**GEOLOGY AND LANDING SITES OF THE ELYSIUM BASIN-TERRA CIMMERIA REGION,
MARS.**

D.M. Nelson^1^, J.D. Farmer^1^, R. Greeley^1^, H.P. Klein^2^, R.O.
Kuzmin^3^,

^1^Dept. of Geology, Arizona State University, P.O. Box 871404, Tempe,
AZ, 85287-1404, ^2^SETI Institute, 2035 Landings Dr., Mountain View, CA
94043, ^3^Vernadsky Institute of Geochemistry and Analytical Chemistry,
Russian Academy of Sciences, Kosygin St., 19 Moscow, 117975, GSP-1,
Russia.

**Introduction:** Of key importance to future Mars missions is the
search for evidence of past or present life. Potential sites for the
2001 lander are limited to low latitudes and elevations \<2.5 km because
of engineering constraints. We have identified landing sites that hold
high priority for astrobiology that fall within these constraints. Three
of the potential sites are in the Elysium Basin-Terra Cimmeria region,
an area that has experienced a prolonged and complex hydrologic history.
We recommend these sites be targeted early for high resolution orbital
imaging and spectral mapping.

**Astrobiology in Mars Missions:** The search for life on Mars, whether
it be extant or extinct, began systematically with the Viking missions
of the 1970s \[1\]. While results of the biology experiments from the
two Viking Landers have been widely interpreted as being due to
inorganic processes \[1\], the Orbiters revealed that early Mars was
more Earth-like and that liquid water played an important role in
shaping the early Martian surface \[2\] making conditions favorable for
exopaleontology \[3-7\].

The Mars 2001 Lander does not as yet have a landing site identified.
However, in keeping with the objectives of the NASAÕs Astrobiology
Institute (NAI), a site with a clear potential for astrobiology must be
a top priority. Such a site should include a region with a definite
hydrological history. This includes locales with valley networks,
outflow channels, lacustrine basins, or outflow plains. Hydrothermal
deposits are also important targets and may be associated with
identifiable features such as volcanic constructs, proclastic flows,
chaotic terrains formed by magma-ground ice interaction, and impact
sites. Although hydrothermal deposits have not yet been discovered, it
is hoped that new high resolution imaging from the Mars Observer Camera
(MOC) and compositional information from the Thermal Emission
Spectrometer (TES) might help locate such features.

**Mars 2001 Site Selection Constraints:** Several important engineering
constraints have been imposed on possible landing sites for the 2001
mission. Latitudes are presently restricted to 3¡N to 12¡S, to ensure
maximum solar power for the longest time. The elevation must be below
+2.5 km but above -3 km. In addition, surfaces must have rock abundances
(estimated from IRTM data \[8, 9\]) of 5-10% coverage to provide high
probability for a safe landing. In addition, dust coverage at the site
needs to be at a minimum so that the lander is not in danger of sinking
into the regolith and that local rocks are not buried. Finally, it is
preferable that sites under consideration have been (or will be) imaged
at high resolution (\<50 m/pixel) either by Viking or by MOC to improve
our assessment of potential engineering safety hazards (e.g., boulders,
steep slopes, etc).

**Geology and Potential Sites of the Elysium basin / Terra Cimmeria
region:** As part of an extended reconnaissance of the equatorial region
of Mars to find sites that have the highest priority for exopaleontology
\[10\], the geology of the southern Elysium basin and north Terra
Cimmeria has been mapped at 1:2M scale \[11\]. The area covered ranges
from 5¡S to 23¡S and 170¡W to 200¡W, and includes Gusev crater, north
MaÕadim Vallis, and Apollinaris Patera. This area is scientifically
interesting and astrobiologically significant as it has been shown to
have had a varied and prolonged hydrologic history with its valley
networks, multiple outflow channels, and evidence of cryospheric melting
near Apollinaris Patera.

In developing a detailed geologic framework for potential landing sites
within Elysium Basin/Terra Cimmeria, we utilized the highest resolution
data available, including both Viking Orbiter and MOC images. Our
visible range mapping will be combined with spectral (and eventually
mineralogical) data obtained from the Thermal Emission Spectrometer as
they become available from the MGS Ô96 mission. Data from the Thermal
Emission Imaging System (THEMIS) instrument, to be obtained during the
2001 orbital opportunity, may also provide important mineralogical data
needed to refine site priorities.

