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Scientists have long struggled to understand what lies at the planet's center. Direct observation of its center is impossible, so researchers must {{U}}(51) {{/U}} to other evidence. In 1889, a German scientist detected a severe earthquake in Japan. Geophysicists concluded that shock waves {{U}}(52) {{/U}} jolts (晃动) from one side of Earth through the center to the other side. Then in 1936, Danish geophysicist lnge Lehmann studied the waves' {{U}}(53) {{/U}} to determine that within Earth's core of molten (熔化了的) iron lies a solid inner core - but {{U}}(54) {{/U}} that core was made of eluded (难倒) her. Other geophysicists quickly determined that Lehmann's inner core was composed mostly {{U}}(55) {{/U}} iron. Since then, Lehmann's discovery has {{U}}(56) {{/U}} conventional Earth science. But now scientists are challenging traditional theory with new and radical {{U}}(57) {{/U}}. 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 {{U}}(58) {{/U}}, 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 {{U}}(59) {{/U}} from the formation of Earth," says Dziewonski. Dziewonski and Ishii speculate that shortly {{U}}(60) {{/U}} 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 {{U}}(61) {{/U}} 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 {{U}}(62) {{/U}} of the planet," Dziewonski says. Dziewonski's idea is tame (温和的) compared to the {{U}}(63) {{/U}} theories of independent geophysicist J. Marvin Herndon. Earth's inner core is made not of iron, he claims, but a {{U}}(64) {{/U}} 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 {{U}}(65) {{/U}} fission-split and crash into one another in a chain reaction. In other words, we may live on top of a gigantic, "natural" nuclear power plant. |