Mars at very low obliquity: Atmospheric collapse and the fate of volatiles

The obliquity of the Martian spin axis is known to undergo quasiperiodic oscillations superposed over chaotic long-term variations. It is probable that within past history there were geologically long periods when the obliquity oscillated around low (10-15°) mean values. During such low obliquity epochs the climate system is controlled by deposition of permanent solid CO₂ deposits in the polar regions. With a simple season-resolved energy balance model, we show that as the atmosphere collapses, surface topography plays a major role in CO₂ condensation and sublimation processes, defining distribution and dynamics of CO₂ deposits. Thick CO₂ deposits are formed at steep pole-facing topographic slopes at moderately high latitudes, not at the poles. The total mass of the deposits is not a function of obliquity, but strongly depends on the pre-history of the climate system. We outline criteria to identify such low-obliquity epochs in Mars history.