**SITE SELECTION FOR THE MGS '01 MISSION: AN ASTROBIOLOGICAL
PERSPECTIVE.**

Jack Farmer^1^, David Nelson^1^, Ronald Greeley^1^, Harold Klein^2^, and
Ruslan Kuzmin^3^, ^1^Dept. of Geology, Arizona State University, ^2^SETI
Institute, Mountain View, CA, ^3^ Vernadsky Institute, Moscow, Russia.

**Introduction:** Major goals of the Mars Global Surveyor Program
include: 1) the search for past or present life, and/or evidence of
prebiotic chemistry, 2) understanding the volatile and climatic history
of Mars, and 3) determining the availability and distribution of mineral
resources. The cross-bridging theme for these goals is the history of
liquid water. Among the most important objectives of the MGS program is
to visit sites that have a high priority for exopaleontology\-- that is,
to explore sites that have a high potential for harboring a Martian
fossil record and/or prebiotic chemistry. Studies of the terrestrial
fossil record reveal that microbial fossilization is strongly influenced
by the physical, chemical and biological factors of the environment
which together, strongly influence the types of information that will be
captured and preserved in the rock record. On Earth, the preservation of
biogenic signatures in rocks basically occurs in two ways: rapid burial
in fine-grained, clay-rich sediments and rapid entombment in
fine-grained chemical precipitates. Entombment by aqueous minerals can
occur as either primary precipitates (e.g., hydrothermal sinters, or
evaporites), or during early diagenetic mineralization (e.g.,
cementation). The key process is the rapid reduction of permeability
following deposition. This creates a closed chemical system that arrests
degradation (oxidation). For long-term preservation, organic materials
must be sequestered within dense, impermeable host rocks composed of
stable minerals that resist chemical weathering, dissolution and
extensive reorganization of fabrics during diagenetic recrystallization.
Favorable minerals include highly ordered, chemically-stable phases,
like silica (forming cherts) or phosphate (forming phosphorites). Such
lithologies tend to have very long crustal residence times and (along
with carbonates, shales), are the most common host rocks for Precambrian
microfossils on Earth. Other potentially important host rocks include
evaporites and ice, both of which have comparatively short crustal
residence times on Earth. These "taphonomic" constraints provide a
fairly narrow set of criteria for site selection. While they are
difficult to apply in the absence of mineralogical information, their
consideration is nevertheless essential if we are to follow a strategy
founded in clear scientific principles.

**Scientific Constraints for Site Selection:** A key objective of MGS is
to identify sites for exopaleon-tology and then collect samples for
return to Earth. Certainly a first step in the process is to target
sites where liquid water was present and could have provided a clement
environment for life. Next is to identify sites where aqueous sediments
were deposited. However, the fact that microbial fossilization only
occurs under specific circumstances means the site selection process
cannot end with these broad criteria. On Earth, most aqueous sedimentary
deposits are actually barren of fossils. Thus, the second step in the
process involves identifying those paleoenvironments that were most
favorable for the capture and preservation of fossil biosignatures (as
noted above). Based on terrestrial analogs, geological environments that
are especially favorable for preserving a microbial fossil record
include \[1,2\]: 1) mineralizing hydrological systems (e.g., surface and
shallow subsurface hydrothermal, mineralizing cold springs in alkaline
lake settings), 2) evaporite basins (e.g., terminal lake basins and arid
shorelines), and 3) mineralizing soils (e.g., sub-soil hard pans
including silcretes, calcretes and ferracretes).

**Engineering Constraints for Site Selection:** The 2001 lander will be
deployed by parachute and use a retro-rocket landing system similar to
Viking. This will constrain landing site elevation to between +2.5 and
--3.0 km, and surface rock abundance to between 5-10%. Because the
lander will be powered exclusively by solar panels, sites are also
limited to equatorial latitudes between 3^o^N and 12^o^S. Finally, it
has been suggested that, if possible, sites be limited to those that are
covered at Viking resolution better than 50 m/pixel. This last
constraint places an especially severe limitation on the number of
scientifically-interesting sites for Astrobiology. However, this
constraint, along with the rock abundances estimated by IRTM, can be
relaxed for sites where supplemental high resolution MOC imaging can be
obtained
(http://mars.jpl.nasa.gov/2001/landing[site/EngConstr.html]{.underline}).

