Recent impacts on Mars exposes subsurface ice

The Mars Reconnaissance Orbiter (MRO) took these images of a 6m crater on Mars in October 2008 (left) and again in January 2009; see this press release for more details. This crater is absent from images acquired in 2007, so it must be due to a relatively recent impact. Note also the bright material in the crater that fades over time. This is to be expected if this were subsurface ice that was suddenly exposed to the surface. Water ice is not stable at the surface of Mars, and will sublimate (vaporize) over time. The MRO spacecraft has discovered several new craters where fresh ice appears to fade over time. This particular crater has a latitude of 43 degrees, which indicates that subsurface ice on Mars extends all the way from from the poles to Mars' mid-latitudes.

Martian gullies at crater's edge

Gullies are often spotted in sloped terrain on Mars, like the ones seen here at the edge of Hale crater on Mars. This image was acquired by the Mars Reconnaissance Orbiter August 3, 2009; that spacecraft has been observing Mars since March 2006. Martian gullies are of great interest, since their dendritic appearance suggests that groundwater might be seeping out and flowing downhill. However, after many years of study, it is still unclear whether wet or dry processes are responsible for sculpting these gullies. Dry processes include boulders or avalanches that might carve out these gullies as rocks and gravel tumble downhill. Also keep in mind that the martian surface is too cold for liquid water to exist there. Nonetheless, any groundwater would absorb salts from the surrounding rock, which might lower its freezing point enough to exist in liquid form. And where there is liquid water, there is also the possibility for microbial life. For more details, see this press release.