Global Constraints on Rainfall on Ancient Mars : Oceans , Lakes , and Valley Networks

Introduction: The last five decades of Martian exploration have shown evidence for standing surface liquid water in Mars' ancient past [1]. The Vi-king, Mars Global Surveyor (MGS), and Mars Reconnaissance Orbiter (MRO) spacecraft have all observed valley networks in the equatorial and southern tropical regions of Mars [2][3]. As well, recent observations by the MRO HiRISE camera show geo-morphological features that have been interpreted as lake strandlines/shorelines [4]. These observations led to extensive research into possible mechanisms for sustaining surface liquid on ancient Mars while also explaining how the current Martian climate could be so hostile to the existence of surface liquid water. Mechanisms include: a once thick CO2 atmosphere that was slowly removed by the solar wind and/or entrainment into the subsurface (as carbonates or CO2-H2O clathrates) [5]; a relatively thin CO2 atmosphere periodically injected, perhaps by volcanism, with additional greenhouse gases such as SO2 [6]; or a temporarily clement climate generated by the impact of moderate or large sized impactors [7][8]. However, the existence and distribution of precipitation depends on more than achieving global mean temperatures above the freezing point of water. The actual global abundance and distribution liquid water in a warmer Martian atmosphere is equally important and has not been previously investigated. Understanding the constraints on precipitation due to atmospheric dynamics and topography will provide insight into the current geological evidence for surface liquid water. We present here the results of our investigation into the patterns of precipitation and evaporation for the types of climates that might have existed during ancient Mars [9]. Modeling Methods: As a first order assessment of the global constraints on precipitation in a warm, Martian paleoclimate, we used an Earth climate model modified to have a Martian topography. The Community Atmosphere Model (CAM) already includes a hydrological model that couples the ocean, land, and atmosphere reservoirs of water. We merely changed the topography in the model to match current Mars topography and thus capture the likely distribution of water in a Martian paleoclimate. For a first order study this is a valid simplification since the large scale atmospheric dynamics are similar on Earth and paleo-Mars due to their similar rotational