There's another theory about the formation of the Valles Marineris:
In summary, the following elements fit into a complete picture consistent with the hypothesis that Valles Marineris on Mars is a canyon formed by the grazing impact of a population of former moonlets of Mars:
*-There has been an excess of objects striking the Martian surface at angles of less than 15 degrees. Such an excess implies a former population of orbiting objects.
*-VM lies near the Martian equator.
*-VM is oriented parallel to the equator.
*-VM was formed from west to east; that is, in a direction prograde with respect to the planet's rotation.
*-The highest surface features in the equatorial region of Mars are the Tharsis volcanoes to the immediate west of VM.
*-Multiple parallel troughs suggest multiple moonlet impacts, or multiple fragments from one moonlet.
*-The sculpted or "wrinkle ridge" appearance inside the canyons is consistent with the roll of an irregular-shaped asteroid.
*-Phobos will follow precisely this scenario when it decays onto Mars in 40 million years.
I wonder what that event could have done to the former atmosphere of Mars, and its water.
To give you an idea: the Chicxulub crater was formed by an object of 10 km (6 miles), similar to the size of Phobos and Deimos. Now imagine a series of similar impacts.
Edited by Abramelin, 23 January 2013 - 09:44 AM.