Scientists think the Valles Marineris formed mostly by rifting of the crust as it got stretched . Individual canyons within the system are as much as 60 miles ( 100 km ) wide . The canyons merge in the central part of the Valles Marineris in a region as much as 370 miles ( 600 km ) wide . Large channels emerging from the ends of some canyons and layered sediments within suggest that the canyons might once have been filled with liquid water .