Space Missions to Mars Mariner 9 The first detailed topographic maps of Mars were produced with data from NASA's Mariner 9 orbiter mission of 1971-1972, in which channel and valley networks, volcanoes, canyons and craters were photographed at reasonable resolution for the first time. Mariner 9 was also the mission that returned pictures of layered (possibly sedimentary) terrain in Mars' polar regions. The channel and valley networks bear such striking similarities to river valleys and deltas on the Earth that they were assumed to demonstrate beyond doubt that at some time in its past history, Mars must have had significant quantities of running water coursing across its surface. The Vikings Five years after Mariner 9, in 1976, NASA's two Viking landers sent back many images of Mars' landscape, showing panoramic scenes of broken boulders distributed over flat dusty plains. Viking also measured the elemental composition of both Mars' atmosphere and surface soils. In combination with the Mariner data, the Viking results have allowed a picture of Mars as a rocky planet with a significant and complex geological history to be built up. The two hemispheres of Mars exhibit very different geological histories. Most of the northern hemisphere consists of almost flat, low-lying plains, showing little cratering. In contrast, the terrain of the southern hemisphere appears more ancient, with cratered highland regions cross-cut by canyons, channels and valley networks. But many questions about Mars remain unanswered, particularly about the fluvial and seismic history of the planet. Pathfinder The Pathfinder mission of 1997 landed on a rocky plain at the mouth of the Ares Vallis in Chryse Planitia (19.33°N, 33.55°W). During its month of operation, it recorded spectacular images of a rock-strewn plain, with tantalising glimpses of rounded pebbles and possible layered structures and hollows within some of the rocks. Chemical and image data for rocks and soil were acquired by the Sojourner rover, a mobile robotic probe. Inferences drawn from these data are that some of the rocks might be sedimentary, possibly even conglomerates, thus implying a significant fluvial history. Paler, almost cream-coloured patches in the soil might be areas leached by fluid action, hard-grounds or deposits left after evaporation. The disordered landscape of partly rounded pebbles and boulders has been interpreted as the type of landscape remaining after catastrophic flooding, further evidence of the stability of liquid water at some time in Mars' past. The Global Surveyor Recent data from the laser altimeter aboard the Mars Global Surveyor have been interpreted as evidence for a very large ocean across Mars' northern hemisphere. This spacecraft is currently in orbit around Mars, producing the most detailed mapping of the surface ever attempted.

Mariner 9 Viking Pathfinder Sojourner Global Surveyor