Active cratering and near-surface water ice on Mars Answered
The Mars Reconnaissance Orbiter has taken some high-resolution pictures of brand new (two months old!) craters, where the impact exposed a layer of water ice just below the surface. Repeated pictures taken since the impact show the ice slowly sublimating, leaving behind the dark regolith.
Read the whole press release and see some of the pictures here: http://www.nasa.gov/mission_pages/MRO/news/mro-20090924r.html