字幕表 動画を再生する 英語字幕をプリント Just about every two years, the planet Mars makes its closest approach to Earth... around 36 million miles. That's when we pack our robotic emissaries off to the Red Planet, timing their launches to spend the least effort to get there. Some fly around it... snapping pictures... Others land ... to sample its surface.... ...a few to crawl around its canyons and craters. 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: Did Mars develop far enough - and stay that way long enough - for life to arise? And, if so, does anything live now within Mars' dusty plains... beneath its ice caps... or maybe somewhere underground? Mars does not give up its secrets easily ... it's almost as if the little planet is embarrassed. Over a century ago, a few observers thought they saw clues that Mars is alive. 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. American astronomer, Percival Lowell, found the lure of these features irresistible. He saw Schiaparelli's channels as artificial canals. He speculated that they carried melting snow from the poles to the dry interior. After all, on Earth, the Suez Canal had recently opened to ship traffic. The Panama Canal was beginning to be dug. The Martian canals, Lowell said, were built by a sophisticated society confronting an environmental catastrophe on the grandest of scales. Those Martians, he thought, must face urgent choice: move water across vast arid regions, or perish on an increasingly dry planet. As the 19th Century gave way to the 20th, Lowell took his case to the public, in a series of three best-selling books. And the public responded with... questions. Who were these Martians, who had the means to remake an entire planet? Some offered schemes for making contact. Giant mirrors would flash greetings... Light beams... Mental telepathy. Many astronomers grew deeply skeptical... but Lowell's vision of a harsh, yet Earth-like planet endured in the public's imagination.. 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. Three more Mariners followed. They found huge dormant volcanoes... the deepest and longest canyon in the solar system...but not a trace of life, present or past. In the mid-1970's, two lander-orbiter robot teams, named Viking, took up residence at Mars. Maybe the Martians were just hiding, so theVikings tested the soil for signs of life. But all the evidence from Viking told us... Mars is not only barren... but in fact hostile to life. It's no wonder. Martian air temperatures range from -20 degrees Fahrenheit to down below -200. 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. On Earth, impact craters erode over time from wind and water... and even volcanic activity. On Mars, they can linger for billions of years. But so can the imprint of riverbeds, lake bottoms and ocean shorelines... And the Viking orbiters saw a lot of them. It's not hard to believe that a great deal of water once flowed here. But where did all the water go? To find out, scientists needed to do real field-geology on Mars. They needed rovers... travelling robots with tools and instruments. In 2004, the heroic twin rovers Spirit and Opportunity bounced down... stood up... and drove off, ready to roam the Red Planet. Opportunity had come to rest in a small crater at a spot called Meridiani Planum... and it didn't have far to travel. For here, in plain view, on the crater wall before it was exposed bedrock... the first ever seen on Mars. Not far away, Opportunity found layered rocks on the face of a cliff. On Earth, rocks like these typically form in sedimentary layers at the bottom of ancient oceans. And at every turn, Opportunity rolled across tiny, smooth, almost perfectly round pellets... called "blueberries" because they appeared dark bluish-brown against Mars' rust-colored surface. Initially thought to be volcanic in origin, these blueberries proved to be iron-rich spherules of exactly the type formed within cavities at the bottom of an ocean. And the rovers has another trick up their mechanical sleeves; they could drill into the rock itself... brush it clean ... and nuzzle a spectrometer right up to the virgin minerals. They found that up to 40% of the rocks were sulfate salt... a tracer for standing water. That wasn't the only evidence. Spirit's broken wheel, dragging behind it, exposed soils saturated in salt. Clearly these rocks formed in water. But how long ago? And, if you needed to find water on Mars right now, where would you look? One answer... the North Pole. From orbit, it seemed to be covered in frozen carbon dioxide - what we call dry ice. But might water ice exist below the surface? Enter Mars Phoenix... a lander that arrived at the North Pole in early 2008. Phoenix used rockets, not airbags, to touch down in what appears to be the floor of an ancient ocean. The lander's descent thrusters blew away the top layer of soil, and Phoenix snapped some pictures. It sure looked like ice. Mission managers instructed the robot to conduct a simple experiment... reach out, dig a trench... then watch what happens. As expected, clumps of white stuff appeared. A couple of days later, it was gone... vaporized. That means