the climate,
atmosphere, and ecosphere of our planet. We are building the
international space station in part to learn how humans can live and
work for long periods in space. The centrifuge facility being
constructed for the space station will provide a unique facility for
basic science related to the effects of microgravity on living
things. We have located 3.6-billion-year-old evidence in a martian
meteorite that suggests a warm wet climate on Mars and hints of
biological activity. We are planning a series of a dozen spacecraft
to Mars, including sophisticated surface rovers and sample return,
much of this work motivated by the search for ancient life on that
planet. We are accelerating the search for other planets, focused
eventually on the discovery of a "pale blue dot," an inhabited planet
in some distant solar system. Generous private donations are supporting
the most comprehensive search ever undertaken for radio signals from
intelligent life in the Galaxy. Life in the cosmos strikes a resonant
chord with the public and the science community alike.
We are here today for the world's first scientific conference on
astrobiology, to help define this field and to develop new
cross-disciplinary programs for the study of life in the universe.
The meeting has been organized around five key questions that
illustrate the breadth of this subject:
	 How does life originate?
	 Where and how are other habitable worlds formed?
	 How have the Earth and its biosphere influenced each other over time?
	 Can terrestrial life be sustained beyond our planet?
	 How can we expand human presence to Mars?
This is just one of many ways of looking at astrobiology. Many studies
can be included under this umbrella. Most of these are not new. What
may be new is the emphasis we are trying to bring to the
interdisciplinary aspects of these studies. We seek to bring
life scientists and physical scientist together, to help break
down the barriers that exist between academic disciplines. If we
can talk to each other, we are bound to gain insight and generate
new ways of looking at old problems. That is the primary purpose of
this astrobiology workshop.
Detection of Planets Orbiting Sun-Like Stars
Geoffrey W. Marcy and R. Paul Butler
San Francisco State University and
University of California, Berkeley
During the past 11 months, astronomers have finally discovered planets
orbiting Sun-like stars. A total of eight planets has been detected by
the Doppler technique, and there are possible planets detected by
astrometry around one other star. Some of the new planets exhibit
properties similar to those in our Solar System. But many of them
have properties that were unexpected. Several planets are more
massive than Jupiter, and some orbit their host star in orbits
smaller than Mercury's orbit. Equally unexpected is that three
of these planets have non-circular orbits. Current theory of the
formation of planetary systems is challenged to account for these
new planetary properties, but several models are emerging,
involving gravitational scattering of planetesimals and viscous
or tidal decay of orbits. The occurrence rate of true analogs of
our Solar System will soon be determined with the detection of
long-period gas giants analogous to Jupiter.
During the upcoming five years, several new techniques for detection
of extrasolar planets will be tested and implemented. Most
immediately, the Keck 10-meter telescope will advance the proven
Doppler and astrometric (MAP) techniques to find smaller giant
planets in surveys of about 400 stars. The Keck surveys will be
sensitive to Saturn-mass and Jupiter-mass planets within 5 AU, as
well as the close-in "51-Peg-like" planets. Ground-based interferometry
will begin this year (at Palomar) to provide astrometric precision of
50 milliarcsec, enabling detection of Neptune-mass planets within
5 AU. A Keck version of this interferometry, using both Kecks, may
achieve precision of 20 milliarcsec, enabling detection of
sub-Neptune planets. Direct detection of planets from the ground is
being explored by several groups using ground-based super-AO
(R. Angel, C. Max). Balloon-borne telescopes may supersede these
efforts cheaply and quickly to make direct detections.
Habitability of Planets
James F. Kasting
Penn State University
The habitable zone (HZ) around a star is defined as the region in which
an Earth-like planet could support liquid water. The continuously
habitable zone (CHZ) represents the overlap of the HZs at two different
instants in time. HZs move outward with time because main sequence
stars get brighter as they age. The inner edge of the HZ is set by
loss of water by way of photodissociation followed by escape of
hydrogen to space. A conservative (i.e., pessimistic) estimate for
the solar flux at which this phenomenon occurs is 1.1 S0, where S0
is the present solar flux at Earth's orbit, 1370 W/m2 (1). The outer
edge of the HZ is set by CO2 condensation, which shuts off the
stabilizing feedback provided by the carbonate-silicate cycle. Within
the HZ, atmospheric CO2 concentrations should increase with orbital
distance as a consequence of this cycle. A conservative estimate for
the solar flux at the outer edge of the HZ is 0.53 S0 (1). In terms
of distance, the HZ for our own Solar System extends from at least
0.95 AU to 1.37 AU, and the 4.6-Gyr CHZ extends from at least 0.95 AU
to 1.15 AU. Corresponding fluxes and distances for other types of
stars are tabulated in ref. (1).
The number of planets expected to lie within the HZ or CHZ around a
given star depends on how planets are spaced and on how rapidly the
star evolves. If planets are spaced geometrically, as they are in our
own Solar System (i.e., according to Bode's Law), then roughly equal
numbers of instantaneously habitable planets are expected around all
types of stars. Early-type stars (type O, B, and A) will have fewer
planets that remain continuously habitable because they evolve in
luminosity much more rapidly than does the Sun. Late-type stars (late
K and M) may have few continuously habitable planets because their HZs
lie within the tidal locking radius of the star. Any potentially
habitable planets are likely to become locked in synchronous rotation,
allowing their atmospheres to condense out on their dark sides. Stars
not too different from the Sun (early F to mid K) have about
a 50% chance of harboring a long-lived habitable planet if Bode's
Law spacing is obeyed (1).
A glaring deficiency in the climate model on which these predictions
are based is that it fails to account for the apparently warm climate
of early Mars (2,3). Either greenhouse gases other than CO2 and H2O
were important, or some other aspect of the Martian climate system
has been poorly represented. New ideas about how to solve this problem
will be discussed if circumstances permit. In any case, the existence
of a warm climate on early Mars implies that HZ around a star is
probably wider than has been calculated to date.
The exciting development in this field is that it now appears
feasible to look for Earth-sized planets around other stars using
a space-based, infrared interferometer and to examine their atmospheres
spectroscopically (4). The practical way to search for life on
extrasolar planets is to look for the 9.6-m band of O3 (5). O3
is a sensitive indicator of atmospheric O2, and O2 is, under most
circumstances, a strong indicator of photosynthetic life. Exceptions
to this rule include planets on either side of the HZ, as such planets
can conceivably accumulate large amounts of O2 abiotically (7). A
planet with O3 in its atmosphere and liquid water on its surface,
however, is very likely to be inhabited. The fact that we are on
the verge of being able to identify such planets suggests that
building such an instrument should be a top NASA priority.
References:
1. J. F. Kasting, D. P. Whitmire, and R. T. Reynolds, Icarus 101:108-128 (1993).
2. J. F. Kasting, Icarus 94:1-13 (1991).
3. S. W. Squyres and J. F. Kasting, Science 265:744-749 (1994).
4. J. R. P. Angel and N. J. Woolf, Scientific American, April, 60-66 (1996).
5. A. Leger, M. Pirre, and F. J. Marceau, Astron. Astrophys. 277:309-313 (1993).
6. J. F. Kasting, H. D. Holland, and J. P. Pinto, J. Geophys. Res. 90:10,497-10,510 (1985).
7. J. F. Kasting, in Proceedings of NASA Ames Bluedot Workshop, D. Des Marais, ed.,
	June, 1996.
The Search for Life on Mars
Christopher P. McKay
NASA-Ames Research Center
Although the Viking results may indicate that Mars has no life today,
the possibility exists that Mars may hold the best record of the events
that led to the origin of life. There is direct geomorphological
evidence that in the past Mars had large amounts of liquid water
on its surface. The Mars meteorites also suggest that early Mars
was wet with conditions suitable for organic material. Atmospheric
models would suggest that this early period of hydrological activity
was due to the presence of a thick atmosphere and the resulting
warmer temperatures. From a biological perspective, the existence
of liquid water by itself motivates the question of the origin of
life on Mars. From studies of the Earth's earliest biosphere we know
that by 3.5 Gyr. ago, life had originated on Earth and reached a fair
degree of biological sophistication. Surface activity and erosion on
Earth make it difficult to trace the history of life before the
3.5 Gyr. time frame. If Mars did maintain a clement environment for
longer than it took for life to originate on Earth, then the question
of the origin of life on Mars follows naturally.
Near-Term Evolution of Earth's Climate
William A. Sprigg
National Research Council
Earth's climate is in constant evolution. Our view of a climatic
optimum has existed for but a brief period in the history of climate
so far as we have been able to reconstruct it. In the near term,
climate will likely continue in this climatic optimum. But "the devil
may be in the details."
Current research to understand monthly to decadal time-scales of
climate variability attempts to unravel the intricate processes of
variability and change. Anticipating the monthly, seasonal, and
interannual variability of climate over the coming decades would
be valuable, indeed. The challenge to do so reaches into many
disciplines of science and engineering.
Climatology entered a new era in the early 1970s. El Nino, an
aperiodic warming of water in the central and eastern equatorial
Pacific Ocean, became a target of intense scientific interest. Long
rooted in Peruvian folklore and economy, El Nio was seen connected
to a complex, interactive ocean-atmosphere system. El Nio became a
rallying point for multidisciplinary study. The hope for seeing
predictability in the system became a reality. And the several
cooperating disciplines matured into a broadly international,
interdisciplinary science. The study of this system continues. Now
have climatologists, in partnership with doctors, epidemiologists,
plant and animal ecologists, found another rallying point for
interdisciplinary research?
Infectious diseases are the world's leading cause of premature death.
While it is widely known that living conditions and sanitation habits
are most important in determining human health, finding the extent to
which weather and climate are a factor is important also. Drought can
lead to famine, weakened immune systems, and disease. And disease
outbreaks have been linked to variations in weather and climate.
Eruption of meningococcal meningitis follows the timing of dry and
wet seasons in the African Sahel. Where carriers, or vectors, of
disease are involved, weather and the ecosystem are principal
factors, as in the 1993 outbreak of hantavirus in the Southwestern
United States. Some vectors, such as the mosquito Aedes aegypti,
the carrier of dengue fever, are geographically limited by temperature
or other environmental condition. Observations, analyses, and forecasts
of weather, climate, and plant and animal ecology can assist in the
control of disease vectors, in activating stepped-up disease
surveillance strategies, and in developing health-care services.
A new interdisciplinary rallying point is suggested. It could begin
by understanding the legacies and lessons learned from studies of El Nino.
Gravity and Biology
Emily R. Morey-Holton
NASA-Ames Research Center
Gravity has been the most constant environmental factor throughout the
evolution of biological species on Earth. Organisms are rarely exposed
to other gravity levels, either increased or decreased, for prolonged
periods. Thus, evolution in a constant 1G field has
historically prevented us from appreciating the potential biological
consequences of a multi-G universe. To answer the question "Can
terrestrial life be sustained and thrive beyond our planet?" we need
to understand the importance of gravity on living systems, and we need
to develop a multi-G, rather than a 1G, mentality.
The science of gravitational biology took a giant step with the advent
of the space program, which provided the first opportunity to examine
living organisms in gravity environments lower than could be sustained
on Earth. Previously, virtually nothing was known about the effects of
extremely low gravity on living organisms, and most of the initial
expectations were proven wrong. All species that have flown in space
survive in microgravity, although no higher organism has ever completed
a life cycle in space. It has been found, however, that many systems
change, transiently or permanently, as a result of prolonged exposure
to microgravity.
Although our knowledge of the biological consequences of spaceflight
has increased significantly, we have only snapshots of biological
changes in multiple species. Humans adapt to spaceflight with fluid
redistribution, altered cardiovascular function, disrupted
visual/sensory/motor orientation, transient anemia, loss of muscle
mass, and site-specific bone loss. Juvenile rats adapt to altered
gravity with changes in many physiological systems resembling the
adaptation process of humans. Suckling rat pups flown in space for
two weeks during early development and then returned to Earth appear
to have a permanent locomotor deficiency. Pregnant rats flown in space
for 11 days and returned to Earth gave birth to pups with exaggerated
head movements in response to directional changes for the first few
postnatal days. Quail eggs fertilized on Earth and incubated on Mir
did hatch, but the hatchlings did not develop the skills necessary
for prolonged survival. Frog eggs fertilized in space showed evidence
of defects that would be predicted to be lethal or at least seriously
harmful in larvae, yet the tadpoles appeared normal externally when
they hatched. However, the tadpole lungs were smaller until they
returned to Earth and began to gulp surface air. Plants are known to
respond to gravity; roots grow in the direction of the gravity vector
and away from sunlight, while shoots grow against gravity and toward
sunlight. The Russians have grown plants for a single generation on
Mir with few notable changes, yet some shuttle experiments suggest
that chromosomal abnormalities may occur.
Biological changes are observed even when cells are isolated from the
whole organism and grown in culture. Most scientists predicted this
would not occur since cells were considered too small to be affected
by gravity changes. Recent shuttle results suggest that flight may
alter the characteristics of cultured cells. For example, spaceflight
may reduce the activity of bone cells, decrease the ability of muscle
cells to fuse upon return to Earth, and decrease the mass of muscle
fibers by decreasing the rate of protein synthesis. These data suggest
that spaceflight directly affects cells and tissues even when isolated
from systemic factors.
Modern molecular techniques and spaceflight hardware are providing
tools for future space research. Recent studies with fish eggs are
providing a new understanding of vertebrate embryonic development in
space through real-time visualization, which ultimately may allow
monitoring gene expression in vivo while the embryos are developing.
Advances in cell membrane biology and identification of molecules
that attach both to the internal cell skeleton and to the external
matrix have focused interest on the potential importance of this
system as the possible transducer of mechanical signals to biological
effects at the cell nucleus. Development of science and hardware for
Space Station and beyond should allow us to better understand the role
of gravity in biology.
In summary, data from flight experiments suggest that some organisms
may only be transiently affected by decreased gravity while others
may require gravitational loading for normal development and even
survival. Gravitational biology research in space has revealed the
inadequacies of our understanding about life on Earth, caused major
revisions in widely held scientific assumptions, and opened new
avenues of scientific inquiry.
Ames and its university and industry collaborators pioneered the field
of gravitational biology, frequently inventing the techniques and
technologies that made biological research in space possible. We have
created an inventory of specialized flight equipment that allows
frequent, low-cost access to space for research. Ames has a
capability unmatched in the world for the study of gravitational
effects on living systems, providing the international science
community with access to instruments and facilities that allow a
broad range of living systems-from single cells through plants and
animals to humans-to be studied across the full range of the gravity
spectrum, from the microgravity of orbital space to hypergravity on
centrifuges. The International Space Station (ISS) will provide
continuous access to space with accommodations for the most
sophisticated array of biological laboratory equipment ever flown.
ISS will enable the first multi-generational studies of living
organisms in extraterrestrial environments. The centrifuge and
gravitational biology facilities being developed for ISS will
provide long-duration laboratory facilities for scientists. The
centrifuge can be used not only as an onboard 1G control but also
to simulate gravity levels on the Moon and Mars, to determine the
gravity level required for stimulating physiological systems, and
to provide data on intermittent gravity as a potential countermeasure.
These Space Station facilities will not only allow researchers to
examine unknown areas of terrestrial physiology to benefit life on
Earth but will also allow scientists to evaluate the potential of
terrestrial life to thrive in environments beyond Earth as a prelude
to exploration of the solar system.
Gravitational biology research conducted beyond Space Station and
during explorations of Mars missions would allow us to address
profound questions about our future: Is terrestrial life limited
to Earth? Or can we-if we choose or if we must-live, reproduce, and
thrive on other worlds?
