Findings of the Mars

Special Regions Science Analysis Group

By the MEPAG Special Regions Science Analysis Group

David Beaty, co-chair (Mars Program Office, JPL/Caltech), Karen Buxbaum,
co-chair (Mars Program Office, JPL/Caltech), Michael Meyer, co-chair
(NASA HQ), Nadine Barlow (N. Ariz. Univ.), William Boynton (Univ.
Ariz.), Benton Clark (LMA), Jody Deming (Univ. Wash.), Peter Doran
(Univ. Illinois, Chicago), Kenneth Edgett (MSSS), Steven Hancock (Foils
Engineering), James Head (Brown Univ.), Michael Hecht (JPL/Caltech),
Victoria Hipkin (CSA), Thomas Kieft (NM Inst. Mining & Tech), Rocco
Mancinelli (SETI Inst.), Eric McDonald (Desert Research Inst.),
Christopher McKay (ARC), Michael Mellon (Univ. Colorado), Horton Newsom
(Univ. NM), Gian Ori (IRSPS, Italy), David Paige (UCLA), Andrew
Schuerger (Univ. Florida), Mitchell Sogin (Marine Biological Lab), J.
Andrew Spry (JPL/Caltech), Andrew Steele (CIW), Kenneth Tanaka (USGS,
Flagstaff), Mary Voytek (USGS, Reston)

July 14, 2006

# 

**This report has been approved for public release by JPL Document
Review Services (CL#06-0854), and may be freely circulated.**

[Recommended bibliographic citation:]{.underline}

MEPAG SR-SAG (2006). Findings of the Mars Special Regions Science
Analysis Group, Unpublished white paper, 76 p, posted June 2006 by the
Mars Exploration Program Analysis Group (MEPAG) at
<http://mepag.jpl.nasa.gov/reports/index.html>.

or

Beaty, D.W.; Buxbaum, K.L.; Meyer, M.A.; Barlow, N.G.; Boynton, W.V.;
Clark, B.C.; Deming, J.W.; Doran, P.T.; Edgett, K.S.; Hancock, S.L.;
Head, J.W.; Hecht, M.H.; Hipkin, V.; Kieft, T.L.; Mancinelli, R.L.;
McDonald, E.V.; McKay, C.P.; Mellon, M.T.; Newsom, H.; Ori, G.G.; Paige,
D.A.; Schuerger, A.C.; Sogin, M.L.; Spry, J.A.; Steele, A.; Tanaka,
K.L.; Voytek, M.A.; (2006). Findings of the Mars Special Regions Science
Analysis Group, Unpublished white paper, 76 p, posted June 2006 by the
Mars Exploration Program Analysis Group (MEPAG) at
http://mepag.jpl.nasa.gov/reports/index.html.

[Correspondence authors:]{.underline}

Inquiries should be directed to David W. Beaty
(<David.Beaty@jpl.nasa.gov>, 818-354-7968), Karen L. Buxbaum
(<Karen.L.Buxbaum@jpl.nasa.gov>, 818-393-1135), or Michael A. Meyer
(<mmeyer@mail.hq.nasa.gov>, 202-358-0307).

# Executive Summary

Introduction and approach

Current Planetary Protection policy designates a categorization IVc for
spacecraft potentially entering into a "special region" of Mars that
requires specific constraints on spacecraft development and operations.

NASA requested that MEPAG charter a Special Regions Science Analysis
Group (SR-SAG) to develop a quantitative clarification of the definition
of "special region" that can be used to distinguish between regions that
are "special" and "non-special" and a preliminary analysis of specific
environments that should be considered "special" and "non-special.\"

The SR-SAG used the following general approach: Clarify the terms in the
existing COSPAR definition; establish temporal and spatial boundary
conditions for the analysis; identify applicable threshold conditions
for propagation; evaluate the distribution of the identified threshold
conditions on Mars; analyze on a case-by-case basis those purported
geological environments on Mars that could potentially exceed the
biological threshold conditions. Furthermore, describe conceptually the
possibility for spacecraft-induced conditions that could exceed the
threshold levels for propagation.

The following represent the results of the SR-SAG study in which
"special regions" are more practically defined, a comprehensive
distillation of our current understanding of the limits of terrestrial
life and relevant martian conditions, and an analytical approach is
presented to consider special regions with current and future
improvements in our understanding. The specific findings of the SAG
reported in the executive summary are in bold.

definition

The existing definition of "special region" (from COSPAR 2002 & 2005,
NASA, 2005) is "... a region within which terrestrial organisms are
likely to propagate, [or]{.underline} a region which is interpreted to
have a high potential for the existence of extant Martian life forms.
Given current understanding, this applies to regions where liquid water
is present or may occur. " The SR-SAG determined that in order to
proceed with identifying special regions, some words needed
clarification. The word propagate is taken to mean reproduction (not
just growth or dispersal). Also, the focus on the word "likely" is taken
to apply to the probability of specific geological conditions during a
certain time period and not to probability of growth or terrestrial
organisms. While the report does concentrate on the salient parameters
of forward contamination and martian environmental conditions, it does
not address the second clause of the definition concerning probability
of martian life, as there is no information.

The study limited itself to special regions that may exist on Mars to
environmental conditions that may exist within the next 100 years, a
period reasonably within our predictive capabilities and within which
astronauts are expected to be on the surface of Mars. The SAG also
considered only the upper five meters of the red planet as the maximum
depth to which current spacecraft could access as a consequence of
failure during entry, descent and landing. Environments deeper than five
meters were considered important as possible habitats for life but were
also considered in need of specific information about the expected
nature of the environment to be accessed and the operational approach
taken by the robotic platform, and therefore should be approached on a
case-by-case basis.

limits to microbial life

The approach of the study group was to find any terrestrial
representative that demonstrated the ability to reproduce under the
worst environmental conditions. Although many factors may limit
microbial growth and reproduction, the known overriding environmental
constraints on Mars are low temperature and aridity, and a surface that
is bathed in ultraviolet and galactic cosmic radiation.

Life on Earth has been able to survive extremely low temperatures, but
for this study, the figure of merit is the ability to reproduce. An
extensive review of the literature on low temperature
metabolic/reproductive studies reveals an exponential decrease in
microbial metabolism, enabling long-term survival maintenance or perhaps
growth. However, experiments and polar environments themselves have
failed to show microbial reproduction at temperatures below -15°C. **For
this reason, with margin added, a temperature threshold of -20^o^C is
proposed for use when considering special regions.**

Although many terrestrial microorganisms can survive extreme
desiccation, they all share the absolute requirement for liquid water to
grow and reproduce. Various measures are used to quantify the
availability of liquid water to biological systems, but the one that was
used to integrate biology and geology for this analysis was water
activity (a~w~). Pure water has a water activity of 1.0, and water
activity decreases with increasing solute concentration and with
decreasing relative humidity. Some example water activities are: sea
water a~w~ = 0.98, saturated NaCl = 0.75, ice at -40^o^C = 0.67. For
this application, water activity has the advantage in that it is a
quantity that can be derived and measured, and applied across multiple
length scales in equilibrium. The lowest known water activity that
allows microbial growth is for a yeast in an 83% (W/V) sucrose solution
where a~w~ = 0.62. **Based on current knowledge, terrestrial organisms
are not known to be able to reproduce at a water activity below 0.62;
with margin, an activity threshold of 0.5 is proposed for use when
considering special regions.**

water on Mars

Water on Mars in best analyzed in two broad classifications: the
portions of Mars that are at or close to thermodynamic equilibrium and
those that are in long-term disequilibrium.

In considering martian equilibrium conditions, the repeatability of
thermal inertia results from data set to data set suggests that
numerical thermodynamic models are generally accurate to better than a
few degrees during most seasons and are even more accurate on an annual
average. Comparison between Mars Odyssey Gamma Ray Spectrometer (GRS)
measurements and theoretical models of ice stability based on these same
thermodynamic numerical models demonstrates excellent agreement between
theory and observation. A critically important value of models is that
they have predictive value down to spatial scales much finer than that
achievable by observational data, and so, although there are macroscopic
processes that can produce distinct departures from equilibrium, the
scale tends to be local to regional, not microscopic.

Where ice is in vapor-diffusive exchange with the atmosphere, the
equilibrium temperature (the frostpoint), is at about -75°C on
contemporary Mars. Ice is not stable with respect to sublimation in
places where diurnal or seasonal temperature fluctuations significantly
exceed -75°C. Thus Mars' ample supply of near-surface water is
stubbornly sequestered in solid form at temperatures below the frost
point, either on the polar caps or in vast high latitude, subsurface
deposits. While the surface of Mars at many low-latitude locations may
exceed 0°C in the peak of the day, the temperature 10-20 cm below those
surfaces remains perpetually below -40°C. Were liquid to hypothetically
form at a higher surface temperature, it would be transported in a
matter of *minutes or hours* to the relatively cold region just below
the surface, and eventually to a permanent polar or subpolar reservoir
by evaporation and condensation. Thus, persistent liquid water at or
near the martian surface requires a significant departure from the
general planetary setting in the form of either long-term disequilibria
(such as geothermal sources) or from short-term disequilibria (an
impactor).

The equilibrium water activity of martian regolith can be calculated as
a function of temperature, using a mean absolute humidity of 0.8
microbar and assuming equilibrium with the atmosphere. In warm regolith,
a~w~ is literally orders of magnitude too small to support life. Water
activity approaches unity at the frostpoint, but at extremely low
temperatures. If however, there is a significant barrier to
equilibration with the atmosphere, there is a possibility of much higher
absolute humidity, and, therefore, significantly higher a~w~ at warmer
temperatures. Desert crusts have been proposed as a potential mechanism
to provide a diffusion barrier, and were considered in this study.
**Although crusts on Mars have been observed at the past landing sites,
and other crust types are hypothetically possible elsewhere, experience
with desert crusts on Earth shows that the effect of a semi-permeable
crust is to retard, not prevent, the achievement of equilibrium**.

**Where the surface and shallow subsurface of Mars are at or close to
thermodynamic equilibrium with the atmosphere (using time-averaged,
rather than instantaneous, equilibrium), temperature and water activity
in the martian shallow subsurface are considerably below the threshold
conditions for propagation of terrestrial life. The effects of thin
films and solute freezing point depression are included within the water
activity.**

While an extensive literature speculates on mechanisms to form liquid
water on Mars at different times in the past and under different climate
conditions, common to all of them is the explicit understanding that
present-day equilibrium conditions do not support the persistence of
liquid water at the surface. Uncertainty exists about whether previous
conditions were persistent or episodic, with some attributing conditions
to be punctuated, due to impact effects, while others envisioning longer
term stable early climates. More recently, orbital forcing has been
recognized as a factor driving climate change, with 50 kyr being the
shortest climate cycle affecting latitudinal precipitation.

The SAG considered possible environments in long-term disequilibrium,
where water and temperature were in equilibrium under conditions at an
earlier time, but for which conditions have changed, and do not hold for
the present. Geological deposits might survive for 10^4^ -- 10^7^ years
by virtue of giving up their water very slowly. The SAG examined several
potential sites for long-term disequilibrium, either theoretical or
actually observed, such as gullies, mid-latitude features of purported
snow/ice deposits, remnant glacial deposits, craters, volcanoes, slope
streaks, recent outflow channels, possible hydrothermal vents,
low-latitude ground ice, and polar caps.

-   **Some---although, certainly, not all---gullies and gully-forming
    regions might be sites at which liquid water comes to the surface
    within the next 100 years. At present, there are no known criteria
    by which a prediction can be made as to which---if any---of the tens
    of thousands of gullies on Mars could become active---and whether
    the fluid involved is indeed water---during this century.**

-   **Because some of the 'pasted-on'--type mantle has a spatial, and
    possibly a genetic, relationship to gullies (which in turn are
    erosional features possibly related to water), the 'pasted-on'
    mantle may be a special region. The mid-latitude mantle, however, is
    thought to be desiccated, with low potential for the possibility of
    transient liquid water in modern times. Because the mid-latitude
    mantle and some kinds of gullies may have a genetic relationship,
    the mantle is interpreted to have a significant potential for modern
    liquid water.**

-   **No craters with the combination of size and youthfulness to retain
    enough heat to exceed the temperature threshold for propagation have
    been identified on Mars to date.**

-   **We do not have evidence for volcanic rocks on Mars of an age young
    enough to retain enough heat to qualify as a modern special region
    or suggest a place of modern volcanic or hydrothermal activity.**

-   **Despite a deliberate and systematic search spanning several years,
    no evidence has been found for the existence of thermal anomalies
    capable of producing near-surface liquid water.**

-   **The martian polar caps are too cold to be naturally occurring
    special regions in the present orientation of the planet.**

The SR-SAG proposes that martian regions may be categorized as
non-special if the temperature will remain below -20°C *or* the water
activity will remain below 0.5 for a period of 100 years after
spacecraft arrival. All other regions on Mars are designated as either
special or uncertain. An uncertain region is treated as special until it
is shown to be otherwise. The SAG found no regions to be special, but
found uncertainty with the gully and possibly-related 'pasted-on' mantle
regions. In this context, the SAG has listed Mars environments that may
be "special" and classified those that have observed features probably
associated with water, those that have a non-zero probability of being
associated with water, and those areas that, if found, would have a high
probability of being associated with water.

A map has been developed that provides a generalized guidelines for the
distribution of areas of concern that may be treated as special regions.

It should be noted that even in a region determined to be "non-special,"
it is possible a spacecraft may create an environment that meets the
definition of "special" or "uncertain." It is possible for spacecraft to
induce conditions that could exceed for some time the threshold
conditions for biological propagation, even when the ambient conditions
were 'not special' before the spacecraft arrived. Whether a special
region is induced or not depends on the configuration of the spacecraft,
where it is sent, and what it does. This possibility is best evaluated
on a case-by-case basis.

In summary, within the upper five meters most of Mars is either too cold
or too dry to support the propagation of terrestrial life. However,
there are regions that are in disequilibrium, naturally or induced, that
could be classified as "special" or enough uncertainty exists to be
unable to declare the regions as "non-special."

[Executive Summary 2](#executive-summary)

[1. INTRODUCTION 9](#introduction)

[1a. "Special Regions"\--History and the Current Problem
9](#a.-special-regions--history-and-the-current-problem)

[1b. This study 10](#b.-this-study)

[1c. How does this study extend the results of PREVCOM?
10](#c.-how-does-this-study-extend-the-results-of-prevcom)

[1d. Future Steps 11](#d.-future-steps)

[2. APPROACH 11](#approach)

[3. CLARIFICATION OF THE EXISTING SPECIAL REGION DEFINITION
11](#clarification-of-the-existing-special-region-definition)

[4. BOUNDARIES FOR THE PRESENT ANALYSIS
12](#boundaries-for-the-present-analysis)

[4a. Time frame 12](#a.-time-frame)

[4b. Maximum depth of penetration by an impacting spacecraft
14](#b.-maximum-depth-of-penetration-by-an-impacting-spacecraft)

[5. IMPLICATIONS FROM MICROBIOLOGY 17](#implications-from-microbiology)

[5a. Introduction 17](#a.-introduction)

[5b. Lower temperature threshold 18](#b.-lower-temperature-threshold)

[5c. Water activity threshold 20](#c.-water-activity-threshold)

[5d. Other possible limits to terrestrial life
23](#d.-other-possible-limits-to-terrestrial-life)

[5e. Discussion 24](#e.-discussion)

[6. WATER ON MODERN MARS 24](#water-on-modern-mars)

[6a. The distribution of water where it is at equilibrium
24](#a.-the-distribution-of-water-where-it-is-at-equilibrium)

[6b. Possible secondary factors that affect a general thermodynamic
model
27](#b.-possible-secondary-factors-that-affect-a-general-thermodynamic-model)

[6b-i. The possible effect of diurnal and seasonal heating/cooling.
27](#b-i.-the-possible-effect-of-diurnal-and-seasonal-heatingcooling.)

[6b-ii. The possible effect of recharge from subsurface water reservoirs
29](#b-ii.-the-possible-effect-of-recharge-from-subsurface-water-reservoirs)

[6b-iii. The possible effect of unfrozen thin films of water
30](#b-iii.-the-possible-effect-of-unfrozen-thin-films-of-water)

[6b-iv. The possible effect of semi-permeable crusts
30](#b-iv.-the-possible-effect-of-semi-permeable-crusts)

[6c. Calculation of water activity on modern Mars.
32](#c.-calculation-of-water-activity-on-modern-mars.)

[7. MARS ENVIRONMENTS IN THERMODYNAMIC DISEQUILIBRIUM
33](#mars-environments-in-thermodynamic-disequilibrium)

[7a. Introduction 33](#a.-introduction-1)

[7b. Gullies 34](#b.-gullies)

[7c. Mid-latitude geomorphic features that may indicate deposits of
snow/ice
38](#c.-mid-latitude-geomorphic-features-that-may-indicate-deposits-of-snowice)

[7d. Glacial Deposits 42](#d.-glacial-deposits)

[7e. Craters 44](#e.-craters)

[7f. Young volcanics 48](#f.-young-volcanics)

[7g. Slope streaks 51](#g.-slope-streaks)

[7h. Recent outflow channels? 54](#h.-recent-outflow-channels)

[7i. The non-discovery of geothermal vents
54](#i.-the-non-discovery-of-geothermal-vents)

[7j. The Possibility of Low-Latitude Ground Ice
55](#j.-the-possibility-of-low-latitude-ground-ice)

[7k. The polar caps. 56](#k.-the-polar-caps.)

[8. REVISION OF THE SPECIAL REGION DEFINITION AND GUIDELINES
57](#revision-of-the-special-region-definition-and-guidelines)

[9. DISCUSSION OF NATURALLY OCCURRING SPECIAL REGIONS
59](#discussion-of-naturally-occurring-special-regions)

[9a. Risk Acceptability 59](#a.-risk-acceptability)

[9b. Special regions on Mars within the temporal and spatial limits of
this analysis
60](#b.-special-regions-on-mars-within-the-temporal-and-spatial-limits-of-this-analysis)

[10. DISCUSSION OF SPACECRAFT-INDUCED SPECIAL REGIONS
62](#discussion-of-spacecraft-induced-special-regions)

[11. ACKNOWLEDGEMENTS 65](#acknowledgements)

[12. REFERENCES 66](#references)

[APPENDIX I. Acronyms and Abbreviations
75](#appendix-i.-acronyms-and-abbreviations)

[APPENDIX II. Derivation of Figure 9.1
76](#appendix-ii.-derivation-of-figure-9.1)

# 1. INTRODUCTION

## 1a. "Special Regions"\--History and the Current Problem

In 2002, COSPAR introduced the term "special region" as a part of Mars
planetary protection policy. Prior to 2002, planetary protection related
requirements for spacecraft going to the martian surface consisted of
two categories that were distinguished by the purpose of the mission:

-   IVa. Landers without extant life detection investigations

-   IVb. Landers with extant life detection investigations.

However, by 2002 exploration results (primarily from the MGS orbiter,
and soon after confirmed by Mars Odyssey) strongly suggested that some
parts of Mars might be more likely than others to attract interest for
extant life investigations and potentially be more vulnerable to the
effects of Earth-sourced biological contamination. This led to the
introduction of the concept of "special regions," which are environments
on Mars that need a high degree of protection independent of the mission
purpose.

In April 2002, a COSPAR planetary protection workshop formulated a draft
definition of "special region" and proposed that a new mission
categorization, Category IVc, be established for missions that come (or
might come) into contact with them. This proposal was presented to
COSPAR at its 2002 meeting, and was formally adopted shortly afterwards
**(http://www.cosparhq.org/scistr/PPPolicy.htm)**. NASA followed up by
incorporating the special regions concept into its policy by means of
modification of NASA Procedural Requirements 8020.12C *Planetary
Protection Provisions for Robotic Extraterrestrial Missions*, which was
issued in 2005.

***[DEFINITON #1.]{.underline}***

In 2005, an NRC committee (referred to as NRC PREVCOM[^1]) completed a
NASA-requested detailed 2-year study entitled *Preventing the Forward
Contamination of Mars* (NRC, 2006). In their analysis of "special
regions," NRC PREVCOM found that in using the current special region
definition, "there is at this time insufficient data to distinguish with
confidence "special regions" from regions that are not special." They
also raised an important issue of scale---"Mars exhibits significant
horizontal and spatial diversity on km to cm spatial scales," but some
of the relevant observational data have a spatial resolution no better
than \~3x10^5^ km^2^. NRC PREVCOM recommended an interim policy in which
all of Mars is considered a "special region."

For further information on planetary protection policy and history
related to Mars, the interested reader is referred to excellent recent
reviews by DeVincenzi et al. (1998) and NRC (2006).

