Retention of an atmosphere on early Mars

The presence of valley networks and indications of high erosion rates in an cient terrains on Mars suggest that Mars was warm and wet during heavy bomb ardment. Various processes that could occur on early Mars were integrated i nto a self-consistent model to determine what circumstances might lead to w arm temperatures during and at the end of heavy bombardment. Included were weathering and burial of CO2 as carbonates, impact erosion: sputtering, and recycling of CO2 back into the atmosphere by burial and heating. The model s suggest that despite losses from the atmosphere by weathering and impact erosion, Mars could retain a 0.5 to 1 bar atmosphere at the end of heavy bo mbardment partly because weathering temporarily sequesters CO2 in the groun d and protects it from impact erosion while the impact rate is declining an d impact erosion is becoming less effective. Because of the low output of t he early Sun, surface temperatures can be above freezing only for a very ef ficient greenhouse, such as that suggested by Forget ann Pierrehumbert [199 7]. With weak greenhouse models, temperatures are below freezing throughout heavy bombardment, and such a large amount of CO2 is left in the atmospher e at the end of heavy bombardment that it is difficult to eliminate subsequ ently to arrive at the present surface inventory. With strong greenhouse mo dels, temperatures are well above freezing. during heavy bombardment and dr op tn close to freezing at the end of heavy bombardment, at which time the atmosphere contains 0.5 to 1 bar of CO2. This can be largely eliminated sub sequently by sputtering and low-temperature weathering. Such a model is con sistent with the change in erosion rate and the declining rate of valley fo rmation at the end of heavy bombardment. Conditions that favor warm tempera tures at the end of heavy bombardment are an efficient greenhouse, low weat hering rates, low impact erosion rates, and a smaller fraction of heat lost by conduction as opposed to transport of lava to the surface.