Paper No. 2
Presentation Time: 8:20 AM


GRANT, John1, WILSON, Sharon Purdy1, CALEF, Fred2 and MSL SCIENCE TEAM, The, (1)Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, D. C, DC 20560, (2)Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109,

Geologic units in Gale crater display a range of relative ages and record a long record of geologic activity since the crater formed in the Early Hesperian. Alluvial activity included formation of alluvial fans flanking portions of the crater walls. An example includes the upper Peace Vallis fan extending from the northern wall of the crater. Topographic data suggests this fan is part of a larger alluvial accumulation extending into the region explored by the Curiosity rover.

Crater statistics in combination with geologic mapping reveal the oldest units in Gale date from the early-to-mid Hesperian and suggest their emplacement began shortly after formation of the crater. The older units likely include the bulk of the deposits associated with the alluvial fans. However, younger ages are also derived for some surfaces, implying a period of geomorphic activity during the later Hesperian or into the Amazonian.

Much of the later activity may relate to exhumation as the deposits comprising Mt Sharp retreated to the current, eroded form near the crater center. However, local surfaces on some fans, including portions of the Peace Vallis fan, may reflect a veneer of sediments on the broader fan form. Erosional relief (e.g., required for topographic inversion of relict distributaries) implies ~5-10 m surface lowering on these surfaces, consistent with a paucity of craters <30-40 m. Coupled with the absence of pedestal craters or other morphologies pointing to exhumation as the sole cause of late activity, these data suggest late occurring, local alluvial activity on some fans. The small areal extent of the units and differences in surface properties make age determination from small diameter craters difficult. Nevertheless, the timing of possible late alluvial activity in Gale correlates with alluvial activity in Margaritifer Terra hypothesized to result from melting snow. At Peace Vallis, any late deposition appears limited to the upper fan, though sediment contributions to the areas explored by Curiosity cannot yet be ruled out. Regardless, associated water would have drained downslope and could have contributed to development of late diagenetic features observed by Curiosity in the rocks at Yellowknife Bay.