Complex Molecules in the Interstellar Medium
Scott Sandford
NASA-Ames Research Center
I will briefly review our current state of knowledge concerning the
composition of complex molecules in the interstellar medium (ISM).
Given the limited time available, it will not be possible to give a
full discussion of the composition of everything in the ISM, and I
will largely restrict myself to the discussion of materials in
interstellar dense molecular clouds, since they are where stars and
planetary systems form. I will also concentrate largely on solids
because they contain a major fraction of the heavier elements in these
clouds and because these materials are the most likely to survive
incorporation into new planetary systems and participate in the subsequent
formation and evolution of life. However, dense clouds are not
well-defined, long-lived entities but are dynamic objects that are
formed from materials in the diffuse ISM and destroyed on time scales of
106-108 years. As a result, materials in space are probably constantly
being mixed between the dense ISM and the more diffuse intercloud ISM,
and some discussion of the materials found in the diffuse ISM is also
merited.
It has been known for some time that the interstellar medium (ISM)
contains gas and dust. This was initially demonstrated in 1930 by
Trumpler, who noted that disparate measurements of interstellar
distances could only be explained by the presence of a general
"absorption" produced by the interstellar medium itself. However,
it was another 30 years before the first molecules were detected by
optical and radio spectroscopy. Of these techniques, spectroscopy at
millimeter and radio wavelengths has proven to be one of the most
powerful techniques for the detection of molecules in the gas phase.
The current list of gas phase molecules uniquely identified in space
using radio techniques contains close to 100 molecules ranging in size
from 2 to 13 atoms. These molecules are believed to be formed by a
combination of ion-molecule reactions and gas-grain surface reactions
in dense molecular clouds. An additional family of carbon-carrying
molecules, polycyclic aromatic hydrocarbons, has also been identified
in the gas phase by their characteristic infrared emission features.
While radio astronomy has amply demonstrated that the interstellar
medium contains molecules and must therefore support ongoing
chemistry, it is only capable of identifying materials in the gas
phase. The composition of the solids in the interstellar medium
remained essentially a mystery until the advent of infrared astronomy,
particularly through the use of infrared spectroscopy. We now know that
a large portion of the heavier elements in both the diffuse and dense
ISM are in molecular form in solid grains and (in the dense clouds) in
ices, rather than in the gas phase. The presence of ices in dense
clouds is not surprising since the grains in these clouds typically
have temperatures of less than 40 K. At these temperatures, most gas
phase atoms and molecules will freeze out onto the grains.
The formation of ice mantles on interstellar grains leads to several
additional chemical processes. First, the condensation process itself
generates new species through gas-grain reactions. Such reactions can
lead to several different classes of molecules. Grains in environments
where H/H2 > 1 will produce hydrides like NH3, H2O, CH4, etc.
Environments where H/H2 < 1 will produce mantles that more closely
reflect the abundance of molecules in the gas phase and will contain
species like O2, N2, CO, and CO2. Recent infrared telescopic and
laboratory studies of the CO ice band indicate that these disparate
types of environments probably do exist in dense clouds. Some of the
more common and better identified constituents of interstellar ices
are H2O, CH3OH, CO, CO2, H2, CH4, XCN, HCO, H2CO, and OCS. In
addition, there exists good indirect evidence for the abundant presence
of a number of other molecules, including N2, O2, and NH3.
A second way in which the presence of ices leads to additional
chemistry is through their interaction with ionizing radiation in the
form of high-energy charged particles and ultraviolet photons.
Exposure of mixed-molecular ices to UV and cosmic ray irradiation and
subsequent warming results in the destruction of some of the original
ice species and the creation of new and more complicated compounds.
Laboratory studies indicate that such processes can be expected to
produce, among other things, ethers, alcohols, nitriles, and
isonitriles, amides, ketones, compounds related to polyoxymethylene
(POM), and hexamethylenetetramine (HMT). The latter is of interest
since it is known to produce amino acids when placed in acidic
solutions. There is already some evidence for some of these compounds
in the interstellar medium. For example, the infrared absorption
spectra of protostars often show an absorption band near 2165 cm-1
characteristic of the CN stretch in a nitrile or isonitrile. This
band is produced in virtually all laboratory ice analogs irradiated
in the laboratory. In addition, it has been demonstrated that more
than 5% of the carbon in the diffuse ISM resides in grains rich in
aliphatic organics. While the exact nature of this material has yet
to be determined, it has a CH2/CH3 ratio of ~2.5, implying a
substantial degree of complexity. The profile of the C-H stretching
feature of this material is quite similar to some of the more
abundant carbonaceous components of meteorites and the refractory
organic residues produced when interstellar ice analogs are
photolyzed in the laboratory.
While these molecules are not complex by the standards of molecules
in living systems, they represent a major step up the ladder of
complexity from the simple 2-, 3-, and 4-atom molecules many
people normally associate with space. The presence of this
molecular complexity may have played a significant role in terms of
the seeding and maintenance of early life on the Earth.
Carbonaceous Chondrites: A Window on
Organic Chemistry in the Early Solar System
J. R. Cronin
Arizona State University
Origin of life hypotheses postulate prior formation of some minimum
set of molecular components by strictly abiotic processes. In most
cases, the spontaneous synthesis of organic compounds of considerable
complexity is proposed. Knowledge acquired over the last 40 years of
stellar, interplanetary, and planetary processes now allows us to
place organic chemical evolution in a cosmochemical context in
which each of these locales plays a unique and significant role
in the development of organic structural complexity. The organic
matter of carbonaceous chondrites provides important insights to
these processes by bridging the gap between stellar and interstellar
chemistry, and the final steps that were played out on the surface of
the early Earth. The study of these primitive meteorites allows us to
see how stellar and interstellar processes are made manifest in/on
planetary bodies and also provides a data base which might prove
useful in extrapolating toward the as yet uncharted territory of a
pre-RNA world.
The organic carbon indigenous to carbonaceous chondrites is found both
in an extended macromolecular phase and in a complex suite of soluble,
low molecular weight compounds. Since the fall of the Murchison
meteorite (1969), analytical work from several laboratories has
contributed to a detailed, although still incomplete, inventory of
these compounds [1]. Many common biomolecules have been found-e.g.,
eight of the 20 protein amino acids; however, meteoritic organic
compounds are, in general, distinctive and show (i) complete or nearly
complete structural diversity, (ii) exponentially declining concentrations
within homologous series, (iii) predominance of branched chain isomers, and
(iv) unusually high contents of 2H (D), 13C, and 15N relative to
terrestrial organic matter.
The finding of substantial stable isotope enrichments in meteoritic
organic compounds suggests that they are, or are closely related to,
interstellar organic matter [2]. Taking into account the fact that
the organic-rich I- and M-type carbonaceous chondrites are uniquely
those that have experienced aqueous processing suggests the following
formation hypothesis: (i) accretion of the parent body from
volatile-rich, icy planetesimals containing interstellar organic
matter, (ii) warming of the parent body and generation of an aqueous
phase in which the interstellar organics underwent various reactions,
and (iii) mild thermal processing with eventual loss of residual
volatiles, leaving the suite of nonvolatile compounds that now
characterize these meteorites.
Meteorites have delivered organic matter to the Earth throughout
its existence, and it is possible that this organic matter represents
an essential step in a continuous process of organic chemical
evolution that began in the presolar molecular cloud and continued
prebiotically and biologically on the early Earth. If so, it is also
possible that meteoritic organic matter may hold clues to the origin
of chiral selectivity, a question fundamental to understanding the
origin of life. The suggestion that UV synchrotron circularly polarized
light emitted by neutron stars might introduce an enantiomeric bias
into the organic matter of interstellar clouds [3] has led us to seek
evidence for remnant enantiomeric excesses in the organic matter of
carbonaceous chondrites. Prior analyses of the optical activity of
meteorite extracts and the enantiomeric ratios of meteorite chiral
compounds have given negative or unconvincing results. We have renewed
the search for enantiomeric excesses, focusing on molecular chiral
centers that (i) might have been formed in the presolar molecular cloud
or within the parent body, (ii) would have avoided racemization during
aqueous alteration of the meteorite parent body, and (iii) are located
in molecules that are unlikely to have been contaminated by corresponding
terrestrial compounds. Analyses of selected chiral amino acids from the
Murchison meteorite suggest L-enantiomer excesses of the order of 5-10%.
In general, the finding of enantiomeric excesses in extraterrestrial
molecules supports the hypothesis that exogenous delivery made a
significant contribution to organic chemical evolution leading to
the origin of life. The finding of these enantiomeric excesses
specifically in a-substituted amino acids may have implications for
the chemistry of a pre-RNA world insofar as it suggests the possibility
that these unusual, but meteoritically abundant, amino acids were early
biomonomers.
[1] 	Cronin J. R. and Chang S. (1993) in The Chemistry of Life's
	Origins (J.M. Greenberg et al., eds.) Kluwer, pp. 209-258.
[2] 	Epstein S. et al. (1987) Nature, 326, 477-479.
[3] 	Bonner W. A. and Rubenstein E. (1987) BioSystems, 20, 99-111.
Evolution of the Early Earth and Its Biosphere
David J. Des Marais
NASA-Ames Research Center
The history of life on Earth is a rich tapestry of adaptation and
innovation which was shaped, at least in part, by the changing surface
environment of our planet. To the extent that all rocky planets have
followed similar evolutionary paths, studies of our own biosphere can
guide us in our search for extraterrestrial life. Understanding the
nature, timing, and causes of long-term changes in the global
environment is a key objective.
Our surface environment reflects a delicate balance between processes
operating within the Earth and those processes which govern the exchange
of matter and electromagnetic radiation between the planet and outer
space. Solar radiation and the heat flow from Earth's interior are the
two most important agents which impinge upon and influence the surface
environment. Over the past 4 billion years, solar luminosity has
increased by almost 30 percent and heat flow has declined about
threefold. These changes affected the atmosphere dramatically. Given
the geologic evidence that liquid water existed 3.8 billion years ago
and that the sun was less luminous at that time, the ancient
atmosphere's greenhouse properties, hence its composition, must have
differed substantially from today. A combination of higher levels of
the greenhouse gases carbon dioxide (CO2) and methane (CH4) seems most
likely. In addition, a higher interior heat flow caused reduced
volcanic species to be emitted at much higher rates, rates sufficient
to overwhelm any source of molecular oxygen (O2). Thus, 4 billion
years ago, the atmosphere had perhaps tens of percent of CO2, a few
tenths of a percent or less of O2, and also nitrogen, water vapor,
and hydrogen.
Plate tectonics, driven by the heat flow engine, transformed the
continents from smaller, thinner, less stable sheets into stabilized,
thickened, and areally more extensive platforms. Early life both
adapted to and exploited the atmospheric changes and the transformations
in crustal configuration. To compensate for a declining source of
reduced species for energy and the synthesis of organic compounds,
life invented photosynthesis. To adapt to the changing atmosphere,
life invented respiration and various strategies for mitigating the
toxic effects of O2 and the declining availability of CO2. Ultimately,
complex cells developed which were the single-celled eukaryotic
forerunners of plants and animals. These cells actually required
a dependable supply of O2.
Important changes over shorter, 100-million-year time scales also
influenced our biosphere's evolution. Tectonic processes, which are
crustal movements driven by heat flow, altered the continental
configurations and global climate. It is already well-known that
continental movements have influenced the isolation and development
of plant and animal species. Even billions of years ago, continental
breakups and collisions influenced the processing of the biologically
important elements carbon (C), sulfur (S), phosphorous (P), and
probably nitrogen (N). Because C and S exist in the crust in both
oxidized and reduced forms, the tectonic processing of their crustal
reservoirs has affected the composition and oxidation state of the atmosphere.
As changes in the global environment become better documented, we
should explore in more detail their implications for the evolution of
the early biosphere. Some interesting correlations are already evident.
The bacteria which tolerate the highest temperatures live in volcanic
hydrothermal environments and are most closely related to the ancient
common ancestor of life. Evidence of O2-requiring eukaryotic cells
first appears about 2.1 billion years ago, at the time when atmospheric
O2 levels increased substantially. The CO2-fixing enzyme in algae and
green plants is more highly selective for CO2 in the face of high O2
levels, than is the "more primitive" bacterial CO2-fixing enzyme.
Biosynthetic pathways which require O2 have apparently evolved relatively
more recently than other, non-O2-requiring pathways. Thus, major trends
in the evolution of the early, unicellular biosphere appears to have
been broadly compatible with presently-known changes in the global
environment.
Perhaps lessons learned from Earth's early biosphere can be applied to
the search for Martian life. Clearly Mars' environment has also witnessed
dramatic changes. For example, as the volcanism waned and surface
temperatures may have declined on Earth, thermophilic bacteria retreated
deeper into Earth's crust and to a few scattered near-surface hot sites
associated with volcanism. Did the same retreat occur on Mars? Did life
survive the retreat?
The recent studies of Martian meteorite ALH84001 established that
mineralogical and chemical (including organic) information can be
preserved even at the submicron scale. What can fossil information
recorded at this spatial scale in terrestrial samples tell us about
ALH84001 or about early life on Earth?
Human activities presently contribute to global environmental change.
Records of past change on Earth should help us to predict the
consequences of our activities, both for Earth's climate and for our biosphere.
Response of Earth's Ecosystem to Global Change
David L. Peterson
NASA-Ames Research Center
The Earth is in the midst of rapid and unprecedented change, much of it
caused by the enormous reproductive and resource acquisition success of
the human population. For the first time in Earth's history, the actions
of one species-humans-are altering the atmospheric, climatic, biospheric,
and edaphic processes on a scale that rivals natural processes. How will
ecosystems, involving those manipulated and managed by humans largely for
human use, respond to these changes? Clearly ecosystems have been
adjusting to change throughout Earth's history and evolving in ways
to adapt and to maintain self-organizing behavior. And in this process,
the metabolic activity of the biosphere has altered the environmental
conditions it experiences. I am going to confine this presentation to
a few thoughts on the present state of terrestrial ecosystems and the
urgency that changes in it is bringing to all of us.
Virtually all of human population growth occurs within the terrestrial
ecosystems, as the ever-expanding human civilization brings the goods
and services of the ecosystems under their control to sustain and
improve the human condition. This fact is exerting tremendous pressures
on Earth's life support capacity. Many of the environmental problems
that challenge human society are fundamentally ecological in nature.
In response to this situation, the Ecological Society of America proposed
the Sustainable Biosphere Initiative (Lubchenco et al. 1991), an ambitious
call-to-arms to focus ecological research in three areas: global change,
biological diversity, and sustainable ecological systems. Recent analyses
by Kates et al. (1990) concluded that humans now dominate roughly half of
the ice-free terrestrial surface, and by Vitousek et al. (1986) estimate
that nearly 40% of potential terrestrial net primary productivity is
being appropriated for human use. One of the consequences of this
alteration of the landscape is to force the remaining millions of
species (plant and animal) into a much smaller and often fragmented
fraction of land and resources.
Much of this human-dominated land is devoted to food production.
Virtually all arable land is now in agricultural production or being
lost from production due to mismanagement. Over the past 50-60 years,
progress in agricultural science and practices (the Green Revolution)
has helped to keep production in pace with population growth. Until
recently! A disturbing trend in some recent analyses has indicated a
flattening of per capita grain production (only eight grains fulfill
the majority of food production) over the past seven years (Brown et
al. 1990). And there are disturbing patterns of inequities in food
delivery due to droughts, distribution, corruption, and severe weather.
