Edwin S. Kite, Jean-Pierre Williams, Antoine Lucas, Oded Aharonson
The single most important control on long-term climate change on Mars is thought to be decay of the CO2-dominated atmosphere, but direct constraints on paleoatmospheric pressure P are lacking. Of particular interest is the climate that allowed rivers to flow early in Mars history, which was affected by P via direct and indirect greenhouse effects. The size of craters embedded within ancient layered sediments is a proxy for P: the smaller the minimum-sized craters that form, the thinner the past atmosphere. Here we use high-resolution orthophotos and Digital Terrain Models (DTMs) to identify ancient craters among the river deposits of Aeolis, and compare their sizes to models of atmospheric filtering of impactors by thicker atmospheres. The best fit is P <= 760+/-70 mbar, rising to P <= 1640+/-180 mbar if rimmed circular mesas are excluded. Surveys tend to undercount smaller craters, so these fits are upper limits. Our work assumes target properties appropriate for desert alluvium: if sediment developed bedrock-like rock-mass strength by early diagenesis, the upper limit is greatly increased. If Mars did not have a stable multibar atmosphere at the time that the rivers were flowing, the warm-wet CO2 greenhouse of Pollack et al. (1987) is ruled out, and long-term average temperatures were probably below freezing, implying that exoplanet habitable-zone calculations that use Mars as a reference point may need to be reconsidered.
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http://arxiv.org/abs/1304.4043
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