A coupled atmosphere–hydrosphere global climate model of early Mars: A ‘cool and wet’ scenario for the formation of water channels

Martian water channels are considered evidence of a climate warm enough to allow the existence of long-term fluvial systems on early Mars during the Noachian and Hesperian boundary (3.85–3.6 Ga). Quantitative inferences of water channel formation from climate models are crucial to develop an accurate understanding of the early Martian environment. We present the results of a newly developed 3-dimensional Paleo Martian Global Climate Model (PMGCM) assuming a CO₂/H₂O/H₂ atmosphere under the ‘Faint Young Sun’ condition (with a solar luminosity of ~75% of the current value) for surface pressures between 0.5 and 2 bar. The PMGCM has a hydrologic cycle module, which includes ocean thermodynamics and water vapor advection, convection, condensation and precipitation processes, as well as calculations of surface fluvial activities (e.g., fluvial activity and sediment transport) at a high horizontal resolution. Our PMGCM results show that the early Martian surface environment could have been ‘cool’ (between ‘warm’ and ‘cold’); namely, the surface temperatures could have been high enough (>273 K) during summertime to allow seasonal melting of snow and ice deposits, and low enough (<273 K) during wintertime to produce considerable snow precipitation and accumulation, under the conditions of a mean surface pressure of approximately 1.5 bar and an H₂ composition of 3%. The results also indicate that a ‘wet’ surface environment should be characterized by precipitation and seasonal melting of snow and ice (neither ‘dry’ nor ‘permanently frozen’ states), and enough fluvial activity and sediment transport could have occurred in the low to middle latitudes to produce Martian valley networks within a relatively short time (less than tens of million years). Therefore, we suggest that a moderate climate, that is, ‘cool and wet’ conditions lying between ‘warm and wet’ and ‘cold and frozen’, best explains the fluvial activity on early Mars.