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.
Dziewonskis idea is tame (温和的) compared to the (13) theories of independent geophysicist J. Marvin Herndon. Earths inner core is made not of iron, he claims, but a (14) 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 (15) 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.
1 A try B leave C turn D point
2 A create B receive C feel D overcome
3 A work B solution C job D patterns
4 A whether B what C why D how
5 A from B within C of D to
6 A followed B dominated C restored D opposed
7 A ideas B demands C phenomena D movements
8 A things B acts C methods D records
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