INTRODUCTION

Mars site selection, or knowing where to go on Mars to accomplish particular landed-science objectives, is fundamental for sound planning of future exploration efforts for exobiology. In the broadest sense, the search for martian life is guided by what we perceive to be the basic requirements for the existence of living systems, or for their preservation as fossils.

Site selection for Mars exobiology will be discussed in terms of the search for extant and fossil life. However, it should be noted that the same rationale that applies to the search for fossils on Mars, also holds for a prebiotic chemical record. On the Earth, this early precursor record has been lost, recycled by weathering and tectonics, and destroyed by emerging life. But such compounds are much more likely to have persisted within the more stable martian crust, particularly if life failed to emerge on Mars. In exploring Mars for precursor organic compounds, the most important targets are the same as those identified for fossil life, namely stable aqueous mineral deposits in ancient cratered terrains. Such deposits may have sequestered and preserved prebiotic organic molecules for several Gyr. Thus, even if martian life never developed, aqueous minerals, and especially the fluid inclusions they contain, remain as primary targets for exobiology.

The fundamental requirement for living systems is liquid water. Without it, metabolic activities of living cells would be impossible. Thus, to a large extent we equate the search for extinct or extant life with the search for liquid water, past or present. Given present martian surface conditions, stable, or nontransient, liquid water today can only be deep beneath the surface where higher temperatures and pressures favor its stability.

But the deep subsurface of Mars is unlikely to be accessible for some time given the logistics and cost of "deep" (>few meters) drilling. Consequently, exobiological site selection is driven by the search for environments favorable either for the preservation of ancient life or for natural processes that bring extant subsurface life into surface environments.

HIGH RESOLUTION ORBITAL IMAGING

In planning for upcoming missions, the most pressing need is the selection and prioritization of targets for high-resolution orbital imaging. Terrestrial analog studies indicate that a resolution exceeding 30 m/pixel will be needed to resolve many important lower-order geomorphic features on Mars. Martian landforms visible at nominal Viking resolution (~200 m/pixel) are at a scale much larger than comparable features on Earth. To what extent this reflects major differences in process or small differences integrated over long time spans, is presently unknown. Thus, present site priorities are likely to need refinement as higher-resolution imaging becomes available for key sites and the criteria for geological processes and duration are reassessed.

Once exobiology sites have been imaged from orbit at high resolution, the goal is to search for appropriate aqueous mineral deposits using visible imaging to refine geological interpretations, and infrared and gamma-ray spectroscopy to interpret mineralogy. Such data will provide a basis for refining site priorities for future landed missions and for developing site-specific exploration strategies to explore for evidence of life. In developing exploration strategies for landed missions, it is important that we incorporate predictions from depositional facies models based on studies of terrestrial analogs, particularly during the early stages of exploration.

DURATION OF HYDROLOGICAL SYSTEMS

Prioritization of targets for orbital imaging is an important aspect of site selection because we will not be able to image all areas of interest at high resolution. In prioritizing sites for orbital imaging, emphasis is placed on geological features considered to be indicative of prolonged hydrological activity, as well as depositional processes that favor the long-term preservation of biological information. Table 1 provides an example of a comparative framework that emphasizes the level of integration of water-related geomorphic features within drainage basins as a basis for assessing the relative duration of hydrological systems in fluvial-lacustrine settings. We are presently limited in using this approach because we are not really sure how analogous martian landforms are to those on Earth. In part this stems from a general lack of high-resolution imaging at critical sites and an inability to resolve many smaller-scale landforms that may be critical to interpretations of process and duration. As higher-resolution imaging (<30 m/pixel) becomes available, it is important that we carry out comparative geologic studies of key sites to evaluate relative duration. Of course, our estimates of duration based on comparative geomorphology will eventually need to be calibrated to absolute time scales based on radiometric dating of martian samples.

HIGH-PRIORITY SITES FOR EXTINCT MARTIAN LIFE

In selecting sites for exopaleontology (i.e., the search for fossils or biomarkers), priority is given to landing sites in ancient terrains where hydrological systems involving liquid water appear to have been long-lived and which exhibit a high probability of having surficial aqueous mineral deposits. We also give preference to mineral deposits that are likely to have had a long residence time in the martian crust, namely, those that are diagenetically stable and resistant to chemical weathering. Examples include silica (as chert) and apatite (as phosphate). It should be noted that fine-grained detrital sediments also provide a suitable host for fossils and organic matter. Especially favored are clay-rich sedimentary deposits formed in environments that were reducing, with rapid sedimentation and compaction, and where permeability was further minimized by early cementation. Given the absence of plate tectonics on Mars and the attenuated hydrological cycle there, ancient terrains (>3.0 Gyr) are probably much more widespread and better preserved than on Earth.