Even as late as 1986, many agricultural analysts were predicting that
the Earth could sustain population growth by another 2 billion to 2030,
thanks to the kinds of technological innovations which typified the
Green Revolution. One of the victims of this landscape fragmentation
and simplification into just a few crop and forest types is the
irreversible loss or decline of biodiversity. Loss of biodiversity
has a multitude of consequences including certain loss of medicinally
useful plants, loss of ecosystem interactions through the modification
of the genetic composition of populations and species, and reduced
biotic control on competing populations with sometimes adverse effects
such as disease outbreaks. Despite the importance of land use change,
no global inventory of land cover, its change, much less the uses to
which land is being put, exists at the scale at which the billions of
individual actions alter the land. This is one of the five priority
science strategy themes of Mission to Planet Earth (NASA HQ, 1996).
One of the principal ways by which the biosphere affects the climate
systems is through feedback of biogenic emissions to atmospheric
chemistry. The exchange of carbon dioxide between the biosphere and
the atmosphere is dominated by oxygenic photosynthesis, and this annual
cycle is clear in continuous measurements of CO2 partial pressure
(Keeling et al. 1989) and in models of net photosynthesis and
respiration (Potter and Klooster 1996). Isotopic analyses indicate that
the cause for the steady increase in CO2 is attributable almost entirely
to the combustion of fossil fuels. CO2 concentrations have fluctuated
throughout history, across glacial and interglacial periods. However,
the recent climb to over 300 ppm is unprecedented in both its magnitude
and its rate of change (Barnola et al. 1987). In general, the biosphere
is in a state of dynamic equilibrium between assimilation and respiration
on an annual time scale. This sudden increase in CO2, sometimes called a
fertilization effect, is projected to have ecosystem effects which range
from competitive advantages for C3 plants over C4 leading to changes in
ecosystem structure and function, increased water and nutrient use
efficiencies leading to lowered C:N ratios in foliage and effects on
herbivore populations, and increased sequestration of carbon in biomass.
There is some carbon budget and transport evidence (Tans et al. 1990)
that a missing sink of carbon exists in the northern terrestrial
ecosystems that as yet remains undiscovered. And recent analyses by
Keeling's group indicate the timing of seasonality in northern ecosystems
is lengthening, implying a potential sudden shift in the range of these
ecosystems (Keeling et al. 1996).
As a global greenhouse gas, the abundance and radiation cross-section of
CO2 makes it the largest contributor to greenhouse warming. However,
several other gases, notably nitrous oxide and methane, though lower
in abundance have much higher radiation cross-sections, and their
concentrations in the atmosphere are largely controlled by ecosystem
processes. Methane is generated in ecosystems from the metabolism of
anaerobic organisms and emitted through several physical and biological
processes. Inundated lands in the tropics, in the polar regions, and in
agriculture (rice fields) account for the major sources, while biomass
combustion contributes a lesser amount. Changes in nitrogen
biogeochemistry, particularly the man-made production of nitrogen
fertilizer which is nearly equal to natural processes, is contributing
to increases in N2O. Nitrous oxide is very stable and long-lived in the
troposphere, where it acts as a greenhouse gas, and when it makes it to
the stratosphere, it removes ozone. Since removal processes are slow,
any increases now will stay in the atmosphere for many years to come.
Human actions alter the nitrogen cycle through fixation of nitrogen in
fertilizer and its over-application to crops in excess of demand
(resulting in leakage of N2O to the atmosphere), through fossil fuel
burning, and through land conversion, exposing soils by removing
plants to take up nitrogen and encouraging nitrification/denitrification
to liberate N2O. The effect of these two gases, plus CO2, is toward
global warming. One of the initial international policy efforts
is to stabilize the global concentrations of these trace gases by the
year 2000 (IPCC 1995).
The combination of shifts in climate (rainfall redistribution;
precipitation timing, duration, state, and chemistry; air temperature
increases; weather extremes; changes in cloudiness) coupled with
alterations in nitrogen availability can lead to many forms of
ecosystem response. In the cold regions, temperature increases lead to
more rapid nutrient turnover and decomposition, increased plant growth,
 and richer foliage (C:N decreases), accompanied by a loss of insulating
moss on the forest floor, potentially leading to release of the large
quantities of storage carbon below ground (Hom 1986). Ecosystem studies
in the northeast U.S., receiving acidic precipitation from upwind power
plants in the Midwest, have found ecosystems respond to this excess nitrogen
by increases in foliar nitrogen content, increased productivity, and
eventually lowered lignin content, making the plants potentially more
susceptible to insect attack (Aber et al. 1989). In California, for
example, positioned at the convergence of two different climate systems
(monsoonal and arctic), changing patterns of precipitation and higher
temperatures would result in the migration of communities to higher
elevations and northern latitudes, replacement of Douglas fir-a high value
commercial species-by Ponderosa pine, and greater susceptibility to insect
and fire effects (California Energy Commission, 1989). As these ecosystems
move up, they also force out relict alpine communities. Species migration
is very slow, hampered by biological enertia, as seen in pack rat middens
in the Grand Canyon, indicating that the present community took almost
2000 years to establish after the end of the last ice age.
The changes I have mentioned thus far represent only a few of the
serious challenges we must confront. A short list would also include:
a) increasing ultraviolet radiation as ozone thins in the stratosphere;
b) species migrations and invasions as alien plants and animals catch a
ride on modern modes of transportation; c) salinization of croplands due
to irrigation; d) disturbance of watersheds leading to freshwater quantity
and quality changes; e) overgrazing of grasslands producing albedo changes
with a positive feedback effect on regional climate and desertification;
f) redistribution of freshwater for irrigation having downstream and
coastal zone problems; g) soils erosion and degradation reducing crop
yield; h) increases in fire frequency leading to savannahization;
i) losses of key ecosystem components, especially birds and pollinators
due to habitat destruction; and j) the intense application of chemical
means to control pests and plant pathogens and to remove vegetation.
The deepening concern in all these changes has led to a socio-political
shift from dealing with scarcity towards concepts of sustainable
development. In this concept, the goal is to leave the Earth's ecosystems
in a state (goods and services) that preserves for future generations what
we have today. In the early seventies, the Club of Rome commissioned a
modeling study that revealed the limits to growth and drew widespread
controversy, criticism, and concern. The concern was amplified in the
second book, Beyond the Limits, (Meadows et al. 1990) with model
projections giving only a fairly short time horizon for humanity's
response, about 30-50 years before a virtual collapse in resources and
in the human population. Their recommendations centered on aiming for
zero growth and sustainability. And Mathews (1983) added to this concern
by discussing how resource competition will likely be the new basis for
national security and the motive behind future wars.
Many analyses trace the present pattern of overuse of resources to the
increasing disparities between rich and poor. The proportion of people
living under crushing poverty is increasing in all countries and seems
to be a paradox in a world with so much productive capacity (Brown 1990).
The poor, most of whom are agricultural workers, are being forced into
more and more marginal lands. One growing area of alarm is infectious
disease. Some components of the ecosystems capable of keeping up with
the rate of change are the microbial communities. As humans have tried
to usurp the power of the genetic machinery to combat disease through
the production of antibiotics and vaccines, they have inadvertently
contributed to the adaptation of the microbial community to more virulent
strains of disease. The disruption of the ecosystems and of the habitats
of rodents and insects, coupled with the failures to control them by
chemical agents, are also enabling vectors that transmit disease to
thrive. The poorest members of the human society are at greatest risk
as epidemics become more common worldwide (Garrett 1994). The problem
is not confined to only the poorest countries or the poor in the First
World, as the  disease epidemic in the U.S. and the E. coli outbreaks in
several large metropolitan regions testify recently. While the attempts
for treatment and vaccination continue to have trouble, it is clear
that understanding the ecological basis for the transmission of
infectious disease can help in the prevention of human exposure to diseases.
In conclusion, what can and should be done? There are innumerable
suggestions and recommendations being made, many of them laced with
socio-political agendas. First, efforts such as the Sustainable
Biosphere Initiative to bring ecological knowledge to bear on these
ecologically-based problems is vital. Second, education is probably a
key factor, especially if accompanied by actions the individual can
make everyday. Third, conversion from fossil fuel dependence to,
perhaps, solar energy is worth exploring. Fourth, a worldwide improvement
in the condition and status of women is badly needed. There is a clear
indication that poverty is being feminized. Fifth, conversion to
sustainable agro-ecosystems is crucial to preserve and rebuild soils
and nutrient supplies and to maintain biological diversity. Overall, a
sustained global commitment and cooperation is necessary to accomplish
these goals.
Literature Cited:
Aber, J. D., et al., BioScience 39(6):378-386 (1989)
Barnola, J. M., Nature 329:408-414 (1987)
California Energy Commission, The Impacts of Global Warming on
California, Interim Report (1989)
Garrett, L., The Coming Plague, Farrar, Strauss and Giroux (1994)
Hom, J., Ph.D. Dissertation, Chapter 1, Univ. of Alaska-Fairbanks (1986)
Intergovernmental Panel on Climate Change (IPCC), Second Synthesis,
UNFCCC, (1995)
Kates, R. W., et al., in The Earth as Transformed by Human Action,
Cambridge Univ. Press, pp. 1-17 (1990)
Keeling, C. D., et al., in Aspects of Climate Variability in the Pacific
and the Western Americas, AGU Geophysical Monographs (1989)
Keeling, C. D., et al., Nature 375:666-670 (1996)
Lubchenco, J., et al., Ecology 72(2):371-412 (1991)
Mathews, J. T., Foreign Affairs (1983)
Meadows, D., et al., Beyond the Limits, Universe Books (1990)
NASA Headquarters, MTPE, Strategic Enterprise Plan 1996-2002 (1996)
Potter, C. A., and S. Klooster, in press, Tellus (1996)
Tans, P., et al., Science 247:1431-1438 (1990)
Vitousek, P. M., Ecology 75(7):1861-1876 (1994)
Vitousek, P. M., et al., BioScience 36:368-373 (1986)
Brown, L., et al., State of the World 1990, Norton and Co. (1990)
Response of Microbial Ecosystems to Global Change
Ken Nealson
Center for Great Lakes Studies
In order to consider the question of global change as it relates to
microbial ecosystems, one must be in the right spatial scale and
recognize some fundamental properties of microbial ecosystems and
microbes themselves. That is, we must ask what is the environment
that a microbe actually sees; when this is done, we can conclude that
large size and complexity are decided disadvantages when global change
occurs. As size decreases, the ability to escape from bulk phase
conditions increases, and except for true ecological calamities, the
changes that dramatically affect the larger eukaryotes probably have
little effect on the microbes on a global scale. When one also considers
the metabolic advantages of a high surface to volume ratio enjoyed by
the bacteria, it is easy to understand why the microbes have remained
small-if it was an advantage to be large, they would be large (given
3.8 billion years)!
The simplicity in structure of microbes has allowed a large radiation
of metabolic types to fill the many niches in which energy can be
gleaned on our planet. Probably from times as old as 3.8 billion years
ago the planet was inhabited with abundant bacteria forming dynamic
ecosystems known as stromatolites, and doing quite well in what by
modern standards could only be considered to be a very harsh
environment. An examination of today's bacteria emphasizes this
point; bacteria are found in environments ranging from -5 C to
over 120 C, from pH values of 0-10, salinities ranging from distilled
water to hypersaline brines. In these structurally simple creatures
there is not much to break!...no complex chromosomes, no meiosis or
mitosis, no nucleus or nuclear membrane, and-with only a few exceptions
like bacterial flagella and some photosynthetic membranes-no complex
structures that can be destroyed by extreme conditions. These
creatures are the "Timex watches" of the biological world and stand in
stark contrast to the eukaryotic "Grandfather clocks" with their
complex structures and organelles that reflect their evolution and
behavior and that can be damaged or rendered non-functional by rather
minor environmental changes.
If one considers survival rather than growth, the point is even more
strongly made. It is rather routine (and was done for years in my
microbiology course at Scripps Oceanographic Institution) to take a
seawater sample of 4-10 C in aerobic waters and culture from it
thermophilic bacteria (those able to grow above 60 C) which will
not grow at temperatures below 50 C, or sulfate-reducing bacteria
that will not grow in the presence of oxygen. While these bacteria
are not abundant, they are nevertheless present in a viable state,
ready to prosper when the right niche is encountered.
Adding to all of this is the rather remarkable metabolic versatility
of microbes, the ability to glean energy for survival and growth from
almost any redox couple that is present on Earth, as long as the
reductant or oxidant is not itself toxic. The versatility is expressed
both in terms of electron donors (fuels) and electron acceptors
(oxidants). Eukaryotes, for the most part, have evolved to use rather
simple carbohydrates (often glucose) as their fuel and oxygen as their
oxidant. The absence of either of these would have drastic consequences
on eukaryotic ecosystems. In contrast, prokaryotes have evolved to
utilize many different electron donors and electron acceptors;
microbial ecosystems would be altered, but not catastrophically, by the
removal of easily metabolized carbohydrates and/or oxygen.
So what types of global change can affect these apparently ubiquitous
and adaptable prokaryotes? To answer this question we may like to look
at the geological evolution of the planet and ask whether any of the
changes we have observed would be dramatic or even calamitous. Here we
can examine a few key variables that may have represented major factors
in microbial ecosystem structure and abundance. I begin with three types
of change that must have been truly dramatic if and when they occurred.
Temperature: If global impacts heated the world oceans to very high
temperatures (greater than 50-100 C) after the evolution of life, it
could have had a major impact on those species that survived. Such
events were apparently common on the early Earth, and it is no surprise
that most of the oldest lineages of extant bacteria are thermophiles or
hypothermophiles. One can argue that these are the survivors of major
events-irrespective of whether they were the organisms that evolved
first. Global change of a few degrees is of very little consequence
to the microbes, who characteristically can survive and grow over a
range of 10-20 degrees with little effect. The effects might well be
on the removal of predators that would be much more susceptible to
temperature changes.
Oxygen: One of the major global changes that occurred in terms of
bacteria was the introduction of oxygen. This compound is toxic by
virtue of the generation of toxic byproducts of oxygen (singlet oxygen,
superoxide, hydroxyl radicals, and peroxides) and must have posed a
major problem for organisms encountering it. Even today, we have many
bacteria whose growth is inhibited by oxygen and others that are
killed by it. However, the introduction of oxygen must have led to
the production of a wide array of electron acceptors that could be
used for metabolism (Fe3+, nitrate, oxidized sulfur compounds, etc.),
and the radiation of bacteria into these energetic "niches," and
finally to the evolution of the ability to use oxygen as the electron
acceptor of choice-as evidenced by the universal use of oxygen for
respiration by eukaryotes.
Eukaryotes: The origin and radiation of the eukaryotes must certainly
have been among the most influential of the global "changes" endured
by the prokaryotes. Imagine the prokaryotic world-metabolic competition,
with simple organisms virtually incapable of predator-prey, and in
reasonable metabolic balance. In this scenario, bacterial stromatolites
probably dominated the face of the planet. When the eukaryotes began to
emerge and blossom, three major effects would be expected:
1) competition for space and metabolites,
2) grazing, and
3) the creation of new niches.
Certainly the inhabiting of the environment by the hungry eukaryotes
would have challenged some of the prokaryotes, but because of size and
metabolic differences, these effects could be expected to be minor. Of
more impact, however, was the appearance of heterotrophic grazers. This
probably resulted in the widespread disappearance of stromatolitic
microbial mat environments due to the grazing of the eukaryotes. Today
such stromatolitic environments are found primarily in hypersaline or
hyperthermal environments, places with conditions too harsh for the
eukaryotic grazers. However, in terms of today's microbial ecosystems,
the tables have turned, and there are few eukaryotes who are not
dependent on symbiosis, tolerant of commensalism, or impacted by
parasitism of prokaryotes. As the eukaryotes radiate, they create a
wide array of new niches.
