Lunar and Planetary Science XXX 1833.pdf

THE ELYSIUM BASIN- REGION OF AS A TARGET FOR MARS EXOPALEONTOLOGY. J. D. Farmer1, D. M. Nelson1, R. Greeley1, H. P. Klein2. 1 Dept. of Geology, Arizona State University, P.O. Box 871404, Tempe, AZ 85287. 2 Dept. of Biology, Santa Clara University, Santa Clara, CA 95053.

Introduction: Exopaleontology and identifying aque- tufa-depositing cold springs in alkaline lake settings), ous mineral deposits are key elements for upcoming Mars evaporitic environments (e.g., terminal lake basins and arid missions. Using set engineering constraints, areas in Ely- shorelines) and mineralizing soils (e.g., hard pan deposits sium Basin, a region with an extended hydrological history, such as silcretes, calcretes, ferracretes, etc.). have been targeted for geologic mapping as possible landing The same types of deposits targeted for Mars Exopale- sites. ontology (as outlined above) also hold great interest for Mars Exopaleontology: Of the other planets compris- understanding the early hydrological and climate history of ing the Inner Solar System, Mars holds the greatest potential Mars. For example, the presence of carbonates and sulfates for having developed life. The Viking missions revealed the is predicted by models for early atmospheric and climate surface of Mars to be devoid of liquid water and the organic evolution on Mars [10-11]. And even if life did not develop compounds necessary for living systems. Life detection on Mars, the same lithological targets could preserve a pre- experiments conducted at the two Viking Lander sites were biotic organic chemical record. On Earth, this early record designed to search for evidence of metabolism in Martian has been lost through aqueous weathering and plate tectonic soils. Results of these experiments have been widely inter- recycling of the crust. The hydrological cycle appears to preted as being due to inorganic processes [1]. In contrast, have died early on Mars and the Martian crust has not mapping by the Viking Orbiters revealed that early Mars been recycled by plate tectonics. Thus, the ancient Martian was more Earth-like and that liquid water played an impor- highlands may preserve a far more extensive record of early tant in shaping the early [2]. Recognition planetary evolution than found on Earth. Indeed, the 4.57 that water was abundant at the surface of Mars early in the Ga age of , ALH84001 [8], indicates that planet’s history has fueled an ongoing interest in the Post- some crustal sequences extend back to the earliest period of Viking search for fossil [3-7]. Martian history. On Earth, there is an extensive fossil record of early High Priority Sites for Mars Exopaleontology: microbial life preserved in Precambrian rocks. If life devel- Ancient Paleolakes. Martian terrains which show evi- oped on Mars, it is likely that the ancient Martian crust also dence of a prolonged hydrological history are of special preserves a record of early life. Present efforts to explore interest in the search for a record of early life. In exploring Mars for a fossil record are embodied in the field of Exopa- for aqueous sedimentary deposits, depositional basins that leontology [7], the conceptual framework of which is mostly may have once held paleolakes (e.g., Crater, 15°S, derived from principles of Earth-based microbial paleontol- 185°W [12]) are a priority because on Earth such environ- ogy. Recent reports of putative biosignatures in Martian ments are often sites of enhanced chemical sedimentation meteorite ALH84001 [8] has stimulated NASA’s interest in which promotes microbial fossilization. exploring for a Martian fossil record. While the biogenicity Mapping and analysis of surface geomorphic features of features in ALH84001 remains controversial, such studies on Mars provide a basis for determining the relative ages of (along with the systematic application of paleontological geological units and for assessing the duration of hydrologi- principles) have helped to extend and refine the current cal systems. However, the most important evidence for strategy for Mars Exopaleontology. determining the nature of past environments is surface min- Criteria for Site Selection: In exploring for a Martian eralogy. The floors of some crater basins on Mars exhibit a fossil record, site selection is crucial. The first step in im- high albedo (e.g., crater, 7°S, 192°W) which may plementing missions to search for past Martian life is to reflect the