Snow or rain on early Mars?

Abstract:

The climate of Mars during the late Noachian 3-4 Ga remains a long-standing mystery. Over the last three decades, the key debate has been whether the valley networks and other fluvial features were carved by rain in a warm, wet Earth-like climate or by snowmelt in a mainly cold climate. Recently, it has become clear that close intercomparison between the geology and 3D climate model predictions can be used to move this debate forward. Here I describe new work comparing three-dimensional simulations under warm and cold climate scenarios with the surface distribution of valley networks. I discuss how the lack of periglacial landforms at equatorial latitudes (the ‘periglacial paradox’) and destabilizing climate influence of Tharsis can be used to rule out scenarios where the surface water inventory was extremely high, indicating a mainly cold, snowmelt-driven scenario for valley network formation is most likely. Finally, I discuss plausible episodic warming mechanisms and the importance of early Mars to our conception of exoplanet habitability.