Bruce Murray Space Image LibraryViking Orbiter view of the northwestern Tharsis plateau Viking Orbiter view of the northwestern Tharsis plateau Viking Orbiter view of the northwestern Tharsis plateau, with the three Tharsis Montes at bottom left (from top to bottom: Ascraeus Mons, Pavonis Mons, and Arsia Mons) and Olympus Mons at top right. The mountains are shrouded in the aphelion cloud belt, a thin layer of water ice crystals that form over the Tharsis Plateau when Mars is furthest from the Sun. This image was taken near midday, when the clouds had yet to sublimate in the afternoon warmth. This image was taken on June 24, 1978, on Viking Orbiter 1's 735 revolution around Mars. NASA / JPL / USGS / Justin CowartOriginal image in PNG format here.