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Mars is a planet of outsized splendor¹. Despite being only half as big and a tenth as heavy as Earth, it bears the solar system’s tallest mountain, longest canyon² and largest crater³. 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 debris⁴ 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?

火星是一个拥有惊人光辉的行星。尽管它的体积只有地球的一半那么大，重量是地球的十分之一，但它拥有太阳系中最高的山峰、最长的峡谷以及规模最大的火山口，在22和12公里宽处，其内部卫星火卫一和火卫二像生动形象的两颗小土豆。科学家们怀疑这两个卫星的形成就像地球上形成独一无二的月球一样，在亿万年前，发生剧烈碰撞时，大量的碎片被送入轨道从而形成。但如果火星卫星的形成与月球的形成过程大体类似，为什么它们的体积如此之小？

The answer may be that they did not form alone. New simulations from Pascal Rosenblatt of the Royal Observatory⁵ 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 dense⁶ inner regions of the disk, those larger moons would have stirred the disk’s sparser⁷ outer reaches, allowing smaller companions like Phobos and Deimos to coalesce⁸ from the ripples⁹. The study appears in the journal Nature Geoscience. [Rosenblatt et al, Accretion¹⁰ of Phobos and Deimos in an extended debris disc stirred by transient moons]

答案或许是它们并不是单独形成的。比利时皇家天文台的帕斯卡罗森布拉特和他的研究团队使用数值模拟了一次火星上的大碰撞事件，以及碰撞所造成的碎片盘演变，展示是如何形成其他的卫星，在几百公里之外。由于碎片的密集程度不同，较大的卫星会在碎片盘内侧吸积形成之后，其产生的引力牵引会搅动碎片盘的外侧，允许形小型卫星，如卫星火卫一和火卫二。这项研究发表在《自然地球科学》期刊上。

In this scenario¹¹, 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 rotation¹², 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 investigations¹³ could test the new hypothesis by looking for clusters of Martian craters¹⁴ 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 2 centimeters of altitude per year to Mars’s tidal pull. It will plunge¹⁵ into the planet in 20 to 40 million years, leaving lonely, far-out Deimos as the last vestige¹⁶ of what may have been a once-mighty system of Martian moons.

在这一假设下，我们今天之所以只看到火卫一和火卫二，是因为那些较大的卫星在它们形成几百万年后被摧毁。它们运行的低、快速轨道超过火星自转，汇成推动潮汐力使它们盘旋而下，撞上星球。（相比之下，地球上的卫星，环绕轨道运行的速度慢于地球自传。允许卫星像螺旋一样外传并生存下来。）未来的调查可以通过寻找由陨落的卫星所形成的火星陨石坑集群，来测试这一新假设。但与此同时，证明火星可以杀死它同伴的证据就在眼前：火卫二的运行轨道是稳定的，但火卫一处在“死亡螺旋”中，由于火星的潮汐力，每年高度降低2厘米。在未来二千至四千万年内，它将被拉向火星，孤独地陨落。非常遥远的火卫二，其最后的遗迹可能是一个曾经辉煌的火星卫星系统。