If this has already been discussed or known about then I apologise for getting excited, but I was trawling through my book (Exploring Mars) and I thought of something intresting.
I was wondering how the Valles Marineris could have formed, I was thinking the usual tectonic movment or erosion etc.
Then it dawned on me, what if a object say the size of Pluto or smaller (a ice planet or a planet with a ice crust) smashed into Mars many millions of years ago, and created the massive scar we see today.
My theory is that the "object" was at such an angle that it didnt hit Mars directly, but "dragged" its way across the surface as it did, gashing out the Canyon, the water and other liquids were vapourised into the atmosphere, and the core of the object broke up.
You can see the stress the collision made where the Volcanos are to the Canyons Left, the huge uplift created massive pressure on Mars crust and rocks were liquified by the intense heat thus creating Olympus Mons etc.
When the object finally broke up it was "bounced" across half of Mars before finally crashing down within the huge crater-field we see to the SE of Valles Marineris.
As for the ice and liquids, this "rained" down onto Mars, carving out the soft Mars rock and gullies we see today.
Anyway it was just a thought I had today, I was wondering if anyone else had thought of this and do you think theres enougth evidence to back my theory, or is it all just a bit inconclusive.