There once were two planets, new to the galaxy and inexperienced in life. Like fraternal twins, they were born at the same time, about four and a half billion years ago, and took roughly the same shape. Both were blistered with volcanoes and etched with watercourses; both circled the same yellow dwarf star—close enough to be warmed by it, but not so close as to be blasted to a cinder. Had an alien astronomer swivelled his telescope toward them in those days, he might have found them equally promising—nurseries in the making. They were large enough to hold their gases close, swaddling themselves in atmosphere; small enough to stay solid, never swelling into gaseous giants. They were “Goldilocks planets,” our own astronomers would say: just right for life.

The rest is prehistory. On Earth, the volcanoes filled the air with water vapor and carbon dioxide. The surface cooled, a crust formed, and oceans condensed upon it. In hot springs and undersea vents, simple carbon compounds bubbled up to form amino acids and peptides. The first bacteria moved through the ooze; then came blue-green algae, spreading across the planet like a watery carpet, drinking in sunlight and exhaling oxygen, giving breath to everything that came after. Geologists call this the Great Oxygenation Event—the most momentous change in the planet’s history. It seems inevitable now: life’s triumphant march toward complexity, toward us. But like most creation stories this one is also a cautionary tale. It has both a Heaven and a Hell.

In 1877, when the Italian astronomer Giovanni Schiaparelli drew the first detailed map of Mars, he imagined the planet as an earthly paradise. He labelled one region Eden, another Elysium, others, on later maps, Arcadia and Utopia. Peering through his telescope on the roof of the Palazzo di Brera, in Milan, Schiaparelli had seen what looked like oceans, continents, and water channels swim into view. “The planet is not a desert of arid rocks,” he wrote. “It lives.” And his successors often took him at his word: the sharper their telescopes, the blurrier their vision. They saw mountains of ice and rivers of snowmelt, William Sheehan writes in his 1996 book, “The Planet Mars: A History of Observation and Discovery.” They saw fertile oases and a moss-green equator. They saw an irrigation system so linear and “trigonometric,” as the astronomer Percival Lowell put it, that it could only be the work of a highly intelligent race. Some even saw a Hebrew word for Almighty—Shajdai—spelled out on the planet’s surface. “True, the magnitude of the work of cutting the canals into the shape of the name of God is at first thought appalling,” the San Francisco Chronicle noted in 1895. “But there are terrestrial works which to us today seem no less impossible.”

By the time humanity got its first closeup view of Mars, a little less than a century after Schiaparelli mapped it, the planet had come to seem like a second, more exotic Earth. Books like “The Martian Chronicles” described a place of eerie desert grandeur, inhabited by slender, tawny beings given to strange hallucinations—Taos without the tourists. And though infrared studies suggested that its surface had seventy times less water than Earth’s driest desert, biologists still hoped for the best. “Given all the evidence presently available, we believe it entirely reasonable that Mars is inhabited with living organisms and that life independently originated there,” a study by the National Academy of Sciences concluded in March, 1965.

Four months later, NASA’s Mariner 4 spacecraft swung past the planet’s northern hemisphere and sent back a series of images. They were a grainy black-and-white—two hundred by two hundred pixels, converted from lines of numbers—but they left a clear impression. Where Arcadia and Elysium lay, there was a desolate waste, pocked with craters. It didn’t look like Earth. It looked like the moon.

The search for life on Mars is now in its sixth decade. Forty spacecraft have been sent there, and not one has found a single fossil or living thing. The closer we look, the more hostile the planet seems: parched and frozen in every season, its atmosphere inert and murderously thin, its surface scoured by solar winds. By the time Earth took its first breath three billion years ago, geologists now believe, Mars had been suffocating for a billion years. The air had thinned and rivers evaporated; dust storms swept up and ice caps seized what was left of the water. The Great Desiccation Event, as it’s sometimes called, is even more of a mystery than the Great Oxygenation on Earth. We know only this: one planet lived and the other died. One turned green, the other red.

Still, we keep going back. Like a delinquent sibling, Mars is all we’ve got—the next Earth-like planet may be in the Tau Ceti system, seventy trillion miles away—and its virtues nearly redeem its vices. Mars has sunlight, water, carbon, and nitrogen. Its surface is no more unpleasant than the inside of a volcanic vent, where bacteria thrive. It may yet have life. On November 26, 2011, NASA sent the world’s most sophisticated mobile science lab to explore it: the robotic rover Curiosity. The project’s scientists were quick to lower expectations: they were just looking for places that might once have been habitable, they said. Yet Mars, even dead, may answer some very old questions about life: What sets its machinery in motion? Why here and not there? Why us and not them?

The command center for NASA’s Mars missions is at the Jet Propulsion Laboratory in Pasadena, California. Hidden in the foothills of the San Gabriel Mountains, along a scrubby arroyo north of Los Angeles, it’s an oddly bucolic setting for an endeavor so cerebral. On the paths between buildings, mule deer wander about, nibbling at potted plants and twitching their ears as rumpled engineers shuffle past, lost in calculation. When I was a boy, my father, who is an electrical engineer, used to do research at Caltech in the summer, and he sometimes took me to J.P.L. on weekends. The place had all the glamour of the space age then, with its glassy offices and swooping pavilions, moon rovers and rocket ships. It hasn’t changed much, but the buildings now have a shopworn air—the fate of all shiny things. The lab’s budget was slashed in the early eighties, its planetary missions nearly scrapped. Only military research has kept J.P.L. alive.

The morning of August 4, 2012—“Landing Day Minus One,” as the NASA engineers called it—began with a briefing from a few of the project leaders. The rover was scheduled to touch down in less than forty-eight hours, after the most complex and technically daring landing sequence in the history of the space program. Most of NASA’s successes are built on repetition. Curiosity’s guidance system dated back to the Apollo days; its supersonic parachute came from the Viking missions of the late nineteen-seventies. But its signature component—a landing system known as the Sky Crane—was brand-new. It hadn’t even been tested on Earth: the Martian gravity and atmosphere could only be simulated on a computer. “A cockamamie device,” one NASA researcher called it, in private. “A lot of us are crossing our fingers and gritting our teeth. Failure, unfortunately, is an option.”