Orographic precipitation in valley network headwaters: Constraints on the ancient Martian atmosphere

We examine the Martian valley networks in the framework of topographic influences on precipitation. We use an analytical model and the Laboratoire de Météorologie Dynamique (LMD) early Mars global circulation model (GCM) to explore the local-scale distribution of orographically forced precipitation as a function of atmospheric pressure. In simulations with 500 mbar and 1 bar CO₂ atmospheres, orographic lifting results in enhanced snowfall upslope of the observed valley network tributaries. Our framework also suggests that a 2 bar atmosphere cannot create the observed valley pattern at the highest-relief valley network, Warrego Valles. As in previous work, the GCM does not generate temperatures warm enough for rain or significant snowmelt in the highlands with CO₂ greenhouse warming alone. Thus while transient periods of unusual warming are still required to melt the deposits and carve the valleys, our model predicts snow deposition in the correct locations. Key Points A model is used to predict the local distribution of snowfall on early Mars Melting these snowpacks would have created the observed distribution of valleys A 2-bar model atmosphere cannot reproduce the distribution of Warrego Valles.