Like strokes from a giant brush, dark streaks decorate the wall of a trough in the Acheron Fossae region of Mars, which lies about 620 miles (1,000 kilometers) north of the huge volcano Olympus Mons. Released February 24, 2010, the picture is among the latest from NASA's Mars Reconnaissance Orbiter program.
Such streaks are thought to be evidence of one of the only geologic processes currently active on the red planet. Scientists think the streaks form when dry sand or fine-grained dust flows down a slope like an avalanche, exposing darker material underneath. (See a picture of an active avalanche on Mars.)