The images revealed river beds , craters , massive extinct volcanoes ( such as Olympus Mons , the largest known volcano in the Solar System ; Mariner 9 led directly to its reclassification from Nix Olympica ) , canyons ( including the Valles Marineris , a system of canyons over about long ) , evidence of wind and water erosion and deposition , weather fronts , fogs , and more .