Testing the impact heating hypothesis for early Mars with a 3-D global climate model

The nature of the early Martian climate has been the subject of debate for decades, with geologic evidence suggesting an environment with prolonged precipitation and flowing liquid water on the surface, while climate models have struggled to reproduce such conditions. In this paper, we test the impact heating hypothesis for warming early Mars as presented in Segura et al. (2008) using a new early Mars version of the NASA Ames Research Center 3-D Mars Global Climate Model. We simulate impacts of asteroids 30-, 50-, and 100-km in diameter into atmospheres possessing 150-mbar, 1-bar, and 2-bar surface pressure conditions, accounting for both radiatively active and radiatively inert water clouds. Based on the scenarios simulated here, we find that the evolution of post-impact initially hot and moist conditions can be characterized in four phases: 1) a rapid radiative cooling phase, 2) a latent heat phase in which cloud formation and the radiative effects of water vapor induce a temporary warm period with significant precipitation, 3) a transition phase in which cooling accelerates due to sublimation at the surface and the lack of available water in the atmosphere for greenhouse warming and in which water vapor begins to contribute less to surface warming than water clouds, and 4) a steady state phase with mean annual surface temperatures below freezing and minimal precipitation. In these post-impact climate scenarios, global average surface temperatures remain above freezing for only 0.043 to 6.25 Mars years, accompanied by 0.23 to 5.8 m of cumulative precipitation (global equivalent) falls during 10 simulated Mars years. Ultimately, periods of warm temperatures and significant precipitation are short-lived and even in the warmest cases, do not support sustained conditions in which valley networks are likely to form in the long run either by liquid precipitation or by seasonal melting of surface ice. Scenarios with high surface pressures and radiatively active clouds experience the longest periods of above-freezing post-impact temperatures and result in the highest mean annual temperatures during the fourth phase (272.8 K in our warmest scenario), highlighting the potential significance of water clouds in the early Martian climate and the importance of their careful physical treatment in models. Future studies addressing sustained warm and wet early Mars conditions should investigate the potential effects of obliquity, initial surface ice distribution, and possible delivery of reducing greenhouse gases on these post-impact climates.