Global modelling of the early martian climate under a denser CO2 atmosphere: Water cycle and ice evolution

R Wordsworth,F Forget,E Millour,J.W Head,J.-B Madeleine,B Charnay

doi:10.1016/j.icarus.2012.09.036

R Wordsworth, F Forget + Show 4 more

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https://doi.org/10.1016/j.icarus.2012.09.036

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Abstract

We discuss 3D global simulations of the early martian climate that we have performed assuming a faint young Sun and denser CO2 atmosphere. We include a self-consistent representation of the water cycle, with atmosphere–surface interactions, atmospheric transport, and the radiative effects of CO2 and H2O gas and clouds taken into account. We find that for atmospheric pressures greater than a fraction of a bar, the adiabatic cooling effect causes temperatures in the southern highland valley network regions to fall significantly below the global average. Long-term climate evolution simulations indicate that in these circumstances, water ice is transported to the highlands from low-lying regions for a wide range of orbital obliquities, regardless of the extent of the Tharsis bulge. In addition, an extended water ice cap forms on the southern pole, approximately corresponding to the location of the Noachian/Hesperian era Dorsa Argentea Formation. Even for a multiple-bar CO2 atmosphere, conditions are too cold to allow long-term surface liquid water. Limited melting occurs on warm summer days in some locations, but only for surface albedo and thermal inertia conditions that may be unrealistic for water ice. Nonetheless, meteorite impacts and volcanism could potentially cause intense episodic melting under such conditions. Because ice migration to higher altitudes is a robust mechanism for recharging highland water sources after such events, we suggest that this globally sub-zero, ‘icy highlands’ scenario for the late Noachian climate may be sufficient to explain most of the fluvial geology without the need to invoke additional long-term warming mechanisms or an early warm, wet Mars.

Highlights

After many decades of observational and theoretical research, the nature of the early Martian climate remains an essentially unsolved problem
3.1 Fixed atmospheric humidity simulations In Forget et al [2012], we described the climate of early Mars with a dense, dry atmosphere
Our results have shown that early Mars is unlikely to have been warm and wet if its atmosphere was composed of CO2 and H2O only, even when the effects of CO2 clouds are taken into account

Summary

Introduction

After many decades of observational and theoretical research, the nature of the early Martian climate remains an essentially unsolved problem. The geomorphological evidence for an altered climate on early Mars includes extensive dendritic channels across the highland Noachian terrain (the famous ‘valley networks’) [Carr, 1996, Fassett and Head, 2008b, Hynek et al, 2010], fossilized river deltas with meandering features [Malin and Edgett, 2003, Fassett and Head, 2005], records of quasi-periodic sediment deposition [Lewis et al, 2008], and regions of enhanced erosion most readily explained through fluvial activity [Hynek and Phillips, 2001]. Some studies have suggested evidence for an ancient ocean in the low-lying northern plains. These include a global analysis of the martian hydrosphere [Clifford and Parker, 2001] and an assessment of river delta / valley network contact altitudes [di Achille and Hynek, 2010]. In the absence of other evidence, the existence of a northern ocean in the Noachian remains highly controversial

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