National Geographic : 1977 Jan
Sifting for Life inthe Sands of Mars By RICK GORE NATIONAL GEOGRAPHICSTAFF UDDENLY the smooth sands of Chryse were not what they had seemed. For al most a year the glittering Viking 1 space craft, followed closely by its identical twin, Viking 2, had been on the wing. Behind lay nearly half a billion miles of space. Ahead lay perhaps the most astounding de tective story in the history of science: the search for life on Mars. But as Viking 1 swung into orbit around that small ruddy planet last June, and its cameras began taking close-ups of its prime landing site on the Plain of Chryse, mission planners at the Jet Propulsion Laboratory in Pasadena, California, grew nervous. Mars was showing a far more rugged and bewilder ing face than had been imagined. It was not that they had expected a gentle planet. Four years earlier Mariner 9 had dra matically dispelled that notion, photograph ing towering volcanoes, some perhaps still active. One, Olympus Mons, is three times as high as Mount Everest (pages 28-9). Mariner had also revealed a gigantic canyon system, Valles Marineris, as much as four miles deep and 150 miles wide, and as long as the United States is wide.* Mariner 9 pictures showed channels that looked like old riverbeds. Some scientists still rejected the idea that water could have flowed on dust-blown Mars. Today the thin Mar tian atmosphere-a hundred times less dense than earth's-is too cold and the atmospheric pressure too low for water to exist for long, if ever, as a liquid. However, if in the distant past rivers had indeed flowed in those channels, mission planners reasoned, there should be a smooth basin of sediments near their mouths. Several of these "river" systems flowed onto Chryse Planitia. Finding a safe place to set their craft down was the Viking team's first priority. So a landing site had been chosen near where those channels broke onto Chryse. Surface Features Tell of Deluges But in late June Viking's close-ups revealed disquieting features not far from where Vi king 1 was scheduled to land, features that left little doubt that Mars had once seen water-incredible amounts of it. Those mys terious channels, Viking photos suggested, had been carved by raging, cataclysmic floods a billion or more years ago. Upland from Chryse lay a great chaotic region of deep cracks and collapsed terrain. Heating from deep within Mars, geologists speculate, had melted a huge mass of subsur face ice. Millions of tons of water had burst suddenly from the ground. Rivers that may have been 15 miles wide and hundreds of feet deep tumbled and crashed down toward Chryse, carving cliffs and gorges near where Viking 1 was to land. Viking geologists now saw a hodgepodge of other features nearby-etched tablelands, deep craters, big pits, and odd knobby protu berances-that they could not readily explain. The cameras of Viking, even at the 950 mile low point of its long elliptical orbit, *See "Journey to Mars" by Kenneth F. Weaver in the February 1973 NATIONAL GEOGRAPHIC.