Overhead view of the central region of the giant Valles Marineris canyon system on Mars, computer- processed from Viking spacecraft imagery & topographic maps of the planet. The Valles Marineris is over 3000 km long & up to 8 km deep, dwarfing the Grand Canyon of Arizona. The canyons were formed by a combination of geological faulting, landslides, & erosion by wind & water. The central area shown here contains layered deposits which may have been deposited by ancient lakes. North is at top. (Image processing by Alfred McEwen & topographic mapping by Sherman Wu, both at USGS, Flagstaff, Arizona).