Evolution of Light- and Gravity-Sensing Genes in Plants
Lewis Feldman
University of California, Berkeley
Organisms perceive and respond to a wide variety of physical stimuli,
including gravity and light. Following perception, translation of
physical stimuli into a biological response occurs via a number of
interlinked steps collectively designated "signal transduction."
When and from where did the gravity signal transduction pathway evolve?
The hypothesis advanced here is that the signal transduction pathway for
gravitaxis (gravitropism) evolved by building on and/or incorporating
signal transduction steps which evolved earlier in connection with
prokaryotes responding to light.
In the earliest prokaryotes, light-absorbing pigments may have been used
to absorb excess light energy (UV?) that was harmful to cells growing in
the sea in surface communities. Absorption of the light could either have
served as a means of directly screening the cell's light-sensitive
machinery from UV, or the light absorbed may have stimulated a
light-avoidance response (negative phototropism), with the organisms
actively moving away from the light. A light avoidance response would
necessitate the development of a transduction pathway to convert the
physical stimulus of light into a response (avoidance).
Of the extant prokaryotes, about half are capable of directed movement,
including mechanisms involving flagella, which may be scattered over the
entire cell or concentrated at one or both ends of the cell. A second type
of motility mechanism characterizes a group of helical-shaped bacteria
called the spirochetes in which several filaments spiral around the cell
under the outer sheath of the wall structure. A third type of mechanism
for motility found in prokaryotes involves the secretion of slimy
substances allowing a gliding movement that may result from the presence
of flagella motors.
During early prokaryotic evolution it is believed that organisms existed
as chemoautotrophs nurtured by the free organic compounds generated in
the primordial seas via abiotic synthesis. With the depletion of these
compounds it is hypothesized that earlier-evolved, UV-absorbing,
energized pigments were coupled with electron transport systems to
drive ATP synthesis. In modern archaebacteria (the extreme halophiles),
a pigment that captures light energy (bacteriorhodopsin) is built into
the plasma membrane, leading to the relatively direct establishment of
a proton gradient.
By about 3.5 billion years ago, photosynthesis was likely
well-established on earth, and it is now believed that these ancient
organisms had metabolic capabilities similar to modern cyanobacteria.
What is the evidence to support this hypothesis that the gravity
signal transduction pathway evolved from and/or built upon elements
of the phototropic pathway?
From unicellular organisms the data are few, but there are a few
supportive facts.
1. 	In many motile unicellular plants, gravitaxis is impaired by
	UV light, suggesting that an efficient gravitactic system evolved only
	after an oxygen/ozone atmosphere was produced, via
	photosynthesis, to screen out the UV light.
2. 	Flagellar responses and phototropic stimuli are coupled in
	algae through calcium ions and calcium-mediated
	membrane phenomena. Calcium and calcium-modulated
	events are central, as well, to the gravitropic
	response in plants, suggesting common steps in
	the two pathways.
Stronger evidence that the gravity signal transduction pathway
developed and evolved from the light processing pathways comes from
studies of higher plants, specifically corn, in which much effort has
focused on calcium and its mechanism of action. Calcium is hypothesized
to have a central role in gravity signal transduction. The processing
of the gravity stimulus is believed to involve a redistribution of
calcium and a subsequent interaction of calcium with target proteins,
such as the calcium-binding protein calmodulin, and calcium-activated
protein kinases.
For studies of calcium action, roots serve as a model system. In most
plants roots typically show a "complete" gravity (gravitropic) response
and grow vertically downward. This directional response occurs in either
light or darkness. However, in many plants if roots are maintained in
darkness their direction of growth is instead nearly parallel to the
soil surface. Subsequently, when these roots are illuminated (light
can penetrate to considerable depths in soil), the direction of growth
alters and the roots now grow downward.
Work has shown that in roots exhibiting this light-dependent gravitropism
that an unusual calcium-regulated protein kinase is involved in the
gravity response. Application of drugs which specifically target this
kinase can inhibit this light-dependent gravity response. However, in
roots of species showing the complete gravity response in darkness
(light-independent gravitropism), the previously used drugs are
without effect and the treated roots exhibit vertical, downward growth.
Using results from these experiments, we propose that the original
gravitropic signal transduction pathway was not only light-requiring
but also incorporated steps of the earlier evolved light transduction
pathway. We consider that the light-independent gravity signal
transduction pathway evolved later. Moreover, while this "new"
pathway still makes use of calcium in transducing the gravity
stimulus, additional calcium target proteins evolved, separate
and independent from those used in light signal transduction.
Vertebrate Development in Space: Clues and Complications
Debra J. Wolgemuth
Columbia University College of Physicians and Surgeons
The development of an animal includes embryogenesis, growth, and
reproduction. The successful completion of this program requires
the capability to survive in and adapt to altered environments. On
Earth, the gravitational field represents a constant force in the
environment which may have influenced the size, shape, and behavior
of animals throughout evolution. The extent to which altered
gravitational environments can affect the normal developmental program
in higher organisms is poorly understood. Space flight, especially for
extended periods of time, represents an altered environment in which
animals will be required to undergo these various aspects of the
developmental program, not only in the virtual absence of gravity but
also in the presence of other unique environmental features such as
radiation and alterations in atmospheric pressure.
Underlying the question of how this altered environment will affect
development is the hypothesis that normal development depends on the
embryo's ability to maintain a programmed temporal and spatial coordination
of morphogenetic events, even under abnormal conditions, and that normal
adult functions require the same plasiticity of responses. Clearly animals
are able to survive for limited periods of time in the microgravity
environment, and limited data from space flight experiments demonstrate
that at least certain aspects of reproduction can occur as well. However,
it may well be that some alterations in morphology or function do occur
in a microgravity environment, and although subsequent development may
appear overtly normal, subtle effects with potential long-term
ramifications may result. All of our analyses to date, without exception,
are hampered by the paucity of experimental subjects, the relatively
short duration of exposure to the space flight environment, and the
lack of biomarkers and endpoints sufficiently sensitive to detect
these effects.
Thus, one of the greatest needs in evaluation of the effects of the space
flight environment on the biology of terrestrial organisms is the
development of appropriate experimental paradigms and sufficiently
sensitive experimental endpoints to test and predict such effects.
In this discussion, we will summarize some of the observations of
vertebrate development in microgravity and consider various stages
of development in vertebrates that might be expected to be differentially
sensitive to altered gravity conditions. While we emphasize mammalian
development, we discuss the suitability of various model systems for
examining such effects in other species. Furthermore, we consider
recent developments in our understanding of the molecular genetic
program regulating embryogenesis and reproduction that could serve
as markers for assessing perturbations of development. A question of
particular importance focuses on the effects of the space environment
on the stability of the eukaryotic genome, with the potential short-
and long-term effects that such perturbation might have.
Understanding the influence of space flight environment on the
developmental processes is important not only in terms of evaluating
possibilities of human survival in space but also in understanding how
animals could adapt to and be influenced by a constant microgravity
environment.
Selected References
Alberts, J. R., Serova, L.V., Keefe, J. R., and Apenasenko, Z. 1986.
Early postnatal development of rats exposed in utero to microgravity.
In: Final reports of U. S. monkey and rat experiments flown on the
Soviet satellite Cosmos 1514. Eds: Mains, R.C., Gomersall, E.W. NASA TM-88223.
Dubrova, Y. E., Jeffreys, A. J., and Malashenko, A. M. 1993. Mouse
minisatellite mutations induced by ionizing radiation. Nature Genetics 5:92-94.
Mains, R. C., and Gomersall, E. W. 1986. Final reports of U. S.
monkey and rat experiments flown on the Soviet satellite Cosmos 1514.
NASA TM-88223.
Pedersen, R. A. 1986. Potency, lineage, and allocation in preimplantation
mouse embryos. In: J. R. Rossant and R. A. Pedersen (Eds.), Experimental
Approaches to Mammalian Embryonic Developmentt. New York: Cambridge
University Press, pp. 3-33.
Wolgemuth, D. J., and Murashov, A. K. 1995. Models and Molecular
Approaches to Assessing the Effects of the Microgravity Environment
on Vertebrate Development. ASGSB Bulletin 8:63-71.
Gravity Sensor Plasticity in the Space Environment
Muriel D. Ross
NASA-Ames Research Center
The ability of the brain to learn from experience and to adapt to new
environments is recognized to be profound. This ability, called
"neural plasticity," depends directly on properties of neurons
(nerve cells) that permit them to change in dimension, sprout new parts
called spines, change the shape and/or size of existing parts, and to
generate, alter, or delete synapses. (Synapses are communication sites
between neurons.) These neuronal properties are most evident during
development, when evolution guides the laying down of a general plan of
the nervous system. However, once a nervous system is established,
experience interacts with cellular and genetic mechanisms and the
internal milieu to produce unique neuronal substrates that define
each individual. The capacity for experience-related neuronal growth
in the brain, as measured by the potential for synaptogenesis, is
speculated to be in the trillions of synapses, but the range of increment
possible for any one part of the nervous system is unknown. The question
has been whether more primitive endorgans such as gravity sensors of the
inner ear have a capacity for adaptive change, since this is a form of
learning from experience.
Gravity sensors are very simple parts of the nervous system. There are
two of them on each side of the head: a utricular macula and a saccular
macula. Each is about the size of a period in newsprint. Each consists of
a test mass of crystallites (otoconia), a layer of two kinds of sensory
cells (type I and type II), and another layer that amounts to a neural
plexus (tangle) of nerve fibers, their complex branches and expanded
sensory endings (calyces), and small-diameter nerve fibers of central
origin. Maculae sense linear accelerations, translational and
gravitational, while other endorgans of the inner ear sense angular
accelerations. Information sent from the sensors to the central nervous
system is used for the reflex control of eye movements, posture, and balance.
It was originally proposed that gravity sensors would prove to be the
Achilles heel of human spaceflight. It was suggested that humans would
suffer continuous intolerable nausea, weakness, and disorientation in a
weightless environment, making human exploration of space impossible.
Spaceflight, however, demonstrated the ability of humans to survive and
thrive in weightlessness after an initial adaptive period of illness or
malaise called "Space Adaptation Syndrome" (SAS). The syndrome was
endured for a period that varied between 1 and 14 days, depending on the
individual. Once returned to Earth, another adaptive period resulted.
These adaptations were attributed to central rather than peripheral
mechanisms since the prevailing theory is that central conflict between
mismatched visual, kinesthetic, and gravity sensor inputs is the cause
of "Space Adaptation Syndrome." In contrast, the hypothesis tested in
the work to be reported here was that the periphery also adapts to a
novel gravitational environment (exhibits neural plasticity) by changing
the number of synapses in the sensory cells. Ability to function
effortlessly in weightlessness signals that adaptation has been completed.
The initial test of this hypothesis was an experiment conducted in
microgravity on rodents as the model for mammalian gravity sensors.
This took place on the first Space Life Sciences Mission, SLS-1, which
lasted nine days. A second experiment was conducted on SLS-2, a 14-day
flight. The basic premise was that synapses in the receptor cells would
change in number in an altered environment. Serial thin sections were
cut from the medial part of the utricular macula of flight animals and
controls, and studied at the level of the transmission electron
microscope. Synaptic bodies (ribbon synapses) were photographed and
counted in 50 serial sections per gravity sensor for the SLS-1
experiment, and in 100 serial sections for each macula for SLS-2.
Data from three rats per experimental group were pooled for SLS-1,
and from two rats per group for SLS-2. Ribbon synapses were further
characterized by the appearance of the central dense body as spherular
or rod-like. Synapses in pairs or in groups were noted. Every seventh
serial section was photographed in its entirety so that the sensory
cells could be numbered and the section used as a map. The sections
crossed the entire medial aspect of the macula. Thus, more than 100
sensory cells were profiled in each section. For each experiment,
~ 6000 synapses in over 1000 hair cells were counted and analyzed
statistically, using analysis of variance (Super ANOVA software)
with the level of significance set at p < 0.05. Graphs were prepared
using DeltaGraph Pro 3 and CANVAS.
For SLS-1, results were obtained from tissues collected ~ 4.5 to 6
hours post-landing (recovery day, or R + 0) and on the ninth
post-landing day (R + 9). The data showed that the means of synapses
per hair cell had increased by ~ 41% in type I cells and by ~ 55% in
type II cells (p < 0.0001 for both) on R + 0. There was a shift toward
the spherular form of ribbon synapse in both types of sensory cells of
flight animals (p < 0.0001), a near doubling of pairs in the flight
rats (p < 0.0001), and an increase by a factor of 12 of groups of
synapses (p < 0.0001). However, there was no significant difference
in synaptic mean counts between flight and control animals on R + 9.
High counts were retained in flight animals, while counts were elevated
to comparable levels in control animals. These findings at R + 9 were
attributed to stress induced by experimental treatments post-flight
that included withdrawal of blood from the tail vein and numerous
injections of radioactive materials required by other experimenters.
The SLS-1 study demonstrated the labile nature of cell-to-cell
communication in gravity sensors but left unanswered the question
whether counts would have been higher if the tissues had been collected
in space. That is, had the gravity sensors already begun to adapt to
earth by 4.5 hrs post-landing? The SLS-2 experiment answered this
question. Macular tissues were collected and fixed in-flight on day 13,
when it was anticipated that the animals would be adapted to
microgravity. The results showed that, in type II cells of in-flight
experimental animals, means of counts had increased by 100% compared
to ground controls (11.4 versus 5.4, p< 0.0001). In contrast, the
mean number of type I cell synapses was not much different from that
obtained in the SLS-1 experiment. There was a 44% increase in the
mean in-flight (p < 0.034) and little change in mean synaptic number
in type I cells post-flight. In the case of type II cells, by R + 0
the mean number of synapses in type II cells had declined to 9.3, a
figure comparable to that obtained at R + 0 on SLS-1 (also 9.3). The
mean number of synapses in type II cells was further reduced
post-flight (to 8.7 at day 14 post-flight). Neither post-flight
mean in flight animals differed significantly from their controls
(controls, 7.5 at R + 0 and 7.2 at R + 14). Additional results
showed that the trends toward spherular synapses, pairs, and groups
were replicated in the maculae collected in-flight.
These results are the first to demonstrate the plasticity of the
sensory connections in the gravity sensors in a model mammalian
system. It is evident that exposure to the altered gravity of space
has a profound effect on communication sites between the sensory cells
and the nerve fibers ending on them. The increase in spherular synapses
as a result of spaceflight is interesting because it has been proposed
that these synapses are the more primitive. That is, spherular synapses
develop into the rod-like form. The larger numbers of pairs and groups
could function to increase the efficacy of the synapses at those
specific sites.
There is additional evidence, not recounted in this abstract, that
handling the rats and/or human interaction with them also affects
synaptic counts in maculae. The reason for this surprising finding,
whether hormonal or simply resulting from increased activity, is
unknown. However, it is clear from these studies that controls for
space experiments (and all others as well) must be carefully thought out.
