A full Mars. In this image the massive volcano Olympus Mons is clearly visible upper left of center, while at center are the three Tharsis Montes volcanoes. All these volcanoes are believed to be extinct. To the right of the Tharsis Montes are the deep canyons of Valles Marineris, a meandering network of channels as long as the United States is wide. Mars has been known historically as a bright, reddish star in Earth's sky. The ruddy hue is due to the presence of iron oxide (i.e., rust) on the martian surface. Mars is often host to white clouds of water-ice crystals (like the high-altitude cirrus clouds on Earth) and experiences seasonal dust storms that can cover the entire planet.