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(1) from a climate that once allowed rivers to flow(2-6) 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(7-9). The Aeolis Dorsa region near Gale crater on Mars contains a high density of preserved ancient craters interbedded with river deposits(11) and thus can provide constraints on atmospheric density at the time of fluvial activity. Here we use high-resolution images and digital terrain models(10) 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(12,13). 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(14): 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 CO2/H2O greenhouse(2) is ruled out, and long-term average temperatures were most likely below freezing.