**Approach Used:** Using composite maps showing the distribution of the
above constraints provided by JPL
(http://mars.jpl.nasa.gov/2001/landing[site/]{.underline}

[EngConstr.html]{.underline}) we have reviewed all Viking imaging data
for sites that meet the engineering constraints defined above. Each site
has been visually examined at the highest Viking resolution available
and prioritized according to the following scheme:

*Highest priority:* Evidence for varied and sustained hydrological
activity; sites where water may have ponded (potential terminal lake
basins with evaporites or fine-grained detrital sediments, inclusive of
impact craters); chaos areas or channels adjacent to volcanic edifices
or impact craters (potential hydrothermal mineralization); floors of
impact craters with central peaks and associated high albedo features
(potential hydrothermal activity and/or evaporites or fine-grained
detrital deposits); with pristine features, deflational areas showing
little or no evidence for aeolian mantling.

*Moderate to high:* As for high, but with aeolian mantling evident as
isolated dunes.

*Moderate priority:* Termini or floors of channels (potential grab-bag
sites) that originate in highland chaos or adjacent to volcanic edifices
(areas of potential hydrothermal mineralization); evidence of general
terrain softening due to aeolian mantling or surface weathering,
although features still visible.

*Low-moderate priority:* Evidence of isolated hydrological activity as
sparse channels located in intercrater highland areas (potential
grab-bag sites?). Heavy to moderate aeolian mantling.

*Lowest priority:* No evidence of hydrologically-related geomorphic
features (e.g., extensive lava flows or pyroclastics), and/or heavily
mantled (or otherwise featureless) terranes.

**Results:** Within the engineering constraints outlined above, Table 1
presents the results of our preliminary global reconnaissance of
potential landing sites for exopaleontology. The overall impact of the
Viking resolution requirement (50 m/pixel) and strict adherence to rock
abundance data eliminated all but one of the highest priority sites we
had identified previously, based only on elevation and latitude
constraints (Table 2). While a number of moderately-high priority sites
remain (Table 1), clearly the highest priority (most scientifically
compelling) sites for Astrobiology lie outside of the Viking resolution
requirements or are marginal in terms of rock abundance.

TABLE 1. High to moderately-high priority sites for Astrobiology. Sites
identified meet all engineering constraints (within 3°N-12°S; rock
abundance 5-10%) and Viking Orbiter Imagery at \<50 m/pixel.

### HIGHEST PRIORITY

**[Site Name]{.underline} [Latitude/Longitude]{.underline} [Site
Type]{.underline} [VO Image Coverage]{.underline}**

Terra Cimmeria 8-11°S 216-220°W 1, 2 760A01-12

Mangala Valles 3-12°S 150-155°W 1, 2 442S-460S image series

#### MODERATE TO HIGH PRIORITY

## MC-11

Xanthe/Da Vinci crater 0-3°N 40-44°W 2 742A01-66

S. Ares Vallis 0-3°N 17-19°W 2 405B19-40

## MC-13

Libya Montes 1-3°N 272-274°W 1, 2 137S02-24

## MC-19

SE Xanthe/Iani Chaos 9-12°S 27-29°W 3 962A21-29

SE Xanthe/Iani Chaos 9-12°S 29-30°W 3 963A21-30

SE Xanthe/Iani Chaos 8-12°S 30-31°W 3 964A21-30

Iani Chaos 0-3°N 13-15°W 3 406B01-18

## MC-23

Apollinaris Chaos 12-4°S 188-190°W 3 372B01-26

NE of Gusev crater 14-9°S 180-181°W 1, 2 386B01-26

Al-Qahira Vallis 15-14°S 194-196°W 1, 2 452A01-08

**TABLE 2.** High to Moderately-High priority sites nearly meeting
present engineering constraints (within 3°N - 12°S), marginal rock
abundance, and Viking Orbiter Imagery \~50 to 100 m/pixel.

