Coevolution of Mars’s atmosphere and massive south polar CO 2 ice deposit

A massive CO 2 ice deposit overlies 1 part of Mars’s primarily H 2 O ice 2 – 4 south polar cap 5 . This deposit rivals the mass of Mars’s current, 96% CO 2 , atmosphere 6 . Its release could substantially alter Mars’s pressure and climate 1 . The deposit consists of alternating CO 2 and H 2 O ice layers to a depth of up to approximately 1 km (refs. 1 , 7 , 8 ). The top layer is an enigmatic 9 – 11 1–10 m covering of perennial surface CO 2 ice 12 called the residual south polar cap. Typical explanations of the layering invoke orbital cycles 1 , 7 . Up to now, models assumed that the H 2 O ice layers insulate and seal in the CO 2 , allowing it to survive high-obliquity periods 7 , 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 CO 2 can instead exchange with the atmosphere through permeable H 2 O ice layers. Using currently observed albedo 14 , 15 and emissivity 16 properties of the Martian polar CO 2 ice deposits, our model predicts that the present massive CO 2 ice deposit is a remnant of larger CO 2 ice deposits laid down during periods of decreasing obliquity that are ablated, liberating a residual lag layer of H 2 O ice, when obliquity increases. Fractions of previous CO 2 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 CO 2 ice deposit stratigraphy, the residual south polar cap’s existence and the presence of a massive CO 2 ice deposit only in the south. We use our model to calculate Mars’s pressure history and determine that the massive CO 2 ice deposit is 510 kyr old.