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Mariner 9 was the first spacecraft to achieve orbit around another planet, reaching Mars on Nov. 14, 1971. The probe arrived in the middle of a dust storm that had enveloped the entire planet. Only three objects extended above the dust. They turned out to be the tallest volcanoes in the solar system, with Nix Olympica (now called Olympus Mons) standing three times higher than Mt. Everest on Earth.
When the storm ended, Mariner 9's camera was able to photograph the entire surface of Mars. The images revealed a very strange, tortured surface. Just south of the giant Olympus Mons, a vast ridge rose 8 kilometers (about 5 miles) above Mars' average surface height and supported several more giant volcanoes. This volcanic plateau was named Tharsis Ridge. Closer to the equator, a grand canyon of Mars cut an area 4,800 kilometers (almost 3,000 miles) long. Mariner 9 also showed that Mars was strangely lopsided, with more volume in its southern hemisphere than in its northern.
Stranger still, Mariner 9's images confirmed that water had once run on the red planet's surface. Mars had huge outflow channels spanning hundreds of kilometers, but mission scientists could not find the water that had carved them. This discovery created a raging debate. Where had the water gone, and how could the now-frozen Mars ever have been warm enough to host liquid water?
Mariner 9 ran out of fuel and was shut down on Oct. 27, 1972. Scientists at the United States Geological Survey eventually turned several thousand of Mariner 9's images into the first accurate planetary map of Mars. They named the enormous canyon Valleris Marineris in honor of Mariner 9's achievements.
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