#### HIGHEST PRIORITY SITES

**[Site Name]{.underline} [Latitude/Longitude]{.underline} [Site
Type]{.underline} [VO Image]{.underline}**

Amenthes Rupes 2.9°S 249.5°W 1, 2 379S45, 47

Apollinaris "chaos" 11.1°S 188.5°W 2, 3 596A35-36; 635A57

Da Vinci crater 1.2°N 39.1°W 2, 3 014A72-80

Ganges Chasma (1) 8.5°S 43.9°W 1, 2 014A29-41

Ganges Chasma (2) 8.8°S 42.5°W 1, 2 014A29-41

Libya Montes (region) 1-3°N 270-280°W 1, 2 377S75-80; 876A02-05

N Memnonia Terra (1) 11.3°S 174.2°W 1, 2 38S10-14; 439S03-09; 440S02-08

N Memnonia Terra (2) 11.2°S 178.2°W 1, 2 437S07-09; 438S02-05

Nicholson crater 0° 164°W 2, 3 387S31-34; 637A47-50

Reuyl crater 9.9°S 192.8°W 2, 3 596A31-34

Shalbatana source (1) 0.2°N 46.3°W 1, 2, 3 897A33-36, 66, 68

Shalbatana source (2) 0.7°N 44.5°W 1, 2, 3 897A33-36, 66, 68

### Key for Site Types

> 1 = grab bag site
>
> 2 = fluvial-lacustrine
>
> 3 = potential hydrothermal

ASTROBIOLOGY PERSPECTIVE FOR MGS '01 SITE SELECTION: J.D. Farmer et al.

**Conclusions and Recommendations:** The Mars Observer Camera (MOC),
presently in orbit at Mars, has been providing very high resolution
images (as good as 1.5 m/pixel) for selected sites on Mars. While these
images are rapidly changing our view of the Martian surface, the present
distribution is nevertheless quite limited. Thus, our present
recommendations are of necessity based on photogeologic evidence
obtained by Viking. To address the site selection concerns of
Astrobiology, we recommend that there be a focus on the highest priority
sites given in Tables 1 and 2. Furthermore, we recommend that these
sites be specifically targeted for high resolution imaging by MOC as
soon as possible, to assist in the process of site prioritization.

The most important powerful information for reconstructing
paleoenvironments (and therefore the most useful information for
refining site selection for Astrobiology) is mineralogy. However,
because most of the orbital data obtained by the '96 mission will not be
available until after landing site selection '01, it is important that
there be an ongoing effort to update site priorities for sample return
missions in '03 and '05. It is also important that mapping studies be
carried out now at the highest priority sites using available data and
that these mapping efforts be updated as new data becomes available.

The inability to identify many smaller geological features presently
impedes geological interpretations and therefore the site selection
process. Targeted high resolution observations by MOC are likely to
significantly advance our knowledge of finer-scale geologic features,
and alter our prioritization sites for 2003 and '05. The Thermal
Emission Spectrometer (TES) is presently mapping the Martian surface in
the mid-IR and is expected to provide important new information about
global mineralogy over the course of next year. For example, during the
pre-mapping phase, TES identified a large deposit of coarse-grained
hematite that is suggestive of aqueous activity. Unfortunately, this
type of mineralogical data is presently too limited in distribution to
provide a framework for site selection. While TES will provide
globally-distributed data at 3 km/pixel, there will still be a need for
follow-on mapping of key sites at higher spatial resolution. This
requirement will hopefully be met by the THEMIS instrument, which is to
be launched in 2001.

**References:** \[1\] Farmer, J.D., D.J. Des Marais, *Lun. Planet. Sci.
XXV*, 367-368, 1994. \[2\] Farmer, J.D. and D.J. Des Merais,
*Exopaleontology and the search for a fossil record on* *Mars*, (in
press).