VERNAL CRATER, SW ARABIA TERRA: MSL CANDIDATE WITH EXTENSIVELY LAYERED SEDIMENTS, POSSIBLE LAKE DEPOSITS, AND A LONG HISTORY OF SUBSURFACE ICE. D.Z. Oehler1 and C.C. Allen1. 1NASA-JSC, Houston, TX 77058, [email protected]; [email protected]. Summary: Vernal Crater is a Mars Science Laboratory (MSL) landing site candidate providing 100 km relatively easy access to extensively layered sediments as well as potential lake deposits. Sediments of Vernal Crater are 400-1200 m below those being investigated by Opportunity in Meridiani Planum, and as such would allow study of significantly older geologic units, if Vernal Crater were selected for MSL. Vernal The location of Vernal Crater in SW Arabia Terra Crater provides exceptional scientific interest, as rampart craters and gamma-ray spectrometer (GRS) data from the region suggest a long history of ice/ fluids in the subsurface. The potential value of this MSL candidate is further enhanced by reports of atmospheric methane over Arabia, as any insight into the source of that methane would significantly increase our understanding of Mars. Fig. 2. Rampart craters (white ovals) in the region of Vernal Finally, should MSL survive beyond its prime mission, Crater. Daytime IR on MOLA topography. Color scale: reds the gentle slope within Vernal Crater would provide a = highs (-1300m); dark blue = lows (-2000m). route out of the crater for study of the once ice/fluid-rich varying amounts of erosion and thus are of varying ages plains. [6,7]. This, in turn, implies that ice or fluids have been in Regional Setting: Arabia Terra is a one of the few the subsurface for extended periods [6,7]. The fact that equatorial regions on Mars characterized by relatively numerous small (1 to 5 km-diameter) ramparts are present high abundances of near-surface hydrogen as measured might additionally suggest the presence of near-surface by GRS (1); this has been taken to suggest the presence ice in the last several million years, when obliquity was of shallow ice or hydrated minerals [1,2]. Vernal Crater higher and ice was stable closer to the equator. [3] lies in SW Arabia Terra, halfway between a proposed Atmospheric methane has been detected over Arabia ancient Arabia basin [4-6] to the east and the massive in concentrations suggesting recent release to the circum-Chryse outflows to the west (Fig. 1). SW Arabia atmosphere [10-12]. Various sources, both biogenic and Terra adjoins northern Meridiani Planum and both abiogenic, have been postulated for this gas [13], and any include extensive layered units that have been interpreted insight into the production and release of methane to the as sedimentary rocks [8,9]. Rampart craters are common Martian atmosphere would be a major advance in our in SW Arabia Terra (Fig. 2); they range in diameter from understanding of Mars. Circum-Chryse Vernal Crater: Vernal Crater (Fig. 3) is a 55 km- Outflows Arabia Terra Daytime IR (THEMIS) Viking 1 Pathfinder PROPOSED ANCIENT SW Arabia BASIN (Dohm et al., 2007) Vernal Crater Opportunity MOLA DEM on Hillshade S. Highlands Layered Unit Fig. 1. Regional setting, proposed ancient Arabia Basin, and circum-Chryse outflows. 1 to 25 km, impact three different geological units (L, S and R of Edgett [9]), and have ejecta blankets with both 20 km rounded and angular morphologies. This latter observation suggests that the ramparts have experienced Fig. 3. Vernal Crater. Daytime IR. diameter structure, centered at 6o N, 355.5o E. It is 20 km MOLA topography on characterized by a well defined IR-bright feature which Daytime IR (contours 25m) corresponds to a layered unit, a gentle slope of ~1.5 m degrees (from about -1350 m in the north to -2200 m in m 5 0 7 0 MOC S06--01229 the south), and a strongly asymmetric rim (Fig. 4). 3 5 1 1 - - Seven geomorphic units in this crater have been m mapped (Fig. 5) using detailed comparisons of THEMIS MOC S19-01964 5 7 VIS/IR, MOLA, and MOC images that were spatially 8 1 rectified in ArcGIS [14]. The northern half of the crater - m Blocky 0 contains Noachian Plains units (NPuu and NPmd) 5 7 Outcrops 1 dominated by features typical of aeolian processes (dunes - Possible meanders Crater low and yardangs). The southern half may expose Noachian (NPmm) -2200m fluvio-lacustrine sediments; it contains few dunes or Possible lake obvious yardangs but has possible meandering and Undulating deposit (NIll) braided stream units (NPmc and NPmm), layered deposits layers (NIlu) corresponding to the IR-bright feature, and a (NIlu) potential lake deposit (NIll) with associated shoreline- Rim high like bedding (NIls) (Fig. 5). These more southerly (-1000 m) MOC M11-03134 deposits are likely to be the units of greatest applicability to MSL science goals related to present and past habitability. Stratigraphically, the