【每日一篇SA】Is Mars Missing a Moon? - Scientific American

Scientific American 60-second Science, July 4, 2016

作者：Lee Billings 翻译整理：SophieMen

原文地址：Is Mars Missing a Moon? - Scientific American

音频下载：160704MartianMoons.mp3

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Transript and Translation

Mars is a planet of outsized splendor. Despite being only half as big and a tenth as heavy as Earth, it bears the solar system’s tallest mountain, longest canyon and largest crater. At 22 and 12 kilometers wide, however, its inner moon Phobos and outer moon Deimos are figurative small potatoes. Scientists suspect both formed much as Earth’s single large moon did, from a massive debris disk ejected into orbit by a giant impact eons ago. But if Mars’s moons formed like Earth’s, why are they so very much smaller?

火星是一个有着迷人光晕的星球。尽管它的大小只有地球的一半、质量只有地球的十分之一，火星却有着太阳系最高的火山、最长的峡谷和最大的火山口。火星的两颗卫星，内侧的Phobos和外侧的Deimos，直径分别是22千米和12千米，被比喻成小土豆。科学家们怀疑这两颗卫星都像地球的唯一卫星月球一样，是由亿万年前在一次巨大冲击中被射入轨道的一个大质量碎片形成的。但是如果火星的卫星的形成方式和地球的卫星一样，为什么相比而言火星的卫星小这么多呢？

The answer may be that they did not form alone. New simulations from Pascal Rosenblatt of the Royal Observatory of Belgium and colleagues show how the debris disk from a giant impact on Mars could have generated additional moons a few hundreds of kilometers in size. After forming in the dense inner regions of the disk, those larger moons would have stirred the disk’s sparser outer reaches, allowing smaller companions like Phobos and Deimos to coalesce from the ripples. The study appears in the journal Nature Geoscience. [Rosenblatt et al., Accretion of Phobos and Deimos in an extended debris disc stirred by transient moons]

答案或许是因为他们并不是独自形成的。比利时皇家天文台的Pascal Rosenblatt和他的同事们进行的新仿真显示，在火星一次巨大冲击产生的碎片形成了几百千米大小的卫星。在碎片的高密度内核形成后，那些大卫星搅动着碎片稀疏的延伸区域，使得更小的如同Phobos和Deimos这样的伴随星在涟漪中合并。这一研究发表在《自然·地球科学》期刊上[Rosenblatt et al., Accretion of Phobos and Deimos in an extended debris disc stirred by transient moons]。

In this scenario, the reason we only see Phobos and Deimos today is that the bigger moons were destroyed a few million years after their formation. Their low, fast orbits outpaced Mars’s rotation, creating a tidal pull that sent them spiraling down to crash into the planet (Earth’s moon, by contrast, orbited slower than our planet’s rotation, allowing it to spiral outward and survive). Future investigations could test the new hypothesis by looking for clusters of Martian craters produced by the infalling moons, but in the meantime, proof that Mars can kill its companions is right before our eyes: The orbit of Deimos is stable, but Phobos is in a death spiral, losing two centimeters of altitude per year to Mars’s tidal pull. It will plunge into the planet in 20 [million] to 40 million years, leaving lonely, far-out Deimos as the last vestige of what may have been a once-mighty system of Martian moons.

在这一场景中，我们今天只能看到Phobos和Deimos的原因是更大的卫星在形成几百万年后被毁灭了。他们在又快又低的绕行中超越了火星自转的速度，形成了一种能够使他们螺旋下降坠落行星的潮汐力（相反，地球的卫星，绕行速度低于行星自转，使得卫星螺旋远离从而一直存在）。未来的研究可能通过寻找火星上由坠落卫星形成的火山口群来对这一新假设进行检验；但是同时，火星能够杀死它的伴星的证据就在我们眼前——Deimos的轨道是稳定的，但是Phobos却在死亡螺旋之中，在火星的潮汐力作用下其高度每年下降2厘米。它将会在两千万年到四千万年内坠入火星，留下Deimos——作为曾经强大的火星卫星们的最后遗迹——孤独而珍贵的存在着。

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Vocabulary

• splendor, \ˈsplen-dər, noun, 1. great and impressive beauty. 2. things that are very beautiful or impressive.

• canyon, \ˈkan-yən, noun, a deep valley with steep rock sides and often a stream or river flowing through it.

• crater, \ˈkrā-tər, noun, 1. a large round hole in the ground made by the explosion of a bomb or by something falling from the sky. 2. the area on top of a volcano that is shaped like a bowl

• figurative,\ˈfi-g(y)ə-rə-tiv, adjective, 1. used with a meaning that is different from the basic meaning and that expresses an idea in an interesting way by using language that usually describes something else : not literal. 2.showing people and things in a way that resembles how they really look : not abstract.

• debris, \də-ˈbrē, noun, 1.the pieces that are left after something has been destroyed. 2.things (such as broken pieces and old objects) that are lying where they fell or that have been left somewhere because they are not wanted.

• far-out, \ˈfär-ˌau̇t, adjective, very strange or unusual.