A Wet , Dry , or Hot Gusev Crater ? : Using Themis and Mer to Test Depostional Hypotheses

Introduction: The unifying theme of the Mars Exploration Program is the search for evidence of past or present liquid water [1]. The underlying assumption behind this is that areas on Mars exposed to liquid water would have had the highest probability of capturing and preserving biomarkers. As a part of this program strategy, two Mars Exploration Rovers (MERs) will launch toward Mars in 2003, with early 2004 landings. Each will carry the Athena science package [2] with instruments suitable for gathering evidence of ancient water on Mars. Thus, one of the considerations in landing site selection has been sites showing possible evidence for past water. Gusev crater (14.64°S 175.36°E) is a site previously interpreted as a lacustrine depocenter for detritus from the 900 km long Ma'adim Vallis fluvial system [3-6]. Photogeologic interpretations cited to support this hypothesis include: fluvial terraces within Ma'adim [7], evidence of stream migration [8], debris lobes or 'deltaic' deposits at the mouth of Ma'adim [3,6], shoreline deposits [6], curvilinear ridges within Gusev analogous to sedimentary structures formed in ice-covered lakes by sub-glacial rotary currents [9]. While these interpretations are consistent with fluvio-lacustrine deposition, it is important to test this hypothesis with new data sets (THEMIS, TES, MOC, and MOLA) and to consider other plausible deposi-tional models. Surface Unit Deposition: Eight surface units mapped within Gusev crater vary in lateral extent from several tens of kms to basin-wide [10]. Subsequent erosion and superposition of strata make original lateral extents difficult to discern. The extent of the LB unit within Gusev, its termination against pre-existing units, and deposition at the lowest elevations within Gusev imply lateral flow during deposition. Horizontal sub-layering within surface units (Figure 1) suggests multiple depositional events. Slope changes between sub-layers suggest lithologic variation, varying degrees of induration, or changes in depositional environment. Observations of unit orientations and extents, however, are not unique indicators of a fluvio-lacustrine deposi-tional environment. Depositional Hypotheses: Here we utilize a new surface unit map, inferred stratigraphic sequence, and new THEMIS observations [10-11] to evaluate three depositional hypotheses for Gusev. Fluvio-Lacustrine. The first model involves depo-sition of sediment under a fluvial and/or lacustrine