re: Valles Marineris - a gigantic dehydration crack?
Interesting thought, but I still think the valley formed from tectonic activity. If you want to see some dehydration, check out this picture. In particular, the crater looks as thou it dried out, causing the ground to shrink. This is in the Northern region of Mars where a shoreline might have existed when Mars had an ancient ocean.
Here are more ‘mud’ cracks in an area that might have been a sea bed of the hypothesized Northern Ocean. Same is true of this picture.