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Lunar and Planetary Science XXXI 1858.pdf

HIGH-RESOLUTION THERMAL-INERTIA MAPPING OF : SITES OF EXOBIOLOGICAL INTEREST. B.M. Jakosky, L.R. Braun, M.T. Mellon, and S.M. Pelkey 1, 1Laboratory for Atmospheric and Space physics, University of Colorado, Boulder, CO 80309-0392, email [email protected].

Introduction. One of the science goals of Mars properties of the surface at the decimeter scale, but exploration is the determination of whether there has mapped at 3-km resolution. Although coverage is not ever been life there. To find out, we will be sending complete yet, with substantial gores in the available spacecraft to sites that have the potential for life and maps, coverage is sufficient to allow geologic interpre- determining their physical, chemical, and mineralogi- tation of many of the features seen on the surface. cal properties. Here, we discuss the physical proper- Site analysis. Below is a site-by-site analysis of ties of locations that hold the potential for life, in or- some of the high-priority sites as described by Farmer der to understand the geological processes responsible et al.: for their formation and to determine their suitability (latitude –8 to –11, longitude 216- for spacecraft exploration. The sites we examine are 220). Channel flowing out from an impact crater, those that have been identified as having had liquid possible site of hydrothermal activity or mineraliza- water, either in the form of surface flows, groundwa- tion. Thermal inertias of the region are moderate, ter, or potential hydrothermal systems or hot springs. around 175-200 J-m-2-s-1/2-K-1 (all thermal inertias will We utilize MGS TES observations of the thermal iner- be given in these units). The channel itself has the tia of the surface as a constraint on the physical prop- same thermal inertia near the crater, increasing to erties. slightly higher value as the channel begins to broaden Sites of exobiological interest. The environ- out downstream, up to 300-400. mental requirements for life include the presence of (-3 to –12, 150-155). This is a liquid water, access to the biogenic elements, and the large outflow channel originating in the Memnonia availability of a source of energy that can drive me- Fossae graben, likely related to and thereby a tabolism. Prior analysis indicates that the biogenic potential hydrothermal site; there are locations where elements are widely available, and that energy can be water may have ponded for extended periods of time. widely derived from water-rock chemical interactions. This is in the middle of a large low-inertia region, Thus, we focus here on sites that have had liquid wa- with regional values being less than about 75. Craters ter, as identified in previous geological analyses and and channels show up as only-slightly higher values, in catalogs of potential landing sites for sample-return still not much more than 100. For this low value, the missions. These sites include valley networks on an- surface must consist primarily of loose, unconsoli- cient terrain, catastrophic , possible dated material, with few exposures of surface rocks or paleolakes in craters or canyons, and possible hydro- crusts. thermal systems. Da Vinci Crater (1, 39). This is a large, flat- Relevance of thermal inertia. The thermal iner- floored crater with possible interior ponding, and is tia is a measure of the physical properties at the 1- to located adjacent to a region of chaos that likely formed 10-cm scale (see abstract by Mellon et al.). It is pri- when water drained from the subsurface. The re- marily sensitive to the particle size of loose, uncon- gional thermal inertias are moderate, in the 200-300 solidated materials, the size and abundance of rocks range. The interior of Da Vinci includes areas that on the surface, and the presence and degree of indura- have values as high as about 500. The adjacent chaos tion of crusts. It is difficult to determine whether the region has strikingly high values of thermal inertia, up present-day properties are a remnant of the original to about 600. surface-forming process or are due to subsequent Ares Valles (0-3, 17-19). Ares Valles is a major processes involving wind or water. Despite this, the outflow channel, emanating from a large region of thermal inertia provides an important constraint on chaotic terrain. The regional thermal inertia ranges the nature of the surface. In particular, the properties from about 250 to 500, with substantial variations at this scale are important for understanding the po- from place to place. Although the high-inertia values tential to safely land a spacecraft, to traverse the sur- are not confined to the channel floor, the channel floor face with a rover, or to collect rock samples. and chaos regions generally have high values; there We use the MGS Thermal Emission Spectrometer are some locations within the chaos that have the measurements to examine the thermal inertia at ~3- same moderate values as the surrounding regions, so km spatial resolution. Thus, we are determining the Lunar and Planetary Science XXXI 1858.pdf

MARS THERMAL INERTIA AND EXOBIOLOGY: B.M. Jakosky et al.

