Justin Kelly wrote:
Personally (probably dude to everyone comparing it to the grand canyon) I have always though Valis Marinaris would have been carved out by water, do we know how? Aside form erosion, is there anything else that could have formed it?

Alex writes:
While there are certainly many competing models positing causes for the formation of the Valles Marineris (VM), the most commonly accepted model involves comparing the features not to the Grand Canyon on Earth, which as you note was primarily carved by running water, but rather to the Great Rift Valley in eastern Africa. VM is interpreted to represent a series of interconnecting canyons (chasma) that were formed by crustal tension cracks/fractures that were progressively widened by tectonic forces and erosive processes.

see http://cmex-www.arc.nasa.gov/VOViews/CANYONS.HTM

There is fairly compelling evidence that fluvial processes (plausibly involving liquid water) and aeolian processes, in addition to mass wasting, have enlarged the chasma over Mars' geologic epochs. Futher, there is suggestive evidence (e.g., interior layered deposits [ILD]) that the VM may have hosted standing bodies of water at points in the past, which would offer promising targets for future lander missions. In fact, some of my colleagues have argued for the ILD's in Melas Chasma as a target for one of the rovers on the 2003 MER mission.

Alex R. Blackwell
University of Hawaii