Just about every two years, the planet Mars makes its closest approach to Earth... around 36 million miles. <br /> <br />That's when we pack our robotic emissaries off to the Red Planet, timing their launches to spend the least effort to get there. <br /> <br />Some fly around it... snapping pictures... <br /> <br />Others land ... to sample its surface.... <br /> <br />...a few to crawl around its canyons and craters. <br /> <br />These probes may pave the way for human explorers... and, perhaps permanent settlers... who'll dig deeper still... in search of answers to our most pressing question: <br /> <br />Did Mars develop far enough -- and stay that way long enough -- for life to arise? <br /> <br />And, if so, does anything live now within Mars' dusty plains... beneath its ice caps... or maybe somewhere underground? <br /> <br />Mars does not give up its secrets easily ... it's almost as if the little planet is embarrassed. <br /> <br />Over a century ago, a few observers thought they saw clues that Mars is alive. <br /> <br />In 1877, the Italian astronomer Giovanni Schiaparelli noted markings... which he saw as a latticework of lines. He called them "canali" in Italian... meaning nothing more than "shallow channels" in English. <br /> <br />American astronomer, Percival Lowell, found the lure of these features irresistible. <br /> <br />He saw Schiaparelli's channels as artificial canals. He speculated that they carried melting snow from the poles to the dry interior. <br /> <br />After all, on Earth, the Suez Canal had recently opened to ship traffic. The Panama Canal was beginning to be dug. <br /> <br />The Martian canals, Lowell said, were built by a sophisticated society confronting an environmental catastrophe on the grandest of scales. <br /> <br />Those Martians, he thought, must face urgent choice: move water across vast arid regions, or perish on an increasingly dry planet. <br /> <br />As the 19th Century gave way to the 20th, Lowell took his case to the public, in a series of three best-selling books. <br /> <br />And the public responded with... questions. <br /> <br />Who were these Martians, who had the means to remake an entire planet? <br /> <br />Some offered schemes for making contact. Giant mirrors would flash greetings... Light beams... Mental telepathy. <br /> <br />Many astronomers grew deeply skeptical... but Lowell's vision of a harsh, yet Earth-like planet endured in the public's imagination.. <br /> <br />That vision was dealt harsh blow in 1964. The Mariner Four spacecraft ventured in for a closer look... And what it saw looked like the Moon. <br /> <br />Three more Mariners followed. <br /> <br />They found huge dormant volcanoes... the deepest and longest canyon in the solar system...but not a trace of life, present or past. <br /> <br />In the mid-1970's, two lander-orbiter robot teams, named Viking, took up residence at Mars. <br /> <br />Maybe the Martians were just hiding, so theVikings tested the soil for signs of life. <br /> <br />But all the evidence from Viking told us... Mars is not only barren... but in fact hostile to life. <br /> <br />It's no wonder. Martian air temperatures range from --20 degrees Fahrenheit to down below --200. <br /> <br />It's also very, very dry. The Sahara Desert on Earth is a rainforest, by comparison. If all of the water vapor in Mars' thin atmosphere fell as snow, it would make a layer of frost not thicker than your fingernail. <br /> <br />On Earth, impact craters erode over time from wind and water... and even volcanic activity. <br /> <br />On Mars, they can linger for billions of years. <br /> <br />But so can the imprint of riverbeds, lake bottoms and ocean shorelines... And the Viking orbiters saw a lot of them. <br /> <br />It's not hard to believe that a great deal of water once flowed here. <br /> <br />But where did all the water go? <br /> <br />To find out, scientists needed to do real field-geology on Mars. They needed rovers... travelling robots with tools and instruments.
