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The daily water ice cycle as determined at Comet 67P/Churyumov–Gerasimenko. During the local day, water ice on and a few centimetres below the surface sublimates and escapes; during the local night, the surface rapidly cools while the underlying layers are still warm, so subsurface water ice continues sublimating and finding its way to the surface, where it freezes again. On the next comet day, sublimation starts again, beginning from water ice in the newly formed surface layer.
Full story: Rosetta reveals comet’s water-ice cycle