Rainfall, snowfall, and fluvial activity on early Mars

Abstract:

Widespread valley networks (VNs) and lakes suggest the presence of surface liquid water on early Mars. However, whether this water was sourced from rainfall and runoff in a “warm and wet” climate or snow accumulation and melting in a “cold and icy” climate is a topic of ongoing discussion, and determining which is these mechanisms is more consistent with the formation of these features would shed critical light on the nature of the early climate.

We present results from our recent and ongoing works in which we implement a 3D Global Climate Model (GCM) to answer the questions: What is the nature of rainfall in a canonical “warm and wet” climate (global mean annual temperature, GMAT, ~273 K)? Is the distribution of VNs/lakes more consistent with the model-predicted distribution of fluvial activity in a “warm and wet” or “cold and icy” climate? And, finally, if rainfall was responsible for VN/lake formation, not snowmelt, what climate characteristics (e.g. GMAT), are required for widespread rainfall to occur in all areas where VNs/lakes are distributed?

We assume present-day topography [e.g. 1] and a relevant Noachian surface water inventory of ~34 m GEL [e.g. 2] and show that:

• In a canonical “warm and wet” climate (GMAT ~273 K), rainfall is negligible and precipitation and fluvial activity would be dominated by snowfall and melting [e.g. 3].
• The distribution of VNs/lakes is more consistent with the distribution of model-predicted fluvial activity in a “cold and icy” climate than in a “warm and wet” climate.
• GMAT >285 K is required for widespread rainfall to occur [e.g. 4].

In summary, we have shown that either (1) snow accumulation and melting was responsible for VN/lake formation or (2) GMAT was >285 K and widespread rainfall was responsible for VN/lake formation.

[1] Smith et al. (1999), Science 284, p1495.

[2] Carr and Head (2015), GRL 42, 2014GL062464.

[3] Palumbo and Head (2018), GRL 45, 2018GL079767.

[4] Palumbo and Head, In review at Icarus Note.