NASA/JPL/University of Arizona
July 20, 2012 — One of the latest from the HiRISE camera aboard NASA's Mars Reconnaissance Orbiter, this image gives us a view into the deep, shadowed gullies gouged into the walls of a crater on Mars. The rugged surfaces of these inner slopes are coated with a thin layer of frost — most likely water ice that's condensed from what small amount of moisture happens to be in the air.
As winter approaches in Mars' southern hemisphere — where this unnamed 9-kilometer-wide (5.5-mile-wide) crater is located — any water in the atmosphere begins to condense as frost onto the planet's surface, starting with shadowy areas that are shielded from direct sunlight during much of the day like the northern inner slopes of craters.
The gullies seen here are anywhere from several to tens of meters across.
Not only are images like this important for scientists monitoring seasonal changes on Mars, they're also wonderful to look at, with dramatic lighting that highlights the harsh landscape of our neighboring world.