## 1b. This study 

[Purpose]{.underline}. At the November 2005 MEPAG meeting, NASA
requested that MEPAG prepare a community-based analysis of the
definition of "special region," and if possible, propose clarifications
that make the definition more useful for mission planning and PP
implementation. MEPAG in turn chartered the Special Regions Science
Analysis Group (SR-SAG) and gave it the following assignment:

-   Propose, if it is possible to reach consensus, a quantitative
    clarification of the definition of "special region" that can be used
    in a practical way to distinguish between regions on Mars that are
    "special," "non-special," and "uncertain."

-   Prepare a preliminary analysis, in text form, of the kinds of
    martian environments that should be considered "special" and
    "non-special." If possible, also represent this in map form.

[Methodology]{.underline}. The SR-SAG consisted of 27 members with
scientific backgrounds in various aspects of microbial survival,
physics, geology, and planetary protection. The group included three
members who also served as part of NRC PREVCOM. The SAG met by means of
weekly teleconferences (with several sub-groups working in parallel) in
December 2005 and January 2006, along with extensive e-mail exchange.
From February 6-8, a three-day Special Regions Workshop was held in Long
Beach, CA to integrate results.

## 1c. How does this study extend the results of PREVCOM? 

We consider the present study to be an extension of the work of the
NRC's PREVCOM Committee (NRC, 2006). Given the phrasing of COSPAR's
definition of special regions, and more importantly, the "specific
examples" listed, NRC PREVCOM brought forward their recommendation that
"until measurements are made that permit confident distinctions to be
drawn between regions that are special on Mars and those that are not,
NASA should treat all direct contact missions as category IVs"
\[missions to special regions, for which they recommended specific
biological cleanliness requirements\]. NRC PREVCOM worked with the
existing definition and elected not to recommend modifications or
qualifications to COSPAR's language[^2]. They advised that the community
should endeavor to expand current understanding through measurement and
analysis in order "to permit confident distinctions to be drawn." This
led to the purpose of the SR-SAG, which was to consider the COSPAR
definition and to propose necessary and appropriate clarifications,
qualifications, and extensions that would allow an improved ability to
recognize special regions (and to allow different people to reach the
same interpretation of the definition).

NRC PREVCOM was explicit in its advice that the Mars Program should
pursue measurements to define special regions. While this study
recognizes that models carry uncertainty and that measurements will be
forthcoming in the course of exploration, we have extended currently
available information through the use of very conservative models and
analysis.

## 1d. Future Steps

Our knowledge about Mars and the limits of life on Earth will continue
to evolve in the coming years. While the analysis reported here has
attempted to make conservative assumptions and add additional margins to
proposed thresholds, the SR-SAG anticipates that findings reported here
may be reviewed and, if necessary updated, several years from now unless
sudden discoveries require an earlier revision.

# 2. APPROACH

The charge to the SR-SAG was to prepare a community-based analysis of
the definition of "special region," and to propose clarifications and/or
guidelines that make the definition both less ambiguous and more
practical. The SR-SAG used the following general approach:

1.  Consider the terms in the existing COSPAR definition, and clarify as
    needed.

2.  Establish temporal and spatial boundary conditions for analysis.

3.  Identify applicable threshold conditions for propagation of
    terrestrial organisms.

4.  Evaluate the distribution of the identified threshold conditions on
    Mars, using both data and models, as appropriate.

5.  Analyze on a case-by-case basis those geological environments
    (including those which are hypothetical) on Mars that could (or
    would if they existed) potentially exceed the biological threshold
    conditions.

6.  Describe conceptually the possibility for spacecraft-induced
    conditions that could exceed the threshold levels for propagation;
    and propose an approach to respond to this possibility.

[A comment about the scientific literature pertaining to water on
Mars]{.underline}.  There is a very large, and what appears at first
glance to be conflicting, literature relating to water on Mars.  This
has created a certain confusion in the community. However, the
conclusions of many of the papers in the literature have qualifications
involving time or circumstances. In order to know how to interpret the
literature, and how to apply it correctly to specifics of the special
region question, the SR-SAG found it necessary to start from first
principles to derive its own understanding of the potential for water on
Mars during the time period of interest. This has given SR-SAG a context
for assimilating and integrating the many relevant details in the
literature.

# 3. CLARIFICATION OF THE EXISTING SPECIAL REGION DEFINITION 

The special region definition (above, DEFINITION #1) consists of two
parts: 1) a defining statement consisting of two clauses, and 2) a
description of where, under the current interpretation, special regions
may occur. The SR-SAG concludes that the first part is still useful, as
long as some of the terms are clarified. The second half needs to be
revised and extended with an updated statement of 'current
understanding.'

The first clause of the defining statement includes the following words,
which need clarification.

-   [Propagate]{.underline}. The verb 'propagate' has two meanings, for
    which synonyms are 'reproduce' and 'spread.' For the purpose of this
    analysis, we have assumed the former meaning only. Although there
    has been extensive discussion that a biological contamination event
    requires BOTH reproduction and dispersion in order to create a
    problem for future explorers, a more conservative position is that
    reproduction alone is sufficient to create questions, and this was
    taken as the point of departure for this study.

-   [Likely]{.underline}. It is assumed for the purpose of this analysis
    that the probability of growth of terrestrial organisms under all
    martian environmental conditions cannot be accurately determined.
    However, the probability that specified geological conditions exist
    within a certain time period can be estimated, in some cases
    quantitatively.

The second clause in the defining statement pertains to possible martian
life forms and their likely locations. Because there is no information
on martian life forms, the hardiest Earth organisms are used as a proxy.
  However, the clause remains as part of the definition since, in the
future, our understanding of potential martian life may change and
affect the parameters defining special regions. As a consequence, the
SR-SAG analysis and this report concentrate on the forward contamination
of Mars with live organisms from Earth. The focus here is on
identification of parts of the martian environment in which viable
terrestrial organisms would be unable to propagate, and establishment of
an objective description of such areas so that appropriate planning and
implementation for planetary protection can occur.

# 4. BOUNDARIES FOR THE PRESENT ANALYSIS

The analysis of martian special regions required certain boundary
conditions to be established as a basis for study. One significant
boundary condition was the time frame to use in the identification of
special regions. Another was a spatial boundary (depth) to be applied to
this analysis of applicable environments in order to understand the
potential of an inadvertent crash to contaminate the subsurface.
Discussion of these two key boundaries---time and depth---is presented
below.

## 4a. Time frame 

With respect to special regions, timeframe issues can be viewed in three
ways---how long to avoid special regions, how long do special regions
exist, or how long until they may exist. Current planetary protection
standards proscribe atmospheric entry by any Mars orbiter for a 50-year
period if spacecraft assembly has not incorporated explicit protocols
for bioburden reduction beyond assembly in a class 100,000 cleanroom.
This time span was selected towards the beginning of Mars exploration
when it was envisioned that the pace of Mars exploration would be
quicker than it has been. Because of the technical challenges of
accomplishing successful Mars missions, their high cost, and the
transition from a "space race" to the more measured pace of
international space cooperation, fewer than 20 missions have been
launched and only about a third of those were successfully implemented
in the three decades since the early Viking missions. Furthermore, from
recent orbiter and rover missions it has become recognized that Mars is
far more diverse than earlier explorations had indicated, with a very
large number of scientific sites now identified for future exploration.
Many cognizant researchers now anticipate that the period of biological
exploration will span the current century, and this study makes no
explicit assumptions about the length of the exploration period.

Based on input from the NASA Planetary Protection Officer, this study
used a 100-year timeframe over which the existence of martian special
regions would be considered and could be encountered by any given
mission. This figure was accepted as a premise for the SR-SAG analysis.
It allowed for the analysis of martian environments to take into account
past and present climate but not to extend to the distant future of
climate change driven by obliquity cycles on Mars. It included
consideration of current naturally occurring special regions, the
possibility that a region could become a special region within the next
100 years (from the date of a mission's arrival) due to a natural event
(e.g., eruption of a volcano), and the timescale for spacecraft-induced
special regions.

How might the environmental conditions on Mars over approximately the
next 100-200 years differ from those of today? The primary factor
controlling long-term climate change on Mars is the variation in the
planetary obliquity (the tilt of its spin axis with respect to its
orbital plane) with time. The martian obliquity has varied between 15
deg and 35 deg during the last 5 million years, with a periodicity of
about 120,000 years (Laskar et al., 2004). This variation is widely
regarded to have been responsible for major climate variations in the
past (e.g., Jakosky and Carr, 1985; Mischna et al. 2003; Haberle et al.,
2003; Head et al., 2003; Head et al., 2005; Mischna and Richardson,
2005; Forget et al., 2006; Head et al., 2006). For example, when the
obliquity is greater than about 30°, the annually averaged saturation
vapor pressure at the martian poles is greater than at the equator, a
condition that drives a major redistribution of both water and CO~2~ on
a planetary scale. At present, Mars has a tilt of 25.2° and is about
halfway through one of these obliquity cycles, though it is presently in
a quiescent period of very little obliquity change. This means that 100
years from now the martian obliquity will be only marginally higher than
at present, which is not of significance for long-term climate change
(Nakamura and Tajika, 2003).

  

The south polar cap does appear to be able to change within a 100-year
time scale.  There are observations showing changes in the CO~2~ ice
cover from one year to another (Malin et al., 2001; Thomas et al.,
2005), observations showing changes on the decade time scale in the
outline of the cap (or equivalently the degree of CO~2~ ice cover), and
observations that show that water ice is exposed where the CO~2~ ice is
disappearing (Titus et al., 2003; Bibring et al., 2004).  In addition,
there are less-direct inferences from the water vapor seasonal behavior
over many decades that suggested the same type of behavior but possibly
with more extreme results (e.g., the entire CO~2~ cap potentially
disappearing in some years).  From a stability standpoint, there is no
reason why the CO~2~ ice can\'t come and go, possibly on the decade to
century time scale (see Jakosky et al. 2005a, Jakosky et al. 2005b).
Whether and how this might affect climatic conditions elsewhere on Mars
is not known. However, we do not have evidence that these south polar
CO~2~ effects are causing significant changes in the planetary
distribution of water.

 

The SR-SAG consensus is that the martian climate 100 years (and 1000
years) from now will likely be essentially the same as it is today.

**[PREMISE.]{.underline} A 100-year time span may be used to assess the
potential for special regions that may be encountered by any given
mission.**

## 4b. Maximum depth of penetration by an impacting spacecraft

While planetary protection concerns itself with all of Mars (surface and
subsurface), not all of Mars is accessible to inadvertent contamination
by robotic spacecraft. Thus, a practical analysis of special regions
must take into consideration the part of the surface and shallow
subsurface that is vulnerable to contamination. For all missions, aside
from planned operations, there is the possibility of accidental
subsurface access as a result of hard impact (i.e., a crash). There can
also be access to the subsurface as a result of intentional hard impact
(e.g., end of mission disposal of hardware or hard landing of entry,
descent, and landing hardware). To directly address these issues, it is
possible to analyze impact scenarios and physical conditions at Mars to
put bounds on the possible contamination depth.

The depth of penetration of a crashing spacecraft is a function of the
following parameters: the angle of impact, the impact velocity, the mass
of the impacting object, and the strength and density of the geological
material being impacted. All of these parameters will vary from mission
to mission. The impact velocity is dependent on the entry velocity (at
the top of the atmosphere) and the ballistic coefficient, which
determines how much the spacecraft will be slowed by the martian
atmosphere. Spacecraft sent to Mars in the future will have a range of
ballistic coefficients, and entry velocity will be different for each
launch opportunity and will also depend on the choice of trajectory. The
penetration depth depends on whether the mass of the spacecraft stays
together or breaks up as it passes through the atmosphere. The impact
angle in a failure scenario would depend on when control of the
trajectory were lost. Finally, the martian surface consists of a mixture
of outcrop (of both igneous and sedimentary rocks), regolith,
accumulated wind-blown dust, and polar cap material, all of which could
have been cemented by ice and/or minerals and would influence the
penetration depth.

The depth of impact can be estimated with crater scaling laws. For
impact into dry granular regolith the following fit to dry sand impact
data is suitable (Holsapple, 1993)

*V=*0.14 *(*1700*/ρ) M*^0.83^ *U*^1.02^ / *G*^0.51^

where *V* is crater volume in m^3^, *ρ* is the regolith density in
kg/m^3^, *M* is the impacting mass in kg, *U* is the velocity in km/s,
*G* is the strength of gravity relative to earth (about 0.38). The term
*(*1700*/ρ)* has been included to extend the original model to densities
other than the nominal sand density of 1700 kg/m^3^.

For impacts into icy material the following weak rock fit is used
(Holsapple, 1993)

*V=0.009 (2100/ρ) M U^1.65^*

Again, the model has been extended with a density dependence. This model
is intended for impacts into targets with strengths averaging about 7.6
MPa over large areas. The laboratory strength of frozen soils and ice
are on the order of 20 MPa at -25°C (Lee *et al.*, 2002), and higher at
lower temperatures, so even allowing for a reduction in strength due to
size effects, this model may overestimate crater sizes to some extent.
This is appropriate for the purpose of estimating maximum depths.

It remains to specify how the crater depth and diameter are related to
the volume. The assumption will be made that the crater is a paraboloid
with a depth-to-diameter ratio of 1/4. This is a typical ratio for the
maximum transient dimensions of a simple crater. It should approximately
agree with the final ratio of an icy crater, but the final crater in dry
granular material would be shallower. The volume of a paraboloid with
depth *H* and diameter *D* is *V=π H D*^2^ /8, and the assumption
*H/D*=1/4 leads to the depth being *H*=0.54 *V*^1/3^.

It would be possible to assume a worst-case scenario for each of the
above variables for a hypothetical fleet of future spacecraft, and from
that to estimate the maximum theoretical crater depth. However, this
would entail a set of stacked probabilities for which the single worst
outcome lacks practicality and usefulness. Because of the broad range of
possible mission scenarios, rather than attempting to seek out the
theoretical maximum, a population of calculated solutions is shown in
Figures 4.1 and 4.2. These diagrams assume a perpendicular impact angle
(the worst case for that variable), and show some of the relationships
involving impact velocity, mass, and target geology on crater depth. The
upper curve in Figure 4.1 represents the case for impact into dry
regolith material with an extremely low average density of 1100 kg/m^3^.

> ![](media/image1.png){width="5.684027777777778in"
> height="3.957638888888889in"}
>
> *Figure 4.1. Crater depth for a spacecraft impacting Mars at 4 km/s (4
> regolith densities shown).*

A relevant scenario is the case of a spacecraft launched on a modern
heavy launch vehicle having a mass for the entry system of about 2400
kg, for which the mass passes intact through the atmosphere and impacts
the surface with a velocity of about 4 km/s. Such a system could create
a crater with a depth of about 5 meters. For other mission scenarios,
these diagrams can be used to estimate the possibility of penetrations
deeper than 5 m. For example, a hypothetical 5,000 kg spacecraft (larger
than we can currently land at Mars) impacting at 4 km/s would have an
estimated maximum penetration depth of about 6.5 meters.

> ![](media/image2.png){width="5.5625in" height="3.9833333333333334in"}
>
> *Figure 4.2. Crater depth for a spacecraft impact into dry regolith of
> density 1100 kg/m^3^ over a range of impact velocities (3 spacecraft
> masses shown).*

In the future, we can expect innovative mission concepts to incorporate
deliberate access of the deep subsurface through hard impacts,
innovative drills, or melt probes. For these, it will be necessary to
analyze the possibility of deliberate access into naturally occurring
special regions as a result of planned exploration into the deeper
martian subsurface. In addition, entry systems at some time in the
future will certainly be configured with different masses, ballistic
coefficients (e.g., to fit in the launch vehicle fairing) or might
arrive at Mars on trajectories with higher atmospheric entry velocities.
For these systems, either detailed analysis of atmospheric deceleration
can be performed or conservative simplifying assumptions can be used
(e.g., no atmosphere) to evaluate impact scenarios and possible
consequences.

**[FINDING]{.underline}. Although naturally occurring special regions
anywhere in the 3-D volume of Mars need protection, only those in the
outermost \~5 m of the martian crust can be inadvertently contaminated
by a spacecraft crash---special regions deeper than that are not of
practical relevance for missions with a mass up to about 2400 kg and
possible impact velocities up to \~4 km/s.**

# 5. IMPLICATIONS FROM MICROBIOLOGY

## 5a. Introduction 

There are many environmental factors to be considered in assessing the
ability of microbial life to grow and reproduce (Table 5.1). As a
starting point for our analysis we considered terrestrial life forms
that might be capable of growth under extreme conditions of the martian
environment, thresholds for environmental factors that would prevent
growth and replication, and the physiological and nutritional
constraints terrestrial microbes must overcome to pose a threat of
widespread forward contamination of Mars over a defined time frame. In
general our strategy has been to find any terrestrial representative (no
matter where it is from) that demonstrates the worst-case scenario. We
are not assigning any special Mars or spacecraft relevance to any of
these organisms or situations, although we are documenting observations
that suggest the metabolic or physiological possibility of reproduction.

*Table 5.1 Some factors that may affect the survival and reproduction of
Earth microbes on Mars*

+-----------------------------------------------------------------------+
| **Factor**                                                            |
+-----------------------------------------------------------------------+
| Water availability and activity                                       |
+-----------------------------------------------------------------------+
| -   Activity of liquid water                                          |
+-----------------------------------------------------------------------+
| -   Past/future liquid (ice) inventories                              |
+-----------------------------------------------------------------------+
| -   Salinity, pH, and Eh of available water                           |
+-----------------------------------------------------------------------+
| Chemical environment                                                  |
+-----------------------------------------------------------------------+
| -   Nutrients                                                         |
+-----------------------------------------------------------------------+
| -   C, H, N, O, P, S, essential metals, essential micronutrients      |
+-----------------------------------------------------------------------+
| -   Fixed nitrogen                                                    |
+-----------------------------------------------------------------------+
| -   Availability/mineralogy                                           |
+-----------------------------------------------------------------------+
| -   Toxin abundances and lethality                                    |
+-----------------------------------------------------------------------+
| -   Heavy metals (e.g., Zn, Ni, Cu, Cr, As, Cd, etc., some essential, |
|     but toxic at high levels)                                         |
+-----------------------------------------------------------------------+
| -   Globally distributed oxidizing soils                              |
+-----------------------------------------------------------------------+
| Energy for metabolism                                                 |
+-----------------------------------------------------------------------+
| -   Solar \[surface and near-surface only\]                           |
+-----------------------------------------------------------------------+
| -   Geochemical \[subsurface\]                                        |
+-----------------------------------------------------------------------+
| -   Oxidants                                                          |
+-----------------------------------------------------------------------+
| -   Reductants                                                        |
+-----------------------------------------------------------------------+
| -   Redox gradients                                                   |
+-----------------------------------------------------------------------+
| Conducive physical conditions                                         |
+-----------------------------------------------------------------------+
| -   Temperature                                                       |
+-----------------------------------------------------------------------+
| -   Extreme diurnal temperature fluctuations                          |
+-----------------------------------------------------------------------+
| -   Low pressure (Is there a low-pressure threshold for terrestrial   |
|     anaerobes?)                                                       |
+-----------------------------------------------------------------------+
| -   Strong biocidal UVC irradiation                                   |
+-----------------------------------------------------------------------+
| -   Galactic cosmic rays and solar particle events (long-term         |
|     accumulated effects)                                              |
+-----------------------------------------------------------------------+
| -   Solar UV-induced volatile oxidants, e.g., O~2~^--^, O^--^,        |
|     H~2~O~2~, O~3~                                                    |
+-----------------------------------------------------------------------+
| -   Climate/variability (geography, seasons, diurnal, and eventually, |
|     obliquity variations)                                             |
+-----------------------------------------------------------------------+
| -   Substrate (soil processes, rock microenvironments, dust           |
|     composition, shielding)                                           |
+-----------------------------------------------------------------------+
| -   High CO~2~ concentrations in the global atmosphere                |
+-----------------------------------------------------------------------+
| -   Transport (aeolian, ground water flow, surface water, glacial)    |
+-----------------------------------------------------------------------+

*Modified after Rummel (2006)*

The Mars environment is extremely cold and dry, and the surface is
bathed in ultraviolet radiation during the daytime and significantly
influenced by galactic cosmic radiation at all times. Because Mars is
cold, but not always, and extremely dry, but perhaps not everywhere, the
concept of "special region" describes those places where environmental
conditions might be compatible with microbial propagation. The
special-region concept allows mission planners to address the
requirements of planetary protection in regions on Mars where
terrestrial Earth organisms might survive and proliferate.