In exploring for evidence of an ancient biosphere, site selection is guided by what we have learned about fossilization processes through studies of ancient (Precambrian) rocks on Earth, and studies of terrestrial environments regarded to be good analogs for the early Earth and Mars. Because the development of a martian biosphere in surface environments is likely to have been interrupted very early (~3.0 Gyr), we believe the microbial record of the Precambrian provides a reasonable proxy for Mars, allowing for obvious differences in geological history and environment.

The Precambrian terrestrial record reveals that the long-term preservation of microbial fossils requires rapid entombment of organisms by fine

FEATURES	DURATION
	SHORTER	LONGER
Channels:	Straight	Meandering
Floodplains:	Absent	Broad
Drainage Network:	Simple	Dendritic
Drainage Basins:	Small Many divides	Large Few divides
Tributaries:	None to first order	Second or higher order
Stream Terraces:	Single level	Multiple level Incised meanders
Deltas:	Absent or small	Large, multi-lobed
Other:	Only young craterspresent	Craters show varyingdegrees of erosion

grained aqueous mineral phases that are stable and which retain biological information through diagenesis. In many Precambrian examples, mineralization occurred very rapidly, prior to cellular decomposition, and probably while organisms were still viable. The best preservation is observed where organic materials were rapidly perfused with fine-grained silica or phosphate.

Other potential host minerals include carbonates, which are less stable, but which also have long crustal residence times. Evaporites, which comprise another group of potential host minerals, are quite soluble and tend to dissolve in an active hydrologic cycle. But the crustal residence times of aqueous minerals on Mars are likely to be longer due to the dry climate and the absence of tectonic overprinting. Thus, while Precambrian evaporites are rare on Earth, they may be abundant on Mars where evidence suggests that the hydrologic cycle was interrupted early. As noted previously, environments that are especially favorable for microbial fossilization include mineralizing subaerial and subaqueous springs, evaporites, and certain hard-pan soils.

Thermal-Spring Deposits
Subaerial thermal-spring deposits are regarded as excellent targets in the search for a fossil record on Mars because of the high biological productivity and pervasive early mineralization typically associated with such systems. Volcanic terrains are widespread on Mars and some possess outflow channels that are likely to have formed by spring sapping. The association of such features with potential subsurface heat sources, such as volcanic constructs or thermokarst features, indicates the possibility for past hydrothermal activity on Mars. Thermokarst features and related chaotic terrains that may have been formed by hydrothermal processes are also prime targets for hydrothermal mineralization and a fossil record. For example, many of the outflow channels comprising Simud, Ares and Tiu Valles systems originate from chaotic terrains of probable thermokarst origin, or from the floors of chasmata related to the vast Vallis Marineris system (e.g. Echus Chasma). Target deposits here include the common thermal- spring minerals, silica, carbonate, and iron oxides, as well as clay-rich hydrothermal alteration halos associated with shallow igneous intrusives.

Dao Vallis-Hadriaca Patera (Latitude: 33.2 deg. S, Longitude: 266.4 deg. W)

This is a broad outflow channel of simple form that originates from an amphitheater-shaped source area on the southern flank of Hadriaca Patera, an ancient highland volcano. The outflow channel is believed to have formed where a localized subsurface heat source melted ground ice. This process is likely to have been associated with sustained hydrothermal activity. The process probably created not only Dao Vallis but similar outflow channels to the south (e.g., Harmakhis Vallis). The large size of the outflow channel suggests that the interval of activity may have been sustained long enough for extensive hydrothermal mineralization, favorable for the preservation of fossils and organic chemical fossils.

Dao Vallis clearly meets several important criteria as a site for exopaleontology and will be a recommended target for high-resolution visible imaging during upcoming missions. Such information is needed to evaluate fully the origin of important small-scale features (such as the knobby terrain on the floor of Dao Vallis, potential spring mounds) and shed light on both the nature of hydrological processes and their duration. But to assess accurately the potential of this site for exopaleontology, high-resolution infrared spectral data are also needed to explore for hydrothermal mineral deposits, such as silica, travertine, or iron-oxide sinters.

Spring outflows may have also transported thermal-spring minerals to the channeled plains of the Hellas Basin, a potential site for landed missions, and also to the Pathfinder landing site in Ares Vallis.

