What is Astrobiology?

Astrobiology is the scientific study of the origin, evolution,
distribution, and future of life in the universe. The study of
astrobiology brings together researchers from historically separate
scientific fields such as microbiology, ecology, astronomy, geology,
paleontology, and chemistry and encourages them to work together to
answer the most fundamental questions science can pose: What is life?
How did we get here? Are we alone in the universe? How can we tell if we
are?

These questions have been asked for generations, but it is only recently
that we have had the technology and the knowledge to address them from a
scientific perspective. Understanding the answers requires that
researchers develop a larger perspective than is possible within any one
field of science. Astrobiology is a collaborative effort, transcending
traditional scientific discipline boundaries.

Researchers begin by studying life on Earth, the only place in the
universe where we know life exists. How did life begin here? How has it
responded to changes in the environment? How has it changed the
environment? What conditions does Earthly life need to exist? The
scientists of the NASA Astrobiology Institute, among others, seek to
answer these questions and many more.

The next step is to look beyond Earth to the possibilities of life
elsewhere. Most of life on the Earth is microbial, and it's likely that
microorganisms will be the type of life we will find elsewhere in the
universe. Astrobiology researchers try to figure out how to search for
microbes and other life on the planets and moons in our Solar System and
on Earth-like planets orbiting other stars. How do you detect evidence
of biology when you can't hold a soil sample in your hand? Does life
leave its mark on a planet so that we can detect its presence remotely?
Is life common? Or is our life-filled Earth rare and unique?

What is life? Where does life exist?

The following background information supports Activities I and II

Astrobiologists also struggle with the question: what exactly does it
mean to be alive? Life as we know it here on Earth exchanges energy and
materials with the environment. Life forms grow, develop, produce waste
products, and reproduce, storing genetic information in DNA and RNA and
passing it from one generation to the next. Life evolves, adapting to
changes in the environment and changing the environment in return. The
basic unit of living things is the cell. Life is based on the chemistry
of carbon and requires liquid water.

The "liquid" part is important. It's very hard to transport important
substances, like nutrients or metabolites, from one place to another
within a solid, and it's hard to control that transport in a gas.
Liquids can do it well.

Water has many qualities that make it an ideal medium for the cellular
biochemical reactions necessary for life. The chemical properties of
water molecules help the other molecules of life, such as DNA, proteins,
structural building blocks of cellular architecture and enzymes that
speed up chemical reactions, and sugars (such as glucose, a common sugar
used for energy), orient themselves in the proper three-dimensional
shapes needed to carry out their functions in the cell. In order to
maintain osmotic balance (and avoid drying out or swelling up), cells
also need dissolved salts such as calcium and potassium. Water has
wonderful capabilities to dissolve the nutrients and salts on which life
depends, and the ability to move these molecules into and out of cells
as it flows.

Water is the only chemical compound that is found naturally on Earth in
all three physical states -- as a gas, a liquid, and a solid. This
property allows water to cycle through evaporation, condensation, and
precipitation, between reservoirs in the oceans, on land, and in the
air. Indeed, water is one of the few substances that can be liquid at
the temperatures and pressures typical of the Earth's surface (mercury
and liquid ammonia are the others). Water will remain liquid over an
extremely large range of temperatures, freezing at 0°C (32°F) and
boiling at 100°C (212°F). Adding salt will lower the freezing
temperature, and adding pressure can raise the boiling point, increasing
the range even more. Plus, it takes a lot of energy to raise the
temperature of water a few degrees. All of which means that temperatures
on Earth can undergo rather large variations before the liquid water
freezes or boils away.

Some microbial and all multicellular life on Earth depends on the water
molecule for survival in another, fascinating way. Many microorganisms
and plants carry out photosynthesis, the biochemical process of creating
sugars and atmospheric oxygen (O2) from CO2 in the air and light energy
from the Sun. Those same microorganisms and plants can then consume the
sugars they create, converting them to usable energy needed for growth
and reproduction. Many other organisms, such as animals (including
humans), later consume the plants as food. The reactions of
photosynthesis use the light from the Sun and liquid water to remodel
CO2 from the atmosphere, forming sugars and breathable oxygen.

