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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  43
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.
What is said about Phobos’ density?
选项 A、It can be learned by the changes in passing-by spacecraft’s trajectory.
B、It is denser than the silicate minerals’ density found on Mars.
C、It proves that there are many holes inside the Phobos.
D、It can judge which scenario of how the Phobos has formed is right.
答案A
解析 事实细节题。第六段末提到轨道的细微变化表明了福伯斯的引力强度，这可以用于计算该卫星的密度。由此可知从航天器轨道的变化可以得知福伯斯的密度，故[A]项正确。第七段中提到福伯斯的密度是half that of the silicate minerals，这说明该卫星的密度更小，故[B]项错误。原文提到对于该卫星密度小最简单的解释是它有很多洞，但不是说密度小就证明其内部真的有很多洞，文中没有说明这一点是被证实了的，故[C]项错误。原文说福伯斯形成的几种可能都符合其密度小这一条件，但不是说从密度小，就可以判断出哪一种可能是正确的，故[D]项错误。
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