presence of evaporite deposits, inclusive of car- identify surface exposures of ancient aqueous sedimentary bonates. The Thermal Emission Spectrometer (TES) ex- deposits on Mars. In particular, we are interested in finding periment, presently in orbit at Mars [13], will provide global deposits of the type that are known be good host rocks for a coverage in the mid-infrared at a spatial resolution of ~3 microbial fossil record on Earth. Studies of fossilization km/pixel. In 2001, higher spatial resolution (~100 m/pixel) processes in modern terrestrial environments indicate that mid-IR orbital mapping will be obtained by the THEMIS the preservation of microbial biosignatures (as either or- instrument for selected sites [14]. These data will provide a ganic-walled cellular remains, biofabrics and/or chemofos- basis for interpreting outcrop-scale mineralogy, testing and sils) typically requires pervasive early diagenetic minerali- refining existing paleoenvironmental models, and for pri- zation which entombs organic materials prior to degradation oritizing sites for the 2003 mission and beyond. [9]. Long-term preservation is enhanced if host sediments Ancient Hydrothermal Systems. Hydrothermal systems have a low permeability and consist largely of stable min- exist wherever groundwater (and/or ice) interacts with sub- eral phases that resist dissolution during later diagenesis surface heat. Hydrothermal systems have been identified as and chemical weathering. Minerals that often meet these important targets for a Martian fossil record [4, 6-7] because requirements include silica, phosphate and carbonate. of the potential for rapid mineralization. On Mars, there are These minerals host most fossil microbiotas found in the a number of geological settings where hydrothermal systems Precambrian record on Earth. Geological environments may have been active earlier in the planet’s history [15]. where rapid mineralization is common include: springs These include many volcanic regions where subsurface out- (e.g., sinter-depositing thermal springs in volcanic terrains, flows of liquid water originated near probable heat sources. Lunar and Planetary Science XXX 1833.pdf

ELYSIUM BASIN AS A TARGET FOR MARS EXOBIOLOGY: J. D. Farmer, et al.

Important targets include: 1) caldera floors and slopes of Conclusions: The search for evidence of past life is a ancient volcanic edifices and associated primary focus of the decade-long Mars Global Surveyor (e.g., Hadriaca Patera/ [16]; Apollonaris Patera Program. To ensure success in this endeavor, it is impera- [17]), and 2) areas of pronounced crustal extension (e.g., tive that we target landing sites where ancient aqueous floors of chasmata associated with Vallis Marineris, and the sedimentary deposits of the types most favorable for the chaos areas and associated channels of capture and long-term preservation of microbial fossils may and west of Apollonaris Patera). Higher rates of heat flow be present. In this regard, studies of the Precambrian fossil in these areas might have promoted melting of the cryos- record on Earth and of fossilization processes in modern phere, along with outflows of subsurface water [18]. The terrestrial environments considered analogs for early Mars, heavily cratered Martian highlands also harbor many provide valuable information for refining site selection smooth-floored basins with associated small strategies for Exopaleontology. valley networks. The formation of impact melt sheets [19- The geology of the highland-lowland transition of the 20] and thermal anomalies formed around uplifted central Elysium Basin/Terra Cimmeria region of Mars suggests a peaks [21] (e.g., Reuyl Crater) may have sustained hy- prolonged hydrologic history and diverse array of ancient drothermal systems and perhaps even paleolakes for pro- aqueous environments. Thus, this area is given a high pri- longed periods. ority in exploring for a Martian fossil record. We have un- Exopaleontology in Elysium Basin/Terra Cimmeria: dertaken detailed geological mapping of this terrain using Locating and mapping sites where aqueous sedimentation is available Viking data. But surface mineralogy is regarded likely to have occurred on Mars marks an important first as crucial for the prioritization of sites for future landed step in site selection for future landed missions to explore missions leading to sample return in 2005. For this reason, for past life. Engineering constraints for the 2001 mission the Elysium Basin/Terra Cimmeria region of Mars is an presently limit landing sites to latitudes between 5oN and regarded as an important target for high resolution imaging 20oS, and elevations below the 2.5 km planetary datum [22]. and spectroscopy during upcoming orbital mapping efforts. Following these guidelines, we have targeted the region References: [1] Klein, H.P., Orig. Life Evol. Biosph. around the Elysium Basin/Terra Cimmeria transition zone 21, 255-261, 1992. [2] Carr, M.H. , 1996. [3] for detailed study and mapping (see poster). Our goal is to McKay, C.P., C.R. Stoker, Rev. Geophys. 