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Saturn’s Death Star-looking moon may have vast underground ocean
Astronomers have found the best evidence yet of a vast, young ocean beneath the icy exterior of Saturn’s Death Star lookalike mini moon.
The French-led team analyzed changes in Mimas’ orbit and rotation and reported Wednesday that a hidden ocean 12 to 18 miles (20 to 30 kilometers) beneath the frozen crust was more likely than an elongated rocky core. The scientists based their findings on observations by NASA’s Cassini spacecraft, which observed Saturn and its more than 140 moons for more than a decade before diving through the ringed planet’s atmosphere in 2017 and burning up.
Barely 250 miles (400 kilometers) in diameter, the heavily cratered moon lacks the fractures and geysers — typical signs of subsurface activity — of Saturn’s Enceladus and Jupiter’s Europa.
“Mimas was probably the most unlikely place to look for a global ocean — and liquid water more generally,” co-author Valery Lainey of the Paris Observatory said in an email. “So that looks like a potential habitable world. But nobody knows how much time is needed for life to arise.”
The ocean is believed to fill half of Mimas’ volume, according to Lainey. Yet it represents only 1.2% to 1.4% of Earth’s oceans given the moon’s petite size. Despite being so small, Mimas boasts the second largest impact crater of any moon in the solar system — the reason it’s compared to the fictional Death Star space station in “Star Wars.”
“The idea that relatively small, icy moons can harbor young oceans is inspiring,” SETI Institute’s Matija Cuk and Southwest Research Institute’s Alyssa Rose Rhoden wrote in an accompanying editorial. They were not part of the study.
Believed between 5 million and 15 million years old, too young to mark the moon’s surface, this subterranean ocean would have an overall temperature right around freezing, according to Lainey. But at the seafloor, he said the water temperature could be much warmer.
Co-author Nick Cooper of Queen Mary University of London said the existence of a “remarkably young” ocean of liquid water makes Mimas a prime candidate for studying the origin of life.
Discovered in 1789 by English astronomer William Herschel, Mimas is named after a giant in Greek mythology.
注释：
elongate: v
表示" 延长；伸长"，means "make long or longer by pulling and stretching;"，如：We plan to elongate the cooperation with that company in Australia. 我们打算延长与澳洲那家公司的合作关系。
cratered: adj
表示" 有坑洞的；多坑的"，如：Artillery cratered the roads. 炮击后大路布满了弹坑。
petite: adj
表示"（女子）娇小的"，means "very small"，如：Night after it left the poor bud, as a petite woman, but Moon. 一夜过后，它只剩下可怜的花苞，像一位娇小的女子，却又含苞待放。
中文简要说明：
土星的卫星「土卫一密玛斯」(Mimas)是18世纪的英国天文学家赫歇尔(William Herschel)首次发现的，不过它的精确外型，要到等1980年NASA的航海家1号探测器(Voyager 1)近距离拍摄才能知晓。一个直径140公里的超巨大陨石坑，几乎是土卫一直径的三分之一，其外型就像是科幻电影《星际大战》(STAR WARS)的行星毁灭武器「死星」(Death Star)，也成为土卫一的非官方外号。天文学家发现，这颗死星的冰冷荒芜外表下，可能有着巨大的地下海洋。
美联社报导，法国领导的研究小组分析了土卫一的轨道和自转的变化，并报告称，冰冻地壳下方20至30 公里，可能有着液态水，或者说是地下海洋。
这组科学团队的资料来自美国NASA的卡西尼-惠更斯探测器(Cassini–Huygens)，数十年来对140 多个卫星的观察数据，卡西尼-惠更斯号是最成功的行星探测任务，不过它已于2017撞向土星，燃烧在大气层中。
有地下海洋的卫星并不少，比如木卫二欧罗巴(Europa)的表面有明显的裂纹，而且坑洞很少，被认为是持续融化的冰层填平了坑洞；土卫二恩西拉多斯(Enceladus)也被认为有地下海洋，卡西尼-惠更斯曾拍摄到土卫二的间歇泉。
但是土卫一不同，它布满陨石坑，没有地下活动的典型迹象。
巴黎天文台的莱尼(Valery Lainey)在电子邮件中表示：「土卫一可能是最不可能找到海洋和液态水的地方。所以这研究是推测出来的，或许它也是潜在的宜居世界。但没有人知道生命的出现需要多少时间。」
莱尼表示，推算土卫一的地下水层(海洋)占据了土卫一体积的一半。然而，鉴于它的直径只有400公里，所以它的总水量也只有地球海洋水量的 1.2% 至 1.4%。尽管土卫一很小，它拥有太阳系中第二大的撞击坑——这就是它与《星际大战》中虚构的死星太空站很相似的原因。
莱尼表示，据信这个地下海洋的年龄在 500 万到 1500 万年前，是绕行土星造成的地质错动形成的热量，融化了地下冰层。
该报告的合著者尼克库柏(Nick Cooper)表示，「非常年轻」的液态水海洋，使土卫一成为研究生命起源的主要候选者。
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