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Finally, in 1965, the triumphant mission of the U.S. spacecraft Mariner 4 brought some reality to musings about Mars. The craft flew past the planet at a distance of only 6,100 miles, transmitting 22 television pictures of a bleak, moonlike landscape, pockmarked by craters and showing no signs of life. Even so, hope persisted. To demonstrate that a Mariner flyby at a distance of thousands of miles might completely overlook a thriving civilization, a young and still unknown Carl Sagan that same year sifted through a thousand pictures of earth shot by a weather satellite orbiting only 300 miles up. In a paper entitled "Is There Life on Earth?" he reported that only one photograph, of a snow-covered superhighway cutting a straight line through a forest, showed any evidence of man's presence on this planet.
When Mariner 9 was successfully inserted into low orbit around Mars in 1971, a planet-wide dust storm obscured its vision for six weeks. After the dust settled, Mariner's cameras revealed a fascinating landscape: towering volcanoes, great canyons, lava flows and a multitude of craters in the red- hued plains. What excited scientists and Mars buffs the most, however, was the unmistakable traces of dry riverbeds and deltas etched into the rock, evidence that water had once flowed freely on the Martian surface. Had life evolved on Mars while water was still ample? And might living organisms still exist there, perhaps in microscopic form?
It was in part to answer such questions that the U.S. Viking 1 and 2 spacecraft, each consisting of an orbiter and a lander, were dispatched to Mars. When they arrived, 45 days apart, in 1976, cameras aboard the orbiters snapped away and remote-sensing devices searched for water vapor in the thin atmosphere and sought out frozen water in the polar ice caps. On the surface, the landers began providing the most accurate measurements yet of Martian surface temperatures, atmospheric density and wind velocity, while the cameras shot more than 4,500 spectacular close-up pictures of the surrounding, rock- strewn landscape. Each lander was also equipped with an arm that scooped up soil samples and fed them to a little onboard biological laboratory, where they were analyzed for any signs of metabolic activity, which would signify life.
; The first soil sample briefly breathed new life into the Mars mystique. After being moistened inside the lab, it suddenly released an unexpectedly high burst of oxygen, setting off a flurry of speculation among scientists on earth. Did the oxygen come from some tiny form of Martian life in the soil? After further tests failed to confirm those first results, scientists reluctantly concluded that the large amount of oxygen had probably been produced by a simple chemical reaction between water vapor and some unidentified oxygen-rich compound in the soil sample.
Some scientists, Soviet and American alike, have still not abandoned hope of finding life or its remnants deeper in the Martian soil. There life forms might have access to water and be protected from the fierce solar ultraviolet radiation that rains down on the surface, virtually unobstructed by the Martian atmosphere. Sedimentary rocks in the ancient riverbeds would be an ideal place to hunt for fossils of organisms that may have lived when Mars was more benign, with a thicker atmosphere, warmer climate and running water on its surface.