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Earths Inner Core Scientists have long struggled to understand what lies at the planets center. Direct observation of its center is impossible, so researchers must___ （1）___ to other evidence. In 1889, a German scientist detected a severe earthquake in Japan. Geophysicists concluded that shock waves ___（2）___ 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 ___（3）___ to determine that within Earths core of molten （熔化了的） iron lies a solid inner core － but ___（4）___ that core was made of eluded （难倒） her. Other geophysicists quickly determined that Lehmanns inner core was composed mostly ___（5___） iron. Since then, Lehmanns discovery has ___（6）___ conventional Earth science. But now scientists are challenging traditional theory with new and radical ___（7）___. For example, Earths center could actually contain an inner core within the inner core, claim Ishii and colleague Adam Dziewonski. Analyzing hundreds of thousands of earthquake wave ___（8___）, 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 ___（9）___ from the formation of Earth, says Dziewonski. Dziewonski and Ishii speculate that shortly ___（10）___ Earth formed around 4.8 billion years ago, a giant asteroid （小行星） smashed into the young planet and nearly melted it. But Earths center didnt quite melt; it ___（11）___ 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 ___（12）___ of the planet, Dziewonski says. | 
