Computer enhanced Viking orbiter 1 mosaic along the Tharsis Ridge, the major volcanic province on Mars. Three volcanoes are aligned along the ridge; from bottom, the most southerly, is Arsia Mons, behind is Pavonis Mons & in the distance Ascraeus Mons. All three rise about 17 kilometres (10.8 miles) above the 10 kilometre (6.2 mile) high ridge. Biblis Patera & Ulysses Patera are the small volcanoes on the left. At right is the western extent of a region of dissecting faults called Hoctis Labyrinthus. Thick cloud is present in the upper portion of the image. The photograph covers an area of about 5 million square kilometres (1,240 miles).