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A candidate explanation for Early Mars rivers is atmospheric warming due tosurface release of H$_2$ or CH$_4$ gas. However, it remains unknown how muchgas could be released in a single event. We model the CH$_4$ release by onemechanism for rapid release of CH$_4$ from clathrate. By modeling howCH$_4$-clathrate release is affected by changes in Mars' obliquity andatmospheric composition, we find that a large fraction of total outgassing fromCH$_4$ clathrate occurs following Mars' first prolonged atmospheric collapse.This atmosphere-collapse-initiated CH$_4$-release mechanism has three stages.(1) Rapid collapse of Early Mars' carbon dioxide atmosphere initiates a slowershift of water ice from high ground to the poles. (2) Upon subsequentCO$_2$-atmosphere re-inflation and CO$_2$-greenhouse warming, low-latitudeclathrate decomposes and releases methane gas. (3) Methane can then perturbatmospheric chemistry and surface temperature, until photochemical processesdestroy the methane. Within our model, we find that under some circumstances aTitan-like haze layer would be expected to form, consistent with transientdeposition of abundant complex abiotic organic matter on the Early Marssurface. We also find that this CH$_4$-release mechanism can warm Early Mars,but special circumstances are required in order to uncork 10$^{17}$ kg ofCH$_4$, the minimum needed for strong warming. Specifically, strong warmingonly occurs when the fraction of the hydrate stability zone that is initiallyoccupied by clathrate exceeds 10%, and when Mars' first prolonged atmosphericcollapse occurs for atmospheric pressure > 1 bar.

Methane release on Early Mars by atmospheric collapse and atmospheric reinflation