A piece of Mars: What happens to dunes as they move over rough terrain? This is what a barchan looks like on a relatively flat surface. If the hills are smaller than the dune, then it does its best to pretend they don’t exist, like the one in this image. It’s 175m (574ft) wide and 190m (623ft) long, with a slipface indicating overall migration to the northeast. (HiRISE ESP_039524_1445, NASA/JPL/Univ. of Arizona)