Sublacustrine Spring Deposits and Carbonate Cements
In arid lake basins on Earth, coarse-grained, nearshore facies are often a locus for extensive carbonate mineralization. This process is of particular interest to exopaleontologists because such mineralization typically enhances the preservation of both microbial fossils and organic matter. For example, in many alkaline lakes in the Great Basin (western United States), micro-organisms living on the surfaces of submerged tufa mounds associated with subaqueous springs, or living interstitially within coarser sediments of lake-margin facies (e.g., fan delta deposits), are commonly entombed by precipitating carbonate minerals. In ancient tufas, evidence of microbial activity is preserved as cellular microfossils and stromatolites, as well as disseminated organic matter. Such deposits are regarded as excellent targets in the search for a fossil record on Mars.

Margaritifer Sinus-Parana Vallis (Latitude: 22 deg. S, Longitude: 11 deg. W)

This site is located within an ancient cratered terrain that has been heavily dissected by several major dendritic valley networks. Channel networks surround a central basin that may have been a depocenter for fluvial-lacustrine sedimentation. Most of the valleys debouch along the southeast margin of the basin. In general, basin-floor sediments appear to be exposed at the surface, although in places hummocky features may be an aeolian mantle.

Formation of the valley networks surrounding the basin was apparently preceded by an early period of mostly larger impacts, evidenced by dissection of the rims of many of the older craters by headward erosion of the channels. The period of hydrologic activity that produced the valleys was followed by a period of smaller impacts, some of which were superimposed on the older craters and valleys. That the intervening period of hydrologic activity that created the valleys may have been of relatively long duration is indicated by the presence of two or more levels of tributaries in several of the longer channel systems and varying degrees of channeling on the rims of the older craters.

Gusev Crater (Latitude: 15.5 deg. S, Longitude: 184.5 deg. W)

A second high-priority fluvial-lacustrine site is Gusev Crater, an impact basin of ~135 km diameter that is located in ancient cratered terrain. The system consists of a single, ~800 km long channel (Ma'adim Vallis) that flowed north, debouching along the southern margin of Gusev Crater. Several different levels of stream terracing, present within the steep-walled canyon, likely record rapid changes in base level. In addition, the lower end of the valley is deeply incised by a much smaller channel that formed by headward erosion late in the history of Ma'adim. Base level changes could have been controlled by drops in the level of a paleolake that resided within the Gusev Crater, or perhaps by local tectonic uplift. These observations support a prolonged hydrologic history for the Ma'adim-Gusev system, although distinction of the processes responsible for the observed changes in base level will require higher spatial resolution than is presently available from Viking.

Just basinward of the terminus of Ma'adim Vallis are lobate deposits that were channeled by late-stage downcutting of outflows from Ma'adim Vallis and subsequently wind eroded. Terracing above and below these deposits suggest they are fluvial-deltaic in origin. The depositional units of the delta are deeply channeled, and stand in high relief above the surrounding crater floor, suggesting they are well indurated. Deltaic and marginal lacustrine deposits are commonly a locus for precipitation of carbonate cements and sublacustrine spring tufas, processes that favor the preservation of fossils and organic matter. In addition, coarser-grained channel deposits may contain fossiliferous clasts derived from older formations upstream. Ma'adim Vallis originates within an extensive chaotic terrain to the south that appears to have formed by thermokarst processes. Thus, throughout its long history, hydrothermal minerals may have been carried to the floor of Gusev Crater from the source areas of Ma'adim Vallis. Refinement and testing of this scenario will require high-resolution visible-range imaging, and infrared spectral data to assist in the search for carbonates or other aqueous minerals.

Evaporites and Lacustrine Shales
Terminal lake basins in arid environments on Earth are usually ephemeral in nature and eventually dry up, depositing their dissolved salts while forming flat playa basins. Evaporite minerals formed in such environments frequently incorporate micro-organisms within fluid inclusions during crystallization. Such deposits have been suggested as potential short-term repositories for viable organisms, or longer-term repositories for cellular fossils and biomolecules. In addition, the fine- grained, clay-rich shales often interbedded with evaporites in these settings provide good repositories for organic matter, particularly where early cementation occurs. Given the numerous paleolake basins that have been identified in ancient terrains on Mars, such deposits hold much interest for exopaleontology.

White Rock (Latitude: 8 deg. S; Longitude: 335 deg. W)

High-priority targets on Mars for evaporites include an unnamed 80-km crater within the Sinus Sabeaus Quadrangle. Numerous channels resembling terrestrial dendritic drainage systems surround the crater basin and may have sustained a paleolake for some undetermined interval of time. The crater floor exhibits patchy albedo of varying intensity. Of particular interest is a spindle-shaped mound of relatively high albedo called "White Rock". This feature exhibits two sets of irregular, wind-eroded fractures and is similar in form to terrestrial yardangs. It has been suggested that White Rock is a playa deposit consisting of chemically precipitated evaporite minerals. This interpretation implies that a hydrological system operated here for a long period of time, first concentrating soluble salts by chemical dissolution and then removing them by evaporation and precipitation. Similar high-albedo features can be found on the floors of other impact craters on Mars (e.g., crater Becquerel, Latitude: 21.3 deg. N; Longitude: 8 deg. W), some showing irregular stratification under high resolution. This suggests that martian evaporites may be fairly widespread.

