A massive CO₂ ice deposit overlies¹ part of Mars’s primarily H₂O ice^2,3,4 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 enigmatic^9,10,11 1–10 m covering of perennial surface CO₂ ice¹² called the residual south polar cap. Typical explanations of the layering invoke orbital cycles^1,7. Up to now, models assumed that the H₂O ice layers insulate and seal in the CO₂, 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 CO₂ can instead exchange with the atmosphere through permeable H₂O ice layers. Using currently observed albedo^14,15 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.

一个大规模的CO ₂冰存款覆盖^1个火星的主要H的部分₂ ö冰^2,3,4-南极帽⁵。该沉积物可与火星目前的96％CO ₂气氛^6相比。它的释放可能会大大改变火星的压力和气候¹。该沉积物由交替的CO ₂和H ₂ O冰层组成，深度可达约1 km（参考文献^1,7,8）。顶层是一个神秘的^9,10,11 1–10 m覆盖多年生表面CO ₂冰¹²称为残留南极帽。分层的典型解释调用轨道周期^1,7。到目前为止，模型假设H ₂ O冰层在CO _2中绝缘并密封，从而使其能够在高倾角时期生存^7,13。但是，这些模型不能定量地预测该矿床的地层学，也不能解释残留的南极帽的存在。在这里，我们提出一个模型，其中矿床的近地CO ₂可以通过可渗透的H ₂ O冰层与大气交换。使用当前观察到的火星极地CO _2的反照率^14,15和发射率¹⁶特性冰沉积，我们的模型预测，目前的大量CO ₂冰沉积是倾斜减少的时期被消融时沉积的较大的CO ₂冰沉积的残余，当倾斜增加时，会释放H ₂ O冰的残留滞后层。以前的CO ₂沉积物的馏分保留为层，因为在过去约510年间，最大倾角的振幅大部分已减小（参考文献¹⁷）。我们的模型同时解释了观测到的大量CO ₂冰层地层，残余的南极帽的存在以及大量CO ₂的存在。冰只沉积在南部。我们使用我们的模型来计算火星的压力历史记录，并确定大量的CO ₂冰沉积是510年前的。