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

Abstract

A massive CO₂ ice deposit overlies part of Mars’s primarily H₂O ice south polar cap. This deposit rivals the mass of Mars’s current, 96% CO₂, atmosphere. Its release could substantially alter Mars’s pressure and climate. 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 enigmatic 1–10 m covering of perennial surface CO₂ ice called the residual south polar cap. Typical explanations of the layering invoke orbital cycles. Up to now, models assumed that the H₂O ice layers insulate and seal in the CO₂, allowing it to survive high-obliquity periods. 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 albedo and emissivity 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.