HIGH-PRIORITY SITES FOR EXTANT LIFE

As mentioned previously, if life exists on Mars today, it is likely to be a chemosynthetic form residing in subsurface habitats where liquid water may be present. Despite the inaccessibility of the deep subsurface during upcoming missions, it is possible that recent outflows of subsurface water have brought such organisms into near-surface environments where they may have been cryopreserved in ground ice. Thus, areas of stable ground ice associated with very recent outflow channels are probably our best hope for discovering extant life. It has been argued, both on empirical grounds as well as theoretical evidence, that ground ice is presently unstable on Mars at latitudes <40 deg. Therefore, as noted previously, exploration for cryopreserved martian life should be focused at higher latitudes.

In prioritizing sites for extant life, we should emphasize the very youngest martian terrains (e.g., preferably those completely lacking impact craters). It has been suggested that micro-organisms may survive in ground ice for possibly millions of years and in evaporite deposits for perhaps hundreds of millions of years. Such suggestions should not be ruled out, despite the likelihood that the normal background radiation in such deposits, integrated over geologic time, may severely restrict long-term organism viability by destroying (via mutagenesis) the genomic integrity of cells. The survival time of dormant, but viable organisms under martian conditions is poorly constrained at present, as are the factors that affect preservation during the transition to the fossil record.

In searching for extant life, present knowledge suggests that we should focus on high latitudes (>40 deg.) where stable ground ice may be present, and especially at sites where ground ice may have formed in association with recent outflows of subsurface aquifers. It follows that mapping the distribution of ground ice using gamma-ray spectroscopy has a high priority for exobiology. It is also possible that surface life may survive within undiscovered liquid water refugia near the surface (e.g., shallow hydrothermal systems), although such environments, if they exist, may be very difficult to locate. It is possible they could be identified using thermal IR. Areas of persistent fogs and frosts at the martian surface, noted previously, may provide indications of near- surface water or water ice that could be accessed by shallow drilling.

Ground ice appears to provide an excellent medium for preserving organisms and biomolecules, but only for short intervals of geological history (perhaps hundreds of thousands to possibly millions of years). Ice may inhibit oxidation of the soil and slow the breakdown of organic materials. But global climate changes, driven by obliquity or other orbital variations, have left the Earth completely ice free on numerous occasions in its history. The present ice caps probably formed during the Miocene and have waxed and waned irregularly since then. The same variations are likely to be true for Mars. Thus, while knowing the present distribution of ground ice on Mars is basic to a strategy to search for extant life there, finding stable ice that is likely to have formed in association with recent outflows of subsurface water, or water-rich pyroclastics, such as lahars, provides the most logical approach.

Ismenius Lacus (Latitude, 44 deg. N; Longitude, 333 deg. W)

Several high-priority targets for cryopreserved organic materials have been located within the Ismenius Lacus Quadrangle of Mars . This terrain lies within the Amazonian-aged "hilly unit" of Deuteronilus Mensae. This area is dominated by numerous mesa-like landforms which are surrounded by debris aprons that resemble terrestrial rock glaciers. The "softening" of mesa rims and associated features suggest the activity of near-surface ground-ice. Mid-winter ice is thought to precipitate from the atmosphere and mix with rock and soil to form masses that slowly creep downslope under the influence of gravity (rock glaciers). The latitude of the area, in combination with the various geomorphic features discussed above, indicate that near-surface ground ice, though varying seasonally, has probably been present here for some time. But the proposed periglacial origin of this terrain needs to be evaluated in more detail using gamma-ray spectroscopy to confirm the presence of ground ice. Unfortunately, young outflow channels that may have delivered a subsurface biota to the ground-ice environment remain to be discovered.

North Polar Cap

Important potential targets for indirect evidence of extant life are the polar regions, with their ice caps and layered terrain. Molecular, and even morphologic, signatures for life could have found their way into polar ices which probably act as cold traps for organic molecules in the atmosphere. This has the added advantage that we already know where polar ice is located. A landed mission to the water-rich north polar cap, likely to be targetted primarily at climatological goals, could also be of considerable exobiological value, provided that it combined capabilities for subsurface drilling with in situ organic analysis and microscopic examination of fine particulates.

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