Gullies are found on many slopes in the middle and near-polar latitudes of Mars. Although they contain no liquid water today, whether and how much water may have been involved in their formation, and any water's origin, is a matter of considerable debate.
The gullies appear relatively young, so a current process, perhaps involving ground ice or condensed volatiles--water or carbon dioxide--may be at work. Regardless, HiRISE images provide insight on gully origins and processes.
In this image, we see gullies within two craters in Terra Sirenum. One closeup shows gully alcoves touching a crater wall. Dark sand fills the alcoves and scattered boulders are apparent. Some bright rocks on the gully walls are visible, perhaps exposed by the processes that carved the gullies.
Another closeup shows gullies that are much larger than those to the north. Intriguing yellowish colors are visible on some of the walls. Determining whether this represents chemical alteration of the walls by water, or exposed, unaltered bright rock, will require further study.
Photo credit: NASA/JPL/University of Arizona