Life in extreme environments

On Earth, life is found anywhere liquid water is present. Only in the
past few decades scientists have realized that "anywhere" includes such
extreme environments as ice-covered Antarctic lakes, hydrothermal vents
on the ocean floor, and porous cracks in deep subsurface rocks. The
organisms that live in these harsh conditions are called extremophiles.
They survive and sometimes thrive in environments once thought too hot,
too cold, too salty, too acidic, too high pressure, too dry, or with too
much radiation for life to exist.

For example, scientists have long known that microbial mats (large
colonies of microbes) are responsible for the beautiful colors observed
in Yellowstone National Park's many hot springs. The water in these
springs tops 90°C (188°F), much too hot to touch. Some hot springs are
also extremely acidic, with pH levels, in a few cases, similar to that
of stomach acid. Yet life thrives in and around them.

In 1977, scientists were stunned to discover abundant life clustered
around hydrothermal vents on the ocean floor thousands of feet below its
surface. The vents form where the Earth's crustal plates crack and
spread apart. Molten rock, or magma, wells up along these cracks,
forming long undersea mountain ranges known as mid-ocean ridges.
Seawater seeps deep into the rock at the cracks, is heated and shoots
back upward through vents nearby, enriched with minerals dissolved from
the rocks along the way. Scientists thought life would be impossible in
the searingly hot temperatures (400°C, 752°F), oppressively high
pressures (thousands of pounds per square inch), complete darkness, and
toxic chemical brew typical near these ocean-floor vents. But the high
pressure keeps the hot water from leaving the liquid state and becoming
a gas. In place of sunlight, microbes living there use chemical
reactions involving hydrogen sulfide, common in the enriched seawater
pouring out of the vent, to generate energy. Other creatures survive by
eating the microbes, or each other. Should the flow of hot, enriched
water slow to a trickle for any reason, the creatures around the vent
would soon die.

Lifeforms discovered at hydrothermal vents include many species of
microbes, mussels, clams, shrimp, and giant tubeworms that can reach ten
feet in length. The tube worms have no stomachs or mouths. They depend
on symbiotic bacteria in their guts for their nutrition, a relationship
that benefits both the worm and the bacteria. Hemoglobin in the worms'
red tips grabs hydrogen sulfide from the water around the vent and
transports it to the bacteria living inside the worm. Using this
hydrogen sulfide as an energy source, the bacteria, in turn, convert
carbon dioxide dissolved in the water into carbon compounds that nourish
the worm.

Researchers have also found bacteria in small pockets of liquid water
embedded twelve feet deep in "solid" lake ice in the McMurdo Dry Valleys
of Antarctica. These valleys are among the coldest, driest places on
Earth, with average temperatures of -20°C (-4°F) and less than 10
centimeters (four inches) of precipitation a year. Small amounts of dirt
(contained within the ice and heated by the Sun) melt small amounts of
the ice surrounding them providing the liquid water needed to support
life. The dirt also provides chemical nutrients for the bacteria that
photosynthesize, grow, and reproduce in the liquid water pockets during
the long Antarctic summer days.

The Rio Tinto in southwestern Spain is another interesting environment
for life. The river has a deep red color, like red wine, because of iron
dissolved in the water. It is highly acidic, with a pH of 2.0 in most of
the river (only slightly less acidic than sulfuric acid). The high
acidity results from chemical reactions between the water and iron and
sulfur minerals in the rocks around the river. Microbes living in the
water also use the iron and sulfur minerals for chemical reactions that
generate energy. Metabolic products from these reactions contribute to
the low pH of the river. Numerous algae and fungi also thrive there.

Scientists have discovered bacteria living in rock 1.5 kilometers
(nearly a mile) underground in an area known as the Columbia River
Basalts in the state of Washington. The bacteria survive in small holes
and cracks in the rock that fill with water from underground aquifers.
They live in complete darkness, at temperatures that approach 40°C
(104°F), because at that depth they are closer to the Earth's hot,
molten core than is life at the Earth's surface. The groundwater reacts
chemically with minerals like olivine and pyroxene in the rock to create
hydrogen gas. The bacteria use this hydrogen, and dissolved carbon
dioxide, to make the methane and other hydrocarbons they use to survive.
The bacteria make a living on rock and water. The scientists who
discovered the bacteria refer to them as \"SLiME\" for Subsurface
Lithoautotrophic Microbial Ecosystem (the word litho refers to rocks;
autotroph refers to an organism that can create its own food).. Indeed,
scientists have calculated that the mass of all the microbes that live
underground equals that of all organisms that live above the surface
(non-microbes, like humans, included).

