A Massive CO₂ Ice Deposit (MCID) that rivals the mass of Mars’ current, 96% CO₂ atmosphere was recently discovered to overlie part of Mars’ southern H₂O cap [1]. The MCID is layered: a top layer of 1-10 m of CO₂, the Residual South Polar Cap (RSPC) [2], is underlain by ~10-20 m of H₂O ice, followed by up to three 100s-meter-thick layers of CO2 ice, separated by two layers of ~20-40 m of H₂O ice [3] (Fig. 1). Previous studies invoked orbital cycles to explain the layering, assuming the H₂O ice insulates and seals in the CO₂, allowing it to survive periods of high obliquity [3,4]. We also model that orbital cycles [5] drive the MCID’s development, but instead assume the MCID is in continuous vapor contact with the atmosphere rather than sealed. Pervasive meter-scale polygonal patterning and km-scale collapse pits observed on the sub-RSPC H₂O layer [1,3] are consistent with it being fractured and permeable to CO₂ mass flux. Using currently observed optical properties of martian polar CO₂ ice deposits [6], our model demonstrates that the present MCID is a remnant of larger CO₂ ice deposits laid down during epochs of decreasing obliquity that are eroded, liberating a residual lag layer of H₂O ice, when obliquity increases. With these assumptions, our energy balance model ex-plains why only the south polar cap hosts an MCID, why the RSPC exists, and the observed MCID stratigraphy. We use our model to calculate Mars’ pressure history and the age of the MCID
