Where is the largest known canyon?

It is on Mars of course. There is no running water to wear away the rock landscape on Mars, so huge valleys can survive for millions of years. A system of enormous canyons called the Valles Marineris (Mariner Valleys) was discovered by the space probe Mariner 9. The canyon is more than 4000 km long and 4 times as deep as the Grand Canyon in USA. Scientists think that these valleys were caused by erosion (wearing away). During storms, the gritty martian dust is blasted by winds reaching up to 450 km/h, wearing away softer rock.