"Extreme" vs. "Normal"

To extremophiles, the conditions in which they live are "normal" and
"common." To them, the conditions we traditionally associate with life
(moderate temperatures, sea level pressures, plenty of sunlight, an
oxygen-rich atmosphere) are "extreme" and deadly. "Normal" and "extreme"
are relative terms.

Conditions that we think of as "extreme" on Earth may be similar to what
is "common" elsewhere in the solar system. Understanding how life
survives in earthly extreme environments can help scientists better
understand how life could exist on other planets and moons.

One thing extremophiles have in common with the rest of us is that they,
too, require liquid water to survive. Consequently, when scientists
think about non-Earthly places where life may exist, they look for sites
where liquid water either is now or was once at some time in the past.

Life on Mars?

Mars today is a frozen, dry world. Its predominantly carbon dioxide
atmosphere is too thin to support liquid water on its surface, and its
surface temperatures are too cold, averaging -65°C (-85°F). Yet its
surface is covered with winding channels that resemble ancient
riverbeds, and there is water ice frozen in the planet's polar ice caps
and subsurface permafrost. Enormous extinct volcanoes indicate Mars was
once tectonically active, even though its core is now too cold to
support volcanic activity. These geological observations, combined with
the data coming back from the Mars Exploration Rovers
(<http://marsrovers.jpl.nasa.gov/home/>) indicate Mars once may have had
a thicker, warmer atmosphere and liquid water standing and flowing on
its surface.

Could life have emerged at that time? Did it find a way to adapt,
evolve, and survive the shift from moderate to extreme conditions? The
frozen deserts of Antarctica resemble the Mars of today. If life can
persist deep in the ice and underground rocks on Earth, perhaps it
survives in the permafrost or polar ice caps of Mars.

An ocean on Europa?

Europa, a moon of Jupiter which is slightly smaller than Earth's Moon,
is one of the smoothest objects in the solar system. A liquid water
ocean thought to be between 50 and 100 kilometers deep surrounds its
rocky interior, and is covered by a layer of water ice a few kilometers
thick.

Few craters from meteorite impacts mar this moon's icy crust, indicating
Europa's surface may be only a few million years old (on old surfaces,
meteorite strikes accumulate and many craters are seen or can be
counted). Cracks and streaks crisscross Europa's surface. Scientists
think water seeps up through cracks in the ice caused by the
gravitational twisting of the ice sheet, creating the streaks observed
on the surface.

Europa is deformed into a slightly oval shape by Jupiter's strong
gravitational pull, in much the same way the Moon's smaller
gravitational pull causes the oceans to bulge in tides on the Earth. But
Europa also feels a gravitational pull from each of Jupiter's other
large moons, and the combination of all these conflicting tugs causes
Europa to twist and flex as it orbits Jupiter. This tidal flexing has
heated Europa's interior, and may explain the ocean under the ice.

A curious analog to Europa's ice-covered ocean can be found in
Antarctica. In 1996, scientists discovered evidence of a lake of liquid
water deep underneath the ice at Russia's Vostok Station about 1,000
kilometers (621 miles) from the South Pole. Dubbed Lake Vostok, the
liquid water sits under about 3,710 meters (12,172 feet) of ice and may
have been isolated for 500,000 to a million years. Lake Vostok, about
the size of Lake Ontario, is thought to be about 484 meters (1,588 feet)
deep, with about 50 meters (164 feet) of sediment at its bottom. No one
knows if there is any kind of life in Lake Vostok. So far, scientists
have not drilled into the lake's water, because organisms carried in the
drilling equipment can contaminate the lake. Contamination by Earthly
organisms is a concern for researchers who want to look for evidence of
life on Mars and in Europa's ocean, too. Techniques for drilling without
contamination will prove invaluable when landers and subsurface probes
are sent to explore Europa and Mars.

