Evolution of the Martian Atmosphere: Results of a Monte Carlo Model of Sputtering

Using a Monte-Carlo model, we determine the sputtering yields for the upper atmosphere of Mars at three different epochs (the present, -2.5 Gyr ago and -3.5 Gyr ago). The carbon yield increases from 1 at 3.5 Gyr ago to 2 at present. These yields imply that sputtering is a major loss process for COz, especially during the early history of the solar system. Over the same period, the yield of oxygen atoms (from all species) increases from 5.5 to 6.5. Based on the uncertainty in the necessary parameters and sensitivity studies with the model, we estimate that these values are good to within 50%. When combined with estimates of impacting fluxes of O+ ions (Luhmann et al. 1992), our estimates suggest that Mars has lost 0.8 bars of C02 and 50 m Hz0 over the last 3.5 Gyr. This is a significant fraction of the CO2 needed to warm Mars sufficiently to allow for liquid water near the surface (Forget and Pierrehumbert 1997; Haberle 1998). Furthermore, both the COZ and the water are large volumes compared to the known Martian reservoirs, indicating the important role sputtering has played in the evolution of the atmosphere of Mars. Subject headings: MARS, ATMOSPHERE; MARS, CLIMATE; ABUNDANCES, ATMOSPHERES; AERONOMY