This HiRISE image shows gullies on a crater wall in the north polar region of Mars.
Although it was late summer when we acquired this image (2010), you can see frost within the gully alcoves. These alcoves are on the poleward facing crater wall and so spend much of the time in shadow. This allows the frost to survive.

The full-image shows that the opposite (south-facing) wall has similar gullies, but no frost during this season. Scientists are analyzing many images like this in order to try and answer the broader question of whether liquid water is responsible for the these gullies or not.

Image is less than 1 km (less than 1 mi) across and is 316 km (197 mi) above the surface. For full images including scale bars, visit the source link.

NASA/JPL/UArizona Larger image