In our investigation, we targeted specific sites that meet criteria
relevant for exopaleontology. The types of sites are (in order of
priority): hydrothermal, lacustrine, and grab-bag. The chaotic terrain
west of Apollinaris Patera and Reuyl crater are potential hydrothermal
regions, Reuyl is also a likely lacustrine environment, and the mouth of
a valley network in north Memnonia Terra is a grab-bag site. Statistics
of these sites are listed in Table 1.

The disrupted, blocky regions west of Apollinaris Patera appear to have
formed where ground water erupted from a subsurface aquifer adjacent to
the volcano \[12\]. Such outflows could have resulted from pressure
increases that occurred during periodic subsurface geothermal heating.
Because Apollinaris was active throughout most of the Hesperian period
\[13\], it is possible that magma-ground ice interaction formed the
chaos and may have sustained hydrothermal activity in the area. On
Earth, hydrothermal deposits have been shown to be excellent
repositories for a microbial fossil record \[4, 14\]. For example,
fossil biosignatures can often be captured and preserved in silica,
carbonate and Fe-oxide sinters \[15-18\]. We have identified a smooth
depression within the chaos as a potential landing site (11.1¡S,
188.5¡W). VO image coverage includes the series (596A35-36; 635A57) at
\~225 m/pixel resolution, and (372B01-26) at \~35 m/pixel resolution,
although rock abundance models suggest that surface dust might be too
deep.

In addition to a hydrothermal site, ponding of water in the Reuyl crater
basin (9.9¡S, 192.8¡W) may have deposited lacustrine shales. Drying up
of such a lake could have left behind evaporites (e.g., halite, gypsum,
and carbonate), which are also important targets for a potential Martian
fossil record. Reuyl crater is a large (\~100 km diameter) relatively
young impact crater which possesses an anomalously large central peak
and floor deposits of high albedo. Impact into an ice-rich lithosphere
could have produced hydrothermal systems within the crater adjacent to
the central peak with outflows ponding to form a lake (possibly
ice-covered). Evaporation of an ice-covered lake would have left aqueous
minerals (hydrothermal deposits, evaporites, etc.) on the crater floor,
perhaps accounting for the high albedo observed near the central peak.
If biota were present, they could have been preserved within these
deposits. Although we consider this an interesting and important target,
there is presently a lack of high resolution orbital coverage (see VO
596A31-34), making it an ideal candidate for high resolution MOC
imaging.

A small valley network system in northern Memnonia (11.2¡S, 178.2¡W) is
the site of a possible lacustrine environment where ancient materials
were transported from the highlands and deposited in a shallow basin in
the lowland transition zone (see 437S07-09; 438S02-25; \~70 m/pixel).
Because the valley cuts into ancient surfaces and materials were
fluvially deposited on Hesperian plains, the period of activity is
suggested to be Late Hesperian, when most other hydrologic activity was
occurring in the area. A landing site near the mouth of the valley
network could provide access to a variety of different rock types
derived from the adjacent highlands making this a Ògrab bagÓ site. In
addition, more basinal sediments could provide access to fine-grained
detridal material (shale) which are good targets for preserved organic
matter.

Unfortunately, with the further constriction of the latitudinal
constraints, several other high-priority sites have been discarded. One
such site is at the mouth of MaÕadim Vallis where it debauches into
Gusev crater (16.2¡S, 184.0¡W), a possible former lacustrine
environment. This channel had been active throughout the Hesperian
period and into the Amazonian \[19\]. Sediments from the oldest highland
units were incised and the material was transported down the channel in
a series of events and emplaced in overlapping deposits on the crater
floor. These materials should have a wide range of ages, be fairly well
sorted in the delta, and might have preserved fossils in possible
shales. Similar sites are at the mouths of Al-Qahira Vallis (15.5¡S,
195.2¡W) and Durius Vallis (15.7¡S, 188.2¡W).

**Conclusions:** The discovery of evidence for past life is a main
driving forced behind the current Mars missions. Because the Mars 2001
Lander is limited with respect to its possible landing sites by a very
restrictive set of engineering constraints, only a few high priority
targets have been identified to date. A geologic map of the Elysium
Basin-Terra Cimmeria region was produced to better understand the nature
of potential landing sites in a terrain of prolonged and varied
hydrothermal activity. The chaos and channels adjacent to Apollinaris
Patera provide access to a potential hydrothermal area, where
groundwater-magma interaction might have provided an environment
suitable for sustaining and preserving life. Reuyl crater is a potential
lacustrine environment where evaporite deposits might have accummulated.
Hydrothermal activity may have occurred near the central peak. Sediments
at the mouth of a valley network system in north Memnonia could provide
a grab-bag of rock materials transported from the southern highlands.
However, images of these sites are either not available at preferred
resolution or are of marginal image quality. Because of this, we
recommend that a high priority be assigned to targeting the chaos west
of Apollinaris Patera, Reuyl crater, and the valley network in north
Memnonia for high resolution MOC imaging and for mineralogical mapping
by TES.

