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During the 1960s and '70s, NASA also sent missions in the opposite direction, toward the outer solar system. Mars, the first target, had long held special intrigue for humanity, though by the 1960s astronomers had decided that life, at least on the planet's surface, wasn't possible. Mars has such a thin atmosphere that any liquid water would vaporize or freeze—and as far as anyone knew then, or knows now, life can't exist in the absence of water. The first few probes to reach Mars, starting with NASA's Mariner 4, in 1965, showed a cratered, moonlike terrain. But six years later Mariner 9 revealed previously unsuspected features, including giant extinct volcanoes, canyons, and, most intriguingly, what looked suspiciously like dried-up river channels. At some time in the distant past, Mars clearly did have more favorable conditions for life to arise. And if it had, it was at least conceivable that microbial remnants could be hanging on, not out in the open air but under the soil.

That's why the twin Viking missions in 1976 carried experiments to look for that life. The Vikings each had an orbiter and a lander; the latter took the first, stunning photographs from the surface of Mars (one panoramic shot presided for a time over the main waiting room in Grand Central Terminal in New York from a billboard). While the orbiters whirled overhead mapping the planet, the landers scraped up some Martian dirt and performed four tests that might betray the existence of microbes. Three of the four were ultimately negative; the fourth was positive, but could be explained away as an unusual but purely chemical reaction (although the principal investigator on that experiment, Gilbert Levin, insisted it shouldn't be).

By the late 1990s, biologists had come to understand that life can flourish in more extreme environments than anyone had thought—deep underground, for example, or in unfrozen salty pockets within ice sheets (it turns out that some earthly bacteria can live in extraordinarily salty water). At the same time, the evidence for abundant surface water in Mars's past had become even stronger thanks first to the wheeled Pathfinder rover, which rolled over the surface in 1997 and photographed rocks that had been transported by huge floods of water. The Spirit and Opportunity rovers that began crawling over Mars in 2004 found evidence of rocks formed in and shaped by once free-flowing water, and in 2006 the Mars Reconnaissance Orbiter began taking the highest resolution pictures ever of ancient channels and possible lakeshores from space. In 2002 the Mars Odyssey orbiter had detected what may be the remains of that water: Its neutron spectrometer revealed massive amounts of water ice (as distinct from the carbon- dioxide, or dry, ice that forms on the surface of the planet's visible ice caps) lying just beneath the surface near the planet's north and south poles. It is here, if anywhere, that Martian organisms might have flourished most recently. This year the Phoenix spacecraft landed near the north pole and began digging with its robotic arm. Shortly thereafter—in a historic breakthrough—Phoenix confirmed the presence of water ice on Mars.

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