☊ 碧瓏司の→ßeℒongŚ

🎁 MP3音檔 (按右鍵可下載聽)：
http://online1.tingclass.net/voas…/2016/20160706sa_space.mp3

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🎁 中英文稿：
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?
火星是一個擁有驚人光輝的行星。儘管它的體積只有地球的一半那麼大，重量是地球的十分之一，但它擁有太陽系中最高的山峰、最長的峽谷以及規模最大的火山口，在22和12公里寬處，其內部衛星火衛一和火衛二像生動形象的兩顆小土豆。科學家們懷疑這兩個衛星的形成就像地球上形成獨一無二的月球一樣，在億萬年前，發生劇烈碰撞時，大量的碎片被送入軌道從而形成。但如果火星衛星的形成與月球的形成過程大體類似，為什麼它們的體積如此之小？
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]
答案或許是它們並不是單獨形成的。比利時皇家天文臺的帕斯卡羅森布拉特和他的研究團隊使用數值模擬了一次火星上的大碰撞事件，以及碰撞所造成的碎片盤演變，展示是如何形成其他的衛星，在幾百公里之外。由於碎片的密集程度不同，較大的衛星會在碎片盤內側吸積形成之後，其產生的引力牽引會攪動碎片盤的外側，允許形小型衛星，如衛星火衛一和火衛二。這項研究發表在《自然地球科學》期刊上。
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 2 centimeters of altitude per year to Mars’s tidal pull. It will plunge into the planet in 20 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.
在這一假設下，我們今天之所以只看到火衛一和火衛二，是因為那些較大的衛星在它們形成幾百萬年後被摧毀。它們運行的低、快速軌道超過火星自轉，匯成推動潮汐力使它們盤旋而下，撞上星球。（相比之下，地球上的衛星，環繞軌道運行的速度慢於地球自傳。允許衛星像螺旋一樣外傳並生存下來。）未來的調查可以通過尋找由隕落的衛星所形成的火星隕石坑集群，來測試這一新假設。但與此同時，證明火星可以殺死它同伴的證據就在眼前：火衛二的運行軌道是穩定的，但火衛一處在“死亡螺旋”中，由於火星的潮汐力，每年高度降低2釐米。在未來二千至四千萬年內，它將被拉向火星，孤獨地隕落。非常遙遠的火衛二，其最後的遺跡可能是一個曾經輝煌的火星衛星系統。
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