On the left you can see the surface of Mars: that’s frozen carbon dioxide — dry ice — covering the ground. The red brick-like pattern to the right of the ice is actually the face of a scarp, a steep cliff. We’re looking almost straight down on it, so it’s foreshortened, but don’t let that fool you; it’s 700 meters (2000 feet) high! [...]
But right there is the plume of a large avalanche, the cloud still rising above the floor! Clearly this was caught within seconds of the landslide hitting the floor of the scarp. The shadow of the plume is clear and obvious below and to the left.
Pretty and impressive.
But why this display? One possible explanation: "it’s spring in the northern hemisphere of Mars, and the warming temperatures are sublimating the dry ice". Which may be causing the slides, when the suddenly gaseous CO2 gets in the cracks of the rocks and dust, thus loosening debris into free-fall. The folks at HiRISE have been looking for this phenomenon and caught it into the act.