## 5b. Lower temperature threshold

It is well documented that microorganisms on Earth live at temperatures
well below the freezing point of pure water, e.g., inside glacial and
sea ice and permafrost. This is possible because certain impurities such
as mineral acids or salts can reduce the freezing point of water. These
impurities can prevent freezing of intergranular veins in ice and thin
films in permafrost and permit transport of nutrients to and waste
products from microbes. Furthermore, from viability and survival
studies, we know that some cells can resist freezing. Survival
strategies include

synthesis of stress proteins; reduction in cell size; dormancy;
sporulation; adaptive modifications to their cellular components (e.g.,
changes in their fatty acid and phospholipids composition); or an
alteration in the "structured" water in their cytoplasm (Russell et al.,
1992; Thieringer et al., 1998). These and other adaptations allow them
to operate more efficiently than mesophilic organisms at low
temperatures. Temperature influences growth rates and cell replication
by affecting the conformation of cellular macromolecules and other
cellular constituents, which in turn control substrate acquisition and
determine the rates of enzymes reactions and metabolism (Russell et al.,
1990). The relationship between temperature and reaction rate (k) can be
described by an Arrhenius equation

*k =Ae^-Ea/RT^*

where Ea is the activation energy; A is a constant, R is universal gas
constant; T is absolute temperature. The activation energy for most
enzymes is usually on the order of 420 kJ/mol. Therefore, although
reactions rates would fall considerably with a drop in temperature,
there is no thermodynamic restriction on growth at low temperatures.
Although thermodynamics predicts some metabolic activity at low
temperatures, the lower temperature limit for cell division is probably
set by freezing of the internal solution of the cell rather than
reduction in enzymatic activity at low temperature. Therefore, we chose
an empirical rather than theoretical approach to setting a lower
temperature limit to cell replication.

In developing a rationale for setting a lower temperature threshold, we
evaluated published reports of microbial activity that provide direct
and/or indirect evidence that microorganisms survive or thrive at
temperatures below -5°C. The studies we evaluated fell into three
groups, direct measurements of cell replication, measurements of
metabolic activity, and indirect measurements of inferred microbial
activity (e.g., N~2~O production in ice cores). Based on a proposal by
Morita (1997), metabolic studies were categorized further into those
providing evidence of 1) survival metabolism, i.e., the extremely weak
metabolism of immobile, probably dormant communities; 2) maintenance
metabolism of communities with access to nutrients and free to move but
still below thresholds for growth; or 3) actual growth and cell division
leading

![](media/image1.wmf)*Table 5.2 Observations of biological activity at
low temperatures*

to propagation. The metabolic activity measured, the methods used, the
temperature limits, and the categories of the responses are listed in
Table 5.2. In addition, several studies have inferred

microbial activity below -20°C from anomalous concentrations or stable
isotope signatures of products of microbial metabolism. For example,
Sowers (2001) proposed nitrification as the likely explanation for peak
concentrations of N~2~O and high d^15^N and low d^18^O of N~2~O in Lake
Vostok ice core from the penultimate glacial maximum, about 140,000
years ago. Price and Sowers (2004) estimated that the rates of biomass
turnover at -40°C correspond to 10 turnovers of cellular carbon per
billion years. Table 5.2 is not exhaustive, but is representative of a
broad and diverse literature on biological activity at low temperatures.

To summarize these data, many groups have demonstrated some metabolic
activity (using various measures and by various techniques) at
temperatures down to -20°C. At the lowest temperatures, activity was
very low (insufficient to support cell replication) and was not
sustained beyond a few weeks. Although reported levels of metabolic
activity at temperatures down to -15°C might support growth, no one has
demonstrated cell replication to occur at or below -15°C. There are no
studies that have systematically looked at growth and replication at 1
degree increments below -15°C. We therefore recommend a lower
temperature threshold of -20°C, below which there is no evidence to
indicate that replication is possible. \[If Earth organisms were to be
discovered in the future that were able to replicate at temperatures at
or below -20°C, this finding would be reevaluated.\]

**[FINDING]{.underline}. Based on current knowledge, terrestrial
microorganisms are not known to be able to reproduce at a temperature
below about -15°C. For this reason, with margin added, a temperature
threshold of -20°C is proposed for use when considering special
regions.**

## 5c. Water activity threshold

Although many terrestrial microorganisms can survive extreme desiccation
in a quiescent state, e.g., as spores, they all share an absolute
requirement for liquid water in order to grow, i.e., to multiply and to
increase their biomass. Various measures are used to quantify the
availability of liquid water to biological systems, depending on the
scientific discipline (e.g., soil microbiology, food microbiology, plant
physiology, plant pathology). Water activity (a~w~) (that is, the
activity of *liquid water*) is related to percent relative humidity (rh)
as follows:

a~w~ = rh/100

when the relative humidity of an atmosphere is in equilibrium with the
water in a system (a solution, a porous medium, etc.). For pure water,
a~w~ = 1.0. Water activity decreases with increasing concentrations of
solutes and as increasing proportions of the water in a system are
sorbed to surfaces, e.g., during desiccation in a porous medium such as
the martian regolith (Table 5.3).

Desiccation (matric[^3]-induced water activity) and solutes impose
related but different stresses on microbial cells. Cytoplasmic a~w~ must
approximate extracellular a~w~ in order to avoid excessive turgor
(osmotic) pressure, plasmolysis, or plasmoptysis (cell explosion);
however, some positive turgor pressure is required for cellular
expansion during growth. Microbes respond to decreasing a~w~ by
accumulating intracellular compatible solutes, a response that has been
well characterized in many different microorganisms and which requires
expenditure of energy for transport or synthesis (Brown, 1976; 1990;
Csonka, 1989; Welsh, 2000).

Low a~w~ in a porous medium has the added effect of decreasing nutrient
availability. As a soil loses water, the water films on the surfaces of
soil particles become thinner and also discontinuous. This limits solute
diffusion and also impedes microbial motility. Solute diffusion is
reduced by a factor of approximately 2 and microbial mobility is
negligible when a soil loses moisture such that a~w~ drops to \~0.99 or
less (Papendick and Campbell, 1981; Wong and Griffin, 1976). Thus, low
matric-induced water activity in a porous medium imposes starvation
conditions due to the diminished solute diffusion and microbial
motility. Filamentous organisms (fungi, algae, cyanobacteria, and
actinomycetes) may overcome this limitation by extending filaments
through air voids in a partially desiccated soil, but this extends their
desiccation

tolerance only to a~w~ of approximately 0.9. In the absence of exogenous
energy sources, bacteria

*Table 5.3 Conditions resulting in various water activities (a~w~) and
microbial responses to (a~w~*) values.

  ------------------ ------------------------------------------------------------
  **Water activity   **Condition or response**
  (a~w~)**           

  1.0                Pure water

  **Solute-induced   
  effects**          

  0.98               Seawater

  0.75               Saturated NaCl solution

  0.29               Saturated CaCl~2~ solution

  0.98 to 0.91       Lower solute-induced a~w~ limit for growth of various plant
                     pathogenic fungi

  0.69               Lower solute-induced a~w~ limit for growth of *Rhizopus*,
                     *Chaetomium*, *Aspergillus*, *Penicillium* (filamentous
                     fungi)

  0.62               Lower solute-induced a~w~ limit for growth of *Xeromyces*
                     (Ascomycete fungus) and *Saccharomyces*­ (Ascomycete yeast)
                     (growth in 83% sucrose solution)

  **Matric-induced   
  effects**          

  0.999              Average water film thickness = 4 µm

  0.9993             Average water film thickness = 1.5 µm

  0.996              Average water film thickness = 0.5 µm

  0.99               Average water film thickness = 3 nm

  0.97               Average water film thickness \< 3 nm ~­~ (\< 10 H~2~O
                     molecules thick)

  0.93               Average water film thickness \< 1.5 nm ~­~(\< 5 H~2~O
                     molecules thick)

  0.75               Average water film thickness \< 0.9 nm ~­~(\< 3 H~2~O
                     molecules thick)

  0.999              Matric-induced a~w~ at which microbial motility ceases in a
                     porous medium

  0.97 to 0.95       Lower matric-induced a~w~ limit for growth of *Bacillus*
                     spp.

  0.88               Lower matric-induced a~w~ limit for growth of *Arthrobacter*
                     spp.

  0.93 to 0.86       Matric-induced a~w~ at which microbial respiration becomes
                     negligible in soil
  ------------------ ------------------------------------------------------------

*Compiled from Papendick and Campbell (1981), Harris et al. (1981),
Griffin et al. (1981), Sommers et al. (1981), Potts (1994).*

might be able to undergo 2-3 rounds of reductive cell division, but this
is not an increase in biomass and thus is not true growth. Desiccation
stress is usually more inhibitory to microbial growth and activity than
a solute-induced water stress with an equivalent a~w~, primarily due to
desiccation-induced nutrient limitation. However, specific solutes may
be toxic to microbes, e.g., sodium ions are inhibitory to some degree to
all microbes if they accumulate intracellularly.

There is no doubt that the majority of hypersaline environments on Earth
harbor significant populations of micro-organisms (for a recent summary,
see Grant, 2004). However, values of a~w~ do not generally fall much
below 0.75, the limiting value obtainable at the saturation point of
NaCl (5.2 M). Halophilic microbes (including members of the Bacteria,
Archaea, and Eukarya) can unquestionably propagate in saturated NaCl
solutions (a~w~ = 0.75). Although the presence of organisms in
concentrated brines of other salts with water activity lower than 0.75
has been observed, there are questions about the nature of their life
cycles, and where and how they reproduce and grow.

For example, microbial communities have been reported in Don Juan Pond
in Antarctica, a small unfrozen Antarctic lake dominated by very large
concentrations of CaCl~2~ during the winter. Total dissolved salts may
exceed 47% (w/v) and the a~w~-value is recorded at 0.45 (Siegel et al.
1979). However, there has been dispute over the evidence for microbial
colonization of this site (Horowitz et al. 1972), and the prevailing
opinion is that life is unlikely to exist at this a~w~ value (Grant,
2004). The algal mat communities develop during the summer in melt water
at the margins of the pond, which is essentially fresh water, and how
this community relates to the low-activity winter brine is uncertain. As
summarized by Grant (2004) "this particular site is long overdue for a
re-examination using direct molecular technologies." Another example is
the MgCl~2~- and KCl-rich Dead Sea brine (a~w~ = \~0.67). However, the
microbes in this brine are likely survivors from brief intervals of
growth following dilution with fresh water (Aharon Oren, personal
communication). A third example is the deep anoxic basins in the
Mediterranean, where the water is nearly saturated with MgCl~2~ (5.0 M,
a~w~ \~0.3) (van der Wielen, 2005). The presence of microbes in this
brine is indicated by 16S rRNA genes and some enzymatic activity.
However, there is no direct evidence of reproduction or growth in the
brine\--the DNA and enzymes could ultimately be derived from microbes
that grew in overlying water with much lower salinity rather than in the
highly concentrated brine.

The lowest solute-induced water activity for which well-documented
growth has been shown is 0.62. This is the case of xerophilic fungi
growing in highly concentrated (83% W/V) sucrose solutions (Harris,
1981). Sucrose solutions as microbial habitats are more relevant to food
microbiology than to naturally occurring environments such as brines or
soils. Nonetheless, this value of a~w~ serves as a useful benchmark.

The lowest matric-induced water activity that allows microbial
proliferation is dictated by solute diffusion and the availability of
nutrients in solution. The lowest matric induced a~w~ enabling growth of
bacteria in culture is approximately 0.88. More importantly, the a­~w~ at
which microbial respiration becomes negligible as a soil loses moisture
is approximately 0.86 to 0.93 (Sommers et al., 1981). Soil respiration
is a culture-independent measure, and thus serves as a good indicator of
the metabolic capabilities of all soil microbes. The actual a~w~ at
which microbial proliferation ceases is, in all likelihood, higher than
this in that soil microbes can respire by endogenous metabolism under
conditions that are too dry for cell proliferation.

Water in contact with ice deserves special attention. The a~w~ of pure
liquid water at any temperature is 1.0 and is not temperature-dependent.
However, the a~w~ of ice is temperature-dependent and declines from 1.0
as temperature decreases. The a~w~ of ice is equal to the water vapor
pressure over ice divided by the water pressure over pure liquid water.
Thus, at T = 0°C, a~w~ of ice = 1.0; at T = -20°C, a~w~ = 0.82, at T =
-40°C, a~w~ = 0.67; and so forth. Note that relative humidity meters
(e.g., Vaisala humicap sensors) read a~w~ and so a relative humidity
meter placed in an atmosphere in equilibrium over pure ice at -40°C will
read 67%.

The water activity of any solution in equilibrium with ice will be equal
to the water activity of the ice and does not depend on which molecules
are in solution or their quantity (Koop, 2002). Physically, the solution
will gain or lose water until the a~w~ is equal between the solid phase
(ice) and the liquid phase (the solution). This allows the a~w~ of
ice-rich regions on Mars to be predicted solely from a measurement of
temperature. Similarly, the eutectic temperature of any solution can be
predicted since that is the temperature at which the a~w~ of ice is
equal to the a~w~ of the saturated solution.

**[FINDING]{.underline}. Based on current knowledge, terrestrial
organisms are not known to be able to reproduce at a water activity
below 0.62; with margin, an activity threshold of 0.5 is proposed for
use when considering special regions.**

## 5d. Other possible limits to terrestrial life

SR-SAG concluded that a number of factors (some listed in Table 5.1,
some not) contribute to a reduction in the probability of propagation,
but for none except temperature and water activity is it possible at the
present time to define practical threshold criteria that would apply to
all terrestrial microbes.

The nutritional requirements for terrestrial microorganisms on Mars were
considered to be key factors in limiting the proliferation of
microorganisms on Mars. Terrestrial microorganisms require exogenous
sources of nutrients, and accessible organic and/or inorganic nutrients
in martian regolith have not been demonstrated (Biemann et al., 1977;
Biemann and Lavoie, 1979). Although terrestrial chemoautotrophs do not
require organic nutrients, they do require exogenous nutrient and energy
sources, not all of which can be obtained in gaseous form. The diurnal
temperature fluctuations shorten durations at temperatures above the
minimum required for growth and require organisms to be capable of
surviving repeated exposure to eutectic freezing. Both elicit a stress
response that diverts resources towards repair of cell damage rather
than cell division. The strong biocidal UVC irradiation on Mars helps to
further constrain the proliferation of terrestrial microorganisms on
Mars by two key processes: (a) UVC irradiation can quickly reduce the
viability of sun-exposed bioloads on spacecraft surfaces, and (b) UVC
irradiation will likely reduce long distance dispersal of the remaining
viable bioloads by imposing a highly lethal non-ionizing radiation
environment on the dispersed microorganisms.[^4] For organisms near or
at the surface, long term exposure to galactic cosmic rays and solar
particle events will certainly increase lethality and reduce viability.

None of these secondary factors have been adequately measured or modeled
for the martian surface or near-subsurface to allow us to set thresholds
about their effect on survival, growth and proliferation of
microorganisms on Mars. However, all combine to lower the likelihood
that Earth organisms will be able to propagate or even spread at the
surface while remaining viable.

**[FINDING]{.underline}. Despite knowledge that UV irradiation at the
surface of Mars is significantly higher than on Earth, UV effects have
not been adequately modeled for the martian surface or near-subsurface
to allow us to set thresholds about their effects on growth and
proliferation of microorganisms on Mars. However, UV may be considered
as a factor that limits the spread of viable Earth organisms.**

## 5e. Discussion

We conclude that thresholds for temperature of -20°C and water activity
(0.5 a~w~) define conditions below which Earth organisms will not grow
or replicate. Such conditions that might exist on Mars must actually
exceed both of these parameters for periods of time sufficient to allow
growth and cell division to occur. We consider these to be very
conservative values. Cell division has never been observed below
sustained temperatures of -12°C, and 0.5 a~w~ is much lower than the
minimum value for matric-induced water activities that allow for
microbial propagation in terrestrial environments. This value is more
conservative (lower) than the lowest solute induced a~w~ known to be
compatible with growth: the unusual case of yeasts growing in a
concentrated solution of sugar. Modeling studies predict that long-term
conditions exceeding these thresholds will not persist long enough to
permit cell division cycles, which may require weeks to years for
completion.

Although it is impossible to assign with certainty values for
probability of growth of an Earth organism on Mars, we can be confident
that assignment of "special region" requires that conditions exceed
minimal temperature and water activity parameters defined above. In
addition, the litany of environmental stressors discussed above further
reduces the likelihood of propagation of terrestrial organisms.

**[FINDING]{.underline}. The most practically useful limits on the
reproduction of terrestrial micro-organisms are temperature and water
activity, for which threshold values (with margin) can be set at -20°C
and 0.5, respectively.**

# 6. WATER ON MODERN MARS

Water on Mars is best analyzed in two broad, distinct classifications:
the parts of Mars that are at or close to thermodynamic equilibrium, and
those that are in long-term disequilibrium.

## 6a. The distribution of water where it is at equilibrium

*[Introduction.]{.underline}* Numerical thermodynamic models of martian
surface and subsurface temperatures have been successfully used for
decades to examine the physical nature of the surface layer (e.g.,
Kieffer et al. 1977) and the behavior of subsurface volatiles (e.g.,
Leighton and Murray, 1966). The repeatability of thermal inertia results
from data set to data set (e.g., Jakosky et al. 2000) indicates that
these models are generally accurate to better than a few degrees during
most seasons and even more accurate on an annual average.

The *absolute* humidity (i.e., the partial pressure of water) varies
with time and location on Mars, but it seldom climbs much above 0.8
microbar. *Relative* humidity is the ratio of this partial pressure to
the saturation vapor pressure of the air or regolith, which is a
function of temperature, varying exponentially with 1/T. Over the large
temperature extremes of a martian day, the relative humidity may go to
100% at night as frost is deposited, and fall to very low values in the
warmth of the day, but the absolute humidity will vary very little.
Where ice is in equilibrium with the observed atmospheric water vapor
pressure on modern Mars (i.e., when it is at the frostpoint), it will
have a temperature of about -75°C (Mellon et al., 2004). This means that
where there is vapor diffusive equilibrium with the atmosphere, ice is
unstable with respect to sublimation at temperatures above -75°C, and
water vapor is unstable with respect to freezing at temperatures below
that.

Mars is warmer at the equator than at the poles. Using factors like the
thermal inertia of the surface material and the solar insolation (which
may include slope effects), it is possible to quantify this, and to
develop planetary-scale maps of parameters like the fraction of the
upper meter that is composed of ice and the depth to the ice table
(e.g., Chamberlain and Boynton, 2006; Mellon and Feldman, 2006;
Aharonson and Schorghofer, in prep.). Such models (e.g., Fig. 6.1) have
a general structure consisting of abundant ice within 1 m of the surface
at high latitude, a mid-latitude belt of ice at a depth of 1-5m , and an
equatorial belt where ice is either deeper than 5 m or absent
altogether. The steady state ice depth depends on thermal properties and
is independent of molecular diffusivity.

![](media/image2.png){width="6.369291338582677in"
height="3.560236220472441in"}

*Figure 6.1. Map of depth to the ice table (depth scale in meters), from
Mellon and Feldman, 2006. Calculated assuming 20 pr um of atmospheric
water vapor scaled by elevation. This depth represents a 100 - 1000 year
average. The solid line is the 6 count/sec isopleth for epithermal
neutrons (see Fig. 6.3).*

Equilibrium thermodynamic models show that the depth to the top of the
ice table increases abruptly at about 50 degrees latitude, in both the
north and south hemispheres (Fig. 6.2). This has been studied
extensively (e.g., Farmer and Doms, 1979; Paige, 1992). It is typical in
model results for the transition from a depth of 5 m to infinite to
occur in less than a degree of latitude. Thus, in these kinds of models
there is no practical distinction between ice table depths of 3 to10m,
which is the maximum depth of penetration for crashes involving
currently, envisioned martian spacecraft.

A critically important value of such thermodynamic models is that they
have predictive value down to spatial scales much finer than that
achievable by observational data. At equilibrium, intensive variables
like temperature and water activity are equal at all scales---this is
one way to define equilibrium. This is essential to interpreting special
regions, since the scale of spacecraft observation (the footprint of the
GRS instrument, for example, is approximately 3 x 10^5^ km^2^) can be
many orders of magnitude larger than the finest scale of relevance for
biology (microns). Note that the degree to which any martian environment
does or does not approach equilibrium does not depend on whether ice is
actually present---a~w~ is a property of both gaseous and solid phases.
Similarly, the magnitude of heterogeneity in T and a~w~ depends on the
effect and scale of geologic processes that can produce departures from
equilibrium conditions (see Section 7 of this report). From our
understanding of the Earth, although there are macroscopic processes
that can produce distinct departures from equilibrium, the scale tends
to be local to regional, not microscopic (for example, one grain in a
rock is not at a meaningfully different temperature than the next
grain).

![](media/image3.png){width="6.0in" height="3.0in"}

*Figure 6.2. Two cross sectional profiles showing the depth to the ice
table (presented by Hock and Paige at the Mars Water Conference,
February, 2006). Calculations are done for at a longitude of 240W in
north and 140W in south.*

*[Is an equilibrium model consistent with observed data?]{.underline}*
The strong general agreement between models of ground temperature and
ground ice, and observations of temperature and hydrogen suggests that
such numerical simulations capture the major portion of the relevant
physical processes that control these phenomena. These models are based
on well-known physical processes of solar heat, radiation, conduction,
etc. They have been validated by analytic solutions and by the general
consistency with spacecraft observations (including planets other than
Mars). The errors in these models tend to be related to missing or
oversimplified secondary physics. For example, emissivity variations
from one region to another due to changes in mineralogy can affect the
kinetic surface temperature and is usually not included in numerical
simulations. The magnitude of these errors can be as much as a few
degrees.

