Coevolution of Mars’s atmosphere and massive south polar CO2 ice deposit

Abstract

A massive CO2 ice deposit overlies1 part of Mars’s primarily H2O ice2–4 south polar cap5. This deposit rivals the mass of Mars’s current, 96% CO2, atmosphere6. Its release could substantially alter Mars’s pressure and climate1. The deposit consists of alternating CO2 and H2O ice layers to a depth of up to approximately 1 km (refs. 1,7,8). The top layer is an enigmatic9–11 1–10 m covering of perennial surface CO2 ice12 called the residual south polar cap. Typical explanations of the layering invoke orbital cycles1,7. Up to now, models assumed that the H2O ice layers insulate and seal in the CO2, allowing it to survive high-obliquity periods7,13. However, these models do not quantitatively predict the deposit’s stratigraphy or explain the residual south polar cap’s existence. Here we present a model in which the deposit’s near-surface CO2 can instead exchange with the atmosphere through permeable H2O ice layers. Using currently observed albedo14,15 and emissivity16 properties of the Martian polar CO2 ice deposits, our model predicts that the present massive CO2 ice deposit is a remnant of larger CO2 ice deposits laid down during periods of decreasing obliquity that are ablated, liberating a residual lag layer of H2O ice, when obliquity increases. Fractions of previous CO2 deposits remain as layers because the amplitudes of the obliquity maxima have been mostly decreasing during the past ~510 kyr (ref. 17). Our model simultaneously explains the observed massive CO2 ice deposit stratigraphy, the residual south polar cap’s existence and the presence of a massive CO2 ice deposit only in the south. We use our model to calculate Mars’s pressure history and determine that the massive CO2 ice deposit is 510 kyr old. The long-term evolution and stratigraphy of the CO2 ice residual southern polar cap of Mars can be explained by a model that includes the active coupling of near-surface CO2 with the atmosphere through the permeable H2O ice layers.