Dark spots and cold jets in the polar regions of Mars: New clues from a thermal model of surface CO₂ ice

Observations of the martian CO₂ ice cap in late winter and spring have revealed exotic phenomena. Unusual dark spots, fans and blotches form as the south-polar seasonal CO₂ ice cap retreats. The formation mechanisms of these features are not clearly understood. Theoretical models suggest that photons could penetrate deep into the CO₂ ice down to the regolith, leading to basal sublimation and gas and dust ejection. We have developed a detailed thermal model able to simulate the temporal evolution of the regolith-CO₂ ice layer-atmosphere column. It takes into account heat conduction, radiative transfer within the ice and the atmosphere, and latent heat exchange when there is a phase transition. We found that a specific algorithm, fully coupling these three components, was needed to properly predict ice sublimation below the surface. Our model allows us to determine under what conditions basal sublimation is possible and thus when and where it can occur on Mars. Our results show that basal sublimation is possible if we consider large pathlengths and very little dust content within the ice. Moreover, the model can explain how dark spots can appear very early after the end of the polar night at high latitudes. We also evaluate the importance of the different parameters in our simulations. Contrary to what was suggested by theoretical models, the role of seasonal thermal waves is found to be limited. Solar radiation alone can initiate basal sublimation, which therefore only depends on the CO₂ ice properties. Three main modes were identified: one where condensation/sublimation only occurs at the surface (in the case of small grains and/or high dust content), one where basal sublimation is possible (large pathlengths and very little dust content) and an intermediate mode where sublimation within the ice may occur. We suggest that these different modes could be keys to understanding many processes that occur at the surface of Mars, like the anticryptic area behavior or the recent reported activity in gullies.