HR: 15:25h
AN: P23D-08    [Abstracts]
TI: The Effect of Crater Obliteration on Inferred Surface Ages on Mars
AU: * Smith, M R
EM: matthers@u.washington.edu
AF: University of Washington, Department of Earth and Space Sciences 4000 15th Avenue NE, Seattle, WA 98195-1310, United States
AU: Gillespie, A R
EM: arg3@u.washington.edu
AF: University of Washington, Department of Earth and Space Sciences 4000 15th Avenue NE, Seattle, WA 98195-1310, United States
AU: Montgomery, D R
EM: dave@ess.washington.edu
AF: University of Washington, Department of Earth and Space Sciences 4000 15th Avenue NE, Seattle, WA 98195-1310, United States
AB: The density of impact craters is currently the only measure of numerical age for terrestrial planets, after conversion with lunar calibration curves. Unlike the Moon, however, the martian surface has been eroded, obliterating some small (<1 km) craters. Recent images from high-resolution satellite-based cameras, such as the Mars Orbiter Camera (MOC) and the Thermal Emissivity Imaging Spectrometer (THEMIS), have identified many small, geologically distinct units, some of which may hold large implications for the geological and climatic history of Mars. Some of these units, such as the light-toned layered deposits, have shown greater susceptibility to erosion than the rest of the martian surface and may not retain small craters over geologic time. We have built upon early attempts to quantitatively account for rates of crater obliteration (combined erosion and deposition) on crater-frequency distributions by developing a model to measure its effect on inferred surface ages over the full range of orbitally observable crater sizes and reported erosion and deposition rates on Mars (0.01 - 10,000 nm/yr). For low long-term rates of obliteration (less than 10 nm/yr), like those observed by the Spirit and Pathfinder rovers, we find that surface modification has a minimal effect on the calculated surface age, even when only small craters are counted. However, for intermediate to high obliteration rates (greater than 50 nm/yr), like those observed by Viking Lander 2 and the Opportunity rover, the change of the calculated surface age can be large, especially when counting only small craters. This sensitivity likely precludes accurate determination of deposit ages by crater counting alone, without considering infilling and erosion. The model, when applied to previous studies using only small craters to date the surfaces of erodable deposits reported as geologically young, can yield surface ages that are older by orders of magnitude than those originally estimated. Consequently, conventional crater counting should be adjusted for obliteration before being construed as evidence for surface ages, especially for surfaces with low densities of small craters.
DE: 5415 Erosion and weathering
DE: 6022 Impact phenomena (5420, 8136)
DE: 6225 Mars
DE: 8136 Impact phenomena (5420, 6022)
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting