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  3. The Martian moon Phobos may have been blasted off its mother planet by a violent impact, or built from fragments of a much large

The Martian moon Phobos may have been blasted off its mother planet by a violent impact, or built from fragments of a much large

admin2013-07-02  30
问题     The Martian moon Phobos may have been blasted off its mother planet by a violent impact, or built from fragments of a much larger moon that was destroyed long ago, according to observations from Europe’s Mars Express spacecraft.

    If confirmed, the result would overturn the prevailing theory that Phobos was once a wandering asteroid(小行星)that got captured by Mars. That theory was based on the fact that visible light reflected off the moon closely matches the spectrum of a common type of carbon-rich asteroid. "For the last 30 years, the scientific community has supported this scenario, " says Marco Giuranna of the National Institute of Astrophysics in Rome, Italy.
    Now, data from the Planetary Fourier Spectrometer on Mars Express reveals that the spectrum of far infrared(红外线的)radiation from Phobos instead resembles that of silicate minerals found on the Red Planet. "The composition is closely related to Mars, " says Giuranna, who helped analyse the data.
    That suggests Phobos may have coalesced(合并)from chunks of the Martian crust that were thrown into orbit after a large object struck Mars — a similar process probably created Earth’s moon.
    Alternatively, Mars may once have had a much larger moon that formed locally out of the same ingredients, and therefore had a similar composition to the Red Planet. If the large moon moved too close to Mars, gravitational forces would have torn it apart, leaving behind a little debris that then came together to form Phobos.
    These debris-coalescence scenarios fit new observations from the Radio Science Experiment on Mars Express, which has been used to measure the motion of the spacecraft as it passes Phobos. Small changes in trajectory(轨道)reveal the strength of Phobos’s gravity, which can be used to work out the moon’s density.
    Phobos turns out to have a density of only 1860 kilograms per cubic metre, which is barely half that of the silicate minerals present. The simplest explanation is that the moon is riddled(打洞)with holes, which fits the reassembly idea. Larger rocks would gather together first, followed by layers of increasingly fine material.
    But Pascal Lee of the Mars Institute in Moffett Field, California, says it may be too soon to rule out a capture scenario, since some of the silicate minerals observed by Mars Express are also seen on carbon-rich asteroids. "The data seem exciting, but from what I have seen, they may still be consistent with Phobos being a captured asteroid, " he told New Scientist.
    He adds that a captured asteroid could also explain Phobos’s low density, if it was broken apart by a large impact and later reassembled from the resulting debris.
    Russia’s upcoming Phobos-Grunt mission could help pin down Phobos’s origin more firmly when it returns samples of the moon to Earth.
Which scenario of Phobos’ formation has the scientific community been supporting for last 30 years?
选项 A、Phobos has coalesced from chunks of the Martian crust.
B、Phobos has been built from fragments of a much larger moon.
C、Phobos was once a wandering asteroid got captured by Mars.
D、Phobos has a similar process which created Earth’s moon.
答案C
解析 事实细节题。第二段中提到如果新的可能被证实，那么福伯斯是火星捕获的一颗漫游的小行星这种理论将会被推翻，而在过去三十年里，科学界都支持这一版本。由此可知[C]项正确。福伯斯是由火星地壳融合而成，是原文提到的第一种新可能，故[A]项错误。福伯斯是由毁灭的一颗更大的卫星的碎片组成，是原文提到的第二种新可能，故[B]项错误。福伯斯的形成过程与月球相同，这指的是福伯斯形成的第一种新可能，不是之前流行的理论，故[D]项错误。
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