The edge of the ice

A piece of Mars: This is the edge of the northern polar cap on Mars. At the top is the bright icy surface, which is abruptly cut by a cliff. The wall of the cliff shows many layers of different materials — the darker ones are old dunes. How cool is it to know that the polar ice cap on another planet is sitting on what used to be an enormous sand sea? It’s like looking at limestone and shale on mountains here on Earth an imagining them once being at the bottom of an ocean. It blows my mind. (HiRISE TRA_000863_2640)