Springtime for sand dunes and polar ice

A Piece of Mars: This 480×270 m (0.3×0.17 mi) scene shows a dark dune peeking out from under its cover of winter frost. In this picture it’s late spring, but still cold up at this latitude – it’s as far north here as the Greenland town of Qaanaaq (pop. 656 as of 2013). The CO2 frost here lingers on shady slopes until summer, preventing the dunes from migrating until it’s gone. (HiRISE ESP_043799_2570 NASA/JPL/Univ. of Arizona)