Titan's thick atmosphere

Titan, Saturn\'s biggest moon, is the only moon in the Solar System with
a thick atmosphere. Multiple layers of clouds and haze completely hide
the moon's surface. Methane clouds float close to the surface. A thick
smog of organic molecules, about 300 kilometers (186 miles) above the
ground, and a thin haze high in the outer atmosphere complete the
picture. These clouds and haze keep nearly all sunlight from reaching
Titan's surface, which remains at -180°C (-292°F). While too cold for
liquid water, scientists speculate that Titan may have lakes or oceans
of liquid hydrocarbons like methane and ethane, a constituent of natural
gas here on Earth. Large rocky landmasses, rather like continents, may
rise out of the "oceans" on Titan's surface. Conditions on Titan may
resemble those on Earth when life first began, which is one reason
astrobiologists find this moon of Saturn so intriguing. The
Cassini-Huygens spacecraft, a joint effort of NASA, the European Space
Agency, and the Italian Space Agency, inserted into Saturn's orbit on
June 30^th^, 2004, and will deploy the Huygens probe to Titan in
December 2004. Huygens will enter the murky atmosphere of Titan, and
eventually descend via parachute onto its mysterious surface, collecting
data all the way.

Impact of extremophiles

The extreme environments of Mars, Europa, and Titan may or may not
harbor life. We do not yet know. Fortunately, scientists can use Earthly
extreme environments to test equipment and techniques they may someday
use to search for life on these and other planets and moons. For
example, scientists have drilled into the rocks under Spain's Rio Tinto,
looking for extremophiles. Subsurface conditions here may resemble those
on Mars, where underground liquid water could exist amid rocks rich in
sulfur and iron. Experience with the drilling machines used at Rio Tinto
will give important insight into the design of drilling machines that
could be sent to Mars in the future.

Earthly extremophiles have forever changed our view of life and the
conditions life needs to survive. Their existence has proven that life
can exist in a broad range of environments. They have allowed scientists
to expand their ideas of where to look for life. It is not enough to
know only about biology when looking for life. Life leaves clues that
biologists can detect -- but some clues will go unnoticed or
unappreciated without the collaboration of chemists, geologists,
ecologists, and biophysicists. Astrobiology provides the umbrella under
which scientists trained in these and other fields collaborate to
understand life here on Earth as well as the possibilities of life
elsewhere in the universe.

For more detailed background information on astrobiology, including a
discussion of the origins of life on the young Earth, the Viking
missions to Mars and the search for life there, and other missions to
Mars, please go to http://nai.arc.nasa.gov/poster.

Please take a moment to evaluate this product at
<http://ehb2.gsfc.nasa.gov/edcats/educational_wallsheet> .Your
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How to use this poster

-   The front of this poster illustrates - in words and pictures -
    fundamental questions addressed by astrobiology: What is life? Where
    is it? How do you find it?

-   Three activities have been developed to explore these themes:

> \- Activity I (in its entirety) is on the back of the poster
>
> \- Activities II and III can be found on this website
> http://nai.arc.nasa.gov/poster

-   The back of this poster also contains a science background narrative
    to support the activities, with additional background for Activity
    III on the website.

-   All materials can be found on the website for easy downloading.

For more information about astrobiology, we recommend the following
websites. They include additional astrobiology curriculum materials.

The NASA Astrobiology Institute The SETI Institute Micro\*scope

http://nai.nasa.gov http://seti.org/epo/
[http://microscope.mbl.edu](http://microscope.mbl.edu/)

Image Credits: Images of Earth, Mars, Europa and Titan reproduced with
permission from NASA/JPL/Malin Space Science Systems. Rio Tinto images
provided by the Centro de Astrobiologia. Image of bacteria from La Duke
hotspring, Yellowstone National Park, by D J Patterson, provided by
micro\*scope
([http://microscope.mbl.edu](http://microscope.mbl.edu/))**;** images of
Lake Bonney and bacteria from its ice courtesy of the Priscu Research
Group, Montana State University at Bozeman; images of rock core sample
and bacteria within courtesy of the Geomicrobiology Journal
16(4):307-324, and Hoi-Ying N. Holman.

Content adapted from \"Life in the Universe\" teaching guides,
copyright: SETI Institute.