Viking orbiter mosaic image of the surface of Mars, showing the giant extinct volcano Olympus Mons, shrouded in cloud. The volcano is the largest in the Solar System, rising 26,400 metres above Mars' average elevation. The top of the cloud blanket, which is thought to be composed of water-ice, lies some 8 km below the volcano's summit. A well-developed region of wave cloud, several hundred kilometres in length, is visible at the upper, left-hand edge of the image (red & white ripples).