单选题
Earth"s Inner Core
Scientists have long struggled to understand what lies at the planet"s center. Direct observation of its center is impossible, so researchers must
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to other evidence.
In 1889, a German scientist detected a severe earthquake in Japan. Geophysicists concluded that shock waves
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jolts (晃动) from one side of Earth through the center to the other side. Then in 1936, Danish geophysicist Inge Lehmann studied the waves
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to determine that within Earth"s core of molten (熔化了的) iron lies a solid inner core-but
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that core was made of eluded (难倒) her. Other geophysicists quickly determined that Lehmann"s inner core was composed mostly
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iron. Since then, Lehmann"s discovery has
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conventional Earth science.
But now scientists are challenging traditional theory with new and radical
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. For example, Earth"s center could actually contain an "inner core within the inner core, claim Ishii and colleague Adam Dziewonski. Analyzing hundreds of thousands of earthquake wave
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, they maintain that the inner core has at its heart a tiny, even more solid sphere (球体). This sphere "may be the oldest fossil
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from the formation of Earth," says Dziewonski.
Dziewonski and Ishii speculate that shortly
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earth formed around 4.8 billion years ago, a giant asteroid (小行星) smashed into the young planet and nearly melted it. But Earth"s center didn"t quite melt; it
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mass as the planet cooled. The core within a core may be the kernel (核心) that endured. "Its presence could change our basic ideas about the
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of the planet," Dziewonski says idea is tame (温和的) compared to the
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theories of independent geophysicist J. Marvin Herndon. Earth"s inner core is made not of iron, he claims, but a
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of nickel and silicon. Herndon has a truly revolutionary notion: Within the nickel silicide (硅化物) inner core is also an "inner" inner core—an 8 km—wide ball of the element uranium. Uranium is radioactive. Herndon thinks the uranium releases heat energy as its atoms
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fission-split and crash into another in a chain reaction. In other words, we may live on top of a gigantic, "natural" nuclear power plant.