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We discuss 3D global simulations of the early Martian climate that we haveperformed assuming a faint young Sun and denser CO2 atmosphere. We include aself-consistent representation of the water cycle, with atmosphere-surfaceinteractions, atmospheric transport, and the radiative effects of CO2 and H2Ogas and clouds taken into account. We find that for atmospheric pressuresgreater than a fraction of a bar, the adiabatic cooling effect causestemperatures in the southern highland valley network regions to fallsignificantly below the global average. Long-term climate evolution simulationsindicate that in these circumstances, water ice is transported to the highlandsfrom low-lying regions for a wide range of orbital obliquities, regardless ofthe extent of the Tharsis bulge. In addition, an extended water ice cap formson the southern pole, approximately corresponding to the location of theNoachian/Hesperian era Dorsa Argentea Formation. Even for a multiple-bar CO2atmosphere, conditions are too cold to allow long-term surface liquid water.Limited melting occurs on warm summer days in some locations, but only forsurface albedo and thermal inertia conditions that may be unrealistic for waterice. Nonetheless, meteorite impacts and volcanism could potentially causeintense episodic melting under such conditions. Because ice migration to higheraltitudes is a robust mechanism for recharging highland water sources aftersuch events, we suggest that this globally sub-zero, `icy highlands' scenariofor the late Noachian climate may be sufficient to explain most of the fluvialgeology without the need to invoke additional long-term warming mechanisms oran early warm, wet Mars.

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