Finally, the differences in increments in synaptic means noted
between type I and type II cells in both experiments can be accounted
for on the basis of the neural circuitry. The existence of microcircuits
in maculae was discovered in ground-based studies and verified by
computer-based 3-D reconstruction methods using transmission electron
microscopy. Type I cells synapse only with expanded nerve fiber
endings called calyces, but type II cells are inserted into microcircuits
that include feed-forward and feedback connections. An hypothesis resulting
from this research is that this siting of type II cells accounts for the
larger increments in synapses shown by these cells in microgravity. It is
postulated that synaptogenesis is an attempt by the system to return the
output to a more normal level in the absence of gravity. This hypothesis
can be tested in later space flights. It is also clear that the unusual
level of synaptic plasticity shown by gravity sensors in space makes
flight experiments a practical way to study the molecular basis of
neural plasticity. Insights obtained should be applicable to neural
plasticity wherever it occurs in the nervous system.
Human Cardiovascular Adaptation to Altered Environments
Benjamin Levine, M.D.
University of Texas
Southwestern Medical Center
The human cardiovascular system has an extraordinary adaptive range. Even
the most unfit person can, within minutes, increase the amount of blood
pumped by the heart and distributed to skeletal muscle by an order of
magnitude. Endurance athletes, who place regular and sustained demands
on the cardiovascular system, may have an additional two-fold capacity
above more sedentary individuals. In contrast, bed rest, space flight, or
other causes of disuse (i.e., illness or injury) result in a reduction in
maximal capacity by 25 - 30%. Thus the cardiovascular system can be viewed
in terms of its acute ability to respond to a sudden increase in metabolic
demand, and a more chronic adaptation to match the range of the system to
the regular requirements that are placed upon it.
In order to understand how the human cardiovascular system adapts to
stress, it is helpful to think about the system as made up of "the
plumbing," comprised of the "pump" (the heart) and "pipes" (the blood
vessels), and the "control system" (the autonomic nervous system, hormones
regulating salt and water balance, and local endothelial derived mediators
of microvascular flow). For acute demands, such as during exercise or
rapid changes in posture (i.e., gravitational vectors), higher order
centers in the brain initiate an increase in the heart rate, termed
"central command." Sensors located in skeletal muscle respond to changes
in both metabolic and mechanical state and send back signals to the brain
reflective of the intensity of effort. The heart itself is a sensory organ
and detects the adequacy of hydration and cardiac filling through
"mechanoreceptors". Pressure sensors or "baroreceptors" in the walls of
the large blood vessels detect the pressure within the vasculature. These
signals are integrated in special centers in the brain which respond by
regulating both the strength and frequency of the heart's contraction
and the resistance of the blood vessels, primarily by neural mechanisms.
Because humans are predominantly upright creatures, with the majority of
the blood volume located below heart level, gravity has a profound effect
on the mechanical distribution of fluid within the cardiovascular system.
When hydrostatic gradients are removed, such as changing from the upright
to the supine position or exposure to microgravity, blood is translocated
from the lower part of the body towards the chest virtually doubling the
amount of blood within the heart. The heart responds to this volume load
by increasing the amount of blood it pumps (Starling's Law of the Heart),
and by initiating both a redistribution and elimination of plasma. Although
Gauer hypothesized in the 1950s that the regulated level for humans was the
upright posture, data from space flight and bed rest studies suggest that
in actuality, the system settles on a pressure and volume approximately
2/3 of the way between upright and supine. It is important to emphasize
that this adaptation is rapid and appropriate, being complete within 48
hours of microgravity conditions. In contrast to bone and skeletal
muscle, for which load and thereby structural integrity is dependent on
gravity and physical activity, the heart is constantly "exercising" with
each heart beat. There does not appear to be any further change in
cardiovascular loading with longer exposures. Because cardiac work is
reduced somewhat in space, the heart appears to atrophy slightly with
a 15-20% reduction in cardiac muscle mass. The blood vessels also appear
to get slightly smaller and stiffer. However this adaptation is a
physiological, rather than a pathological response and does not appear
to be associated with impaired function. In fact, the maximal capacity
of the cardiovascular system, as reflected by maximal exercise performance,
is well maintained during short term space flight. For longer term space
missions, such as the 84 day Skylab mission, some astronauts actually
improved their fitness, probably because the rigorous exercise
requirements of the mission exceeded their pre-flight training practices.
There is no evidence to date of any physiologically meaningful alteration
in the cardiovascular control systems in space.
Although the adaptation to microgravity is physiologically appropriate,
immediately upon return to a 1G environment, it becomes dysfunctional.
Nearly 2/3 of astronauts suffer from orthostatic intolerance, a
condition whereby they are unable to stand continuously for 10 minutes,
after return to earth. In addition, maximal exercise capacity is reduced
by approximately 25%. These problems appear to result because the heart
empties precipitously when head-to-foot gravitational forces are restored
and blood pools below the heart. Because the heart is slightly smaller
and stiffer than it was before adaptation to space, this emptying is
relatively greater than it was before flight. Reflex mechanisms attempt
to compensate by increasing heart rate and vascular resistance. However
in some individuals who are prone to orthostatic intolerance, their blood
vessels appear unable to constrict to the degree necessary to compensate
for the empty heart. It is not yet clear whether this trait is a result
of a unique adaptation to space flight, or an inherent characteristic of
these individuals. Regardless of the mechanism, orthostatic tolerance is
restored rapidly - virtually all astronauts are able to remain upright
within 24 hours after return. Maximal exercise capacity is restored to
pre-flight levels within a week, at least after short term space flight.
The data for long term space flight are more scarce, though the Skylab
experience would suggest that detraining is not dependent on the duration
of the flight.
In summary, despite the dire predictions of early physiologists, the
cardiovascular system adapts well to space. Blood pressure and organ
perfusion are well maintained and maximal exercise capacity is preserved.
There is no evidence for a progressive deterioration of cardiovascular
performance with longer term space flight and on theoretical grounds, the
heart should tolerate even longer missions required to support a mission
to Mars without trouble. Although transiently upon return to earth there
may be a problem with orthostatic intolerance, this does not seem to be
chronic and astronauts re-adapt to earth as quickly as they adapted to
space. The cardiovascular system is not likely to be a significant
impediment to long term space flight.
Biological Responses to Exposure
to the Space Radiation Environment
Amy Kronenberg
Lawrence Berkeley National Laboratory
The environment in space is different from that on Earth in many ways. One
of the environmental factors that is unique is the space radiation
environment. This environment may play an ongoing role in the evolution of
life in the universe in addition to being an important consideration for
human exploration in space. The radiation environments encountered in space
are different from the natural background radiation on Earth and are complex
in nature. Both bacteria and eukaryotes have evolved a series of mechanisms
to respond to similar damages on Earth.
The space radiation environment is largely comprised of positively charged
particles such as protons and helium ions and negatively charged
electrons. The Earth's magnetic field serves as a shield against
charged particle radiations. The primary sources of radiation in space
are traditionally classified into trapped particle radiation, galactic
cosmic radiation, and solar particle radiation. The trapped particle
radiations are located in a series of radiation "belts" that surround
the Earth and are comprised primarily of protons and electrons.
The galactic cosmic radiation (GCR) is the major radiation in
interplanetary space and is comprised primarily of charged particle
radiation. Of the different charged particles that make up the GCR,
87% are protons, 12% are helium ions, and 1% are heavier ions that are
of high energies. Iron is the most important of these heavier ions due
to its abundance and the amount of energy each iron ion can deposit as
it interacts with matter.
The sun contributes in a dynamic way to the space radiation
environment. Solar particle events (SPE) are large intermittent emissions
of protons, helium, and sometimes heavier ions from the sun. The frequency
of SPE varies within the solar cycle. In addition to providing protection
against GCR, the Earth's magnetic field also shields terrestrial life
against intermittent SPE.
The different charged particle radiations in space have biological
consequences that are dependent upon the spatial distribution of the
energy they impart to the cell or tissue. As charged particles pass
through matter, they slow down. The energy deposited is a function of
the energy of a given particle, and the greatest amount of energy
deposited by a particle comes at the very end of its track. Protons
are generally sparsely ionizing radiations until they reach the very
end of their tracks, while the heavier ions are densely ionizing at the
beginning of their path and become more so as they slow down.
Cells, tissues, and whole organisms have evolved a myriad of responses
to the types of damage that can be caused by charged particle
radiations. The early responses to charged particle exposures include
recruitment of DNA repair machinery, changes in gene expression, the
initiation of programmed cellular responses such as apoptosis, and
alterations in the tissue microenvironment.
Included among the initial types of damage that result in DNA following
exposure to charged particle radiations are base damage, single-strand
breaks, and double-strand breaks. Bacteria and eukaryotes have highly
complex systems for the repair of these alterations, which can also
result from naturally occurring errors in replication, changes that
result from ongoing oxidative metabolism, and exposure to chemicals
of various kinds. Following exposure to sparsely ionizing radiations,
base damage and strand breaks are widely scattered throughout the cell
nucleus. For densely ionizing radiations such as iron ions, the
alterations are often clustered. Most of the initial damage to DNA
is efficiently removed prior to the next cell division cycle by processes
including nucleotide excision repair and double-strand break repair. It
has been suggested that the clustered damages associated with densely
ionizing radiation exposures may recruit multiple repair pathways to
the site of damage. The resulting "traffic jam" may lead to an alteration
of the efficiency of repair.
The programmed cell death pathway may be important in maintaining tissue
integrity in the space radiation environment by removing damaged cells
before they can divide. Virtually all cells in multicellular organisms
have evolved a system of altruistic suicide that is often referred to
as programmed cell death or apoptosis. This process is critical to
normal development and tissue homeostasis. Apoptosis is regulated by
a series of proteins that function as either effectors or suppressors
of the process. Through a complex series of heterodimeric and homodimeric
interactions, these proteins act as a "rheostat" to promote or suppress
apoptosis. The amounts of the various components of the "rheostat" are
altered in response to exposure to both sparsely and densely ionizing
radiations. One of the key players in the regulation of the apoptotic
process is the p53 gene, which has been referred to as the "guardian
of the genome" (Lane, D. P., Nature 358(6381):15-6). p53 is expressed
in response to ionizing radiation exposures and serves to down-regulate
one of the natural suppressors of apoptosis, bcl-2. Cells which have
lost the ability to initiate the normal apoptotic pathway appear to
accumulate mutations more readily following exposure to sparsely and
densely ionizing radiation.
In the context of organized tissues, there are additional factors that
mediate responses to charged particle radiations. The extracellular
matrix (ECM) and the cytokine environment provide signals that help
maintain tissue integrity and normal development of the organism. The
ECM influences cell morphology, mediates proliferative capacity, and
regulates gene and protein expression. The ECM can promote cancer
progression and incidence. Tissues respond to charged particle radiations
by rapidly remodeling the ECM. The time course and characteristics of
the remodeling are different in different tissue compartments. The
long-term effects of this remodeling are fertile areas of investigation.
In conclusion, biological systems have evolved a series of mechanisms
at the cell and tissue level to respond to the types of damages that
occur as a consequence of exposure to the various components of the
space radiation environment. In large part, these responses evolved
to protect the integrity of the organism.
Science and Habitability Goals for Mars Exploration
Michael B. Duke
Lunar and Planetary Institute
The rationale for human exploration of Mars is built around the concept
that humans will be required to address fundamental scientific questions
on Mars and that Mars will be required to address fundamental questions
of humanity's future. Recently, the findings of possible evidence for
ancient life on Mars has focused the science goals on the search of
environments where life might have arisen. Without changing the nature
of the generalized strategy for scientific exploration of Mars, the
scientific questions, the needs for robotic exploration, and the
potential roles for humans on Mars are coming into better focus. In
order to support human scientists on Mars in a safe and productive
environment, initial steps will need to be taken along the path toward
a permanent martian outpost. Although addressed in large part by
technology, the habitability goals also can be stated in terms of
scientific questions to be addressed during the robotic and early
human phases of Mars exploration. This paper describes goals and
objectives for Mars science and human habitation, with emphasis on
the possible contributions of humans on Mars.
Artificial Gravity for Human Missions
Laurence R. Young
Massachusetts Institute of Technology
Artificial gravity will be considered as an alternative to currently
inadequate countermeasures for long-duration space flight, of the type
required to explore Mars.
If we are to commit to a spinning vehicle, the physiological questions
get down to two types-one for a full-time rotating habitat and one for
intermittent stimulation-a kind of gravity gym.
For full-time rotation, the first question is what level of acceleration
at foot level is the minimum required to maintain normal function?
Various means of achieving the artificial gravity will be considered,
but we initially just concentrate on the size and speed needed to give
sufficient centripetal acceleration. We can be reasonably certain
that 1 G will suffice, but is it needed? Will a half-G do? Or can we
avoid deconditioning by spinning continuously at a level of 0.38 Gs
to match the Martian gravity?
We are likely to achieve the earliest answers with experiments on
animals. We already know that rats which have been centrifuged
during their space flight don't show the major deterioration in
bone, muscle, and cardiovascular response seen by their free-floating
brethren-but that is only at 1 G. The current International Space
Station plans include a module for a centrifuge to carry up to eight
modules for rodents, fish, and eggs but will not accommodate the
primates which many feel are needed to adequately model human responses.
Intermittent artificial gravity stimulation, on the other hand,
presents a number of potential advantages. As part of our normal
circadian rhythm, the very G-dependent processes which result in
fluid loss and bone deconditioning are probably turned off during
our normal sleeping hours. On the other hand, extended periods of
bed rest produce effects on the skeleton, muscles, and cardiovascular
system which are similar to those occurring in space. This simulation
of space flight is made more accurate if the bed rest is conducted
with a six-degree head-down tilt to accelerate the shift of fluid
toward the head and if the subject lies partially immersed, although
dry, in a high-tech waterbed.
A combination of short-duration and long-term studies in ground
centrifuges and rotating rooms can be useful in answering many of
the key questions concerning the application period, frequency, and
intensity of centripetal acceleration. They will also be useful in
determining the seriousness of the problem of dual adaptation-to the
rotating and non-rotating environments.
Importantly, they may shed light on the physiological importance of a
gravity gradient across the body if one is to truly proceed with
rotators having a radius comparable to the subject's height. These
ground gravitational physiology studies are essential for effective
use and interpretation of the space variable-gravity centrifugation
tests to be mentioned.
Even if the main physiological issues are adequately addressed by a
rotating space vehicle, important human factors issues remain. The
specific constraints of artificial gravity remain at the crux of the
research in terms of a practical solution to interplanetary travel.
If we are right about the ability of astronauts to adapt and maintain
the adaptation, it may be possible to achieve a measure of protection
from an intermittent artificial g exposure with a radius as low as 4 m,
rotating at 10 rpm to produce about half a G at foot level.
Destination Mars: An Astronaut's Perspective
Scott Parazynski, M.D.
NASA Johnson Space Center
The recent discovery of possible fossilized life forms within a Martian
meteorite has rekindled interest in human exploration of the planet.
While the global scientific community must carefully study this physical
evidence as well as await the results of 4 space probes to Mars in the
coming years, others are looking ahead to the next logical step of
sending humans to the planet. Such a feat, while a tremendous scientific
undertaking, is nearly within the grasp of current technology.
The travel distance involved and the harsh Martian environment itself
impose great challenges to human physiology as well as to spacecraft
design. Mission planners face challenges from the long-duration
microgravity environment, complex life-support systems, interplanetary
radiation, and the psychological stressors involved in such an endeavor.
	While the human body readily adapts to weightlessness,
	significant deconditioning of the musculoskeletal and cardiovascular
	systems occur that could preclude safe landing and operations under
	the 1/3 G environment of Mars. Further, the long-term effects of
	microgravity are poorly studied, although a Russian cosmonaut colleague
	has spent 438 consecutive days on-orbit and has returned to normal
	daily activities here on Earth. This re-adaptation was due to his
	intensive exercise regime, including both resistive and aerobic
	training every day of his flight. Space travel longer than this might
	require "artificial gravity," but these systems might be prohibitively
	complex and expensive. Studies from the soon-to-be-built International
	Space Station will address the problem of deconditioning in-depth, and
	help define improved countermeasures.
