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

Kathleen E Scanlon,François Forget,Jean‐Baptiste Madeleine,James W Head,Robin D Wordsworth

doi:10.1002/grl.50687

Abstract

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.

Highlights

[1] We examine the Martian valley networks in the framework of topographic influences on precipitation
It should have been even more important for early Mars if the late Noachian–early Hesperian atmospheric pressure was at least a few hundred millibars: more water can precipitate from a warmer atmosphere, and the altitude dependence of the surface temperature is greater in a thicker atmosphere due to increased thermal coupling between the atmosphere and surface, such that snow and ice deposits are more stable at high altitudes [Forget et al, 2013; Wordsworth et al, 2013]
Orbital Laser Altimeter (MOLA) topography data and climate fields from the Laboratoire de Météorologie Dynamique (LMD) generic model adapted to early Mars [Wordsworth et al, 2011; Forget et al, 2013; Wordsworth et al, 2013] as input to this model

Summary

Introduction

[2] Because liquid water is unstable to freezing and sublimation under current Martian surface conditions, the valley networks incising some of the oldest landscapes on Mars are often cited as evidence that the Martian climate was once warm and wet. Whether they require that ancient Mars experienced high surface pressure or temperatures warm enough for rainfall has been debated, [e.g., Wallace and Sagan, 1979; Carr, 1983; Squyres and Kasting, 1994; Craddock and Howard, 2002; Gaidos and Marion, 2003]. Orbital Laser Altimeter (MOLA) topography data and climate fields from the Laboratoire de Météorologie Dynamique (LMD) generic model adapted to early Mars [Wordsworth et al, 2011; Forget et al, 2013; Wordsworth et al, 2013] as input to this model

Methods

Results

Discussion

Conclusions