The revival of life on Mars

摘要

Few, if any, major scientific quests have taken such frequent, diametrically opposed changes in prospect and direction as has the search for life on Mars. The erratic courses and their supporting rationales are traced: from Percival Lowell's astonishing pronouncement of intelligent beings on the red planet; to their denouement by Mariner 4; to the strong contraindications of any form of Martian life relayed by Mariners 6 and 7; to the discovery of a different, perhaps once-habitable, Mars revealed by the detailed orbital images, including the first evidence of ancient rivers, taken by Mariner 9; to the still-controversial claim of the detection of microbial life by the 1976 Viking Missions; to NASA's subsequent prohibition against any Mars life detection experiments; to the recently emerged consensus, propelled by findings of Pathfinder, the Mars Exploratory Rovers and the continuing discoveries of life in extreme environments on Earth, that past or extant life on Mars is likely. Against this background, the future Mars missions' experiments bearing on the life issue are reviewed. The case is made that none of these experiments, as currently planned, still subject to the prohibition against direct life detection experiments, can resolve this paramount and fundamental question that bears so heavily on the origin and distribution of life, and our place in the universe.