The robotic landers scooped up Martian soil and conducted experiments to look for evidence of organic material, photosynthesis and respiration. The experiments at first seemed to show evidence of biological activity in the soil, but most scientists eventually concluded it was only a chemical reaction.

The orbiters photographed most of Mars' surface and provided Martian weather reports. The pictures and data revealed a below-freezing, dry, barren planet battered by dust storms with gusts exceeding 60 m.p.h. (100 kilometers per hour).

There were pictures of landslides, large impact craters, the 3,000-mile- (4,800-kilometer-) long network of canyons named Valles Marineris, and volcanoes including Olympus Mons -- the solar system's tallest volcano towering 85,000 feet (25,000 meters) above surrounding terrain.

The Viking missions detected evidence of water ice in the northern polar cap and large areas of permafrost. The orbiters detected trace amounts of water vapor in the Martian atmosphere.

The orbiters provided more detailed looks at large outflow channels first discovered by the Mariner 9 orbiter during 1971-1972. Scientists concluded that even the smallest channels photographed by Viking orbiters were formed when underground water once flowed to the surface.

Viking data revealed that Mars' thin atmosphere is mostly carbon dioxide and could not support human life. The atmosphere was also found to have low levels of nitrogen, argon, oxygen and several other gases.

Another discovery made by Viking was the notorious "Face on Mars" that some contend was built by an alien civilization, but which scientists generally insist is merely due to the way shadow and light fell on a rock formation.