#67p

The spacecraft saw ice on 67P’s surface, but almost none of it was visible on the part of the comet illuminated by the sun, it was almost always on the night side. When the comet rotated around, the same areas would have ice on them night after night, but they’d be ice free during the day. This indicates that there is an ice cycle to the comet. During the day, the surface of the comet is heated and any exposed ice sublimates away. As the comet rotates around to night, the heat pulse from the day diffuses its way into the ground, causing ice at depth to sublimate. That H2O then reaches the nearest cold trap – the comet’s surface, facing out to space, where it freezes again. The ice then sits there until it reaches the day side, where it sublimates away again. Although there was not much ice visible on the surface at any given time, this cycle replenished each area so that it could gradually lose water vapor to space on its pass around the sun. -JBB Image credit: http://www.esa.int/Science_Exploration/Space_Science/Rosetta/Rosetta_reveals_comet_s_water-ice_cycle%20