Top Left. CTX mosaic context for top right panel (black rectangle) and panels in Extended Data Fig. 2 (blue dots labelled A–H). Top Right. Regions of dark H2O layer are exposed by LS 297 (HiRISE image ESP_013775_0931). The regions correspond to kilometre-scale depressions in the H2O layer1. Bottom panels show time progression of modelled thermal profile for a 20 m thick H2O layer overlying a semi-infinite CO2 layer that becomes exposed at LS 301. The basal temperature of the H2O reaches the CO2 sublimation temperature between LS 335 and LS 183 the following year. LS 300: last day of seasonal CO2 coverage. LS 301: first LS bare H2O is exposed. LS 316: maximum surface temperature reached. LS 335: basal temperature of H2O layer reaches CO2 sublimation temperature. LS 355: surface CO2 condenses. LS 90: winter solstice; thermal wave decaying. LS 183: last LS basal temperature of H2O layer is at CO2 sublimation temperature. LS 270: summer solstice; basal temperature of H2O layer ~1 K below CO2 sublimation temperature.