there is not a simple or unique relationship of thermal Crater (-10, 193). This crater, west of inertia to location. Apollinaris Patera, has an unusually large central (1-3, 272-274). This site contains peak, and ponding of water may have occurred on the ancient highlands and small valleys, some crater floor; thus, it is a site for possible evaporites of which lie adjacent to the impact craters and are and lake deposits. The region surrounding the crater sites of potential hydrothermal activity. The boundary has variable thermal inertia, ranging from low (~50) between the highlands and the Isidis basin to the north to moderate (~250) values. The crater interior has marks a sharp transition in thermal inertia from pre- strikingly higher thermal inertias, up to ~600. dominantly moderate values in the highlands (250- Shalbatana (-1 to 5, 43-46). This is a flood chan- 350) to high values in Isidis (up to 600). The valleys nel typically 10 km wide, emanating from a 50-km- and channels in the highland area do not show up wide region of chaotic terrain. The bulk of the sur- distinctly in thermal inertia at this resolution. rounding region has moderate thermal inertias (~200), Southeast Xanthe (-9 to –12, 27-31) and Iani although the region to the west and southwest of the Chaos (0-3, 13-15). This is in the out- source chaos has slightly higher values (up to ~350). flow channels to the east of . The The source of Shalbatana has increased thermal iner- surrounding terrain has moderate thermal inertias, tias, up to perhaps 500; the channel has elevated ther- ranging from about 250-350. The chaos terrain has mal inertias, although they are not as high as the variegated thermal inertia, ranging from 250 up to chaos itself. greater than 500 and varying in some cases from one Discussion, interpretation, and wild speculation. 3-km spot to the next by substantial amounts. Clearly, there is no single, systematic, and unique Crater (-9 to –14, 180-181). Gusev Crater description of the surfaces that would be of interest is a Noachian-aged crater that contains possible inte- from the exobiological perspective. While thermal rior lakebed deposits; an -aged channel, inertias tend to be elevated in the water-related chan- Ma’Adim Vallis, emanates from the channel to the nels and chaos deposits, there is substantial variabil- and southwest. The region has a thermal inertia ity. in the range of ~200-250. The interior of Gusev Cra- In general, the low-thermal-inertia surfaces are ter has values that range from this up to nearly 500; likely to consist predominantly of loose, unconsoli- the highest values are located near the southern edge dated fine materials, with particle sizes smaller than of the crater interior, with slightly lower values at the 100 microns. One would expect that these sites would west end and values similar to the adjacent terrain in be relatively safe to land on and easy to traverse, due the northeast. Ma’Adim Vallis is not distinctly differ- to the lack of rocks. Of course, this also means that ent from the adjacent terrain. they are not great places for sampling rocks that Al Qahira Vallis (-14 to –15, 194-196). This is an would tell us the most about the geological and poten- Hesperian-aged channel cutting through Noachian- tial biological history of the areas. aged plains. The regional thermal inertia varies The high-thermal-inertia surfaces are expected to somewhat between about 200 and 350, while crater contain more or larger rocks, although the rock abun- interiors in this region have higher values. The floor dance cannot be easily distinguished from indurated of the channel is in some places distinct, with higher crusts or bonded surfaces from thermal inertia alone. thermal inertias ranging up to ~600, and in other We expect, though, that these surfaces will be of more places having the same thermal inertia as the adjacent interest for sampling. The flip side, however, is that terrain. these sites are likely to be the more difficult areas to Amenthes Rupes (3, 250). The site of interest is on safely land a spacecraft and to traverse with a rover. the southern edge of a degraded impact crater, near Of course, detailed analysis must be done on a the mouth of a channel that flowed into the crater; pixel-by-pixel basis in order to understand the proper- possible groundwater and fluvial-lacustrine processes ties of a specific landing site, and we must keep in may be represented in deposits here. The surrounding mind that the thermal inertia measurements represent region has moderate thermal inertias (primarily the average properties over a 10- to 20-square- around 200), although the region to the north and east kilometer area and that substantial variability may can have substantially higher values. The crater inte- occur at smaller scales. rior and the adjacent channel are poorly sampled by the available coverage; however, the crater interior appears to have higher values, up to about 400, while the channel itself does not distinguish itself in thermal inertia (same value, ~200, as surrounding terrain).