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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  24
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 is the message Pascal Lee intends to convey according to the passage?
选项 A、Phobos’ density shows that a capture scenario can be excluded.
B、Some of the silicate minerals observed are also seen on carbon-rich asteroids.
C、He supports the idea that Phobos was an asteroid captured by Mars.
D、There is not enough evidence to prove which scenario is right.
答案D
解析 推理判断题。第八段中Pascal Lee说排除火星捕获说可能为时尚早，观察数据也符合福伯斯是被捕获的小行星之说。由此可以判断出，[D]项正确。但这并不表示Pascal Lee支持哪种说法，故[C]项错误。因福伯斯的密度而排除捕获说，这与Pascal Lee表达的意思相反，故[A]项错误。观察到的一些硅酸盐矿物在富碳小行星上也有，这是Pascal Lee提供的证据说明还不能排除捕获说，不是他最终要表达的信息，故[C]项错误。
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