**References:** \[1\] Klein, H.P., *Orig. Life Evol. Biosph.* 21,
255-261, 1992. \[2\] Carr, M.H. *Water on Mars*, 1996.

\[3\] McKay, C.P., C.R. Stoker, *Rev. Geophys. 27*, 189-214, 1989. \[4\]
Walter, M.R., D.J. Des Marais, *Icarus* 101, 129-143, 1993. \[5\] Klein,
H.P., and J.D. Farmer, . Status of the search for life on Mars, pp.
65-71, *in* G. Seth Shostek (Ed.), *Progress in the Search for
Extraterrestrial Life*, 1995. \[6\] Farmer, J.D., D.J. Des Marais, *Lun.
Planet. Sci. XXV*, 367-368, 1994. \[7\] Farmer, J.D., *Palaios* 10(3),
197-198, 1995. \[8\] Palluconi, F.D, and H.H. Kieffer, *Icarus, 45*,
415-426, 1981. \[9\] Christensen, P.R., *Icarus, 68*, 217-238, 1986.
\[10\] Farmer, J.D., et al., this volume. \[11\] Farmer, J.D., et al.,
*Lun. Planet Sci XXX*, Abs #1833 (CD-ROM), 1999. \[12\] Carr, M.H., *J.
Geophys. Res,. 84*, 2995-3007, 1979. \[13\] Scott, D.H., et al.,
*Geologic Map of Apollinaris Patera*, I-2351,USGS, 1993. \[14\] Farmer,
J.D. and D.J. Des Merais, *Exopaleontology and the search for a fossil
record on* *Mars*, (in press). \[15\] Cady, S.L., J.D. Farmer, pp.
150-170, *In* G. Bock, and J. Goode (Eds.), *Evolution of Hydrothermal
Ecosystems on Earth (and Mars?)*, 1996. \[16\] Farmer, J.D., et al.,
*Geol. Soc. Amer., Abstr. Prog.* 27(6), 305, 1995. \[17\] Farmer, J.D.,
D.J. Des Marais, pp. 61-68, *in* L.J. Stal, P. Caumette (Eds.),
*Microbial Mats: Structure, Development and Environmental Significance*,
1994. \[18\] Walter, M.R. pp. 112-127, *in* G. Bock, J. Goode (Eds.),
*Evolution of Hydrothermal Ecosystems on Earth (and Mars?)*, 1996.
\[19\] Kuzmin, R.O., et al., *Geologic Map of the Gusev crater-MaÕadim
Vallis region, Mars*, (in press).

ELYSIUM/CIMMERIA LANDING SITES: D.M. Nelson et al.

**TABLE 1.** Landing site characteristics.

**[APOLLINARIS CHAOS]{.underline} [REUYL CRATER]{.underline} [MEMNONIA
VALLEY]{.underline}**

**Center Lat.** 11.1¡S 9.9¡S 11.2¡S

**Center Long.** 188.5¡W 192.8¡W 178.2¡W

**Lat. Range** 10¡-12¡S 9¡-11¡S 10¡-12¡S

**Long. Range** 187.5-189.5¡W 192¡-194¡W 177¡-179¡W

**Elevation\*** +1 to +2 km -1 to 0 km +1 to +2 km

**Landing Ellispe** 10 x 20 km 10 x 20 km 10 x 20 km

**Slope** low low low

**Rock Abundance** 0-5% 0-5% 0-5%

**Unconsolidated\*\*** dusty moderate dust dusty

**Material ^+^** some mantling low mantling low mantling

**Site Type** hydrothermal hydrothermal valley network

lacustrine lacustrine grab-bag

outflow lacustrine

lowlands

**Image coverage** 596A35-36; 635A57; 372B01-26 596A31-34 437S07-09;
438S02-05

**(m/pixel)** \~225; 255; 35 \~225 \~62-66

\* USGS 1:15M Topographic Map, I-2160, 1991

\*\* Based on composite maps from
http://mars.jpl.nasa.gov/2001/landingsite/EngConstr.html

**^+^** Based on VO image observation