	Interplanetary space places astronauts at significant risk of
	irradiation from solar particle events (SPEs) as well as low-level
	galactic cosmic rays. Moreover, even extravehicular activity (EVA)
	on the planet's surface is not without risk, since Mars has essentially
	no magnetic field to shield the space walking astronaut. SPE forecasting
	from Earth, as well as "space weather" instrumentation onboard the
	spacecraft will be essential during the expedition. Enroute to Mars,
	EVA should be minimized, and the spacecraft will need to be designed
	for maximum shielding from radiation.
	Crew selection and compatibility is a key consideration for
	travel to Mars. Not only must the crew have a wide skill base
	(scientific, engineering, medical) to enable self-reliance, but
	also they will need extensive compatibility screening preflight
	and considerable psychological support in flight. Notably, a busy
	training and science schedule in transit and frequent communications
	with family and friends back on Earth will be a formula for success.
	Results from long duration space crews, submariners, Antarctic
	scientists and Biospherians support a team approach with a well-defined
	command structure.
	Until recently, planned Mars trajectories have involved hundreds of
	days in interplanetary space and have relied on conventional rocket
	technologies. A developmental tunable exhaust plasma rocket, utilizing
	nuclear electric propulsion, reduces the transit time to the order of
	90 days each way. Additionally, system redundancy and the power
	capabilities of such a vehicle would allow a powered abort capability
	for return to Earth.
	A "split-sprint" mission is favored, the first spacecraft
	being a payload tugboat (instrumentation, supplies,
	fuel, etc.) that would establish the Martian outpost
	and potentially generate supplies needed for the
	return home. Once the outpost is fully established
	robotically, a human-operated speedboat will
	transfer the crew from low-earth orbit to Mars.
	Depending on phasing, Martian stays could range
	from 30 days to 2 years.
	Assembly of the craft including the nuclear
	reactors will occur in low earth orbit for safety.
	The vehicle will be comprised of at least 3 redundant
	plasma rocket engines and 3 redundant nuclear reactors
	due to their criticality as well as the propulsion and
	power needed for such a mission. Other critical life-support
	systems, including environmental control and
	rendezvous systems must be designed to be highly
	reliable, modular, and fault-tolerant.
	Shielding from SPEs and other sources of
	radiation will be provided by liquid hydrogen fuel
	tanks surrounding the crew modules, as well as an
	induced magnetic field from the plasma rockets themselves.
	Due to the short transit times enabled with the
	tunable exhaust plasma rocket, complex
	countermeasure systems like artificial gravity
	will not be required. Daily exercise en route
	to preserve musculoskeletal and cardiovascular
	health will be necessary, as well as to preserve
	neuromuscular coordination once the crew arrives on Mars.
In the not-too-distant future, the global scientific
community may require human investigation of the planet
Mars. Lessons learned from Skylab, the Space Shuttle Program,
Mir, and the planned International Space Station, as well as
technological advancements in rocketry, will take us there.
Observations of Planetary System Formation
W. J. Welch
University of California, Berkeley
At this stage, the title means observations of the formation and evolution
of disks around young stars and protostars. The reality of disks is
certain, with the imaging of Beta Pic and the HST pictures of ionized
disks around a large percentage of stars in the Orion Nebula. Evidence
about these structures around the youngest objects, especially those
that are embedded, is more indirect. Infrared excesses from T Tauri
stars (which are, of course, visible) imply so much dust that it must
be in a flattened distribution to let out the starlight. Infrared
excesses are detected at wavelengths as long as millimeter waves.
Roughly half the classical T Tauri stars show this emission and have,
by implication, disks. The strength of this emission implies masses as
large as 0.1 Msun. The slightly more massive Ae/Be stars show similar
statistics. Among the most deeply embedded stars, disks are less
frequent, suggesting development of the disks as part of the emersion
from the dense clouds.
The gross infrared and millimeter wave excess spectra can be explained
by simple disk models in which temperature and density are power laws
of the radius. They can also be explained as emission from small
surrounding envelopes, or a combination of both. Maps made with millimeter
wavelength interferometers at a few arc-seconds resolution do not resolve
them but show that at least the longest wavelength radiation must be
coming from very compact structures, disks. The spectral shape of the
emission at long wavelengths frequently is flatter than that of emission
from dust in the interstellar medium, and this may imply that there is
evolution of the grains, growth in the particle size.
Spectral emission, particularly from CO, is often detected at a few
arc-seconds resolution from the disks. Striking velocity gradients are
seen across the disks, which may be understood as Keplerian rotation,
or alternatively, continued infall-or even possibly outflow.
Theoretical estimates of the likely sizes of the disks predict radii of
about 100 AU, requiring resolutions of better than one arc-second to map
them and sort out the various uncertainties in their structures noted
above. Current developments of mm interferometers are beginning to
enable this level of resolution. Recent maps of the two objects HLTau
and L1551/IRS5 show interesting structures on the scales of 40 AU, a
disk-like structure for the former and a binary system for the latter.
Prospects for the next few years are for maps with resolutions as good
as 15 AU. Gas kinematics should then become more clear. Temperature
distributions should help clarify the local chemistry. There may be
some evidence of the formation of major planets and cometary belts,
and comparative studies should tell us about the course of evolution.
A most important ground-based development will be the Millimeter-Wave
Array planned by the NRAO. It will have a resolution of about 7
milli-arc seconds at a wavelength of 0.5 millimeter. This corresponds
to a linear scale of about 1 AU in the Taurus Clouds, and it may detect
the formation of important structures in protostellar disks.
Ultimately, the best sensitivity and resolution for this work would come
from the development of an imaging interferometer which could operate at
wavelengths between 50 and 100 microns. This is where the peak of the
emission occurs for these planetary (or pre-planetary) systems. This
part of the spectrum is totally inaccessible from the ground. Indeed,
it is not too early to contemplate the construction of an imaging
interferometer to be placed in space to operate at these wavelengths.
Theory of Planetary System Formation
Patrick Cassen
NASA-Ames Research Center
Observations and theoretical considerations support the idea that the
Solar System formed by the collapse of tenuous interstellar matter to a
 disk of gas and dust (the primitive solar nebula), from which the Sun
and other components separated under the action of dissipative forces
and by the coagulation of solid material. Thus, planets are understood
to be contemporaneous by-products of star formation. Because the
circumstellar disks of new stars are easier to observe than mature
planetary systems, the possibility arises that the nature and variety
of planets might be studied from observations of the conditions of their
birth. A useful theory of planetary system formation would therefore
relate the properties of circumstellar disks both to the initial
conditions of star formation and to the consequent properties of planets
to those of the disk. Although the broad outlines of such a theory are
in place, many aspects are either untested, controversial, or otherwise
unresolved; even the degree to which such a comprehensive theory is
possible remains unknown.
The main features of the theory adopted by most researchers are as
follows. A small fraction of the material in cold, interstellar clouds
becomes gravitationally unstable to collapse, either by gradual
evolution or by a sudden compression induced by some disturbance.
Collapse occurs rapidly enough (within a million years) to preclude
the loss of much of the cloud's angular momentum, so the collapsed
configuration takes the form of a centrifugally supported disk, in
which most of the original angular momentum is retained. Dissipative
forces within the disk promote the accretion of most of the material to
its center to form a star; lesser fractions of material spread out in the
remaining disk or are ejected back into space by energetic processes near
the star. Solid objects grow in the disk due to the coagulation of dust
and ice grains (both interstellar survivors and newly condensed).
Although gasdynamic forces may cause most of these objects to be accreted
into the growing star, or even ejected in the wind, some become large
enough for gravitational forces to dominate their motions, at which
point they become potential planetary system survivors. Growth to
planetary size occurs through collisions made possible by the
perturbations of orbits induced by mutual gravitational scattering
and perhaps by collective interactions with nebular gas.
In the case of the Solar System, the outer planets apparently grew
fast enough to capture and retain substantial amounts of hydrogen and
helium from the solar nebula. The final planetary masses were probably
attained within 100 million years, although the cleanup of debris
persisted longer. In the context of this theory, the asteroid belt is
the result of a failed planet, the accumulation of which was frustrated
by the gravitational influence of Jupiter. Comets are believed to be
planetesimals accumulated in the Uranus-Neptune region, scattered to
great distances by encounters with the outer planets.
Several components of the theory enjoy solid support from observations
and/or rigorous theoretical analysis. For instance, the basic tenets of
star formation theory have been verified by observations which have
recently detected direct evidence for collapsing gas in star-forming
regions and established the prevalence of circumstellar disks around
young stars. The idea that disk evolution proceeds through the accretion
of material onto the young star is confirmed by measurements of the
accretion luminosity. The fact that dissipative angular momentum transport
in a disk leads to distributions of mass and angular momentum
characteristic of a star and planetary system has an unequivocal
physical basis. Numerical simulations have demonstrated that the
characteristics of the terrestrial planets are typical of the
configurations ultimately attained by a swarm of dynamically evolving
planetesimals possessing the total mass and angular momentum of the
planets they eventually form.
Nevertheless, there are major gaps at every step in the theory, and
recent developments have emphasized the need to remain undogmatic about
even the most entrenched assumptions. For instance, it has been commonly
supposed that the distribution of mass in the present Solar System is not
a perverse distortion of that of the primitive solar nebula in the
planet-forming era. Although it was acknowledged that the distribution
of sedimented solids need not be exactly the same as that of the gas
(the latter being the predominant component), the discovery of
Jupiter-mass planets very close to their parent stars has focused
attention on the theoretically identified possibility of extensive
planetary migration: the radial motion of a planet due to its gravitational
interaction with its parent disk. A consequence of this phenomenon could
be the effective obliteration of evidence of the original mass
distribution in the disk. (In a similar vein, the recent suggestion
that meteoritic material was thermally processed near the Sun and
subsequently ejected to great distances in the nebula contains the
implicit and radical notion that even the distribution of coagulated
solids had little to do with the original distribution of mass in the
nebula.) Thus, understanding the precise way in which planetary
material becomes decoupled from nebular gas is central to interpretations
of disk properties in terms of potential planetary systems.
In addition to these issues, one can identify several other specific,
unresolved questions of the most fundamental order, relevant to the
characteristics of planetary systems: (1) How are the conditions that
lead to the birth of a single star surrounded by a disk distinguished
from those that lead to the formation of a multiple star system? (2)
What are the specific mechanisms responsible for the transport of
angular momentum in circumstellar disks, and how efficiently do they
act? (3) What processes determine the mass of a gas-rich planet? (4)
What determines the ultimate fate of nebular gas that is not
incorporated into planets? (5) What processes govern the final
distributions of volatile substances among the planets? For most
of these questions, definitive observational tests will be difficult
to identify. Well-designed, computationally intense studies, which
incorporate dissipative processes in a quantitatively realistic way,
will be required to address them.
(This abstract is based in part on the article "Origin of the Solar
System," by P. Cassen and D. S. Woolum.)
Primitive Materials
D.E. Brownlee
University of Washington
During formation of the solar system, most of the mass of the solar
nebula was incorporated into the Sun and planets or was ejected back
into the interstellar medium. Only a tiny fraction survived as small
bodies capable of preserving "primitive materials", solids not
severely altered since the early history of the solar system. Solar
system repositories of primitive materials include the comets, and
some asteroids and small satellites. Information on primitive materials
can be obtained by telescopes and by close-up study with spacecraft.
Primitive materials are very fine grained and the most detailed
information is obtained by laboratory analyses of meteoritic samples,
either meteorites or collected samples of interplanetary dust. In the
future, missions like STARDUST will directly return samples of known
comets and asteroids. The solar system's small bodies carry records on
the materials, conditions, time scales and processes that occurred
during planetary formation. They also contain the best clues on the
form and processing of the key biogenic elements in the stages that
preceded planet formation and the development of life.
While primitive materials provide a direct window of historical
information on the formation of planets and early evolution of the
biogenic elements, the window is some-what obscured and confusing.
In all cases primitive materials have been stored inside bodies of
substantial size and processes within these bodies, to various degrees,
overprint original material properties. Common alterations include
thermal metamorphism, aqueous alteration, shock and compaction. In
many cases it is difficult to distinguish original properties from
secondary ones. Outstanding exceptions are presolar grains such as SiC
and graphite that survived parent body, nebular, and interstellar
environments with many of their properties intact.
Primitive materials in the solar system have a considerable range of
diversity. Primitive asteroidal meteorites show a wide range in
mineralogy, grain size, oxidation state, and systematic deviations
from solar elemental composition. Reflection spectra indicate a strong
radial gradient of material types within the asteroid belt. This is in
part due to spatial variations of particle composition in the solar
nebula, consistent in principle with differences in different meteorite
types. The radial variation is also influenced by mysterious, strong
heating of bodies in the asteroid region that had variation with solar
distance. Wide variation of spectral reflectance also exist in the
Kuiper and Centaur objects although it is unknown how much if this due
to intrinsic material properties and how much is due to space weathering
effects. Accounting for parent body alteration effects, it is clear that
broad pre-accretion variations occurred in particle types in various
regions of the solar nebula. In part these are due to mixing of presolar
grains, nebula condensates and solids modified by various nebular
processes. These studies of preserved solar system solids can be
directly compared to analogous solids observed in circumstellar
regions surrounding other stars.
The role of primitive materials in the development of life on Earth
is unknown and perhaps unknowable. The bulk of the Earth was
constructed from materials that did not have primitive
elemental composition but it is likely that much of the Earth's carbon,
nitrogen and water were delivered by primitive bodies from the outer
solar system. These bodies carried a wealth of complex organic
material produced in the interstellar medium, in the solar nebular
and inside parent bodies. For large bodies this molecular material
is destroyed upon impact but for dust and meteorite sized objects
much of it survives accretion to play possible roles in the origin
and early evolution life.
Planetary Perspective on Life on Early Mars and the Early Earth
Norman H. Sleep
Stanford University
Kevin Zahnle
NASA-Ames Research Center
Impacts of asteroids and comets posed a major hazard to the continuous
existence of early life on Mars as on the Earth. The chief danger was
presented by globally distributed ejecta, which for very large impacts
takes the form of transient thick rock vapor atmospheres; both planets
suffered such impacts repeatedly. The exposed surface on both planets
was sterilized when it was quickly heated to the temperature of condensed
rock vapor by radiation and rock rain. Shallow water bodies were quickly
evaporated and sterilized. Any surviving life must have been either in
deep water or well below the surface.
On Earth, global thermal excursions are buffered by the heat capacity of
the oceans, especially the latent heat of condensation. Such buffering
does not occur on Mars; therefore, relatively small (70 km diameter),
relatively frequent impacts heat the surface everywhere to the melting
point. But for impacts large enough to vaporize the oceans (400 km
diameter), thermal buffering serves only to prolong the disaster. Very
high surface temperatures are maintained for thousands of years while
the oceans recondense and rain out. Obviously survival in open water
is more likely on Earth, where the energy required to boil massive
oceans is enormous.
But survival in deep subsurface environments is more likely on Mars
because (i) Mars' lower background heat flow and lower gravity (the
latter permitting open porosity at greater depths) allow deeper
colonies, and (ii) the thermal heat pulse from a major impact is
briefer and so penetrates less deeply. A third factor that favors
ultimate survival on Mars rather than Earth is that the bigger planet
statistically experiences more and bigger impacts. Impacts in the 200
km diameter range are known to have occurred on the Moon and Mars, and
impacts in the 400 km range are likely on Earth but may not have happened
on Mars. Even the largest bodies that may have hit Mars-400-km
diameter-would have sterilized only the upper few hundred meters
of the subsurface.
