【双语阅读】Lunacy and astronomy 天文也疯狂

Lunacy and astronomy
天文也疯狂
In praise of moons
夸夸卫星
They’re number two. They try harder
他们不被看好，但在努力证明自己

TO BE parochial on an astronomical scale sounds like a tall order. Yet people manage it all the time. When they think about the possibility of life in other parts of the universe, they think of it happening on planets. But this is simply local prejudice. Moons, which Earthlings tend to see merely as adornments to the night sky, are just as good a place to look for life as planets are, perhaps even better. Which is why astronomers are trying to spot the moons of planets around other stars (see article). And life is just a part of it. To anyone not hung up on hierarchies of who-orbits-whom, moons are in all manner of ways more various and more intriguing than planets.

要说在广阔天文学领域存在着狭隘理念听起来不可思议，但人们还真一直都有这种想法。每当谈及地外哪里可能有生命的存在时，人们讨论的范围都是都是在行星之间。而卫星仅仅被视为夜空中的装饰。这纯粹是种偏见，其实在这些卫星上和行星一样，都有存在生命的可能，甚至在卫星上的可能性还更高。为此，天文学家正努力在星空中寻找行星卫星，也顺带寻找地外生命。有些人则不纠结于行星、卫星谁绕着谁转的主次关系，对他们来说卫星比行星更多样，也更具吸引力。

It is true that moons are, on average, small. But big moons can be larger than small planets. And what they lack in size moons make up for in variety and activity. Their surfaces range from the stark cratering of Earth’s moon through the cantaloupe-skin of Triton, which circles Neptune, to the cue-ball-smooth ice of Europa and the lurid leprous sulphur of Io, both companions of Jupiter. And though the three rocky planets that are not the Earth—Mercury, Venus and Mars—appear to be geologically quiescent, that is not true of many moons. Spacecraft passing both Triton and Enceladus, a satellite of Saturn, have seen geysers that dwarf those of Iceland or Yellowstone, and the volcanoes of Io are in constant tumult.

总的来说，卫星体积都比较小，但也有也比小行星体积更大的卫星存在。小体积的卫星种类更丰富，运动更活跃。它们表面有的千疮百孔，比如月球；有的像是瓜皮，比如海卫一；有的则布满跟溜溜球一样光滑的冰，比如木卫二；更有甚者表面铺满硫磺，看上去就像得了麻风病一样恐怖，比如木卫一。九大行星中除地球外有三个石质行星——水星、金星和火星，他们在地质活动方面都表现平静。但对多数卫星来说则恰恰相反，飞行探测器经过海卫一和土卫二时发现，这两颗行星上的地热喷泉要比冰岛和黄石公园的同类大得多；而海卫二上的火山更是日以继夜的活动着。

Planets do have an edge when it comes to atmospheres. Venus, Earth and Mars all boast thick enough envelopes of gas to have weather; among the moons only Saturn’s Titan has anything to speak of in that department. But in the matter of oceans—widely seen as a near-necessity for life—the moons do well. Venus and Mars long ago misplaced any seas they may once have had, leaving Earth the only oceanic planet. Earth’s oceans, though, are smaller than those beneath the icy crust of Europa—and Europa’s siblings Callisto and Ganymede may well have subsurface oceans, too, as may Triton. Titan, meanwhile, has rivers, clouds, rains and lakes of methane.

行星的边缘都有大气层：比如金星、地球和火星都孕育了足以产生多样天气的大气层。而在卫星中，只有土卫六有类似的大气。但卫星上也存在万物赖以生存的海洋。金星和火星上面的海洋似乎在很久以前放错了地方至今不复存在，地球是目前唯一有海洋的行星。但是要知道，木卫一的冰壳面积比我们地球的海洋还要广阔，而木卫三和木卫四表面下面也可能存在海洋；同样，在海卫一、土卫六表面下野可能有液态甲烷组成的河流、云层和降雨。

In other star systems, moons have many advantages as abodes for life. One of the discoveries that has come with the ability to detect planets around other stars is that planetary systems are knockabout places in their early years. A lot of small planets get flung out of their orbits into outer darkness. This is less likely to happen to such a body if it is the moon of a large planet, so moons may survive preferentially. And moons can be hospitable where planets are not. An Earth-sized planet too close to its star will end up with one face permanently illuminated and one eternally dark. But a moon going around a large planet in such an orbit is saved from such a fate; as it orbits its parent orbiting the star all of its surface will, from time to time, be bathed in the star’s welcome light.

在其它星系系统中，卫星们有许多作为生命住所的优势。依靠进步的科技，在探测外星系行星时，人们发现其实太阳系早期其实是一个大杂烩。在宇宙形成初期，许多小行星都脱离轨道而被抛向无尽的黑暗里。但许多依附了较大行星的卫星则不会被抛走，而得以幸存。卫星比行星更加友好。地球大小的行星如果太靠近恒星则会使得一面永久光明、另一面永久黑暗。但卫星由于绕行星运转得以避免这种厄运。因为卫星的母星在绕着恒星周转，这样的话，卫星的整个表面都可以按周期沐浴到太阳光了。

In the heavens as it is not on Earth
天空中和地球上可不一样

Moons orbiting planets far away from their stars may also be more hospitable to life than Earth-sized planets would be in similar orbits. A big planet gives off heat. That warms its moons. And further warmth is available from tides. The tides Earth’s moon raises on its parent can be powerful things. Those raised by planets on their moons are more powerful still, since a planet is bigger, and thus has stronger gravity, than its moons. It is the heat produced by constant tidal kneading of their interiors that makes Enceladus spout geysers and Io volcanoes when planets of their size would sit inert.

远离恒星的行星卫星在相似轨道下比起和地球体积差不多的行星更加适合生命生长。大行星会散发热量，给它的卫星取暖，而潮汐也能进一步带来热能，地球上的潮汐由月球引起，力量强大。而由行星引力导致的卫星潮汐则会更加可观，这也因为行星比卫星体积更大，引力更强。持续的潮汐使得星体内部得到运动，就像土卫二的喷泉和海卫一的火山就是由此产生，本来这些星体的活性还是比较平静的。

No one can say whether life is more likely on moons than on planets—but astronomy stands ready to answer such questions over the coming decades. Humans assume that the way things are on Earth is the way things must be. Science is there to help them widen those narrow minds.

目前，还没人能够确定卫星比行星更有可能存在生命，但天文学的进步将会在未来给出这个问题的答案。人类总假设地球的生命模式是惟一的。但科学正是为了去除偏见，增广人类的视野。