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.
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