The layers are from the rock underneath the ice, part of a huge plain over the north pole called Planum Boreum (literally “northern plain”). The rock is covered in a thick sheet of permanently frozen water ice on average a couple of kilometers thick. In the winter carbon dioxide freezes out of the air on Mars and forms a dry ice deposit a meter or so thick on top. This hides the troughs in images, but ones taken in northern summer show them clearly.
Katabatic winds blow through the ice cap: Cold, denser air at higher elevations that flows down to lower elevations. The cap is higher in the center than at the edges farther south, so the wind flows south.
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