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 ___________ (51) to other evidence.
In 1889, a German scientistdetected a severe earthquake in Japan．Geophysicists concluded that shock waves___________ (52) 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'___________ (53) to determine that within Earth's core of molten (熔化了的)iron lies a solid inner core-- But___________ (54) that core was made of eluded (难倒)her, Other geophysicists quickly determined that Lehmann's inner core was composed mostly ___________ (55) iron．Since then, Lehmann's discovery has ___________ (56)conventional Earth science.
But now scientists are challenging traditional theory with new and radical___________ (57)．For example, Earth's center could actually contain an 'inner core within the inner core,' claim Ishii and colleague Adam Dziewonski.
yzing hundreds of thousands of earthquake wave___________ (58), 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 ___________ (59) from the formation of Earth,' says Dziewonski.
Dziewonski and Ishii speculate that shortly___________ (60) 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___________ (61) mass as the planet cooled．The core within a core may B．e the kernel (核心) that endured．'Its presence could change our basic ideas about the___________ (62) of the planet,' Dziewonski says.
Dziewonski's idea is tame (温和的) compared to the ___________ (63) theories of independent geophysicist J．Marvin Herndon．Earth's inner core is made not of iron, he claims, but a ___________ () 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 kmowide ball of the element uranium．Uranium is radioactive．Herndon thinks the uranium releases heat energy as its atoms ___________ (65) 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．
第 51 题