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At today’s presentations at the 50th Lunar and Planetary Science Conference in Texas, scientists showed images and data [pdf] suggesting that many of the Martian gullies found on cliff faces are formed when the dust layer protecting underlying snow gets blown away and the exposed snow/ice then melts.
The image on the left was taken by the high resolution camera of Mars Reconnaissance Orbiter (MRO) in 2009, and has been cropped to post here. The white streaks are what they suggest is exposed ice/snow.
From their paper [pdf]
We interpret these observations to indicate that the light-toned materials are exposures of subsurface, water-ice mantle materials that are very similar to … [materials] being exhumed due to slumping caused by sublimation or seasonal frost processes. … The bright white/yellow appearance of these materials in [MRO] near-infrared imagery is consistent with the presence of dust (relatively clean water ice typically appears relatively blue) that partially covers, or is mixed in with ice deposited as snow, within the pasted-on materials.
A dust content of 1,000 parts per million by mass (p.p.m.m.) is sufficient for melting to occur for a wide range of snow properties and atmospheric pressures, and can occur under current conditions in the mid-latitudes. Modeling suggests that a 5-cm dust lag layer is enough to ‘protect’ the underlying snow from the Sun, and that for melting to occur, rapid exposure of the preserved snowpack is necessary.
Their presentation also included images of this same location, taken across three Martian years and showing the white streaks appearing and then disappearing.
The formation process of these gullies remains one of the primary geological mysteries that Mars presents to us. Is water involved? Ice? Brine? Does this material come from below, leaking out? Or is it placed on the surface as frost?
All of these have been suggested as an explanation. All have problems that make them poor answers.
What these scientists are suggesting is that, rather than water or brine leaking out from below ground or frost being laid down on the surface, the gullies form when subsurface ice/snow is exposed to the Sun and melts. They note that “Subsurface ice can be permanently stable at latitudes poleward of 25° due to slope effects, especially on pole-facing slopes.”
In other words, in latitudes higher than 25 degrees latitude, there could be a lot of snow and ice just below the surface on these slopes, covered by dust so that it doesn’t melt. With time small areas get exposed, melt away, and the subsequent disturbance causes small slides that produce the gullies. This conclusion seems reasonable, considering the number of other studies that suggest the presence of a ice table on Mars, below the surface. Moreover, this cliff is part of a major canyon draining into the eastern side of Hellas Basin, the basement of Mars, where there is ample other evidence of the presence of historic water and modern ice.
As always, I think like a colonist on Mars. These cliffs now appear to me as prime real estate, places to look for easily accessible water on Mars. That they are near Hellas Basin increases their value, as the low elevation of this basin means it will have a thicker atmosphere which will ease colonization. If we can find some caves in this region, we might then have all the ingredients for building that first Martian colony.