Atmospheric loss since the onset of the Martian geologic record: Combined role of impact erosion and sputtering

Chemical and geomorphic evidence suggests that Mars' atmosphere has undergo ne significant loss or modification since the onset of the Martian geologic record. Analyses of proposed loss processes have been unable to individual ly account for loss of enough atmosphere to have supported the presence of liquid surface water at the formation time of the oldest observed surface u nits. Here, we give a simple calculation of the combined effects of major a tmospheric loss processes. Considering only impact erosion and using result s from an analytical model by Melosh and Vickery [1989], we obtain an expre ssion for surface pressure as a function of local crater density. Based on tabulated crater densities, the process of atmospheric cratering can accoun t for a loss of 50-90% of the Martian atmosphere since the onset of the geo logic record. Stable isotope fractionation measurements suggest that loss o f similar to 90% of atmospheric species to space has occurred via solar win d pick-up-ion sputtering [Jakosky and Jones, 1997]. Combined, similar to 95 -99% of Mars' atmosphere could have been lost to space. Adsorption of CO2 i n the regolith and sequestration in the poles as ice or clathrate could acc ount for most of the remaining loss. These calculations suggest that the lo ss processes of impact erosion, sputtering, and sequestration are together capable of explaining the inferred transition from an early atmosphere on M ars capable of supporting stable liquid surface water to today's climate.