Methane release on Early Mars by atmospheric collapse and atmospheric reinflation

Abstract

A candidate explanation for Early Mars rivers is atmospheric warming due to surface release of H₂ or CH₄ gas. However, it remains unknown how much gas could be released in a single event. We model the CH₄ release by one mechanism for rapid release of CH₄ from clathrate. By modeling how CH₄-clathrate release is affected by changes in Mars' obliquity and atmospheric composition, we find that a large fraction of total outgassing from CH₄ clathrate occurs following Mars' first prolonged atmospheric collapse. This atmosphere-collapse-initiated CH₄-release mechanism has three stages. (1) Rapid collapse of Early Mars’ carbon dioxide atmosphere initiates a slower shift of water ice from high ground to the poles. (2) Upon subsequent CO₂-atmosphere re-inflation and CO₂-greenhouse warming, low-latitude clathrate decomposes and releases methane gas. (3) Methane can then perturb atmospheric chemistry and surface temperature, until photochemical processes destroy the methane. Within our model, we find that under some circumstances a Titan-like haze layer would be expected to form, consistent with transient deposition of abundant complex abiotic organic matter on the Early Mars surface. We also find that this CH₄-release mechanism can warm Early Mars, but special circumstances are required in order to uncork 10¹⁷ kg of CH₄, the minimum needed for strong warming. Specifically, strong warming only occurs when the fraction of the hydrate stability zone that is initially occupied by clathrate exceeds 10%, and when Mars’ first prolonged atmospheric collapse occurs for atmospheric pressure >1 bar.