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  3. The search for water—and possible life—on Mars got a boost this week as scientists announced evidence of an ancient ocean on the

The search for water—and possible life—on Mars got a boost this week as scientists announced evidence of an ancient ocean on the

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问题     The search for water—and possible life—on Mars got a boost this week as scientists announced evidence of an ancient ocean on the planet’s northern plains.
    The massive, sediment-filled basin surrounded by "shoreline-like" features was first spotted when scientists were mapping Mars’s surface using images from the late-1970s Viking missions.
    The features made theories of an ancient Martian ocean quite popular—until NASA’s Mars Global Surveyor highlighted some seemingly insurmountable problems. The Arabia and Deuteronilus "shorelines" are thousands of kilometers long. But they undulate like a long wave, rising and falling in height as much as several kilometers(more than a mile)along their length. "That doesn’t seem to jibe with the idea that they are shorelines, because shorelines form at sea level," said lead author Taylor Perron, a postdoctoral fellow in Harvard University’s Department of Earth and Planetary Sciences.
    But there really was an ocean there, Perron and colleagues now suggest in a finding that adds to a number of recent studies bolstering evidence of Mars’s watery past. The researchers believe that Mars’s poles, along with the axis the planet spins on, have moved about 1,850 miles(3,000 kilometers)during the past two or three billion years. The process, known as "true polar wander", can cause dramatic topographic changes in a planet’s surface—in this case making the once-flat shorelines rise and fall over enormous distances. "When the spin axis moves relative to the surface, the surface deforms, and that is recorded in the shoreline," said co-author Michael Manga, professor of earth and planetary science at the University of California, Berkeley.
    On the Martian equator one finds the Tharsis rise—a massive bulge that holds the colossal volcano Olympus Mons. "The largest mountain in the solar system is sitting almost right on the equator, and that’s right where you should expect such a large surface load to be," Perron said. "That tells you that if Mars experienced any true polar wander after the creation of Tharsis, it should reorient in such a way that keeps Tharsis at the equator—a circle 90 degrees from Tharsis."
The current locations of the poles, along with calculations of their previous positions based on the deformation of the shorelines, suggest just such an alignment.
    The findings are sure to spark debate among many professionals in search of a liquid "smoking gun" on the red planet. Jeff Moore, a planetary geologist at NASA’s Ames Research Center who is unaffiliated with the study, finds it plausible that early Mars had large standing bodies of water—either liquid or frozen. "But while the new study presents a possible explanation for why the shorelines don’t follow a constant contour, it’s not a conclusive one," Moore said.
What is the main idea of the passage?
选项 A、Evidence of an ancient ocean on Mars.
B、Researchers find out many problems related to Mars.
C、Mars has experienced a true polar wander.
D、Tharsis rise proves the existence of shorelines on Mars.
答案A
解析
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