A similar impact on Earth would boil the oceans and sterilize the
subsurface down to 1 km depth. Only thermophile organisms could have
survived impacts of asteroids large enough to leave heat or boil the
entire terrestrial ocean (around 300-km diameter) on the Earth.
Studies of terrestrial microorganisms indicate that the last common
ancestor may have been a thermophile and therefore the survivor of
such a catastrophe. This organism needs to be distinguished from first
common ancestor, which may well have originated in a different
environment and adaptively radiated to occupy thermophile niches.
There was probably time for this, as the intervals between dangerous
impacts are millions of years on both the Earth and Mars.
An additional refugium that is poorly understood is ejection of
weakly shocked material to space by impacts and its return to its
home planet or transfer to a neighboring planet. Overall, early Mars
was safer from impacts than the early Earth and probably was habitable
before the Earth-Moon system formed. Extant martian life in the
subsurface is possible. Exploration for martian life and interpretation
of martian fossils needs to be planned with the possibility that martian
and terrestrial life are related by the space transfer of organisms.
Origin of Protocells
David W. Deamer
University of California, Santa Cruz
An important question that must be answered if we are to understand
life's origin fully is whether the living state arose a priori from
pre-existing cellular structures. The alternative is that living
molecular systems were first present as solutions or adsorbed films,
with cellular life developing only at a later evolutionary stage. If
the self-assembly of amphiphilic molecules into membranes preceded the
origin of life, it is plausible that the earliest living systems may
have had access to encapsulated environments. The following concepts
and experimental results support this conjecture.
Bilayers assemble from a variety of amphiphilic compounds.
Although contemporary cell membranes incorporate phospholipids as the
primary component of the lipid bilayer, it is not necessary to think
that complex lipids were required for early cellular life. In fact,
simpler amphiphilic molecules can also assemble into bilayer membranes
(1). These include single-chain amphiphiles such as soap molecules,
glycerol monooleate, oxidized cholesterol, and even detergents like
dodecyl sulfate mixed with dodecyl alcohol. It seems likely that
primitive cells incorporated lipid-like molecules from the environment,
almost as a nutrient, rather than undertaking the more difficult process
of synthesizing complex lipids de novo.
Amphiphilic molecules are present in carbonaceous meteorites.
The presence of amino acids in carbonaceous meteorites supports the
possibility that amino acids were available in the prebiotic environment.
Amphiphilic molecules have also been demonstrated in such meteorites and
furthermore have the ability to self-assemble into barrier
membranes (2, 3). This observation suggests that amphiphiles were present
on the early Earth and available for incorporation into boundary membranes
of primitive cellular organisms.
Bilayer permeability strongly depends on chain length of the component
amphiphilic molecules.
We tend to think of the lipid bilayer as being a nearly impenetrable
barrier to ionic solutes and other large, polar molecules. This leads
to a conundrum when we try to imagine how early forms of cellular life
could have functioned in the absence of highly evolved transport enzymes
that translocate ionic nutrients and metabolites across the bilayer
barrier. It is true that lipid bilayers of contemporary cell membranes
present a significant permeability barrier that is necessary for normal
cell functions, particularly those related to bioenergetics of ion
transport and chemiosmotic ATP synthesis. However, recent results show
that shortening lipid chain length from 18 to 14 carbons increases the
permeation of ionic solutes by several orders of magnitude (4). This
level of permeability is sufficient to encapsulate large molecules such
as proteins and polynucleotides, yet still allow external substrate to
reach an encapsulated enzyme (5). It follows that early cell membranes
could have been composed of shorter chain lipids that provided access
to nutrients for macromolecules undergoing growth and replication in an
encapsulated microenvironment.
Macromolecules can be encapsulated in bilayer vesicles under simulated
prebiotic conditions.
Another conceptual problem has been to imagine how vesicular lipid
bilayers could capture macromolecules, given that the bilayer must
present a nearly impenetrable barrier if the macromolecules are to be
maintained within the membrane-bounded volume. There is a reasonably
straightforward answer to this question. If dispersed lipids are first
mixed with macromolecules such as proteins or nucleic acids, then
subjected to drying and wetting cycles to simulate a tide pool
environment, the macromolecules are readily captured in membrane-bounded
vesicles (6).
Lipid bilayers grow by addition of amphiphilic compounds present in the
bulk phase medium.
It is not sufficient for a primitive cell to replicate its macromolecular
components unless the boundary membrane can also increase in area to
accommodate the internal growth. Recent experimental results from
liposome model systems have provided a useful perspective on primitive
membrane growth processes (7).
Encapsulated polymerases can synthesize nucleic acids.
Polynucleotide phosphorylase (5, 8), and the enzymes required for the
PCR reaction (9) have now been encapsulated in liposomes and shown to
synthesize nucleic acids.
What would be required for the next step, to a true replicating system
of encapsulated polynucleotides that can undergo cell-like growth? In
concept, the answer is straightforward. Externally added substrates
must be supplied to the captured polymerase, and a mechanism for
simultaneous addition of membrane components to the vesicles must
also be available. Furthermore, there should be a regulatory coupling
between growth of the internal molecules and growth of the membrane.
Last, protein enzymes cannot be used as catalysts because they are not
reproduced in such a system. Instead molecules resembling ribozymes
should be components of the system, incorporating both genetic
information and the polymerase activity. In practice, there is still
no way to deal simultaneously with all of these requirements. However,
a few years ago it would have been inconceivable that we would soon
reach a point at which intravesicular synthesis of nucleic acids
became a reality. It seems likely that a laboratory version of an
encapsulated replicating system of molecules capable of growth and
perhaps evolution will be achieved in the
next decade.
References
1. Hargreaves, W. R., and Deamer, D. W. 1978. Liposomes from ionic
single-chain amphiphiles. Biochemistry 17:3759-3768.
2. Deamer, D. W. 1985. Boundary structures are formed by organic
components of the Murchison carbonaceous chondrite. Nature 317:792-794.
3. Deamer, D. W., and Pashley, R. M. 1989. Amphiphilic components
of carbonaceous meteorites. Orig. Life Evol. Biosphere 19:21-33.
4. Paula, S., Volkov, A. G., Van Hoek, A. N., Haines, T. H., and Deamer, D. W.
	1996. Permeation of protons, potassium ions and small polar
molecules through phospholipid bilayers as a function of membrane
thickness. Biophys. J. 70:339-348.
5. Chakrabarti, A., Breaker, R. R., Joyce, G. F., and Deamer, D. W. 1994.
Production of RNA by a polymerase protein encapsulated within phospholipid
vesicles. J. Mol. Evol. 39:555-559.
6. Deamer, D. W., and Barchfeld, G. L. 1982. Encapsulation of
macromolecules by lipid vesicles under simulated prebiotic
conditions. J. Mol. Evol. 18:203-206.
7. Walde, P., Wick, R., Fresta, M., Mangone, A., and Luisi, P. L. 1994a.
Autopoietic self-reproduction of fatty acid vesicles.
J. Am. Chem. Soc. 116:11649-11654.
8. Walde, P., Goto, A., Monnard, P.-A., Wessicken, M., and Luisi, P. L.
1994b. Oparin's reactions revisited: Enzymatic synthesis of
poly(adenylic acid) in micelles and self-reproducing vesicles.
J. Am. Chem. Soc. 116:7541-7547.
9. Oberholzer, T., Albrizio, M. and Luisi, P. L. 1995. Polymerase
chain reaction in liposomes. Current Biology 2:677-682.
Biological Perspective on the Early Earth
and the Chemistry that Spawned Life
Norman R. Pace
University of California
Our perspective on the nature of the geochemical processes that gave
rise to life on Earth has improved enormously over the past decade.
Astronomical and geochemical studies provide an increasingly clear
view of the character of the early Earth, at the time of the origin
of life. Microfossil work is increasingly sophisticated in its
interpretations and has provided solid evidence for life at 3.5
billion years ago or before. Perhaps most dramatically, we can now
infer with increasing confidence the biochemical nature of the
earliest life, which also provides information on the environment
in which that life came to be. The record that we interpret for the
nature of the earliest life is a quantitative one, phylogenetic trees
based on molecules that are found in all of modern-day life and so must
have been present in the earliest genetic cell. Together, the biological
and physical information paint a harsh cradle for life, a violent world
of volcanism and hydrothermal interaction with the molten bones of the
forming planet. We must couch our speculation on the origin of life in
terms of these findings.
The Phylogenetic Record
The geophysical record tells us that the early Earth was hot and
volcanic, but it tells us nothing about the nature of the earliest
organisms. By analysis of molecular sequences, we can infer maps of
the course of evolution. In this analysis, "homologous" (of common
ancestry) genes from different organisms are compared, and the number
of differences in their nucleotide sequences is counted. The greater
the number of differences between the genes, the more evolutionarily
separated are the pairs of organisms. The "evolutionary distance,"
the fraction of sequence differences between pairs in a collection
of sequences, can be used to infer "phylogenetic trees," maps of
evolutionary diversification. Because of their slow rates of evolutionary
change, small-subunit (16S or 18S) ribosomal RNA (rRNA) sequences have
been used most extensively for phylogenetic classification of all life.
The molecular phylogenetic studies provided, for the first time, a
clear view of the evolutionary scope of life on the grand scale. The
figure shows a phylogenetic tree based on rRNA sequence comparisons.
It can be viewed as a road map of evolution. Evolutionary distances
between organisms (i.e., the number of sequence-changes separating
them) are represented by the lengths of line-segments (see scale bar
in the figure). The phylogenetic results show that known biodiversity
falls into three primary groupings, or 'Domains': Archaea (formerly
archaebacteria), Bacteria (eubacteria), and Eucarya (eukaryotes).
Although both are prokaryotic (i.e., lack a nuclear membrane), Archaea
and Bacteria are phylogenetically distinct from one another. Indeed,
there is strong evidence indicating that Archaea and Eucarya are more
closely related to each other than either is to Bacteria: the origin
of the extant evolutionary lineages, the root of the tree of all life
(indicated as "origin" in the figure), appears to separate the Bacteria
from the other two forms.
Some of the lessons gleaned from the universal tree of life contradict
long-held beliefs. For instance, the recognition of the deep
evolutionary divergence between Archaea and Bacteria shattered the
entrenched notion of evolutionary unity among prokaryotes. The
phylogenetic results also show that the eukaryotic nuclear line of
descent, Eucarya, is as old as the prokaryotic lines, Archaea and
Bacteria. The idea that the eukaryotic cell arose relatively recently
(1-1.5 billion years ago was a common assumption based on no credible
data), from the fusion of two prokaryotes, proved essentially incorrect.
To be sure, the rRNA-based data confirm that mitochondria and
chloroplasts are of endosymbiotic bacterial origin, as postulated
long ago (note in the figure the association of the mitochondrial
and chloroplast lineages with Bacteria). The organellar lineages
emerge from subgroups of the Bacteria, however, so their evolution
must have occurred relatively late in the history of life. Some
microbial eukaryotic lineages seem never to have had mitochondria
and chloroplasts so may have diverged from the eukaryotic line of
descent prior to the incorporation of the organelles.
Although phylogenetic trees are based on modern-day organisms, we
can, in principle, infer properties of ancestral organisms by
"parsimony":  if two organisms have a common trait, then it is
likely that their common ancestor had the same trait. With that
perspective, we can infer that the last common ancestor of all life
possessed DNA, a rudimentary transcription system, ribosomes similar
to the modern ones, and many other traits-all the commonalities of
today's life. More specialized traits of the earliest organisms also
are suggested by the phylogenetic results. In the figure, organisms
shown in bold are high-temperature organisms, extreme thermophiles.
It is noteworthy that all of the most deeply diverging lineages in
both Bacteria and Archaea are thermophiles. This suggests that their
common ancestor, the common ancestor of all life, was thermophilic as
well. Additionally, all of these types of organisms have physiologies
consistent with a geothermal setting. All thrive by metabolizing
compounds such as hydrogen, sulfur, and iron.
The Setting for the Origin of Life
We can now imagine, based on solid results, a fairly credible scenario
for the terrestrial events that set the stage for the origin of life.
 It seems fairly clear, now, that the earliest Earth was, in essence, a
molten ball with an atmosphere of high-pressure steam, carbon dioxide,
nitrogen, and the other products of volcanic emission from the
differentiating planet. The cooling curve of the molten Earth would have
reached the critical point of water by roughly 4.2-4.4 billion years ago;
then it would have rained, depositing during an unknown time period 3-4
kilometers of water over the hot crust. It seems unlikely that any
landmass would have reached above the waves, to form the "tidal pools"
invoked by some theories for the origin of life. This is because the
early crust would have been dynamic and thin and consequently probably
incapable of supporting a continental mass sufficient to reach the
surface of the overlying ocean. Instead, the conspicuous feature of
that early Earth would have been the zone of interaction between the
water and the cooling crust. Water would have taken the main role of
conducting heat from the interior of the Earth, as it does now along
the Mid-Ocean Ridge; the hydrosphere would have been in dynamic
interaction with the forming crust through convective hydrothermal
circulation. We can imagine that this early hydrothermal circulation
would have stirred up a thick sludge of volcanic elements, probably
highly enriched with reactive kerogens and organic tars extracted by
convection from the differentiating planetary mass. It was in this
sludge, a perfect setting for complex organic surface-chemistry
supported by pyrite and basaltic glasses, that life probably came
to be. This physical scenario is probably a common one in the formation
of rocky, iron-rich planets. If so, then the origin of life may be a
common consequence of planetary origins. Perhaps the question is not
the probability of the origin of life but rather the probability that
life, having arisen, survives and comes to dominate a planet.
The Chemistry of the Origin of Life
We know very little about the course of chemical events that gave rise
to life's molecules. Since the setting was geothermal, the most fruitful
arena for experimentation would seem to be the interactions of iron,
sulfur, and hydrogen with carbon compounds. This notion is also
indicated by the biological record: iron and sulfur are the basis
for many universally conserved biological catalysts which, because of
their universality, probably were employed by the earliest life. Since
evolution builds upon the pre-existing, iron and sulfur are indicated as
main proto-physiological catalysts. For the same reason, I think it
unlikely that clays or like minerals played much role in the origin
of life. If they had, we might expect materials such as aluminum and
silicate to be widely distributed in biology. Instead, these compounds
are rare.
Considerable attention has focused on RNA as the first catalyst with
genetic relevance, but the evidence for this is hypothetical: RNA is
the only (known) biological molecule that is capable of serving as
both catalyst and genetic information. As argued previously, however,
the fragile chemical nature of RNA is inconsistent with the
hydrothermal fluids that bathed early life. There probably was no
"first self-replicating catalyst." Rather, following the tenets of
biology, it seems much more likely that RNA, DNA, and the other
molecules of cells co-evolved, as generally is the case with interacting
organisms today.
Intelligent Life in the Universe
Frank Drake
SETI Institute
The recent discoveries of other planetary systems and inconclusive
evidence for ancient life on Mars have supported the long-held view
that intelligent life on Earth was the result of common processes which
should have occurred in many planetary systems. The most substantial
controversies remaining concern the frequency with which biotas produce
intelligent, technology-using species, and the lengths of time that
technological civilizations produce detectable manifestations of their
existence. It would appear that these two controversies can only be
resolved through the detection of extraterrestrial intelligent life.