> ![](media/image4.png){width="6.486614173228347in" height="3.45in"}
>
> *Figure 6.3. Map of epithermal neutrons, which are very sensitive to
> subsurface hydrogen and water ice, from the GRS instrument on Mars
> Odyssey (Mellon and Feldman, 2006). Only summer data from both
> hemispheres are used (winter CO~2~ frost obscures the ice signature by
> adding hydrogen poor mass atop the soil - seasonal CO~2~ can be as
> much as a meter or more at high latitudes). Beyond a threshold
> boundary of 6 counts/second, ice detection falls off rapidly toward
> the equator. This boundary is more diffuse in the northern hemisphere
> than in the southern hemisphere.*

Comparison between Mars Odyssey Gamma Ray Spectrometer (GRS)
measurements (indicating the presence of subsurface hydrogen and
subsurface ice) and theoretical models of ice stability based on these
same thermodynamic numerical models demonstrates excellent agreement
between theory and observation (Mellon et al. 2004).

## 6b. Possible secondary factors that affect a general thermodynamic model 

### *6b-i. The possible effect of diurnal and seasonal heating/cooling.* 

The martian surface is subject to diurnal and seasonal heating and
cooling that can cause significant temperature variation. These
temperature fluctuations are attenuated in the shallow subsurface. As
shown in Figure 6.4, the scale of this attenuation depends on the
thermal inertia of the surficial material. When no subsurface ice is
present (e.g., Fig. 6.4a), subsurface heating/cooling beyond a few
degrees occurs only in the upper 2 m or so. However, when a subsurface
layer of ice is present (Fig. 6.4b), it has the effect of wicking away
the heat\--the high thermal conductivity of ice resists the further
propagation of the thermal wave, and significant heating can be
restricted to much shallower depths (0.5 m in this example). Although
Mars has an ample supply of near-surface water, it is stubbornly
sequestered in solid form at temperatures below the frost point, either
on the polar caps or in vast high latitude, subsurface locations
(Leighton & Murray 1966).

The surface of Mars at many low-latitude locations may exceed 0°C in the
peak of the day (Fig. 6.3), an observation that has been offered as
possibly enabling the presence of liquid water. However, as discussed
above, given the extremely low vapor pressure of water in the martian
atmosphere, this temperature is 75° above the frostpoint, so it would be
impossible for new water to condense, and any previously present ice or
water would quickly sublime or evaporate. Once in the vapor phase at
these elevated temperatures, water in the shallow subsurface would tend
to diffuse either upward to the atmosphere, or downward to a colder
place. The thermal minimum in the subsurface would function as a cold
trap. Cyclical heating and cooling of the uppermost

  ----------------------------------------------- -----------------------
                                                  *Figure 6.4: Example
                                                  subsurface temperature
                                                  profiles for (a) a
                                                  homogeneous subsurface
                                                  and (b) a layered
                                                  subsurface, from Mellon
                                                  2004. Each curve is a
                                                  diurnal average
                                                  temperature profile
                                                  superimposed at 25-day
                                                  intervals for a full
                                                  martian year. Both
                                                  cases are for 55° south
                                                  latitude for a thermal
                                                  inertia of 250 J m^-2^
                                                  K^-1^ s^-1/2^ and an
                                                  albedo of 0.25. For the
                                                  layered case the
                                                  thermal inertia is
                                                  increased at and below
                                                  50 cm to 2290 J m^-2^
                                                  K^-1^ s^-1/2^ to
                                                  correspond to densely
                                                  ice-cemented soil. The
                                                  magnitude of the
                                                  temperature oscillation
                                                  is reduced by almost a
                                                  factor of 5 at or below
                                                  the ice table.*

  ----------------------------------------------- -----------------------

martian crust would therefore result in progressive desiccation. Maps of
locations that receive the most heating (e.g., Fig. 6.5) are
equivalently the places that have been the most desiccated. In addition,
it is worth noting that cyclical diurnal and seasonal warming causes
*rapid* sublimation, while a cold fluctuation brings only *slow* ice
accumulation, simply because the atmosphere does not supply a
significant source of water.[^5]

Even though the temperature maxima may exceed 0°C at the surface, it is
possible to show from a map of the mean surface temperature (e.g.,
Mellon, 2004) and the general shape of the temperature attenuation
curves (Fig. 6.4) that the temperature 10-20 cm below those surfaces
remains perpetually below -40°C.

![](media/image5.png){width="6.5in" height="2.863779527559055in"}

> *[Figure 6.5.]{.underline} Peak surface temperature on Mars (from
> Haberle 2001). The warm areas correspond to the most arid spots.*

### *6b-ii. The possible effect of recharge from subsurface water reservoirs*

As discussed above, at localities where the regolith is permeable to gas
(which is certainly the case for most or all of Mars), there will be
vapor-diffusive exchange between the atmosphere and ice within this
volume. This exchange involves two-way mass transfer from ice into
vapor, and from vapor into ice. This process leads to the formation of
an ice table, where there can be a high concentration of ice below the
equilibrium point, and none above it. This is a stable condition, and
one that can last indefinitely. As discussed by Clifford (1991, 1993),
near-surface ground ice can also be replenished by reservoirs of H~2~O
in the deeper subsurface. The existence of deep reservoirs of H~2~O at
equatorial latitudes on Mars has been postulated by a variety of authors
based on a variety of arguments. Water vapor from such reservoirs could
migrate up the geothermal gradient to the thermal minimum in the shallow
subsurface, and from there sublime into the atmosphere.

The presence or absence of ice in the shallow martian subsurface depends
primarily on the stability of ice. Subsurface vapor plumes will stay in
a vapor form unless the temperature is below either the frostpoint or
the dew point---above that, neither ice nor water will form. It does not
matter whether vapor is being contributed from one source or two. In
short, the ice exists where it exists because it is cold, even when it
is replenished. If any location gets warm enough to approach the
biological threshold (-20C or more) then the thermal gradient will work
in the opposite direction, driving water both up and down. Seasonal
variation doesn\'t change this conclusion - diffusion will occur rapidly
under summer conditions (warm to cold) as compared to wintertime (cold
to colder), so the dominant direction of flow will always be out of the
thermally fluctuating zone. (This is why the surface layer stays dry in
the subpolar regions on Earth).

A case where subsurface recharge might matter to the analysis of the
upper 5 meters is when a surfacial crust of very low permeability is
present, and the rate of recharge from below significantly exceeds the
rate of diffusive loss to the atmosphere. This could cause the partial
pressure of water in the shallow subsurface to go up, which would in
turn cause the frost point to increase. This situation is discussed in
detail in Section 6b-iv of this report, but in summary, we have no
evidence that such permeability barriers exist on Mars, and arguments
can be developed for why they are geologically implausible.

### *6b-iii. The possible effect of unfrozen thin films of water*

Since there is known to be water in the martian atmosphere (about 8
microbars), as well as water cycling at some rate between crustal and
atmospheric reservoirs of water, it is inevitable that thin films of
water are present on mineral grains in the dry parts of the martian
crust. The 'stickiness' of water is well-known to experimentalists who
operate high-vacuum equipment on Earth. The bad news is that there is no
way to make a direct measurement of the thickness of thin films of water
in different martian environments. However, the good news is that the
activity of water in the thin films, of whatever thickness, can be
calculated from the relative humidity of the atmosphere in equilibrium
with the thin film. As shown below, the activity of water, the
temperature, or both are less than the biological thresholds across the
entire martian surface and shallow subsurface.

### *6b-iv. The possible effect of semi-permeable crusts*

On Earth, soil crusts can provide a significant permeability barrier,
through which the rate of fluid flow can be lower than the rates of
resupply or fluid loss on either side of the barrier. In such cases,
water can be trapped in a transient way, even when it is out of
equilibrium with the atmosphere.

[Observed crusts on Mars]{.underline}. Crusts are common at the martian
landing sites visited through 2005. Observations to date show them to be
relatively weak and friable. Viking "duricrusts" at Chryse Planitia
(Viking-1) were readily broken by digging action, and those at Utopia
Planitia (Viking-2) were disaggregated simply by shaking them in the
acquisition scoop (Clark et al., 1982). Many crusted materials have been
seen at both MER landing sites, but all seem to be easily broken as the
wheels pass over them (Richter et al., 2004 and Richter et al., 2006).
To date, no examples of high-strength crusts have been discovered at any
of the five landing sites. Although we have no data on the permeability
of any of these crusts, because of their friability, discontinuous
nature, and porosity, they do not appear to be particularly impermeable.

[Terrestrial analogs]{.underline}. Since other types of crusts, possibly
more impervious, might exist elsewhere on Mars, it is important also to
consider other kinds of crusts known from terrestrial experience.
Surface crusts associated with soils on Earth are classified as
biological, chemical, or physical (Soil Survey Staff, 1999).

-   [Biological crusts]{.underline} are composed of mosaics of
    cyanobacteria, green algae, lichens, mosses, microfungi, and other
    bacteria (Belnap et al. 2001).

-   [Chemical crusts]{.underline} are largely formed where water
    containing dissolved salts, commonly carbonate, sulfate, and
    chlorides, accumulates in shallow depressions allowing evaporation
    and precipitation at the surface. Common settings for chemical crust
    include dry lakebeds or sabkas. Salt crust may also form at the soil
    surface from capillary rise of salt-rich soil moisture.

-   [Physical crusts]{.underline} primarily result from the formation of
    aggregates from a reconstituted, reaggregated, or reorganized layer
    of mineral particles. Common types include structural (e.g.,
    raindrop impact), depositional (surface flooding), freeze-thaw, and
    vesicular. Aggregates can range from \~10^-2^ to 10^2^ mm in
    diameter, the larger aggregates due to the formation of soil
    structure.

-   Another type of soil crust is the strongly cemented subsoil layer
    where the soil matrix has been cemented by the extensive
    accumulation of carbonate, salt, and silica (e.g., duricrust,
    caliche).

Common attributes among all types of surface crust is that they
generally enhance surface sealing, provide surface stability, limit wind
and water erosion, increase aggregation of binding of soil particles,
and are commonly \< 10 cm thick.

Because it is so common to desert regions on Earth, perhaps the best
terrestrial crust-forming analog for the martian surface is a type of
physical crust referred to as vesicular crust. This is associated with
reg soils or desert pavements, features ubiquitous to nearly all arid
deserts (McFadden et al., 1998). Vesicular crusts typically underlie a
single surface layer of cobbles or gravel. Desert crusts are primarily
derived from the long-term accumulation of aeolian dust (particle
diameters \<0.1 mm) and require 10^3^ to 10^5^ years to form (McDonald
et al., 1995). The density of this type of crust ranges from about 1.5
to 1.9 g/cm^3^ and they are commonly 3-10 cm thick (McDonald 1994).

[Permeability of desert crust]{.underline}. The measured saturated
hydraulic conductivity of desert crust typically ranges from 0.75 to 0.5
cm/hr. Actual conductivity is typically lower than that measured at
saturated conditions due to trapped air within the crust (McDonald et
al., 1996; McDonald 2002, Young et al. 2004; Meadows et al., 2005).
Terrestrial desert crusts are not completely impermeable because a wide
range of processes, primarily dispersive stress and tensional release
result in the formation of voids, pores, and fractures that prevent the
continuous sealing of the soil matrix. Even the most cemented layers
(e.g., caliche, duricrust) always have fractures that limit the
horizontal and vertical extent of cementation and sealing. Although the
formation of crust fractures is exacerbated by biological processes
(e.g., root propagation), dispersive or tension stress is first required
to promote development of fractures. On the surface of Mars, processes
such as freeze-thaw, formation of ice, and ground shaking due to seismic
activity and meteorite impact are likely to enhance formation and
propagation of fractures in crusted materials.

Episodic retention of moisture beneath desert crust on Earth has been
observed to happen when the rate of water recharge (on Earth, primarily
via rain) exceeds the rate of water loss (i.e., vapor loss through the
crust). For example, measurements in soil beneath physical crusts in
hyperarid deserts show that the crust enhances moisture retention and
leads to a lower soil temperature (\~3-6°C) relative to soils that lack
physical crusts, typically for a period of several months (McDonald,
writ. comm. 2006). However, such anomalies are dynamic, and typically
decay on a time scale of a year.

For such cases, with time the mean rate of diffusive gain will equal the
mean rate of diffusive loss, and equilibrium will be attained (although
both rates will be lower than when crust is absent). Although other
crust-related processes may be discovered in the future, SR-SAG
concludes that to within its standard of confidence, this is not an
environment that will lead to the presence of liquid water.

**[FINDING]{.underline}. Although soil crusts on Mars have been observed
at the past landing sites, and other crust types are hypothetically
possible elsewhere, experience with desert crusts on Earth shows that
the effect of a semi-permeable crust is to retard, not prevent, the
achievement of equilibrium.**

## 6c. Calculation of water activity on modern Mars. 

Persistent liquid water at or near the martian surface thus requires a
significant departure from the general planetary setting in the form of
either long-term disequilibria (caused either by geothermal sources of
heat and water or by vestigial sources from prior climates that have
survived for 10^4^-10^7^ years by virtue of giving up their water *very*
slowly) or from short-term disequilibria (from the influence of a
spacecraft on a cold, icy site or a transient event such as a meteorite
impact). If any surface or shallow subsurface location on Mars were to
get warm enough to reach the biological threshold (-20C or more) then
the heating would drive water both up (to the atmosphere) and down (to a
colder place). Seasonal variation doesn\'t change this conclusion -
diffusion will occur rapidly under summer conditions (warm to cold) as
compared to wintertime (cold to colder), so the dominant direction of
flow will always be out of the thermally fluctuating zone. This is the
reason why on Earth we observe that the surface layer stays dry in the
subpolar regions.

![](media/image6.png){width="4.5680555555555555in"
height="3.154861111111111in"}

*[Figure 6.6]{.underline}. Water activity of present-day Mars vs.
temperature in equilibrium with the present-day atmosphere, assumed to
have a water partial pressure of 0.8 μbar. The region of concern for
life propagation is shown in the upper right. The water activity of pure
ice is less than one over the range of temperatures on Mars (e.g., 0.46
at -90*°*C). The water activity of materials on Mars at subfreezing
temperatures is always less than the value of the water activity of pure
ice. This is a small correction and is not shown here.*

On Earth, life can exist and propagate in soils we might casually
consider "dry," often surviving on thin films of water in capillaries or
at grain boundaries. Water activity (a~w~) was introduced in the
previous section as a quantitative measure of dryness, and a water
activity threshold was established below which terrestrial life is not
know to be able to reproduce. Water activity, a measure of availability
of water, is defined as the relative humidity in the pores of the soil
(although expressed as a decimal fraction rather than a percentage). At
a microscopic level, it is typically surface tension associated with the
concave geometry of water or ice droplets that holds vapor pressure
below nominal saturation, resulting in water activity values less than
unity. Alternatively, salt content can lower the saturation vapor
pressure as well as the melting point. Water activity is thus a proxy
for the specific physics and bioavailability of thin films of water
needed for microbe propagation, and obviates the need to consider
specific soil properties or the presence of brines. Moreover, if soil is
in equilibrium with the surrounding atmosphere, then water activity can
be determined directly from the atmospheric relative humidity. In a
single, easily determined parameter, we can capture the detailed
microscopic interactions of microbes, water, and soil.

Figure 6.6 shows the equilibrium water activity of martian soil as a
function of temperature, derived by assuming the absolute humidity to be
0.8 microbar, in equilibrium with the atmosphere. In warm soil, a~w~ is
literally orders of magnitude too small to support life -- there simply
is not enough water to sufficiently dampen the soil. Water activity
approaches unity at the frostpoint, but at a temperature far too low to
support life. The box in the upper corner of Figure 6.6 delineates the
conditions under which terrestrial life could propagate, far from the
water equilibrium.

**[FINDING]{.underline}. Where the surface and shallow subsurface of
Mars are at or close to thermodynamic equilibrium with the atmosphere
(using time-averaged, rather than instantaneous, equilibrium),
temperature and water activity in the martian shallow subsurface are
considerably below the threshold conditions for propagation of
terrestrial life. The effects of thin films and solute freezing point
depression are included within the water activity.**

# 7. MARS ENVIRONMENTS IN THERMODYNAMIC DISEQUILIBRIUM

## 7a. Introduction

Section 6 of this report argued that where Mars is at or close to
long-term thermodynamic equilibrium, the threshold conditions for
propagation of terrestrial organisms are not met anywhere at the martian
surface and shallow sub-surface. However, there remains the possibility
that some parts of Mars are not at equilibrium. For the purpose of this
analysis, we distinguish short-term disequilibrium (i.e., the changes in
heating that occur on a daily or annual cycle), and long-term
disequilibrium (changes that happen as a result of geologic processes
with a time constant longer than 1 year). Long-term disequilibrium
conditions are the subject of this section.

All over the planet the daily and annual temperature cycles result in
heating and cooling within the outermost skin of Mars. Within this
process, the positive and negative excursions from equilibrium offset
each other---on average, the material is at equilibrium. This has been
described as a 'dynamic equilibrium.' In such an environment, any liquid
that might form at the higher temperature would be transported in a
matter of *hours* to one of the cold ice reservoirs by the process of
evaporation and condensation. This would have the effect of leaving the
surface perpetually desiccated. For the purpose of identifying and
evaluating environments in long-term disequilibrium, some workers (e.g.,
Hock and Paige, 2006) have set up an 'annual equilibrium' criterion, and
then look for excursions from that.

One of the implications of the laws of thermodynamics is that systems
tend to move towards equilibrium. Thus, an environment in long-term
disequilibrium is one where water and temperature were in equilibrium
under conditions at an earlier time, but those conditions have changed,
and do not hold for the present. Geological deposits formed under such
conditions will seek the modern equilibrium. As discussed below, there
are several examples where the path [could]{.underline} take such
deposits through a liquid water field as they adjust to new conditions.
Long-term disequilibrium environments might survive for 10^4^--10^7^
years by virtue of giving up their water *very* slowly.

Long-term disequilibrium develops in response to certain geological
processes. These processes operate at different rates, at different
times, and exhibit different kinds of geologic and geomorphic
manifestations. In evaluating martian environments, there are two ways
to proceed, both of which are used in this report:

-   [Description of processes]{.underline}. These constitute 'theory'
    for integrating a series of observations (example: geothermal vent).
    In some cases, there may be multiple working hypotheses to explain a
    given observation. There are also processes that are currently
    hypothetical, because the predicted observation has not yet been
    recorded (but in some cases is the focus of active search).

-   [Description of geomorphologic features]{.underline}. The
    acquisition and analysis of orbital images generates these kinds of
    basic data, but the linkage of the observations to the inferred
    geological processes is interpretive and further aided by supporting
    data from other approaches (example: dissected mid-latitude mantled
    terrain).

## 7b. Gullies

Martian middle- and high-latitude gullies are geomorphic features whose
age and origins are not fully understood, and there is a very real
possibility that their genesis involved liquid water. The geomorphology
and stratigraphic relations of these landforms to adjacent features
suggest that some might be so young that they could be sites at which
liquid water can occur, at least for brief periods, on the martian
surface today.

[Description]{.underline}. Middle and high latitude gullies were first
described by Malin and Edgett (2000). The largest examples can be seen
in Mars Odyssey THEMIS and Mars Express HRSC images, but the vast
majority of these landforms are small enough that they are best
recognized and described using images of better than 7 m/pixel, such as
those from the Mars Global Surveyor MOC (Figure 7.1).

Gullies always have a channel and usually exhibit an apron, unless it
has been buried. Channels are commonly banked, and in some cases,
meandered, but straight (not banked or meandered) examples also exist.
Some gully channels are leveed. Gullies often (but not in all cases)
exhibit an alcove above the channel; these form by undermining,
collapse, and dry mass movement of debris (Malin and Edgett, 2000).
Gully channels commonly originate at a point about 200--800 m below the
local surface outside of the depression in which the feature occurs, and
the alcove, if it is present, occurs above the point at which the
channel begins (Malin and Edgett, 2000; Gilmore and Phillips, 2002;
Heldmann and Mellon, 2004). Gully aprons, in some cases, are made up of
dozens to hundreds of individual flow lobes. The majority of the tens of
thousands of gullies identified in spacecraft images occur in the walls
of craters, troughs, valleys, pits, and depressions. However, some
variants on the theme are found on dune slip faces, crater central
peaks, and the mountains surrounding Argyre Planitia (Malin and Edgett,
2000; Baker, 2001; Reiss and Jaumann, 2003).

