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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 climatethat once allowed rivers to flow to the cold and dry conditions of today. Theminimum size of craters serves as a proxy for palaeopressure of planetaryatmospheres, because thinner atmospheres permit smaller objects to reach thesurface at high velocities and form craters. The Aeolis Dorsa region near Galecrater on Mars contains a high density of preserved ancient craters interbeddedwith river deposits and thus can provide constraints on atmospheric densityaround the time of fluvial activity. Here we use high-resolution orthophotosand digital terrain models from the Mars Reconnaissance Orbiter to identifyancient craters in Aeolis Dorsa that date to about 3.6 Gyr ago and comparetheir size distribution with models of atmospheric filtering of impactors. Weobtain an upper limit of 0.9$\pm$0.1 bar, rising to 1.9$\pm$0.2 bar if rimmedcircular mesas - interpreted to be erosionally-resistant fills of floors ofimpact craters - are excluded. We assume target properties appropriate fordesert alluvium: if sediment had rock-like rock-mass strength similar tobedrock at the time of impact, the upper limit increases by a factor of up totwo. If Mars did not have a stable multibar atmosphere at the time that therivers were flowing - as suggested by our results - then the warm and wetCO2/H2O greenhouse is ruled out, and long-term average temperatures were mostlikely below freezing.

Low paleopressure of the Martian atmosphere estimated from the size distribution of ancient craters