A piece of Mars: On the left is high ground, covered with dunes (or maybe they’re ripples) running from upper left to lower right. On the right is low ground, covered in deeply eroded dunes (ripples?) running almost from left to right. They were probably created at two different times by winds that changed direction in the intervening time. The set on the right is probably much older. (HiRISE ESP_041991_1715, NASA/JPL/Univ. of Arizona)