![](media/image7.jpeg){width="3.9305555555555554in" height="3.34375in"}

*[Figure 7.1.]{.underline} Typical mid-latitude gullies on the wall of a
crater located at 39.0°S, 166.1°W. This is a sub-frame of MOC image
E11-04033; it covers an area \~3km wide. The upper left corner of the
image is the surface outside the crater; topography slopes down toward
the lower right.*

Where Malin and Edgett (2000) lumped all of the gullies in this range of
settings into a single group, Edgett et al. (2003) suggested that today
they should be split into sub-groups, and their differences may imply
differences in how they form and whether a volatile is involved. The
study of gullies is on-going, and little research has yet addressed the
geomorphic details that distinguish, for example, the gullies formed on
dunes versus crater walls versus crater central peaks and the mountains
rimming Argyre. Some gully-like forms occur at equatorial latitudes, but
they do not exhibit all of the relevant morphologic criteria that
distinguish the middle and high latitude landforms. Specifically, the
equatorial features are (a) the straight, narrow avalanche chutes and
attendant talus deposits that have formed on some steep slopes among
light-toned, layered rock outcrops in the Valles Marineris and
associated chaotic terrains, and (b) the abundant alluvial fans
occurring within a unique impact crater, provisionally-named Mojave,
located at 7.6°N, 33.1°W (Williams et al., 2004).

[Planetary Distribution]{.underline}. Nearly all cases occur at a
location poleward of 30° latitude in both hemispheres (Figure 7.2).
Gullies occurring equatorward of 30° are very rare; the majority of
these are poleward of 27° and mainly located on the north walls of
Nirgal Vallis (Malin and Edgett, 2000; Edgett et al., 2003; Balme et
al., 2006).

[Possible relationship to water]{.underline}. The origin of the gullies
has been much discussed and debated over the past six years, but no
single explanation has yet satisfied all investigators interested in the
subject. The majority of published results regarding middle and high
latitude gullies have centered on the hypothesis that liquid water is
involved, and that the geomorphic expression of the banked channels,
tributary channels, meandering channels, and flow lobes in apron
deposits are all clues regarding the rheologic properties of water-rich
debris flows that have come through a given gully channel on more than
one occasion (Malin and Edgett, 2000; Hartmann et al., 2003).

![](media/image8.png){width="5.838976377952756in"
height="2.736220472440945in"}

*[Figure 7.2.]{.underline} Martian gully locations identified in MGS MOC
images by K. Edgett and M. C. Malin through September 2005. Simple
cylindrical projection map.*

Among the liquid water hypotheses are those that invoke groundwater
(Malin and Edgett, 2000; Goldspiel and Squyres, 2000; Mellon and
Phillips, 2001; Gilmore and Phillips, 2002; Heldmann and Mellon, 2004),
melting of ground ice (Costard et al., 2001), and melting of a
surface-covering snow pack (Lee et al., 2001; Christensen, 2003). Most
publications about gullies on dune slip faces also involve water (Reiss
and Jaumann, 2003; Mangold et al., 2003; Miyamoto et al., 2004a),
although it is acknowledged that CO~2~ or dry granular flow might also
or instead have contributed. In cases where water is invoked to explain
the origin of gullies, discussion has included the notion that pure
water (Heldmann et al., 2005) or brines (Burt and Knauth, 2002) may have
been the agent responsible for the observed landforms. The gullies occur
in a wide range of settings, most of them very far from volcanic
regions, suggesting that igneous-induced hydrothermal processes are not
likely involved (Malin and Edgett, 2000).

Alternate hypotheses center on genesis by release of carbon dioxide that
had been trapped below ground (Musselwhite et al. 2001; Hoffman, 2002),
but Stewart and Nimmo (2002) concluded that it is very difficult to trap
sufficient quantities of CO~2~ beneath the surface. Others suggested
that the gullies formed by dry, granular flow, thus not requiring
participation of a volatile such as water or CO~2~ (Treiman, 2003;
Shinbrot et al. 2004), but this hypothesis does not explain banked or
leveed channels, aprons consisting of many flow lobes, or association of
channel heads with specific rock layers.

[Age]{.underline}. The age of the gullies is central to the concern as
to whether these landforms represent "special regions." The critical
issue is whether liquid water can come to the surface at the head of a
gully channel and run down to and deposit material in the gully apron
today or sometime during the next 100 years.

Estimates for the age of gullies are based on their general geomorphic
and stratigraphic youth and their lack of superimposed impact craters.
Pulling these observations together, Malin and Edgett (2000) concluded
that the gullies could be less than 1 million years old, but this
estimate was based on virtually no information regarding the absolute
age of any particular landform. Reiss et al. (2004) examined small
impact craters superimposing aeolian megaripples in Nirgal Vallis, which
are also superposed by gully aprons. In their research, Reiss et al.
(2004) concluded that the gullies in Nirgal Vallis---assuming the
approach to deriving absolute ages from impact crater size-frequency
distributions provides quantities that approach the true age of features
on Mars---must be younger than 3 million years and might even be younger
than 300,000 years in age. Other investigators have focused on the role
of obliquity excursions, and whether gullies might only be active under
different temperature and pressure conditions than exist today (e.g.,
Costard et al., 2001; Christensen, 2003; Bermann et al., 2005). However,
the work of Heldmann et al. (2005) argued that modern pressure and
temperature conditions are a better fit when the measured run-out
distances of gully channel/apron complexes are considered, suggesting
that the gullies are episodically active today.

The small sampling of gullies available in MGS MOC images at the time
Malin and Edgett (2000) published their results suggested that all
gullies are quite young relative to the martian geologic time scale.
Among that small sample, gully channels and aprons were seen to cut or
superpose landforms that are otherwise considered to be relatively
young, such as aeolian dunes, aeolian megaripples, and patterned ground
similar to that found in terrestrial periglacial settings. Furthermore,
some gullies have dark floors, indicating that dust does not settle and
persist on these surfaces for periods longer than a martian season. The
absence of dust on dark channel floors might be attributed to aeolian
redistribution on a sandy surface, or to recent movement of material
through the gully channel by non-aeolian processes (e.g., runoff of a
liquid).

MGS MOC has continued to collect new images of gullies, almost daily,
since 2000, so the sample is much larger today. In the larger sample
very good examples of gully aprons and channels that have been cut by
fissures and faults, peppered with small impact craters, or superimposed
by windblown sand have now been recognized (Edgett et al., 2003).
However, these "old" gully examples are in the minority, and in most
cases where craters superpose the gully surfaces, multiple craters
occur, suggesting that they are secondary to a larger impact that
happened elsewhere. In other words, the number of craters on the
landforms associated with a gully is not necessarily a good indicator of
age.

The MOC team has recently
(http://www.msss.com/mars_images/moc/2005/09/20/dunegullies/) described
a case in which repeated imaging by MOC revealed the formation of a new
gully on a sand dune slip face (Hellespontus region, west of the Hellas
Basin). The gullies formed sometime between 17 July 2002 and 27 April
2005 (Malin and Edgett, 2005). The setting of this gully (a sand dune
field on the floor of a crater) is distinctly different from that of the
mid-latitude gullies that form on crater walls, and strongly suggests
that there is more than one gully-forming process. Further observation
of Mars is clearly needed. The fact that the Hellespontus gully-forming
event occurred during the current decade shows that it is possible that
some material can flow through at least some gully systems in the modern
era, but it also raises a flag that says: is it possible for new gullies
to form at a location where no gully was observed before?

There are no obvious geomorphic criteria that can be used to predict
which gullies might become active in this century. The MGS MOC is
currently being used to monitor the hundreds of gully sites (tens of
thousands of individual gullies) to look for additional evidence of
change, including features that might indicate whether water is present
and/or has flowed down the relevant slopes in recent years. Features
being sought include new gullies, or channels within a pre-existing
gully complex, and gullies in which bright material---perhaps ice or
salts---has appeared. Other than the fact that gullies are restricted to
certain kinds of slopes, we do not have a means of predicting where a
gully may form in a location where one did not previously exist. Thus,
for the purpose of planetary protection, concern should extend to
gully-forming regions, not just to the specific preexisting features.
The scale of these regions, however, is as yet undefined.

**[FINDING.]{.underline} Some---although, certainly, not all---gullies
and gully-forming regions might be sites at which liquid water comes to
the surface within the next 100 years. At present, there are no known
criteria by which a prediction can be made as to which---if any---of the
tens of thousands of gullies on Mars could become active during this
century, or whether a new gully might form where there isn't one
today.**

## 7c. Mid-latitude geomorphic features that may indicate deposits of snow/ice

The middle martian latitudes exhibit a variety of surficial geomorphic
features that suggest to some investigators that ice-bearing materials
were deposited over much of the surface at these latitudes, perhaps
during a prior obliquity excursion. Although many questions remain, a
small and slowly growing literature describes evidence that ice-rich
materials may once have mantled middle latitude terrain, covering
intercrater plains, crater walls, and other landforms. Two features, in
particular, are relevant here:

1.  A ubiquitous mantle that was deposited and has since become
    roughened by erosion in geologically recent time. The texture of
    this mantle is latitude-dependent. For the purpose of this report,
    this deposit is referred to as the [mid-latitude
    mantle]{.underline}.

2.  Accumulations of materials most commonly found on poleward-facing
    slopes of mid-latitude topographic features, such as crater walls
    and massifs. For the purpose of this report, this deposit is
    referred to as [pasted-on mantle]{.underline}.

[Mid-latitude mantle]{.underline}. A layered deposit, estimated to be
1--10 m thick, mantles much of the surface of Mars between 30 and 60º
latitude in the northern and southern hemispheres (Kreslavsky and Head,
2000; 2002; Mustard et al., 2001).  The terrain once described as
"softened" in Viking-era literature (e.g., Squyres and Carr, 1986) is,
at MGS MOC scale, actually "roughened terrain" (Malin and Edgett, 2001).
Examples of the texture of the mantle are shown in Figure 7.3.

A progression from smooth-surfaced mantle to roughened and pitted
mantles has been observed and described briefly by Malin and Edgett
(2001) and Mustard et al. (2001). In some places, the erosion reveals
that the mantle is layered (Milliken and Mustard, 2003). Because of the
latitudinal relationship, Kreslavsky and Head (2000) hypothesized that
the smoothing was due to a climate-controlled deposition of ice and
dust, and Mustard et al. (2001) proposed that the roughening resulted
from sublimation of ice from a mixture of ice and dust that settled from
the atmosphere to produce the mantle. As discussed earlier in this
report, thermodynamic models indicate that in the mid-latitude regions
shallow ice is unstable under current atmospheric conditions (Mellon and
Jakosky, 1995; Mellon et al., 2004), potentially accounting for the
desiccation interpreted to have occurred there (e.g., Head et al.,
2003).

![](media/image9.jpeg){width="6.384027777777778in"
height="3.173611111111111in"}

*[Figure 7.3]{.underline}. Examples of mid-latitude roughened mantled
surfaces in each hemisphere. (A) Northern hemisphere example from MOC
image SP2-51906, near 30.0°N, 323.5°W. (B) Southern hemisphere example
from MOC image M00-03091, near 40.1°S, 171.6°W. Both images are
illuminated from the left.*

The mid-latitude mantle material exhibits several distinct morphologies
that change as a function of latitude. Lower latitudes of \~30 -- 45°
are characterized by regions of smooth, intact mantle adjacent to
regions where the mantle has been completely stripped from the surface,
whereas higher latitudes of \~45 -- 55° commonly exhibit a knobby
surface texture indicative of incomplete removal of the material
(Milliken and Mustard, 2003; Milliken et al., 2003). Latitudes poleward
of \~55° exhibit the least dissection and removal of the material,
suggesting the mantle deposit has experienced less erosion at these
latitudes and may still be ice-rich beneath a thin layer of ice-free
dust (Mustard et al., 2001; Milliken and Mustard, 2003). The Mars
Odyssey GRS data also detect an increased abundance of H^+^ within the
upper \~1m of the surface at latitudes higher than \~55°, supporting the
hypothesis that these regions might currently include near-surface water
ice.

The latitude-dependence of the mid-latitude mantle, its variations in
morphology with latitude, and the symmetry between the northern and
southern hemispheres suggest the deposition and removal of this layer is
related to global changes in climate (Mustard et al., 2001; Head et al.,
2003). These deposits do not appear to be forming today, but instead
appear to have formed as a result of past geologic processes during
earlier periods of higher obliquity. The mantle blankets all
pre-existing surfaces, independent of topography and surface
composition, suggesting it originated by airfall deposition of dust
cemented by ice precipitated from the atmosphere during favorable
climate conditions. Periods of high obliquity have been proposed to
cause changes in the martian climate that result in both an increase in
atmospheric dust loading and a net transport of water from polar to
mid-latitude regions, providing a mechanism for multiple cycles of
deposition and removal of ice-rich layers that is linked to orbital
variations (Head et al., 2003). The paucity of superposed craters,
together with the correlations with recent periods of higher obliquity,
suggested to Head et al. (2003) that the mantle was emplaced between 2
and 0.5 million years age, and has been undergoing sublimation and
desiccation for the last half million years. In summary, these surficial
ice deposits formed in an earlier geologic period, when Mars had a
different pattern of surface insolation, and are no longer in
equilibrium for the current orbital configuration.  However, there is no
evidence for melting over much of this region, and active layers are not
predicted (Kreslavsky et al., 2006). 

![](media/image10.wmf){width="6.468897637795275in"
height="3.239763779527559in"}*[Figure 7.4]{.underline}. Map showing the
locations of MGS MOC images in which occur different erosion styles of
the mid-latitude mantling materials on a Viking MDIM base (from Milliken
and Mustard, 2003). Colors: Localized removal (yellow), Knobby/wavy
texture (cyan), Scalloped texture and total mantle cover (red).*

['Pasted-on' mantle]{.underline}. Among the wide variety of distinctive
landforms found at middle latitudes, the one that is of concern here
because of its apparently youthful age is that of mantles of material
that appear to have been preferentially preserved on poleward-facing
slopes in craters and on massifs and hills in both martian hemispheres
(Figure 7.5). Described colloquially among researchers in the Mars
science community as "pasted-on" material, these mantling deposits were
initially noted by Malin and Edgett (2001) who speculated that they bore
some resemblance to accumulations of snow left behind on colder,
more-frequently shadowed surfaces. However, the materials are not
light-toned like snow. Mars Odyssey THEMIS VIS images provided wider
fields of view than MGS MOC, and thus greater vistas showing these
"pasted-on" accumulations became readily apparent, leading Christensen
(2003) also to note that these accumulations seem to most commonly occur
on poleward-facing slopes at mid-latitudes, and to expand upon the Malin
and Edgett (2001) speculation that these might represent remnants of old
snow accumulations.

Christensen (2003) and Milliken et al. (2003) proposed that the
pasted-on mantle is a mixture of dust and ice (snow), and further
proposed that they might be the source of water that creates
middle-latitude gullies. However the mutual relationship between gullies
and the pasted-on terrrain is not simple, since there is a wide range of
geomorphic attributes of mid-latitude slope-mantling materials, there is
a wide range of mid-latitude gullies, and the relationships between them
vary. For example, some gullies do not occur in association with such
mantles. No one has yet published a detailed study on whether or how the
two types of landform are related globally, although Milliken et al
(2003) showed that there is a strong correlation between viscous flow
features, dissected mantle terrain, and gullies within the ±30 -- 50°
latitude zones. Also, where gullies and mantles occur together, gullies
cut the mantles and in some cases head at locations higher up the slope
than the margin of the mantle/accumulation. It is possible that some
gullies may be the final erosional product left by the melting of a
prior mantle that is no longer present. In any case, our present
understanding of whether there is a genetic relationship between gullies
and mantles is missing some important details. Much work on the topic
remains to be done.

![](media/image11.png){width="3.55625in" height="3.158333333333333in"}

> *[Figure 7.5]{.underline}. A southeast-facing slope with a mantle of
> 'pasted-on' terrain. This image is located near 38.1°S, 265.0°W in a
> depression at the head of Harmakhis Vallis. Material such as this has
> been interpreted by some (e.g., Christensen, 2003) as a deposit of
> snow or ice beneath a residue of dust that is protecting the material
> from further sublimation. This is a sub-frame of MOC image S14-01956;
> sunlight illuminates the scene from the upper left.*

[Is there a part of Mars that currently has an active
layer?]{.underline} Permafrost is ground remaining frozen (temperature
below the freezing point of water) for more than two consecutive years.
An active layer in permafrost regions is defined as a near-surface layer
that undergoes freeze-thaw cycles due to day-average surface and soil
temperatures oscillating about the freezing point of water. A "dry"
active layer may occur in parched soils without free water or ice but
significant geomorphic change through cryoturbation is not produced in
these environments.

We have enough information to be able to conclude that a wet active
layer is currently absent on Mars. Kreslakvsy et al. (2006) used recent
calculations on the astronomical forcing of climate change to assess the
conditions under which an extensive active layer could form on Mars
during past climate history. Their examination of insolation patterns
and surface topography predicts that an active layer should form on Mars
in the geological past at high latitudes as well as on pole-facing
slopes at mid-latitudes during repetitive periods of high obliquity in
the geological past. They examine global high-resolution MOLA topography
and geological features on Mars and find that a distinctive latitudinal
zonality of the occurrence of steep slopes and an asymmetry of steep
slopes at mid-latitudes can be attributed to the effect of active layer
processes. They conclude that the formation of an active layer during
periods of enhanced obliquity throughout the most recent period of the
history of Mars (the Amazonian) has led to significant degradation of
impact craters, rapidly decreasing the steep slopes characterizing
pristine landforms. However, their analysis indicates that an active
layer has not been present on Mars in the last \~5 Ma, and that
conditions favoring the formation of an active layer were reached in
only about 20% of the obliquity excursions between 5 and 10 Ma ago.
Conditions favoring an active layer are not predicted to be common in
the next 10 Ma. The much higher obliquity excursions predicted for the
earlier Amazonian appear to be responsible for the significant reduction
in magnitude of crater interior slopes observed at higher latitudes on
Mars.

**[FINDING.]{.underline} Because some of the 'pasted-on'--type mantle
has a spatial, and some suggest a genetic, relationship to gullies
(which in turn are erosional features possibly related to water), the
'pasted-on' mantle may be a special region. The mid-latitude mantle,
however, is thought to be desiccated, with low potential for the
possibility of transient liquid water in modern times. Because the
mid-latitude mantle and some kinds of gullies may have a genetic
relationship, the mantle is interpreted to have a significant potential
for modern liquid water.**

## 7d. Glacial Deposits

[Low-Latitude.]{.underline} The topic of glaciation---even at equatorial
latitudes--- has been discussed and debated for more than 3 decades
(e.g., Williams, 1978; Lucchitta, 1981; Kargel and Strom, 1992; Head and
Marchant, 2003; Neukum et al., 2005, Forget et al., 2006). New
spacecraft data, and evidence from terrestrial analogs, have provided
insight into these types of deposits in the equatorial region of Mars.
Head and Marchant (2003) have presented evidence that the large lobate
deposits on the northwest flanks of the Tharsis Montes volcanoes might
have resulted from the accumulation of ice and snow, its flow outward to
form glaciers, followed by the cessation of ice accumulation, collapse
of the glaciers and the production of distinctive glacial deposits that
remain today. Similar deposits occur around the base of the Olympus Mons
scarp and are interpreted by some to represent debris-covered glacial
deposits (e.g., Milkovich et al., 2006), by others as landslides (e.g.,
Carr et al., 1977). Climate modeling shows that during periods of higher
obliquity, water is mobilized from the polar regions, carried by the
atmosphere to the tropics, rises along the western flanks of Tharsis,
and is preferentially deposited as snow and ice as the rising moist air
is adiabatically cooled (Forget et al., 2006).

One of the major impediments to the glacial interpretation of these
deposits in the past has been the lack of occurrence of eskers,
drumlins, and other indications of classic wet-based glaciation (e.g.,
Zimbelman and Edgett, 1992). Recently analyzed terrestrial analogs of
the huge deposits at Arsia Mons (\~180,000 km^2^) (Head and Marchant
2003) and Pavonis Mons (Shean et al., 2005) show that glaciers typical
of polar latitudes on Earth (cold-based glaciers, where the glacier
flows over permafrost and deforms internally to the glacier, rather than
with melting at the base) is a more appropriate analog to these tropical
features on Mars. Thus, even when these glaciers were forming on Mars
several tens to hundreds of millions of years ago (Head et al., 2005;
Shean et al., 2006), there was little to no melting associated with
them. Although we cannot determine whether there may be some residual
ice at depth in these deposits, because of their age it would certainly
be below a thick sublimation till---residual shallow ice is highly
unlikely.

Geomorphic features that may have involved liquid water in these regions
include features interpreted by some investigators to be impact craters,
pingos, mud volcanoes, and the possible basal melts from ice caps.
However, under current climatic conditions at mid to high latitudes
where the supply of water from a shallow source may be present, the
water source would be quickly exhausted, as surface recharge is
virtually impossible.