27, 189-214, produce geologic maps of this region at scales of 1:2M and 1989. [4] Walter, M.R., D.J. Des Marais, Icarus 101, 129- 1:500K to better understand the hydrological history and to 143, 1993. [5] Klein, H.P., and J.D. Farmer, . Status of the develop mission concepts that will address the goals of search for life on Mars, pp. 65-71, in G. Seth Shostek (Ed.), Exopaleontology during landed missions in 2001 and be- Progress in the Search for Extraterrestrial Life, 1995. [6] yond. Farmer, J.D., D.J. Des Marais, Lun. Planet. Sci. XXV, 367- The Elysium Basin/Terra Cimmeria region possesses 368, 1994. [7] Farmer, J.D., Palaios 10(3), 197-198, 1995. many of the important geological features previously identi- [8] McKay, D.S., et al., Science 273, 924-930, 1996. [9] fied as important for Mars Exopaleontology. Small valley Farmer, J.D., D.J. Des Marais, J. Geophys. Res., (Submit- systems and larger outflow channels incise ancient high- ted). [10] , J.B., et al., Icarus, 71, 203-224, 1987. lands, trending downslope from south to north. The dis- [11] Schaefer, M.W, Geochim. Cosmochim. Acta 57, 4619- rupted, blocky regions () west of Apollonaris 4625, 1993. [12] Kuzmin, R.O., et al., Geologic map of the Patera appear to have formed where ground water erupted MTM 15182 and MTM 15187 quadrangles, Gusev crater- from a subsurface aquifer adjacent to the volcano [23]. Ma’adim Vallis region, Mars, (In press). [13] Christensen, Such outflows could have resulted from pressure increases P.R., et al., J. Geophys. Res. 97, 7719-7734, 1992. [14] that occurred during periodic subsurface geothermal heat- http://emma.la.asu.edu/THEMIS/index.html. [15] Farmer, ing. Hydrothermal activity may have persisted in this chaos J.D, pp. 273-299, in G. Bock, and J. Goode (Eds.), Evolu- region for a prolonged period. As noted previously, terres- tion of Hydrothermal Ecosystems on Earth (and Mars?), trial hydrothermal deposits often capture and preserve fossil 1996. [16] Squyres, S.W., M.H. Carr, Science, 231, 249- biosignatures in silica, carbonate and Fe-oxide sinters [24- 252, 1986. [17] Robinson M.S., et al., Icarus 104, 301-323, 27]. In addition, ponded water in the Gusev and Reuyl cra- 1993. [18] Masursky, H., et al., NASA Tech. Man. 88383, ter basins could have deposited lacustrine shales and 459-461, 1986. [19] Brakenridge, G.R., et al., Geology 13, evaporites (e.g., halite, gypsum, and carbonate), which are 859-862, 1985. [20] Newsom, H.E. Icarus 44, 204-216, also important targets for a potential Martian fossil record. 1980. [21] Newsom, H.E., et al., J. Geophys. Res. 101, In developing a detailed geologic framework for poten- 14,951-14,955, 1996. [22] S. Saunders, personal comm., tial landing sites within Elysium Basin/Terra Cimmeria, we 1998. [23] Carr, M.H., J. Geophys. Res. 84, 2995-3007, are utilizing all currently available data, including Viking 1979. [24] Cady, S.L., J.D. Farmer, pp. 150-170, In G. Orbiter images, albedo and thermal inertia data (to estimate Bock, and J. Goode (Eds.), Evolution of Hydrothermal Eco- rock abundance and amount of dust accumulation). Our systems on Earth (and Mars?), 1996. [25] Farmer, J.D., et visible range mapping will be combined with spectral (and eventually mineralogical) data obtained from the Thermal al., Geol. Soc. Amer., Abstr. Prog. 27(6), 305, 1995. [26] Emission Spectrometer as they become available from the Farmer, J.D., D.J. Des Marais, pp. 61-68, in L.J. Stal, P. MGS ‘96 mission. Data from the Thermal Emission Imag- Caumette (Eds.), Microbial Mats: Structure, Development ing System (THEMIS) instrument obtained during the 2001 and Environmental Significance, 1994. [27] Walter, M.R. orbital mission may also provide important mineralogical pp. 112-127, in G. Bock, J. Goode (Eds.), Evolution of Hy- data needed for refining site priorities. drothermal Ecosystems on Earth (and Mars?), 1996.