What on Mars?

A piece of Mars: What on Mars is this (the scene is 600×450 m, or 0.37×0.28 mi)? It can be hard to tell. The lines are ridges of windblown dunes or ripples, the dark gray stuff is active sand blowing between the dunes, and the underlying bedrock is pale tan. But if your eyes can’t make sense of it all, just sit back and enjoy the pretty patterns of Mars. (HiRISE ESP_037161_1785, NASA/JPL/Univ. of Arizona)