In any case, the planetary and Martian discoveries encourage further
searches for extraterrestrial intelligent life and have served to
motivate increased contributions of resources to searches and the
magnitude of the searches themselves.
Analyses of possible methods to detect extraterrestrial technology
still tend to favor radio searches, and thus most current searches
continue to search for radio transmissions, primarily at microwave
frequencies. However, modest searches at optical and infrared
wavelengths are in progress and may grow in ability in the future.
It is clear that there is no logic which can conclusively identify
the search method most likely to succeed, and thus all reasonable
searches are worthy of support.
Progress in radio searches has been enormous over the 36 years that
such searches have been carried out. The search capabilities of SETI
systems have grown very nearly exponentially over that time, with a
capability-doubling time of about 2/3 of a year. The search systems
of today are about 100 trillion times more powerful than those of 1960.
This improvement has benefited greatly by the construction of large
radio telescopes. More important has been the application of modern
computer technology to produce affordable multichannel radio receivers,
with contemporary search systems being capable of monitoring more than
100 million channels simultaneously. Of great importance has been the
development of computer hardware and software which can search for a
variety of intelligent signal forms in the data produced by the
multichannel system. Also of great importance has been the
implementation of search systems in which two radio telescopes,
widely separated, are used in approaches which allow radio frequency
interference to be quickly and reliably identified. This is costly
but very effective in dealing with the major impediment met in
current searches, terrestrial radio interference.
A recent unfortunate development in this field has been the total
withdrawal of financial support by U.S. federal agencies. This withdrawal
has ignored the strong support within the scientific community for such
research and appears to be driven by politics outside the scientific
community. However, a positive development has been the development of
massive financial support by private individuals and foundations; this
support has been adequate to continue existing programs and even to
improve them.
The search for manifestations of extraterrestrial technology still
appears to be one of the more promising and much less costly methods to
detect extraterrestrial life. If successful, it obviously will provide
much more interesting data than will just the discovery that life exists
on another planet.
There is still room to expand existing search systems greatly without
requiring new technological developments. This can come about through
the purchase of still more systems similar to those already in place.
One of the most important steps which could be taken to enhance search
capability would be the provision of large radio telescopes dedicated
to SETI searches. These could be relatively inexpensive, since they
would not need the range of capabilities normally sought in radio
telescopes. Their construction would lead to faster searches, and
higher quality searches, since there would be adequate opportunity
to optimize all aspects of the telescope and its receiver system for
SETI. It would also make searches much more cost effective by eliminating
the costs involved in moving complicated search systems, and their
operators, to remote sites. In the long run, a goal should be to build
and operate large SETI systems on the far side of the moon, the only
place in the solar system which is shielded from human radio transmissions
at all times.
Exploring Mars for Evidence of Past or Present Life:
Roles of Robotic and Human Missions
Jack D. Farmer
NASA-Ames Research Center
During the coming decade, robotic field science will play a fundamental
role in exploring Mars for evidence of past life and/or prebiotic
chemistry. To create a context for such exploration, we especially
need to understand the mineralogy and chemistry of the Martian surface.
We have learned that the preservation of biological signatures in rocks
on Earth is favored by rapid mineralization processes that are restricted
to a comparatively small number of geological settings. Thus, a detailed
knowledge of surface mineralogy will provide valuable clues about past
Martian environments as a necessary context for future exobiological
exploration.
Information about past climate and volatile history resides in
mineralogy, and specifically the mineralogy associated with aqueous
sedimentary processes. The targets of choice in exploring for past
life on Mars are therefore aqueous sedimentary deposits, particularly
those in ancient terranes that date to the early clement period in
Mars' history when liquid water was present at the surface. Site
selection for landed missions will be critical because we cannot
expect to land just anywhere on Mars and find the kinds of deposits
that are likely to preserve evidence of past Martian life or climatic
history. Even if life never developed on Mars, the same kinds of
target deposits are likely to harbor a prebiotic chemical record that
would be crucial in understanding the origin of terrestrial life.
Achieving a scientific consensus on the question of past Martian life
is likely to require multiple sample returns, followed by extensive
interdisciplinary studies carried out in labs on Earth.
To find the best places to explore, we will need to optimize site
selection by carrying out high spatial resolution mineralogical mapping
from orbit, with ground truthing of surface mineralogy using well-placed
and appropriately instrumented robotic rovers. Remote sensing studies of
geological terranes on Earth suggest that spatial resolutions of
< 30 m/pixel for visible range imaging and < 100 m/pixel for
multispectral imaging may be required to accurately identify
high-priority targets on Mars for ancient climate and life studies.
Increased landing precision and highly mobile rovers are technological
priorities that will ensure we reach high-priority targets during
nominal mission times. Precise mobility requirements will vary with
the science goals of each mission, reflecting a balance between
landing precision and the size of target deposits. Landing targets
will need to be pre-selected from orbital imaging, and the higher
the spatial resolution the better for precisely locating the best
landing sites. To optimize for a high science return, it is likely
that rovers will need to be able to traverse distances exceeding
their own landing errors (perhaps multiple kilometers or more during
nominal mission times). In order to target samples for remote analysis
by Earth-based investigators and for pre-planning traverses using
virtual terrain models, rovers will also require instrumentation
for analyzing the composition of rocks at a distance (e.g., "spot"
 spectrometers).
Considering the small amount of material that will be brought back
to Earth, in situ mineralogical analysis will be crucial for selecting
the best materials for sample return. Rock surfaces are likely to be
covered by dust or weathering rinds, and accurate compositional analysis
will require access to freshly exposed interior rock surfaces.
Microscopic imaging of rock surfaces will provide valuable microtextural
and mineralogical information to assist in targeting smaller areas on
rock surfaces for more detailed compositional analyses. Given the
minimal sample preparation required, spectral analysis (e.g., by
infrared or laser Raman spectroscopy), especially in combination
with reflectance techniques for elemental analysis (e.g., alpha
proton X-ray spectroscopy or X-ray fluorescence), provides an
especially favorable approach to in situ mineralogical and organic
analysis for early missions.
The exploration for extant Martian life will require a fundamentally
different approach from that used for ancient climate studies and
Exopaleontology. The Viking missions revealed the surface of Mars to
be inhospitable for life as we know it, owing primarily to the
absence of liquid water. However, it has also been suggested that
life could exist in the subsurface of Mars (perhaps tens to hundreds
of kilometers depth), where an extensive hydrosphere could be present.
It is also possible that areas of rising ground water may provide
shallow subsurface oases capable of sustaining life. We know that
on Earth, subsurface environments are a haven for a wide variety of
heterotrophic microorganisms that do not necessarily require a direct
connection to the surface environment for their survival. During the
upcoming decade of exploration, we could initiate systematic orbital
searches for such "oases" using high spatial resolution multispectral
remote sensing to explore for anomalous concentrations of water vapor,
methane, or other reduced gases, as well as thermal anomalies suggestive
of near-surface hydrothermal systems or fumeroles. This kind of
exploration would be especially interesting if targeted at low elevation
areas where atmospheric density is higher and where crustal thinning is
likely to have occurred, increasing the heat flow to the surface (e.g.,
 the floors of chasmata).
Given the present technological challenges of deep drilling, robotic
platforms are likely to provide very limited access to the Martian
subsurface. As presently envisioned, rover-based drilling systems are
unlikely to penetrate much deeper than a few meters. The implementation
of a subsurface exploration program to search for extant Martian life
could require drilling to kilometer depths. Drilling to such depths
will quite likely require a human presence. It follows that extensive
exploration of the Martian subsurface to search for deep subsurface
water and an extant microbiota may provide the most compelling reasons
for carrying out human missions to Mars.
A fundamental issue facing the scientific community and public at large
is planetary protection. We are embarking upon a new decade of Mars
exploration with a clearly identified objective of sample return. Aside
from the concerns associated with forward contamination of Mars,
particularly in connection with missions aimed at detecting extant
life, the threat of back-contamination raises issues of broader
concern. These issues have yet to be fully addressed, and concerns
over such things as the reliability of sample containment technologies,
the effects of sterilization on the science return of missions, and the
added costs to missions must be clearly understood and transformed into
an effective policy. Planetary protection issues could be a sleeping
giant that, once awakened, could dictate the future of Mars exploration.
Astrobiology Workshop Attendees
Acevedo, Sara		SETI Institute
Allamandola, Louis	NASA Ames Research Center
Anderson, Phillip	Princeton University
Arnold, James		NASA Ames Research Center
Bada, Jeffrey		Scripps Institute of Oceanography
Baldwin, Betty		NASA Ames Research Center
Baldwin, Ken		University of California, Irvine
Berry, William		 NASA Ames Research Center
Bielitzki, Joseph	NASA Ames Research Center
Billingham, John	SETI Institute
Borucki, William	NASA Ames Research Center
Boyd, Jack		NASA Ames Research Center
Briggs, Geoff		NASA Ames Research Center
Brinton, Henry		NASA Headquarters
Brownlee, Donald	University of Washington
Bullock, Mark		University of Colorado
Bunch, Ted		NASA Ames Research Center
Cabrol, Nathalia	National Research Council
Carr, Michael		US Geological Survey
Cassen, Patrick		NASA Ames Research Center
Chahine, Mous		Jet Propulsion Laboratory
Chan, Roland		NASA Ames Research Center
Chandler, David		Boston Globe
Chang, Polly		Stanford University
Chang, Sherwood		NASA Ames Research Center
Chatfield, Robert	NASA Ames Research Center
Christiansen, Marvin	Lockheed-Martin Engineering and Sciences
Chyba, Christopher	Princeton University
Clark, Benjamin		Lockheed-Martin
Cohen, Malcolm		NASA Ames Research Center
Coleman, Paul		Universities Space Research Associates
Condon, Estelle		NASA Ames Research Center
Connell, Kathleen	Aerospace States Association
Corbin, Barbara		NASA Ames Research Center
Cordova, France		University of California, Santa Barbara
Coulter, Gary		Colorado State University
Cowing, Keith		American Institute of Biological Sciences
Cronin, John		Arizona State University
Cudaback, David		University of California, Berkeley
Cullers, Kent		SETI Institute
Cuzzi, Jeff		NASA Ames Research Center
D'Antoni, Hector	NASA Ames Research Center
David, Leonard		Space News
Davidson, Keay		San Francisco Chronicle
Deamer, David		University of California, Santa Cruz
Dean, William		NASA Ames Research Center
DesMarais, David	NASA Ames Research Center
DeVincenzi, Donald	NASA Ames Research Center
DeVore, Edna		SETI Institute
Doyle, Lawrence		SETI Institute
Drake, Frank		SETI Institute
Dressler, Alan		Carnegie Observatories
Duke, Michael		Lunar and Planetary Institute
Ehrlich, Anne		Stanford University
Elachi, Charles		Jet Propulsion Laboratory
Estes, Jack		University of California, Santa Barbara
Farmer, Jack		NASA Ames Research Center
Feldman, Lewis		University of California, Berkeley
Franklin, Lewis		Stanford University
Fuller, Charles		University of California, Davis
Ganapol, Barry		University of Arizona
Ganong, W. Francis	University of California, San Francisco
Gibson, Everett		NASA Johnson Space Center
Gore, Warren		NASA Ames Research Center
Greeley, Ronald		Arizona State University
Greenwood, MRC		University of California, Santa Cruz
Haberle, Robert		NASA Ames Research Center
Hargens, Alan		NASA Ames Research Center
Harper, Lynn		NASA Ames Research Center
Haynes, Norman		Jet Propulsion Laboratory
Hines, Michael		NASA Ames Research Center
Hollenbach, David	NASA Ames Research Center
Holley, Daniel		San Jose State University
Holton, Emily		NASA Ames Research Center
Hubbard, Scott		NASA Ames Research Center
Hutchison, Anne		NASA Ames Research Center
Kasting, James		Penn State University
Kauffman, Stuart	Santa Fe Institute
Kronenberg, Amy		Lawrence Berkeley Laboratory
Langhoff, Stephen	NASA Ames Research Center
Lederberg, Joshua	Rockefeller University
Levine, Ben		Institute for Exercise and Environmental Medicine
Levinthal, Elliot	Stanford University
Levison, Harold		Southwest Research Institute
Liang, Shoudan		Penn State Unviersity
Lissauer, Jack		NASA Ames Research Center
Lomax, Terri		Oregon State University
Luhmann, Janet		University of California, Berkeley
Lusignan, Bruce		Stanford University
Lyons, Richard		Lockheed-Martin Engineering Services
MacElroy, Robert	NASA Ames Research Center
Marcy, Geoffrey		University of California, San Francisco
Marlaire, Michael	NASA Ames Research Center
Martin, Frank		Lockheed Martin
May, James		Center for Science, Technology and Information Resources
McDonald, Henry		NASA Ames Research Center
McDonald, Ian		Penn State University
McKay, Christopher	NASA Ames Research Center
McKay, David		NASA Johnson Space Center
Mead, Susan		NASA Ames Research Center
Meyer, Michael		NASA Headquarters
Morrison, David		NASA Ames Research Center
Munechika, Ken		Moffett Federal Air Field
Nealson, Ken		Center for Great Lakes Studies
Nicogossian, Arnauld	NASA Headquarters
Nuth, Joseph		Goddard Space Flight Center
Pace, Norman		University of California, Berkeley
Parazynski, Scott	NASA Johnson Space Center
Parkinson, Bradford	Stanford University
Pendleton, Yvonne	NASA Ames Research Center
Penwarden, Michael	Computer Life Magazine
Pepin, Robert O.	University of Minnesota
Peterson, David		NASA Ames Research Center
Pierson, Tom		SETI Institute
Pilcher, Carl		NASA Headquarters
Pohorille, Andrew	University of California, San Francisco
Pool, Sam		NASA Johnson Space Center
Potter, Christopher	NASA Ames Research Center
Reightler, Ken		Lockheed-Martin
Rosen, Robert		NASA Ames Research Center
Ross, Muriel		NASA Ames Research Center
Rothschild, Lynn	NASA Ames Research Center
Rummel, John		Marine Biological Laboratory, Woods Hole
Russell, Phillip	NASA Ames Research Center
Sadeh, Willie		Colorado State University
Sandford, Scott		NASA Ames Research Center
Sargent, Anneila	California Institute of Technology
Schild, David		Lawrence Berkeley National Laboratory
Schulte, Mitch		Washington University
Shostak, Seth		SETI Institute
Shu, Frank		University of California, Berkeley
Skiles, Jay		NASA Ames Research Center
Sleep, Norman		Stanford University
Soffen, Jerry		Goddard Space Flight Center
Souza, Ken		NASA Ames Research Center
Sprigg, William		National Research Council
Stofan, Andrew		Lockheed Martin
Strawa, Tony		NASA Ames Research Center
Supulver, Kim		University of California, Santa Cruz
Tarter, Jill		SETI Institute
Toon, Brian		University of Colorado
Townsend, William	NASA Headquarters
Vandonzel, Judy		NASA Ames Research Center
Wade, Charles		NASA Ames Research Center
Weiler, Edward		NASA Headquarters
Welch, Jack		University of California, Berkeley
Wetherill, George	Carnegie DTW
Wiersema, Juliet	SETI Institute
Wolgemuth, Debra	Columbia University
Wong, Carla		NASA Ames Research Center
Woolum, Dotty		California Institute of Technology
Young, Lawrence		Massachusetts Institute of Technology
Zabor, Susan		NASA Ames Research Center
Zahnle, Kevin		NASA Ames Research Center
Zubrin, Robert		Pioneer Astronautics