![](media/image12.jpeg){width="4.054166666666666in"
height="2.5236111111111112in"}![](media/image13.jpeg){width="1.8756944444444446in"
height="2.473611111111111in"}

*Figure 7.6a: An example of a feature hypothesized to be a glacial
deposit (Head et al., 2005). Located in Promethei Terra at the eastern
rim of the Hellas Basin, at about latitude 38º South and longitude 104º
East. From HRSC (Credits: ESA/DLR/FU Berlin (G. Neukum).
http://www.esa.int/SPECIALS/Mars_Express/SEMN3IRMD6E_0.html.*

*Figure 7.6b: A second example of a feature hypothesized to be a glacial
deposit (image provided by Michael Carr). A lobate flow that appears to
have been funneled between a gap between two obstacles in the fretted
terrain of the Deuteronilus Mensae region (40N, 25E,THEMIS V12057009).
The flow has been interpreted by some to be ice-rich, although this
interpretation is not unique. From the superposed craters it is
estimated to be tens to hundreds of millions of years old.*

[Middle- and high-latitude]{.underline}. The mid-latitude regions
exhibit additional geomorphic features for which some researchers have
suggested that ice was or still is a part of the material. These include
kilometer-scale flow features, interpreted by some as indicating they
flowed in a viscous manner, found on some pole-facing slopes (Milliken
et al., 2003), aprons surrounding massifs and mesas in the Deuteronilus,
Protonilus, and Promethei Terra regions, and lineated valley floor
materials and concentric crater floor features that exhibit distinct
erosional textures and morphologies that have been a topic of discussion
since the Viking era (e.g., Berman et al., 2005; Pierce and Crown, 2003;
Carr, 2001; Squyres and Carr, 1986; Squyres, 1979; Zimbelman et al.
1989). The lineated valley floor material was once considered to be the
product of creep or flow of ice-rich material (e.g., Squyres, 1979).
Although Malin and Edgett (2001) noted that lineated

floor materials also occur in completely-enclosed troughs, from which no
material can flow, Head et al (2006a, b) have shown that there is ample
evidence for accumulation zones, zones of convergence and folding, and
ablation zones very similar to terrestrial debris covered valley
glaciers and glacial landsystems on Earth. Superposed craters, however,
indicate that these deposits have not been active for tens to hundreds
of millions of years (e.g., Mangold et al., 2003). Although these
features are typically located in the 30°--50° latitude range, effective
maps are not yet available.

While there is no consensus about middle or equatorial latitude ice
(e.g., discussions of glaciers and other flows at middle latitudes), the
presence of high-latitude ice on Mars, particularly in the north polar
cap and associated with the south polar residual cap, is unquestioned.
The key issue for the purpose of biological propagation is whether any
of it ever exceeds a temperature of -20°C. In the high-latitude areas we
see no geomorphic evidence for melting, and there are theoretical
grounds for believing that is not possible for melting to occur
naturally at present.

![](media/image14.jpeg){width="0.8354330708661417in"
height="0.7661417322834646in"}

*Figure 7.7. Location of lobate deposits (yellow) on the NW margin of
Olympus Mons and the Tharsis Montes (from Scott and Tanaka, 1986)
interpreted by Head and Marchant (2003), Shean et al. (2005) and
Mikovich et al.(2006) to be tropical mountain glaciers.  See Scott and
Tanaka (1986) for further description of additional units. Lobate debris
aprons and lineated valley fill are concentrated in the 30-50° North and
South latitude bands (e.g., Squyres, 1979; Squyres and Carr, 1986).*

## 7e. Craters

[Description, Distribution, Age]{.underline}. Impact events can excavate
to depths where ice and/or liquid water may exist, and can heat the
surrounding region for a significant time, creating an environment that
is out of thermodynamic equilibrium with its planetary setting. During
martian history, although cratering events have certainly exceeded the
threshold conditions for biological propagation, the heating is
transitory, and the thermal anomalies are erased with time. Since
craters are everywhere on Mars, the challenge is to identify which, if
any, craters have the potential to retain enough heat to still harbor
liquid water within 5 m of the surface at the present time. Impact
craters occur randomly both in time and location on the martian surface.

[Water in the target volume]{.underline}. The first question to address
is which craters accessed volatile-rich material in the surface. Impact
craters display a relationship between their depths of excavation (d)
and their diameters at the time that maximum depth is reached (i.e., the
transient crater diameter, D~t~). For small bowl-shaped craters ("simple
craters"; typically \<7-km-diameter on Mars (Garvin et al., 2000)), the
observed crater diameter (D) is approximately the transient crater
diameter and the excavation depth is \~1/5 of this diameter (d ≈ D/5).
For larger ("complex") craters, d ≈ D~t~/10, but the currently observed
rim diameter is larger than the transient diameter due primarily to wall
collapse (Melosh, 1989). Several empirically derived relationships
between D and D~t~ for complex craters have been suggested; one example
is that described by Croft (1985):

D~t~ = D~sc~^0.15^D^0.85^

where D~sc~ = simple-to-complex transition diameter ≈ 7 km on Mars.
Using this relationship, we find that

d ≈ 0.13 D^0.85^

![](media/image15.jpeg){width="4.35625in" height="2.826388888888889in"}

*[Figure 7.8]{.underline}. One of the freshest large craters. This
crater is 11.5 km in diameter, is located at 13.70N 29.52E, and shows a
well-developed central peak (where models indicate that a long-lived
hydrothermal system could operate).  The adjacent craters are all
covered with ejecta deposits, indicating those craters are older.  The
roughness of the floor is due to wall collapse during formation. Note
the well-developed fluidized ejecta blanket to the right. V10297019
(THEMIS daytime visible), north is to the right.*

Fresh impact craters on Mars often display a layered (i.e., "fluidized")
ejecta morphology, which is widely believed to form by vaporization of
subsurface volatiles during crater formation (see review in Barlow,
2005), although laboratory experiments suggest that similar morphologies
could be produced simply through the interaction of an expanding ejecta
curtain and the atmosphere (e.g., Schultz and Gault, 1979). The current
best models for layered ejecta blanket formation suggests that the
surface material must contain at least 15-20% ice before the layered
ejecta patterns begin to form (Woronow, 1981; Stewart et al., 2001).
Crater studies provide information about the entire history of a region,
whereas GRS provides a snapshot of where the ice is today (within the
upper meter of the subsurface). Thus, we do not expect these two
approaches to agree in detail.

-   Although regional variations occur, the smallest craters that show a
    layered ejecta morphology in the ±30° latitude zone are typically
    between 3 and 5 km in diameter, which corresponds to excavation
    depths of 600 m to 1 km. This suggests that the uppermost martian
    crust in this latitude zone has been largely devoid of ice over the
    course of martian geologic history. Is there a systematic change in
    the onset depth as a function of time? Barlow (2004) argued using
    ejecta characteristics that there is no long-term variability in
    volatile concentrations at the depths encountered by these craters. 
    However, recent HRSC data (Reiss et al., 2005) suggest that there
    may be an indication of an increase to the depth of the
    volatile-rich layer over time. These crater-related observations are
    independent of assumptions about interaction between the atmosphere
    and the deep volatile reservoirs.

-   In the \~40-60 latitude range, the following observations suggest
    that ice-rich material has been close to the surface in the past, a
    conclusion broadly consistent with modern GRS data: Onset diameters
    for single layer craters are \<1-km-diameter; pedestal craters
    display even smaller diameters and double layer craters are also
    suspected at these small crater diameters (resolution makes it
    difficult to distinguish single layer from double layer at these
    small diameters).  However, research on craters between 50 and 500m
    diameter in the lineated terrain at 45°N suggests that ice is not
    currently present near the surface (McConnell, 2006). Note that
    older maps of crater onset diameters using low-resolution Viking
    data (200 m/pixel resolution) give somewhat different results than
    maps based on higher-resolution MOC, THEMIS, and HRSC imagery.

[Crater-induced heating and hydrothermal systems]{.underline}. The
amount of heating associated with the impact-induced shock wave depends
on the transient crater diameter and the location within or outside of
the crater (Pierazzo et al., 2005). Part of the kinetic energy
associated with incoming bolides is converted into impact melt and part
of it goes into heating the target material during crater formation
(Ivanov and Deutsch, 1999). Numerical modeling indicates that this
heating is highest under the transient crater floor, within the region
where central peaks and central pits form. In martian craters larger
than 30 km, another significant heat source is the uplifted geotherm
under the central uplift (Abramov et al., 2005a, b). Hydrothermal
systems are expected to be particularly active within the central
peak/central pit regions and in association with impact melts along the
crater floor. Heat within the ejecta blanket and near the crater rim
dissipates more rapidly, particularly with the cold surface temperatures
prevailing on present-day Mars.

Water supply is a critical issue for the formation of an impact
generated hydrothermal system. Even small impact craters emplaced in ice
can produce liquid water (Stewart et al., 2001). Using the relationship
above between crater diameter and depth of excavation, for areas with
ice at a depth greater than 600 m, craters up to 3 km in diameter, where
basement uplift is not an issue, are unlikely to produce a hydrothermal
system of any extent or duration. Larger craters could potentially
access liquid aquifers at depth.

Numerical models also indicate that impact melt sheets and central
uplifts/pits associated with larger complex craters can produce active
hydrothermal systems that can survive on time scales of up to \~10^6^
years (Ivanov and Deutsch, 1999; Newsom et al., 2001; Rathbun and
Squyres, 2002; Abramov and Kring, 2005a, b) The lifetime of an
impact-induced hydrothermal system is related to crater size and the
geothermal gradient. The maximum potential lifetime for a hydrothermal
system is the conductive cooling time for the hot rock produced by an
impact. For example, a recent calculation by Abramov and Kring (2005a),
estimates a 67,000-year lifetime for a hydrothermal system in a 30 km
diameter crater on Mars.

Based on the above general considerations, SR-SAG proposes the
conservative guidelines in Table 7.1 for the maximum amount of time a
crater environment has the potential to retain enough heat in the
shallow martian subsurface to exceed the threshold conditions for
microbial propagation, assuming a supply of water is available. In
general, the thermal lifetimes for craters smaller than 3 km in diameter
are too short to be significant for the purpose of special region
analysis. In addition, very young and fresh craters larger than 30 km in
diameter have not been identified (Table 7.2).

> ***Table 7.1. Approximate relationship between crater size and maximum
> duration of heating***

  --------------------------- -------------------------------------------
  Crater Size (diameter)      Time for which crater environment has
                              potential to retain enough heat to exceed
                              threshold conditions

  3 km                        100 years

  10 km                       1,000 years

  30 km                       100,000 years
  --------------------------- -------------------------------------------

> *Table 7.2. Freshest large craters in the northern hemisphere of Mars
> identified to date*

  ------------ -------------- ------------ ------------------- --------------
  **LATITUDE   **LONGITUDE    **DIAMETER   **CENTRAL           **EJECTA\***
  (N)**        (E)**          (KM)**       STRUCTURE**         

  7.03         117.19         18.0         Central Peak        MLERSRd

  7.16         174.41         9.6          Floor Pit           MLERS

  8.93         43.82          10.9         Summit Pit          MLERS

  12.10        169.24         5.9          Summit Pit          SLERS

  13.70        29.52          11.5         Central Peak        MLERS

  16.95        141.70         13.6         Floor Pit           MLERSRd

  19.51        141.18         9.2          Floor Pit           MLERS

  20.01        246.68         7.9          None                SLERSRd

  23.19        207.76         28.3         Central Peak        MLERSRd
  ------------ -------------- ------------ ------------------- --------------

> Notes: \*Ejecta classifications from Barlow et al. (2000).

[Cratering frequency]{.underline}. A 10-km-diameter crater is expected
to form on Earth approximately every 10^5^ years (Morrison et al.,
1994). The impact rate on Mars is considered to be \~1.3 times higher
than the terrestrial impact rate due to Mars' proximity to the asteroid
belt, but the impact energy will be less because the typical asteroid
impact velocity is 8.6 km/s on Mars compared to \~17 km/s on Earth.
Adjusting the terrestrial impact probability plot for Mars suggests that
one 40-km-diameter crater could be expected to form approximately every
million years on Mars and a 10-km-diameter crater would statistically
form every \~30,000 yrs. Thus, although the probability that a crater of
sufficient size has formed recently enough to still retain an active
hydrothermal system is very low, there is a small chance that such a
crater exists.

[Identification of the most recent large craters]{.underline}. Young
craters can be identified by the following characteristics:

1)  Sharp rim and crater depth approximately equal to those values
    expected for a pristine crater of equivalent size.

2)  No superposed features on either the crater or its ejecta blanket.
    Such superposed features would include dunes, floor deposits,
    tectonic/fluvial features, or small impact craters.

3)  Ejecta blanket and interior morphologies that are sharp and well
    preserved.

4)  The crater and its ejecta blanket displaying thermally distinct
    signatures in daytime and/or nighttime infrared views.

Very few impact craters \>5-km-diameter display these characteristics.
Table 7.2 lists northern hemisphere craters identified from MOC, MOLA,
and THEMIS analysis which appear to be among the freshest craters on the
planet (from data of Malin and Edgett, 2001; Smith et al., 2001, and
Christensen et al., 2004; interpretation by Barlow). These craters have
a similar rating of extremely fresh using the criteria listed above,
none are named, and this list will clearly keep growing as our
exploration of Mars proceeds. We do not have precise ways of dating
craters on Mars; however, the approximate age can be estimated from the
degree of degradation of the crater morphology (Barlow, 2004). Based on
this kind of analysis, we do not have reason to believe that any of the
craters in Table 7.2 are as young as the limits specified in Table 7.1.

**[FINDING.]{.underline} No craters with the combination of size and
youthfulness to retain enough heat to exceed the temperature threshold
for propagation have been identified on Mars to date.**

## 

## 7f. Young volcanics

[Description, distribution]{.underline}. Volcanism or magmatic intrusion
may be capable of generating special regions on Mars by warming up
near-surface rocks and melting ground ice. Some key examples of fluvial
channels in spatial and temporal association with volcanic eruption
products have been documented on Mars (e.g., Burr et al., 2002;
Mouginis-Mark, 1990; Mouginis-Mark and Christensen, 2005). Subsurface
magmatic intrusions (e.g., dikes and sills) have also been implicated in
fluvial activity in volcanic regions (e.g., Tanaka et al., 1998; Head et
al., 2003b). Since the time period of interest for special regions
consideration is from now until 100 years (from the date of a mission's
arrival) in the future, we need to consider both recent volcanic rocks
that might still be warm and the possibility of future volcanic
eruptions.

A first-order assessment can be made from global geologic mapping of
Mars and model-dependent absolute ages derived from impact crater
densities. The youngest volcanic materials on Mars are considered to
have formed during the Late Amazonian Epoch, in which the number craters
per 10^6^ km^2^, or N(1), is less than 160 (Tanaka, 1986). These Late
Amazonian volcanic materials were originally mapped with Viking images
at 1:15,000,000 scale (Scott et al., 1986-87). That mapping effort led
to identification of relatively young volcanic rocks associated with the
Tharsis Montes and Olympus Mons. In addition, young deposits of the
Medusae Fossae Formation have an uncertain origin but could be volcanic
(e.g., Scott and Tanaka, 1982). Improved mapping at 1:15,000,000 scale,
based on Mars Orbiter Laser Altimeter (MOLA) topographic data and early
Thermal Emission Imaging System (THEMIS) data, covers the northern
plains of Mars, or about a third of the planet. Based on this mapping,
the youngest volcanic province on Mars is map unit A[EC]{.smallcaps}~3~
from Tanaka et al. (2005), the distribution of which is shown on Figure
7.9.  These materials include a broad, lightly cratered region of lava
flows and vents associated with Cerberus Fossae (Plescia, 1990, 2003;
Keszthelyi et al., 2000); although some investigators have proposed that
the materials are not volcanic (Murray et al. 2005) or while volcanic,
may be exhumed and therefore not necessarily young (Malin and Edgett
2001).

Magmatic intrusions would provide additional sources of near-surface
heat. Basaltic eruptions are generally emplaced by dikes about 1 m wide
on Earth, but may exceed 100 m in width in association with flood
volcanism (e.g., Wada, 1994). Intermediate and silicic magma bodies on
Earth may amount to several times the volume of extrusive products
(e.g., Tanaka et al., 1986). Thus vent areas of silicic and basaltic
flood lavas on Mars may remain warm for particularly long periods of
time and perhaps generate geysers and hydrothermal springs where water
recharge occurs. Such activity can prevail for as long as hundreds of
thousands of years for particularly large, shallow magma bodies. For
these reasons, any volcanic vent regions displaying evidence for
anomalously warm surface temperatures should be considered special
regions. However, investigations of surface temperatures from MGS TES
and ODY THEMIS data have not revealed any such locales on Mars thus far.

![](media/image16.jpeg){width="6.424409448818897in"
height="3.1244094488188976in"}

*Figure 7.9. Shaded-relief map of Mars (simple cylindrical projection)
showing the distribution of the youngest volcanic rocks on Mars (map
unit A[EC]{.smallcaps}~3~ from Tanaka et al. 2005).  These volcanics are
of Late Amazonian age, which includes geologic time from the present to
about 300-500 Ma.  Although there is no current indication that any of
these rocks are as young as 1000 years, we cannot say with certainty
that the activity in this volcanic region has ceased, and that future
eruptions are impossible.*

[Age]{.underline}. Volcanic products on Mars are largely considered to
be mafic (e.g., Greeley and Spudis, 1981; many Mars meteorite studies),
which are dark in color. The relatively youngest volcanic materials are
therefore assumed to be relatively dark (see Figure 7.10) because of the
progressive accumulation of thin coatings of dust that collect after
dust storms. Age can also be estimated from cratering density. Viking,
MOC, HRSC, and THEMIS data demonstrate that no such dark,

uncratered surfaces occur on Mars (i.e., there are no black lava flows,
like the \~900 year old flows from Sunset Crater in northern Arizona).
The highest resolution images, from MOC, show the volcanoes and flows of
the Tharsis Montes and Olympus Mons, for example, to have been covered
by mantles of fine-grained material (i.e., dust), that, in most cases,
has been there long enough for the materials to become somewhat
indurated and sculpted by wind erosion (Malin and Edgett 2001). The
youngest volcanic materials might be those associated with the Cerberus
Fossae/southeastern Elysium Planitia and Marte and Athabasca Valles
systems, south of the Elysium Rise, but these, too, are mantled with
dust.

The absolute ages of these possibly volcanic surfaces are estimated by
crater density and models of crater production rate (Hartmann and
Berman, 2000; Hartmann and Neukum, 2001). Such age determinations have
to be considered with caution. First, there is debate at present as to
whether or not secondary craters dominate the populations of
sub-kilometer craters (McEwen et al., 2005), which have been
incorporated when evaluating the ages of the youngest surfaces (Hartmann
and Neukum, 2001).

![](media/image17.jpeg){width="4.43125in" height="2.9506944444444443in"}

*[Figure 7.10]{.underline}. Three distinct, apparent volcanic surfaces
in Elysium Planitia progressively younger from right to left (appearing
rugged and cratered, bright and partly incised, and dark, respectively).
The youngest unit is Late Amazonian (in the Tanaka et al 2005 map); the
middle unit is Late and Middle Amazonian, and the cratered unit is Late
Hesperian (Cerberus Fossae 3 and 2 and Utopia Planitia 2 units,
respectively). This shows the episodic nature of eruptions in volcanic
regions. Image width 3.7 km., part of MOC image R11-01377.*

Second, the error in the model ages is uncertain and considered to be a
factor of 2 (Hartmann and Neukum, 2001). Third, the geology of the dated
rocks may be complex in some cases. If the crater counts include
multiple outcrops of lava flows and other volcanic materials, then the
counts provide a mean age. Some surfaces may be much older than others
within the counted area. Crater obliteration processes, particularly for
softer materials such as volcanic ash, may result in surface ages that
are much younger than the rock emplacement ages.

In addition, no known "warm" surfaces---that is, those that can be
attributed to volcanic or magmatic heating---have been detected on Mars
from spacecraft imaging or thermal remote-sensing datasets.

[The possibility of eruptions within the next 100 years]{.underline}. To
establish a volcanic recurrence rate, information is needed for multiple
eruptions. Geologic mapping and crater dating investigations, however,
have not yet resolved this to any useful precision. For example, for the
materials interpreted to be geologic units consisting of multiple,
overlapping lava flows, it is uncertain in detail how many eruptive
episodes were involved. Thus the duration of the Late Amazonian
constrains the mean recurrence interval for volcanic materials of this
epoch. Its duration ranges from 150 to 1100 Ma, given the factor of 2
age uncertainties (see Hartmann and Neukum, 2001, Fig. 14). The
recurrence interval is the inverse of the ages in years times 100 for a
100-year period, or 9.1 x 10^-8^ to 6.7 x 10^-7^. If, however, the
materials of the Cerberus Fossae region and other volcanic materials
were to have a recurrence interval of 10 Ma, equivalent to a younger
estimate of its age from sub-kilometer crater densities (Hartmann and
Neukum, 2001), then the probability of recurrent volcanism for a hundred
year period at any of these sites is on average \<10^-5^. Volcanic
episodicities related to longer time periods have also been discussed by
Wilson et al. (2001) and Neukum et al. (2004).

