Rainfall on Noachian Mars: Nature, timing, and influence on geologic processes and climate history

Abstract

The formation of martian geologic features, including degraded impact craters, valley networks, and lakes, has been interpreted to require a continuously “warm and wet” Noachian climate, with above-freezing surface temperatures and rainfall. More specifically, it has been argued that a change in the nature of rainfall in the Noachian, from a diffusive rain splash-dominated erosional regime to an advective runoff-dominated erosional regime, is the best explanation for the observed temporal differences of erosion style: the degradation of craters has been interpreted to be due to rain splash throughout the Noachian, while the formation of valley networks and lakes has been interpreted to be due to more erosive activity and more abundant fluvial activity at the Noachian/Hesperian transition. However, the presence of a long-lived “warm and wet” climate with rainfall is difficult to reconcile with climate models which instead suggest that the long-lived climate may have been “cold and icy”, with surface temperatures far below freezing, precipitation limited to snowfall, and most water trapped as ice in the highlands. In such a “cold and icy” climate scenario, fluvial and lacustrine activity would only be possible during transient warm periods, which could produce “warm and wet” conditions for relatively short periods of time. In this work, we (1) review the geomorphic evidence for Noachian rainfall and the various rainfall-related erosional regimes, (2) explore climate model predictions for a “cold and icy” climate and the potential for short-lived “warm and wet” excursions, and (3) attempt to characterize the transition from diffusive to advective erosional rainfall regimes through analysis of atmospheric pressure and rainfall dynamics with the goal of providing insight into the nature of the Noachian hydrological cycle and thus, the Noachian climate. We conclude that (1) if rainfall occurred on early Mars, raindrops would have been capable of transferring sufficient energy to initiate sediment transport regardless of atmospheric pressure, implying that rain splash would have been possible throughout the Noachian, and (2) in contrast to previous findings, maximum possible raindrop size does not depend on atmospheric pressure and, as a result, simple parameterized relationships suggest that rainfall intensity (rainfall rate) does not depend on atmospheric pressure. Therefore, our results, based on the implementation of a simple parameterized relationship for rainfall intensity, predict that there would not have been a transition from rain splash-dominated erosion to runoff-dominated erosion related solely to decreasing atmospheric pressure in the Noachian. This finding is not consistent with the hypothesis of Craddock and Lorenz (2017) that the long-lived Noachian climate was “warm and wet” with rainfall throughout the Noachian and that rainfall intensity changed as a function of atmospheric pressure declining through time; our findings do not preclude the possibility that early Mars was predominantly “cold and icy”. Remaining unknown is the mechanism(s) for the observed geomorphic transition in erosion style, and whether melting of surface snow/ice and runoff during a punctuated heating episode in an otherwise “cold and icy” climate could explain the formation of the valley networks and lakes in the absence of rainfall. We conclude by outlining future work that introduces more advanced methodology to further explore a possible relationship between rainfall intensity and atmospheric pressure.