youngest units (in the northern Fig. 4. Vernal Crater. MOLA topography on Daytime IR. half of Vernal Crater) appear to lie a minimum of four Color indicates topography: red = highest elevation; dark hundred meters below those investigated by Opportunity blue = lowest. Yellow rectangles show areas in Meridiani [9]; the older and deeper units to the south of detail for Figs. 7-9. (the layered unit and possible lake/ shoreline deposits) would be 900 to 1200m (respectively) below the sediments being investigated by Opportunity. Landing Ellipse and MRO Footprints: The NPuu recommended landing ellipse and footprints for HiRISE, CRISM, and CTX from Mars Reconnaissance Orbiter (MRO) are shown in Fig. 6 [15]. The gentle slope in Vernal Crater should provide relatively easy access to all NPmd the geomorphic units and the 750 m of exposed section. NIll All MSL engineering constraints, including that for maximum dust coverage, are met in this site [15 ]. NIls NIlu NPmm NPmc Region of Interest NIll NIlu 20 km Landing Ellipse 6-10 km HiRISE 10 x 10 km CRISM NIlu NIlsNIls NIllNIll 30 x 30 km CTX Fig. 6. Vernal Crater (SW Arabia Terra) - MSL Fig. 5. Geomorphic units in Vernal Crater on Inverted Landing Ellipse and MRO footprints on Daytime IR. Daytime IR. MOC footprints and rectangles as in Fig. 4. Discussion: The area characterized by bright IR location). These outcrops have flat tops and somewhat response in daytime images (Fig. 3) corresponds to a curved edges, and are individually separated by rounded, slight ridge in the topography and is comprised of several incised regions. The flat tops and curved edges seem packages of layered sediments. Each package consists of more likely to have been produced by aqueous erosion approximately ten to a few tens of light-toned layers (Fig. than by wind erosion. The belt of outcrops is separated 7, and Figs. 4-5 for location). The correspondence of the from features to the south by a relatively wide, curving layered sediments to a ridge suggests that they are path that is smooth except for the presence of dunes; this indurated and somewhat resistant to erosion. Their wavy may represent the remnant of a meander path. Finally, fabric might suggest fluid-rich deposition and/or soft patches of inverted channels resembling braided stream sediment deformation. This undulating fabric is not deposits occur in association with these blocky outcrops. likely to be attributable to lava, since few volcanic Taken together, these features are suggestive of fluvial processes in the southern half of Vernal Crater. At the southernmost end of Vernal Crater, remnants of crater lake deposits could be located in the area of the major topographic low (Fig. 9, and Figs. 4-5, for location). rs e ay ed) L on m -t m 00 ht 75 0 g 8 - 2 (li - 1 Dark sand ? in lows its os ep D ke 0.25 km La MOC M11-03134 m 0 ? 00 its Fig. 7. Vernal Crater. Undulating layers (NIlu). - 2 os ep D ne features or fissures are known from which lava could eli or emanate [9]. Thus, the layers seem likely to be of Sh aqueous origin and their induration and resistance to erosion might be attributable to cementation by either 2 km MOCMOC S06-01229 S0-01229 interaction with hydrothermal fluids or by evaporation. This layered unit has similarly bright (though not Fig. 9. MOC image with possible lake (NIll) and identical) responses in both daytime and nighttime IR. shoreline deposits (NIls) on MOLA topography. Detailed comparison of spatially rectified MOC, THEMIS VIS, and THEMIS IR imagery suggests that the The fact that the lowest elevation in the crater is just nighttime IR response is due to the packages of light- north of the highest part of the rim (Fig. 4) may not be toned layers while the daytime response originates from coincidental. While the asymmetric rim may have focused dark sand resting in lows within the layered unit wind currents such that the area of the topographic low North of the layered unit and about midway within the would have been preferentially eroded and exhumed, the crater is a distinctive, east-west trend of blocky outcrops asymmetric rim additionally would have shadowed the that resembles incised meanders (Fig. 8 and Figs. 4-5, for region immediately to the north during periods of high obliquity; and this shadowing could have preserved any ? ers ice present in that portion of an ancient crater lake. Thus, and me Vernal Crater’s asymmetric rim, with its highest part in ed cis the south, may have set the stage for preservation of a lake In with relatively long-lived ice in this particular crater. Braided Such a long-lived lake may be the type of setting in which stream? organic remains of either a prebiotic chemistry or possible early Martian microbes (if life ever developed on Mars) th? might be preserved.