Image: Gullies of Matara Crater
Gullies on Martian sand dunes, like these in Matara Crater, have been very active, with many flows in the last 10 years. The flows typically occur when seasonal frost is present.
In this image from NASA's Mars Reconnaissance Orbiter we see frost in and around two gullies, which have both been active before. (View this observation to see what these gullies looked like in 2010.) There are no fresh flows so far this year, but HiRISE will keep watching.
The map is projected here at a scale of 50 centimeters (19.7 inches) per pixel. [The original image scale is 50.3 centimeters (19.8 inches) per pixel (with 2 x 2 binning); objects on the order of 151 centimeters (59.4 inches) across are resolved.] North is up.