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请教0408 阅读13题
Europa is the smallest of planet Jupiter's four largest moons and the second moon
out from Jupiter. Until 1979, it was just another astronomy textbook statistic. Then
came the close-up images obtained by the exploratory spacecraft Voyager 2, and within
days, Europa was transformed--in our perception, at least--into one of the solar system's
most intriguing worlds. The biggest initial surprise was the ahnost total lack of detail,
especially from far away. Even at close range, the only visible features are thin, kinked
brown lines resembling cracks in an eggshell. And this analogy is not far offthe mark.
The surface of Europa is almost pure water ice, but a nearly complete absence of
craters indicates that Europa's surface ice resembles Earth's Antarctic ice cap. The
eggshell analogy may be quite accurate since the ice could be as little as a few kilometers
thick--a tree shell around what is likely a subsurface liquid ocean that, in turn, encases a rocky core. The interior of Europa has been kept warm over the cons by tidal forces
generated by the varying gravitational tugs of the other big moons as they wheel around
Jupiter. The tides on Europa pull and relax in an endless cycle. The resulting internal heat
keeps what would otherwise be ice melted almost to the surface. The cracklike marks on
Europa's icy face appear to be fractures where water or slush oozes from below.
Soon after Voyager 2's encounter with Jupiter in 1979, when the best images of
Europa were obtained, researchers advanced the startling idea that Europa's subsurface
ocean might harbor life. Life processes could have begun when Jupiter was releasing a
vast store of internal heat. Jupiter's early heat was produced by the compression of the
material forming the giant planet. Just as the Sun is far less radiant today than the primal
Sun, so the internal heat generated by Jupiter is minor compared to its former intensity.
During this warm phase, some 4.6 billion years ago, Europa's ocean may have been liquid
right to the surface, making it a crucible for life.
13. It can be inferred from the passage that astronomy textbooks prior to 1979
(A) provided many contradictory statistics about Europa
(B) considered Europa the most important of Jupiter's moons
(C) did not emphasize Europa because little information of interest was available
(D) did not mention Europa because it had not yet been discovered
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发表于 2005-4-20 11:39:00