it can't be salt. Or frozen carbon dioxide, which is stable in the cold dry temperatures of the Martian pole. So it had to be water... the first ever directly seen on Mars. In fact, Phoenix probably saw droplets of liquid salty water form and merge on its legs. And there are other indications that the North Pole was actually warm enough in the recent past for water to become liquid. Beginning in November of 2006, Mars Reconnaissance Orbiter flew over the pole, sending out radar pulses to peer beneath the surface. These data have revealed that the ice cap, just over a mile thick, formed in a succession of layers as the climate alternated between warm and cold. Martian mood swings like this happen because the planet's spin is not stabilized by a massive moon, like that of Earth. Mars can really wobble, with one pole tilting toward the sun for long periods. Whatever water's here now, is tiny compared to what once was. There's a growing body of evidence that early in its history, Mars was on track to become positively Earth-like. But something went horribly wrong. Not long after its birth, the planet's molten interior welled up to spawn volcanoes and hot springs... spewing gas that formed an early atmosphere. As the atmosphere thickened, carbon dioxide began to trap heat from sunlight; in other words, the classic greenhouse effect. Temperatures rose high enough to allow liquid water to flow on the surface. What then caused these waterways to evaporate... and the planet to grow cold and dry? In search of the answer, the Mars Global Surveyor spacecraft was brought down to a low altitude... and it made an intriguing detection. MGS documented the presence of a small magnetic field emanating from the planet. It's incredibly weak compared to that of Earth. Our magnetic field is generated by molten rock deep in Earth's core that rises and falls into a huge region below the outer curst, called the mantle. That has turned our planet into an electric dynamo... the rising and sinking motion within, combined with the spinning motion of the planet, generates a strong magnetic field. This field is strong enough, and extends far enough into space, to deflect the wind of high-energy solar particles... protecting Earth and its atmosphere. In its early years, Mars too appears to have had a robust magnetic field. Rocks in some of the older craters bear a strong imprint of this field.... While newer ones indicate a much weaker field. So what caused Mars' magnetic field to drain away... exposing the planet to solar winds? The answer lies in its violent past... a past that today is still written on the landscape. This is a simple elevation map of Mars' surface, from data gathered by the Mars Odyssey spacecraft. The south pole, colored in orange and red, is piled high with ice that comes and goes with the seasons. Moving off these southern highlands, we make our way north. The landscape is pocked with craters. The largest and oldest ones have slowly faded, softened by windblown dust. We now pass into the Tharsis region, along the equator. Here, on a vast high altitude plateau, are a series of enormous volcanoes... ...including the largest one in our solar system, Olympus Mons. Just to the East is the great Valles Marineris. It's named for the Mariner Nine mission that found this vast gash in the Martian landscape. It's about 2500 miles long and up to 400 miles wide. Dust devils sweep along the plains above it... And dust blows up the valley slopes. We can see this incredible landform in a realistic rendering of the data. Giant landslides have caused the walls to slump off and pile onto the valley floor. Feeding into this valley: a maze of side channels. Scientists think these and other tributary features were formed when water underground flowed into the main basin, and the land above collapsed. Wider parts of the canyon are regarded as possible landing sites for human missions. They offer flat surfaces... and possibly access to liquid water that may remain below the surface even today. From the layers of rock that line the canyon walls... and layers of dust on the canyon floor... future science prospectors can fill in details of the planet's history. The theory is that the great Valles Marineris formed early in Mars' history, as the planet cooled. Its huge sides were pulled apart as the Tharsis region, just to the west, began to rise up. But there seems to have been another, much larger event that changed Mars' course forever. Travel north, down the slopes of Mars's great volcanoes. The elevation drops as we dive into what appears to have been an immense ocean. With this vast basin in the north, and the very high elevations of the South, Mars is a lopsided planet. And here's why: Early on, when the Solar System was young, Mars was apparently hit by at least 15 large asteroids. One of them was large enough to turn most of Mars' northern hemisphere into one huge impact feature. Earth, too, was walloped by objects this big. But because Mars is small, only about half the size of Earth - and has only about a third Earth's surface area - ... the effect would have been much more profound... and long-lasting.