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Published May 2014 | Supplemental Material
Journal Article Open

Low palaeopressure of the martian atmosphere estimated from the size distribution of ancient craters


The decay of the martian atmosphere—which is dominated by carbon dioxide—is a component of the long-term environmental change on Mars from a climate that once allowed rivers to flow to the cold and dry conditions of today. The minimum size of craters serves as a proxy for palaeopressure of planetary atmospheres, because thinner atmospheres permit smaller objects to reach the surface at high velocities and form craters. The Aeolis Dorsa region near Gale crater on Mars contains a high density of preserved ancient craters interbedded with river deposits and thus can provide constraints on atmospheric density at the time of fluvial activity. Here we use high-resolution images and digital terrain models from the Mars Reconnaissance Orbiter to identify ancient craters in deposits in Aeolis Dorsa that date to about 3.6 Gyr ago and compare their size distribution with models of atmospheric filtering of impactors. We obtain an upper limit of 0.9 ± 0.1 bar for the martian atmospheric palaeopressure, rising to 1.9 ± 0.2 bar if rimmed circular mesas—interpreted to be erosionally-resistant fills or floors of impact craters—are excluded. We assume target properties appropriate for desert alluvium: if sediment had rock-mass strength similar to bedrock at the time of impact, the paleopressure upper limit increases by a factor of up to two. If Mars did not have a stable multibar atmosphere at the time that the rivers were flowing—as suggested by our results—then a warm and wet CO_2/H_2O greenhouse is ruled out, and long-term average temperatures were most likely below freezing.

Additional Information

© 2014 Macmillan Publishers Limited. Received 31 August 2013; Accepted 10 March 2014; Published online 13 April 2014. We thank I. Daubar, J. Dufek, B. Ehlmann,W. Fischer, V. Ganti, I. Halevy, J. Kasting, K. Lewis, M. Manga, R. Ramirez, M. Rice, A. Soto and R. Wordsworth for preprints and discussions. We thank the HiRISE team and the CTX team. This work was financially supported by an O.K. Earl Fellowship (to E.S.K.) and by the US taxpayer through NASA grants NNX11AF51G (to O.A.) and NNX11AQ64G (to J-P.W.). Author contributions: E.S.K. designed research, picked craters, carried out the data-model comparison and drafted the main text. J-P.W. wrote the forward model of impactor-atmosphere interactions. A.L. built the digital terrain models and wrote the corresponding Supplementary text. O.A. supervised research. All authors contributed to the interpretation of the results and to the revisions.

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