The formation and stability of buried polar CO2 deposits on Mars

Shallow Radar soundings of the south polar layered deposits (SPLD) have revealed buried CO2 ice with as many as three distinct layers separated by thinner layers of water ice (Bierson et al., 2016). This layering is suggestive of formation by cyclical processes such as obliquity, eccentricity and advance of perihelion. If obliquity is the main driver, then there were many opportunities for CO2 deposits to have formed within the last 3.5-4 Myr. To persist, however, CO2 deposits must be followed by the deposition of an insulating layer of porous water ice that can seal in the CO2 to survive the obliquity maxima that follow. We suggest that the existing deposits were formed within the last 350 kyr. Each obliquity minimum was quickly followed by a period in which perihelion occurred in the northern summer (southern winter) solstice. Under these conditions, an enhanced deposition of low porosity, fine-grained water ice could occur on the winter pole. A similar series of depositions and burials by water ice could have occurred between 2.75 and 2.2 million years ago, but these deposits were unlikely to have survived the many high obliquity swings that occurred between 2.2 Ma and 400 ka. The formation and burial of CO2 ice could also have occurred in similar fashion earlier during the last  ∼ 100 Myr in which the southern polar layered deposits were formed. Although at current times the geothermal flux may be too low to cause basal melting of CO2 deposits, in earlier times, deposits formed in deeper scarps and valleys could have led to basal CO2 melting and sequestration into the regolith. By extension, basal melting and sequestration in the regolith may provide an explanation for the fate of the thick CO2 atmosphere (pCO2 ≳ 0.5 bar) implied by climate models of early Mars.