At first, the data sent back to earth by two Mariner spacecraft more than 60 million miles away seemed to offer as little hope as the lunar rocks that life would be found elsewhere in the solar system. Flying past the planet Mars, the small, instrument-packed spacecraft detected no evidence of nitrogen, an indispensable ingredient of life on earth. Probing the upper reaches of the Martian atmosphere, they failed to find anything like the ozone shield that protects the earth's surface from the sun's deadly rain of ultraviolet radiation. Even their stunning close-up...
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