In summary, the recurrence and extent of volcanic eruptions on Mars
appears to be sufficiently low in the most recently active regions that
the occurrence of special regions due to either recent or renewed
volcanism seems unlikely. However, without more precise geologic mapping
of eruption deposits and their age dating, it is not possible to
determine exactly the recent volcanic recurrence interval of local
volcanic areas on Mars that may be of interest to future landed
missions. For example, Athabasca Valles, proposed by some to be one of
the youngest fluvial and volcanic systems on Mars, was seriously
considered as a candidate landing site for the Mars Exploration Rovers
(e.g., Golombek et al., 2003), yet its detailed geologic history remains
elusive.

[Potential to exceed propagation thresholds]{.underline}. Since volcanic
heat is lost with time, only extremely young volcanics have the
potential to exceed the propagation thresholds. An obvious question,
therefore is how much time is too much? A simple calculation was done
using an infinitely thick slab with an initial temperature of 1000°C
cooling in an environment with a surface temperature of -40°C, and the
following parameters: thermal conductivity of the surface = 2.5 W/m/K,
heat capacity of the surface = 800 J/kg/K, and density of the surface =
2600 kg/m^3^. As is typical of solutions to the heat flow equation, the
initial cooling is very rapid, but after the first few tens of years,
the cooling slows considerably. The temperature of the surface drops to
less than -20°C within about 1000 y**.** Although it is beyond the scope
of this report to analyze the possible cooling histories of all
configurations of eruptive volcanic rocks, the 1000-year figure is a
useful practical guideline. Volcanic rocks older than that cannot have
retained enough heat exceed the -20°C propagation limit within the upper
5 m of the martian crust. This limit is very conservative in the sense
that the upper five meters will almost certainly be dry as well as cold.
It is more difficult to put a limit on the amount of time for loss of
water from the system, but this is not necessary since both threshold
conditions must be satisfied for propagation. More detailed treatments
of the interactions of magma and ice on the Earth and Mars can be found
in Wilson and Head (2002) and Head and Wilson (2002), respectively.

Volcanic rocks younger than 1000 years old have not been discovered on
Mars, so we do not have evidence for shallow volcanic-related special
regions.

**[FINDING.]{.underline} We do not have evidence for volcanic rocks on
Mars of an age young enough to retain enough heat to qualify as a modern
special region or suggest a place of modern volcanic or hydrothermal
activity.**

## 7g. Slope streaks

[Description and Age.]{.underline} Dark- and light-toned slope streaks
(Figure 7.11) occur in dust-mantled regions, particularly Arabia Terra,
Tharsis, and various hills and slopes in the regions between Tharsis and
Elysium in the northern hemisphere (Figure 7.12). Sullivan et al.
(2001), Schorghofer et al. (2002), and Aharonson et al. (2003) have
presented basic summaries on the subject. Slope streaks typically
originate at a point on a slope that is often associated with a
roughness element, such as a small knob. The streaks then continue
downslope for hundreds of meters or more. They usually have a narrow,
well-contained shape that spreads out into a single fan or multiple,
sometimes anastomosing or braided, forms as it proceeds down the slope.
Slope streaks have sharp boundaries, usually (but not always) with a
nearly constant brightness throughout the feature, and their albedo
contrast with the surrounding terrain has been seen to decrease over
time.

![](media/image18.jpeg){width="6.052083333333333in"
height="2.7944444444444443in"}

*Figure 7.11. Slope streak examples (from Phillips and Chyba, 2006).
Typical slope streaks (a, MOC image M16-00596, 27N 133 W, 5.9 m/pixel)
have a point source and a wedge-shaped, sometimes digitate, appearance. 
More complex examples (b, image M16-00596, 27N 133 W, 5.9 m/pixel; c,
image E02-00308, 0N, 165W, 4.4 m/pixel) have an intricate, braided,
sometimes anastomosing shape. These features are widely interpreted to
be the result of dry dust avalanches, but one working hypothesis for
what triggers them to form involves water.*

Slope streaks are relevant to the present analysis regarding special
regions because they form in response to modern geologic processes, and
some investigators have proposed that these features might involve the
action of liquid water (Schorghofer et al., 2002; Ferris et al., 2002;
Miyamoto et al., 2004b). New slope streaks have been identified in
comparisons between Viking and MOC images, and new streaks have even
observed to have formed during the Mars Global Surveyor mission, on
timescales as short as \~100 days (Malin and Edgett, 2001; Sullivan et
al., 2001; Schorghofer et al., 2002; Aharonson et al. 2003; Miyamoto et
al., 2004b).

[Possible Relationship to Water]{.underline}**:** There are two current
working hypotheses for their formation mechanism.

-   The dry models suggest that the streaks form through dust movement.
    In the dry process, oversteepening slopes caused by air fall
    deposits of dry dust eventually collapse, forming a dust avalanche
    (Williams, 1991; Sullivan et al., 2001; Miyamoto et al., 2004). In
    support of the dry model: all light and dark slope streaks occur in
    low thermal inertia areas mantled by accumulations of dust thick
    enough to be evident as such in MGS MOC images.

-   The second class of models involves water, but the role of water
    varies between models. Early models such as Ferguson and
    Luchitta (1984) suggested that the dark streaks could be stains on
    the surface produced by wet, briny debris flows. These flows could
    form when a slope intersected an aquifer, allowing the periodic
    release of fluid from a wet subsurface layer. Schorghofer et
    al. (2002) suggested that water could lubricate avalanches, or the
    sublimation of near-surface ice could trigger mass movements. Ferris
    et al. (2002) proposed a model involving groundwater springs which
    infiltrate and saturate the surface, creating the dark streaks.
    Miyamoto et al. (2004b) concluded that the streaks are not debris
    flows, but could not eliminate the possibility that water is
    involved in the streak-forming process.

![](media/image19.jpeg){width="6.629166666666666in"
height="3.352777777777778in"}

*Figure 7.12. Map of the distribution of dark slope streaks (from
Aharonson et al., 2003)*

Despite the various hypotheses in the literature, two key observations
indicate that wind is a controlling factor for at least some slope
streaks: 1) examples have been seen where dust devil tracks turn into a
slope streak once a crater rim has been crossed; 2) a population of
slope streaks is clearly emplaced preferentially on west-facing slopes
(Baratoux et al., 2006)---this correlates with wind direction. The
abundance of slope streaks decreases sharply at about 33°N and they are
absent at higher latitude. Baratoux et al. (2006) interpret this as an
indication that poleward of 33°, the ubiquitous dust is ice-cemented,
and is thus not amenable to flow, whereas equatorward of 33° the dust is
desiccated and flows easily. For these reasons, it is unlikely that
substantial quantities of liquid water are involved in slope streak
formation.  There is, however, an open question on the role of trace
amounts of transient H~2~O in triggering these dust avalanches, for
example by changing cohesion or porosity of the dust matrix.

## 7h. Recent outflow channels?

[Description, Age]{.underline}. One of the prominent aspects of martian
geology is the evidence for ancient outflow channels. Could the process
that created them continue to the present era? The youngest such feature
that has been dated is Athabasca Vallis (10N, 157E)---Burr et al. (2002)
give the age of the latest flood at Athabasca as 2-8 My. Some other
channels nearby are only slightly older. Furthermore, there are also
some young channels just to the southeast of Olympus Mons
(Mouginis-Mark, 1990) that Basilevsky et al. (2006) date at 20 My. These
examples indicate the presence of deep groundwater (or ice) that might
have episodically erupted to the surface along faults (or caused by
dikes). Although this type of feature may have continued to form until
the geologically recent past, we do not have evidence that any are
active today, or have reason to predict that any will be active within
the next 100 years.

## 7i. The non-discovery of geothermal vents

An important objective of the THEMIS infrared investigation on Mars
Odyssey has been the search for temperature anomalies produced by
surface cooling or heating due near-surface liquid water or ice, or
hydrothermal or volcanic activity. THEMIS has mapped virtually all of
Mars at night in the infrared at 100-m per pixel resolution, and has
observed portions of the surface a second time up to one Mars' years
later. An analysis has been performed of all of these images to search
for maximum temperatures greater than those expected from rocks or
bedrock alone (220K), and no example has been found in any image of a
temperature that requires an internal heat source (Christensen, writ.
comm., 2006).

The THEMIS database has also been searched for spatial patterns that
might indicate evaporative cooling associated with near-surface water,
or to recent volcanic activity or hydrothermal heating. The THEMIS
nighttime images exhibit very high spatial variability owing to
variations in surface properties such as rock layers, exposed bedrock,
sand and granule dunes (Christensen, writ. comm., 2006). These thermal
variations greatly complicate the search for anomalous patterns
associated with sub-surface water. To overcome the effects of these
surface temperature complexities, a search for seasonal changes in
surface temperature was undertaken in an effort to isolate changes due
to dynamic heating or cooling processes from those due to thermophysical
properties. This process involved the use of histogram adjustments of
the overlapping portions of two images to remove the additional
complicating temperature changes produced by the two-hour variation in
local time of the Odyssey orbit, and by the differences in season
between the different observations. In addition, numerous image pairs
have been converted to thermal inertia using the THEMIS Standard Thermal
Model as a quantitative means to remove the time of day and seasonal
effects. To date no significant changes have been detected, but this
analysis will continue throughout the Odyssey extended missions. Based
on the analysis thus far, there is no evidence in the THEMIS thermal
images for the existence of near-surface liquid water or ice that is
close enough to the surface to be capable of producing measurable
thermal anomalies.

The MGS MOC investigation has also addressed this issue, by focusing on
imaging of relatively young volcanic and impact landforms and searching
for evidence that activity is occurring today---such as eruption,
production of flows (mud or lava), and the like. No such features have
been found in MOC or other orbiter images.

**[FINDING.]{.underline} Despite a deliberate and systematic search
spanning several years, no evidence has been found for the existence of
thermal anomalies capable of producing near-surface liquid water.**

## 7j. The Possibility of Low-Latitude Ground Ice

The Mars Odyssey GRS data (Fig. 6.3) clearly reflect high-latitude ice
in the vicinity of both poles, but also some significant positive
equatorial anomalies in places like Arabia Terra. A number of authors
have concluded, in large part based on the instability of ice within a
meter of the surface (the volume within which the GRS instrument can
detect ice), that these anomalies are related to hydrated minerals
(e.g., Basilevsky et al., 2003). For example, the high H in low
latitudes is entirely consistent with the observations of hydrated
minerals found by MER and by OMEGA, and there is no evidence of layering
of H in the low latitude anomalies.

However, there has also been some discussion of the possibility that
these anomalies represent low-latitude ground ice. For example, it has
been suggested that when the south polar cap has exposed water ice, the
water vapor content of the martian atmosphere could climb and might make
ground ice stable to low latitudes (e.g., Jakosky et al. 2005).  It may
be possible for ice to be emplaced onto the surface or into the top
meter during these episodes.  While the ice is not stable today, it has
been suggested that ice that had been previously deposited might still
exist in a transient state while in the process of disappearing. 

However, diffusion processes controlling potential ice deposition in the
equatorial regolith are thought to occur on 100 to 5000 year time scales
(Feldman et al., 2005). If the time scale of uncovered polar water ice
is shorter than 100 years it is difficult for significant amounts water
ice to accumulate. Polar cap exposure on longer time scales, such as
would result from orbital oscillations, might result in higher
concentrations of equatorial ground ice, but also would result in a
different distribution of high-latitude ground ice than is observed
today (Mellon et al., 2004). In addition, \"transient\" ice would be
stable and in equilibrium with the atmosphere only in regions and depths
where subsurface temperatures are typically below the new frost point
(for even a 10 fold increase in atmospheric water the new frost point
would only be 212 K).  Subsequent replenishment of the CO~2~ cap on the
south polar region and subsequent reduction of atmospheric water vapor
would result in removal of now unstable ice by sublimation on the same
time scale as its emplacement. Thus, presently observed equatorial
concentrations of hydrogen are more likely to be the result of one or
more hydrated minerals and not due to transient water ice (Feldman et
al., 2005).

[The possibility of massive subsurface ice deposits]{.underline}. There
has also been discussion in the literature of the possibility of massive
subsurface ice deposits on Mars, even at low latitude, representing the
remains of a frozen ocean (postulated by Clifford, 1993; possibly
observed by Murray et al., 2005). The site described by Murray et al.
(2005) is in southern Elysium (around 5ºN latitude and 150ºE longitude),
where they reported geomorphic evidence consistent with a
presently-existing frozen body of water, which they interpreted as the
remains of surface pack-ice. The ice slab is interpreted to have an age
from crater counts of 5 ±2 Ma, an original mean thickness of 45 m, and a
present-day mean thickness of 30 m (with a significant part of the
difference representing an overlying sublimation lag). The essential
question, for the purpose of this analysis, is not whether such
interpretations are correct or not, but whether if they are correct this
could constitute a niche environment in which the biological thresholds
for T and a~w~ could be exceeded within the upper 5 m. As discussed in
Section 6 of this report, the ice itself is not intrinsically special,
and if in fact it is preserved at all in the shallow subsurface, it
would be in a place where the temperature is low enough that a~w~ is
below the biological threshold.

## 7k. The polar caps.

In the initial COSPAR definition of special region (see Definition #1),
the polar caps are mentioned as an example of a special region. Using
findings of this analysis and the proposed guidelines in this report,
the polar caps would no longer fit the definition nor serve as valid
examples. Jakosky et al. (1993) (as well as others) have calculated that
at the present obliquity, the maximum summer temperatures typically
reach about 200 K at the pole (Figure 7.13). This temperature is
consistent with the amount of atmospheric water vapor observed in the
atmosphere during northern polar summer, which is about 10 precipitable
microns (pr *μ*m) with a 10 km scale height. More water vapor is
observed in the atmosphere during the north polar summer (\~100 pr
*μ*m), which is consistent with a surface ice temperature of \~210K.

![](media/image20.jpeg){width="6.5in" height="4.853543307086614in"}

> *[Figure 7.13]{.underline}. Maximum summer time temperature of the ice
> surface at the pole (similar north and south). The scatter is due to
> eccentricity and perihelion variations. The largest surface
> temperature is roughly 205-210K. From Jakosky et al (2003), using
> models in Jakosky et al (1993).*

Contributing to the perpetual low temperature is not only the latitude
(hence low sun angle) but also the high conductivity of solid ice. Polar
ice is highly conducting material (e.g., Paige and Keegan, 1994). Paige
and Keegan determined the thermal inertia of the polar ice caps and
found them to be in the neighborhood of 2000 (MKS units), which is
consistent with solid ice. While snow and firn might produce more
insulating material, both measurements of thermal inertia and models
(Arthern et al., 2000) indicate that the polar ice is, indeed, highly
conducting, and the annual skin depth is around 10m. That means that
seasonal changes must heat up a slab 10m thick to affect a significant
change in surface temperature. The argument is particularly compelling
at the north pole, which currently experiences cool summers at aphelion
(and will continue to do so for many thousands of years). The south
polar cap, despite receiving more summer sunlight, is protected by a
layer of highly reflective CO~2~ ice, which holds the surface
temperature at a constant 145K. While these models include the high
conductivity and heat capacity of ice, none of these models take into
account the additional latent heat loss when the sublimation rate is
high---this would cause the present models to be overly optimistic
regarding the potential for warming the polar cap material.

The warmest places on the polar caps might be steep slopes where the
strata of the polar layered deposits are visible and the ice may be
actively retreating. Equatorward facing slopes or low latitude ice would
indeed receive more insolation. This may account for the higher
atmospheric water vapor if the slopes reach temperatures of 210-212K.

Even local heating of the surface of this ice to temperatures greater
than -20°C is not thermodynamically possible in today's climate. As
described above, Mars' large mass of polar ice is at an annual mean
temperature of \~-110°C (at least 50°C lower than the coldest ice on
Earth) and is known to have high thermal conductivity.  Consider, for
example, local warming due to a patch of dark particulate material
(analogous to the source of cryoconite holes on Earth (NRC, 2006;
p.75-78)).  Assuming albedo of 0.15, nominal thermal properties, and 85N
latitude, model calculations indicate that the ice temperature will
never exceed -43°C.  Alternatively, consider the heat balance required
to maintain ice at -20°C. At this temperature, radiation to the sky
would be \~220W/m^2^, and the latent heat of sublimation would be nearly
60 W/m^2^ for a total of 280W/m^2^. Conductive losses to the subsurface
would be of comparable magnitude, depending on the specific model. But
even with a perfectly clear sky and a low (dark) albedo of 0.15, a flat
surface at 85 N in the peak of the day in midsummer only receives \~210
W/m^2^ insolation.

**[FINDING]{.underline}. The martian polar caps are too cold to be
naturally occurring special regions in the present orientation of the
planet.**

# 

# 8. REVISION OF THE SPECIAL REGION DEFINITION AND GUIDELINES 

[]{.mark}

SR-SAG concluded that it could fulfill its assignment concerning
reducing ambiguity in the term "special region" by retaining the
original COSPAR definition and by adding an updated set of
clarifications and implementation guidelines (DEFINITION #2). These
guidelines are as quantitative as possible based on the SR-SAG analysis,
and are intended to allow the definition to be interpreted in a common
way by the many different interest groups who are stakeholders in Mars
exploration.

***[DEFINITION #2.]{.underline}***[^6]

# 9. DISCUSSION OF NATURALLY OCCURRING SPECIAL REGIONS 

## 9a. Risk Acceptability 

A key term in the definition of special region (see Definition #1, #2)
is "*likely* to propagate." "Likely" implies a probability. The lay
usage of the word "likely" is that it implies a probability level of
50%. However, that is clearly not the intent of COSPAR for this
application---something significantly lower, but non-zero, would be
consistent with probability thresholds used elsewhere in PP policy.
According to this definition, not every martian environment that has
non-zero probability to exceed the threshold conditions for propagation
is special; only those for which the magnitude of that potential reaches
"likely" would qualify.

Risk consists of a probability and an adverse consequence (for example,
the propagation of terrestrial organisms placed in a certain martian
environment). A crucial question is the degree to which this risk is
acceptable. SR-SAG considered this issue in some detail. A process
referred to as "expert elicitation" is commonly used in risk analysis
studies to determine a consensus risk tolerance level. As an analogous
example, this kind of a process might be used to determine the
acceptable level of risk that there is an exogenous termite in a
shipment of imported lumber. In these kinds of cases, although it would
be desirable to have the risk be as low as possible, setting the risk
level too low typically introduces unacceptable consequences in other
areas. In order to determine the difference between acceptable and
unacceptable risk, it is helpful to set quantitative risk standards.
Such risk standards have been used in martian planetary protection for
many decades for parameters like orbital lifetime and trajectory
biasing. However, SR-SAG ultimately chose not to propose a consensus
quantitative risk standard, for at least two reasons:

-   The probability that T and a~w~ will exceed their threshold values
    in a given martian environment within 100 years of mission arrival
    cannot be quantitatively determined for all martian environments.

-   Even if T and a~w~ exceed their threshold values in a specific
    martian environment, the probability of propagation of a population
    of mixed terrestrial microbes cannot be quantitatively estimated.
    There are too many additional factors involved that are poorly
    understood. We have minimal ability to do this even on Earth.

For these reasons, SR-SAG opted instead to use a qualitative
determination of acceptability of risk (to this group) for different
martian environments (Table 9.1). This risk acceptability includes the
likelihood that the environment exists at all, or will exist within 100
years (e.g., the probability that volcanic activity will commence at a
specific site); the risk that if the environment exists propagation is
possible (gullies exist, but can organisms propagate there within 100
years?); and the risk that an extreme hypothesis that is difficult to
test and impossible to reject will turn out to be correct. One of the
essential aspects of exploration is that there is an element of the
unknown---that is why the exploration is justified in the first place.
Although there is risk in encountering the unknown, we have learned from
our experience on Earth that as scientific knowledge progressively
increases, it is possible to develop strategies for carrying out
exploration activity in a relatively safe (although not risk-free) way.

It is important to point out that the composition of the SR-SAG team was
chosen to be representative of the Mars scientific exploration
community. The team is entirely composed of Mars scientists, who are
intensely interested in the future exploration of Mars. Although the
judgments embodied in Table 9.1 describe SR-SAG's comfort zone, there
are clearly members of the community who are both more and less
conservative than this consensus position. The same is true of other
stakeholders in Mars exploration, and SR-SAG makes no representation
regarding their views. Should a more conservative risk posture be
desired, there are several implementation options available, including
sterilizing all spacecraft sent to Mars, sterilizing spacecraft sent to
certain places on Mars, placing certain places on Mars off-limits, and
not flying spacecraft to Mars at all.

## 9b. Special regions on Mars within the temporal and spatial limits of this analysis

[Environments that definitely exceed the threshold
conditions]{.underline}*.* SR-SAG cannot identify any regions on Mars in
which the threshold conditions for propagation are exceeded at the
present time.

[Environments of Concern]{.underline}*.* Using the guidelines described
in this document (Definition #2), there are martian environments (based
on understanding as of April, 2006) that might exceed the threshold
conditions for propagation of terrestrial organisms. They are summarized
in Table 9.1. Of these, only the mid-latitude pasted-on mantle and the
gully-forming regions are thought to be of high enough concern for
exceeding the threshold conditions during the next 100 years that the
SR-SAG believes they should be treated as special regions for the
purposes of planetary protection.

*Table 9.1. Listing of Mars environments of concern as possible special
regions*

+-----------------------+-----------------------+-----------------------+
| **Classification of   |                       |                       |
| martian environments  |                       |                       |
| by their potential to |                       |                       |
| exceed the threshold  |                       |                       |
| conditions in         |                       |                       |
| temperature and water |                       |                       |
| activity for          |                       |                       |
| microbial propagation |                       |                       |
| (within the boundary  |                       |                       |
| conditions of the     |                       |                       |
| analysis).**          |                       |                       |
+-----------------------+-----------------------+-----------------------+
| **A. Observed         | **B. Observed         | **C. Not known to     |
| features for which    | features for which    | exist, but if         |
| there is a            | there is a low, but   | examples could be     |
| significant (but      | non-zero, probability | found, would have a   |
| still unknown)        | of a relationship to  | high probability of   |
| probability of        | modern liquid water** | association with      |
| association with      |                       | modern liquid         |
| modern liquid water** |                       | water.**              |
+-----------------------+-----------------------+-----------------------+
| -   Recent gullies    | -   Low-latitude      | -   Volcanic          |
|     and gully-forming |     slope streaks     |     environments      |
|     regions           |                       |     young enough to   |
|                       | -   Low-latitude      |     retain heat.      |
| -   "Pasted-on"       |     features          |                       |
|     mantle            |     hypothesized to   | -   Impact            |
|                       |     be glaciers       |     environments      |
|                       |                       |     young enough and  |
|                       | -   Features          |     large enough to   |
|                       |     hypothesized to   |     retain heat       |
|                       |     be massive        |                       |
|                       |     subsurface ice    | -   Modern outflow    |
|                       |                       |     channels          |
+-----------------------+-----------------------+-----------------------+

[]{.mark}

Gullies and pasted-on mantle of concern here were formed at the same
latitudes and on slopes where insolation conditions might (although
this, to, is highly uncertain and inconclusive) be conducive to their
development. The relationship between gullies and pasted-on mantle is
complex\--there might be more than one type of each, and they may have
asynchronous, or no, relationship. These are clearly an area for future
research attention. The following relationships are worth noting:

-   There is a very real possibility that the gullies were formed by the
    action of liquid water, and the possibility (although less-well
    documented and studied as of the present time) that "pasted-on"
    mantle provided that water (at least in some cases).

-   The gullies and pasted-on mantle both occur at middle latitudes.

[Map distribution.]{.underline} Based on the analysis above, several
boundaries of primary significance to interpreting the possible presence
of special regions on Mars can be shown in map format (Figure 9.1; for
additional details on the construction of this map, see Appendix II).

*[Figure 9.1.]{.underline} Map of the stability of ice in the shallow
martian subsurface, as shown by synthesis of results from three
thermodynamic models presented at the Mars Water Conference (2006) by
Aharonson and Shorghofer, Mellon and Feldman, and Chamberlain and
Boynton. For details, see Appendix II.*

**BOUNDARY A**: The 6 count/second isopleth (using summer data only)
from the GRS instrument (Figure 6.3).

**BOUNDARY B**: The most equatorward position of the limit of ice
stability at a depth of 5 m, using the three planetary scale
thermodynamic models presented at the Mars Water Conference in Feb.
2006. Each of these models uses somewhat different methodology and
somewhat different input parameters, so the derived results are somewhat
different (although they are all based on the same physics). The SR-SAG
is not in a position to judge which, if any, of these models is correct.
However, they all have the same general form, with an equatorial belt
where none of them show that ice is stable within 5 m of the surface,
north and south polar zones where all of them agree there is continuous
ice within 1 m of the surface, and an intermediate zone where ice is
discontinuous or within 1-5 m of the martian surface. For the purpose of
Figure 9.1, the equatorial and polar zones were mapped only where all
three models agreed; thus, the mid-latitude zone also incorporates all
of the model-dependent uncertainty.

**BOUNDARY C**: The observed distribution of gullies and mid-latitude
mantles has a relatively well-defined equatorward limit (see Figures 7.2
and 7.4). The map distribution of the roughened mantles of Mustard et
al. (2001) is known (Figure 7.4), but the "pasted-on" mantles of
Christensen (2003) have not been sufficiently mapped. As a practical
matter, the distribution of gullies and roughened mantles is very
similar. Because of the possibility that either could be an environment
where the threshold conditions for microbial propagation are exceeded,
this boundary is drawn to encompass both.

The map areas between these boundaries lead to the following
interpretations:

-   The equatorial belt (i.e., the region equatorward of all three map
    boundaries) is a region within which ice is thermodynamically
    unstable within 5 m of the martian surface at the present time.
    Gullies are absent. Although shallow liquid water could be present
    in a disequilibrium environment, such environments have yet been
    identified. The youngest volcanics on Mars are present within this
    region (see Figure 7.10), but there is no evidence that they are
    young enough to retain enough heat support modern liquid water. To
    date, despite ongoing searches, no thermal anomalies have been
    identified.

-   Between Boundaries B and C, ice is thermodynamically unstable within
    5 m of the martian surface at the present time, and gullies,
    pasted-on mantle, and mantles are locally present. The geologic
    processes associated with gullies and pasted-on mantle is still a
    subject of active research, but there is a significant possibility
    that they could be either continuously or episodically active, and
    their activity could cause the threshold conditions for temperature
    and water activity to be exceeded. Because these features are only
    locally present within this region (especially on certain kinds of
    crater walls), there are large areas where there is no evidence of
    either gullies or mantles. Note that gullies and mantles are not
    limited in their map distribution to the area between Boundaries B
    and C---both extend to considerably higher latitude than Boundary B.

-   Between Boundaries A and B is a region within which there is
    evidence (from models) that ice is stable at a depth from 1-5 m, or
    within which it is discontinuously or seasonally present. Such
    discontinuities can be caused by variations in albedo, variations in
    thermal inertia, slope effects (e.g., poleward-facing slopes get
    less sun that equatorward-facing slopes). The longitudinal variation
    in the position of the Boundary B is primarily related to variations
    in thermal inertia of the martian near-surface materials. The
    worst-case crash scenarios within this region are potentially
    capable of penetrating to ice.

-   The area poleward of Boundary A represents the polar regions, where
    there the GRS data have been interpreted show essentially continuous
    shallow frozen ground and ice caps at each pole. It is too cold for
    naturally occurring liquid water at any time during the year.

# 10. DISCUSSION OF SPACECRAFT-INDUCED SPECIAL REGIONS

The analysis and discussion to this point has focused on identification
of naturally occurring environments on Mars within which terrestrial
life forms might be able to propagate. To complete the analysis,
consideration was given to whether the arrival of a spacecraft could
induce a special region on Mars even when one did not exist at the
landing site beforehand. Spacecraft are capable both of generating heat,
and carrying liquid water, both of which could have an effect on the
threshold conditions for propagation. Most importantly, in regions where
there is (or may be) ice present, it is conceivable that local heating
would result in the formation of liquid water for some amount of time.
If conditions were created that exceeded the temperature and water
activity thresholds for significant periods, replication of
spacecraft-borne terrestrial bioburden could not be ruled out.

A series of representative scenarios for different mission types,
including orbiters, landers/rovers, balloons, and drill missions were
considered, for both nominal and non-nominal (crash) scenarios. Also
considered were different operational factors: descent engine exhaust
plumes, power/heat sources, roving into special regions, on-surface
activities (e.g., sampling -- scoops, drills, rock abrasion tools, melt
probes) and burn-up/break-up scenarios for discarded descent hardware.

The complexity of considering all the potential scenarios with respect
to induced special regions led to the conclusion that general
categorization is not possible or practicable. The recommendation,
therefore, is that analysis of spacecraft-induced special regions for
Mars missions should be done on a case-by-case basis, with mission
project teams being required to produce something equivalent to a "Mars
environmental impact assessment" as part of the early stage mission
planning. \[Such an analysis may form part of the support documentation
for the mission certification request.\]

The use of RTGs (radioisotope thermal generators) on spacecraft will
necessitate an extended analysis, since they can act as a perennial heat
source, creating temperatures local to the RTG above the threshold
temperature condition. In the case of a non-nominal landing, resulting
in the co-location of near-surface ice, an RTG and contaminated
spacecraft parts, it would be possible to have a disequilibrium
condition for many months, exceeding the proposed threshold conditions
for propagation of terrestrial organisms.

A key factor in the assessment of spacecraft-induced environmental
changes is the minimum duration for the conditions to be above the
thresholds before qualifying as a special region and therefore a PP
concern. At one extreme, the pressure or friction associated with
sampling, for example with a scoop, may cause melting of ice in the
timeframe of seconds or less. At the other extreme, longer duration
increases in temperature associated with spacecraft activity can be
predicted, for example deliberate or accidental contact of a perennial
heat source with the surface. In this scenario, the local ice will act
as a heat sink, dissipating heat rapidly, but there may be a longer
period of time (up to years) where the temperature and/or water activity
thresholds are exceeded.

It is recognized that microbial propagation is not an instantaneous
process---a finite amount of time above the temperature and water
activity thresholds is necessary in order for the mechanics of
biological propagation to take place. As an example, food spoilage in a
refrigerator demonstrates that microbes are active at low temperatures
but are slow growing, with the amount of time for spoilage dependent on
temperature. Unfortunately, for the analysis of growth rates pertinent
to spacecraft-induced special regions, there is not the same volume and
quality of data to draw from as for the analysis of the temperature and
water activity thresholds presented in Section 5 above; and, while there
are abundant data on growth responses of organisms to temperature,
little is available in the range of interest to this problem. It was
possible, however, to search the literature for the highest documented
growth rates at the temperatures of interest and report here on the
growth rates of whatever microorganism was the subject of that
literature. Organisms isolated from polar environments seem to be the
most cold tolerant that have been tested, whereas microorganisms
responsible for food spoilage under refrigeration seem to *grow* the
best at cold but not extreme temperatures. We found that the highest
documented growth rates at the temperatures of interest (-15°C to +5°C)
were based on an early study of *Sporotrichum carnis* (Ratkowsky et al
1982, Haines 1931). We concluded that significant replication of this or
any other terrestrial microorganism would not occur if the temperature
excursion to a maximum of -5°C did not exceed 22 hours, to 0°C did not
exceed 3 hours, and to 5°C did not exceed 1 hour. Subsequent work on
growth rate models (Ratkowsky et al 1983), based on estimates for the
doubling time of terrestrial organisms in ideal culture at 5°C, support
these conclusions. \[For an analysis pertinent to conditions for
possible microbial growth at Mars, it is worth noting that the boiling
point of water at martian atmospheric pressure of 8.6 mbar is 5°C;
therefore, there is no need to extend this kind of analysis to
temperatures higher than shown in Table 10.1.\] The proposed times in
Table 10.1 may be overly restrictive in that they are based on organisms
in the exponential growth phase and do not take into account the latency
that would precede growth and division once adequate temperatures were
attained.

> *Table 10.1 Proposed times for which localized spacecraft-induced
> environments may exceed the temperature threshold of -20°*C with no
> cell replication resulting

  ----------------------------------- -----------------------------------
  Not-to-exceed temperature of        Elapsed time before replication of
  spacecraft-induced environment      terrestrial organism could occur

  -5°C                                22 hours

  0°C                                 3 hours

  5°C                                 1 hour
  ----------------------------------- -----------------------------------

It was further concluded that a cumulative limit for qualification as a
special region could NOT reasonably be set. While some organisms are
able to \"bank\" metabolic activity and benefit from serial exposure to
favorable conditions, little or no data is available evaluating the
affect of freeze thaw cycles that range from +5°*C* to -20°*C* or below
on microbial metabolism. Indeed, it is reasonable to expect that
repeated freeze/thaw cycles would have cumulative negative effects on
the majority of organisms; the SR-SAG was not able to translate this
into a quantitative implementation guideline that is based on
experimental data. Further conservatism is present in this
recommendation since over this period of time the other stressors in the
martian environment would mitigate against faster replication. Microbial
psychrophily is an active area of research, and as additional studies
targeting physiological responses and cold adaptation provide new
information, future assessments can define additional boundaries
including cumulative limits.

**[FINDING]{.underline}. It is possible for spacecraft to induce
conditions that could exceed for some time the threshold conditions for
biological propagation, even when the ambient conditions were in
equilibrium before the spacecraft arrived. Whether a special region is
induced or not depends on the configuration of the spacecraft, where it
is sent, and what it does. This possibility is best evaluated on a
case-by-case basis.**

# 11. ACKNOWLEDGEMENTS 

Early drafts of this report were presented at three
conferences/workshops, and the discussions at those meetings helped
clarify many issues: The Mars Water Workshop (Feb. 23-24, 2006; at Ames
Research Center), the Mars Planetary Protection Working Group (March
2-3, 2006; at Ames Research Center), and the Mars Exploration Program
Analysis Group (MEPAG) meeting (April 19-20, 2006; Monrovia, CA). The
following Mars scientists contributed data or ideas to this analysis:
Oded Aharonson (Caltech), Ralph Milliken (Brown University), Cynthia
Phillips (SETI Institute), Aaron Zent (ARC), Matt Chamberlain (Planetary
Science Institute), Michael Carr (USGS Retired), Aharon Oren (Hebrew
University of Jerusalem), Phil Christensen (Arizona State University),
Bruce Jakosky (University of Colorado, Boulder). Valuable review
comments were received from Margaret Race, Deneb Karentz, Pericles
Stabekis, John Rummel, Gerhard Kminek, Christopher Chyba, Steve
Clifford, and Michael Carr. Although the reviewers listed provided many
constructive comments and suggestions, they were not asked to endorse
the conclusions of the final report.

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# [APPENDIX I]{.underline}. Acronyms and Abbreviations

# 

ARC -- Ames Research Center

COSPAR -- Committee on Space Research

GRS -- Gamma Ray Spectrometer, an instrument on the on the 2001 Mars
Odyssey mission (http://grs.lpl.arizona.edu/content/learning/aboutgrs/)

HEND -- High-Energy Neutron Detector; part of the Gamma Ray Spectrometer
suite of instruments

HRSC -- High Resolution Stereo Camera, an instrument on the 2003 Mars
Express spacecraft.

IR -- Infrared

MDIM -- Mars digital image model, constructed from roughly 4600 Viking
Orbiter images.

MEPAG -- Mars Exploration Program Analysis Group

MER -- Mars Exploration Rover, a NASA lander launched in 2003

MGS -- Mars Global Surveyor - a NASA orbiter launched in 1996

MKS -- System of metric units based on meters, kilograms, and seconds.

MOC -- Mars Orbiter Camera, an instrument on the 1996 Mars Global
Surveyor spacecraft.

MOLA -- Mars Orbital Laser Altimeter; an instrument on the 1996 Mars
Global Surveyor spacecraft
(http://ltpwww.gsfc.nasa.gov/tharsis/mola.html)

NASA -- National Aeronautics and Space Administration

NRC -- National Research Council

NRC PREVCOM -- Was a committee of the National Academy of Sciences that
was therefore

independent of NASA

PP -- Planetary Protection

RTG -- Radioisotope Thermal Generator

SR-SAG -- Special Regions -- Science Advisory Group

THEMIS -- Thermal Emissions Imaging System

UV -- Ultra Violet

VIS -- Visible

# [APPENDIX II.]{.underline} Derivation of Figure 9.1 

Using the following process, a map of the martian shallow equilibrium
ice was developed.

-   The data from the GRS instrument on Mars Odyssey are widely accepted
    as a clear indication of high latitude shallow ground ice. For the
    purpose of mapping, only summer data from both hemispheres are used
    (winter CO~2~frost obscures the ice signature by adding hydrogen
    poor mass atop the soil - seasonal CO~2~ can be as much as a meter
    or more at high latitudes). Using an arbitrarily selected threshold
    value of 6 counts per second, the region of permanent shallow ground
    ice can be shown. [Note #1]{.underline}: Poleward of that boundary,
    there are fewer than 6 c/s (the epithermal neutrons go down as H
    goes up). [Note #2]{.underline}: The position of the 6 c/s threshold
    value should be considered blurred on a 600km scale due to the GRS
    neutron footprint.

-   Several equilibrium thermodynamic models for Mars have been
    calculated at a planetary scale. These models predict the
    distribution of ice equatorward of the GRS 6 c/s threshold. Because
    the GRS can\'t detect deeper than about a meter, this gives us a way
    to model the ice distribution to deeper depths. At the 2006 Mars
    Water Conference
    (<http://es.ucsc.edu/~fnimmo/website/mars2006.html>), the models
    developed by three independent research teams (Chamberlain and
    Boynton, 2006; Mellon and Feldman, 2006; Aharonson and Schorghofer)
    were presented. These models use somewhat different methodology and
    inputs, but the results have a very similar structure. In each case,
    there is a north and south mid-latitude belt that can be thought of
    as discontinuous ice, and/or a zone of ice less detectable by GRS
    due to its depth, along with an equatorial belt with no near-surface
    ice. The position of the mid-latitude belts is somewhat different in
    the three models. To allow for appropriate conservatism, a boundary
    was drawn (Boundary B on Figure 9.1) that encompasses the most
    equatorward indication of ice in any of the models. Thus, Boundary B
    also incorporates model-dependent uncertainty.

-   Equilibrium thermodynamic models show that the depth to the top of
    the ice table increases abruptly at about the position of the dashed
    line. This has been studied extensively (e.g., Farmer and Doms,
    1979; Paige, 1992). It is typical in model results for the
    transition from 5m to infinite to occur in less than a degree of
    latitude. In order to further represent appropriate conservatism,
    the north and south dashed lines were each shifted one degree of
    latitude towards the equator, so as to encompass possible ice within
    5 meters of the surface.

[^1]: NRC PREVCOM was a committee of the National Research Council (of
    the National Academies of Science) that, at NASA's request, examined
    planetary protection measures for Mars. Subsequent to accepting its
    statement of task, an NRC committee operates independently of its
    sponsoring agency.

[^2]: The NRC PREVCOM's Statement of Task included the language that "to
    the maximum possible extent, the recommendations should be developed
    to be compatible with an implementation that would use the
    regulatory framework for planetary protection currently in use by
    NASA and the Committee on Space Research (COSPAR)." The full NRC
    PREVCOM statement of task is given on pp. vii-viii, NRC PREVCOM
    (2006).

[^3]: matric effects are those induced by the adhesive and cohesive
    properties of water in contact with a solid surface

[^4]: The UV irradiation on Mars is significantly higher in the UVC
    region (190 -- 280 nm) than on Earth due to a generally thinner
    atmosphere and the lack of an extensive ozone layer (Applebaum and
    Flood, 1990; Cockell et al. 2000; Kuhn and Atreya, 1979; Patel et
    al., 2002). On Earth, the ozone layer attenuates all UV irradiation
    below 290-300 nm, i.e., no UVC wavelengths reach the Earth's
    surface. The presence of UVC irradiation on Mars creates an
    environment at the surface that exhibits a total UV flux (200 - 400
    nm) that is up to three orders of magnitude more biocidal than on
    Earth (Cockell et al. 2000; Patel et al., 2002). Recent models
    suggest that the high UVC flux on Mars can act to reduce the
    viability of some sun-exposed microbial cells on spacecraft surfaces
    by greater than six orders of magnitude in as short a time as a few
    tens of minutes to no more than several hours (Newcombe et al.,
    2005; Schuerger et al., 2003; 2006). In addition, the downwelling
    UVC will penetrate pits, cracks, and other microscopic topographical
    features on spacecraft materials, resulting in some of the more
    sheltered microorganisms becoming inactive in reasonably short
    periods of time (Schuerger et al., 2005). However, the biocidal
    effects of UVC cannot reach deeply embedded bioloads, cannot
    penetrate UV-absorbing materials, and cannot affect bioloads on
    internal components of spacecraft.

[^5]: By analogy (for those of us old enough to remember) it takes only
    a short time to defrost a freezer, but a relatively long time for
    the ice to accumulate again.

[^6]: Spacecraft-induced special regions are mentioned in DEFINITION #2
    but are developed in Section 10.