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Never mind planets. The solar system’s icymoons may well be the best places to hunt for aliens 别管其他行星了。太阳系中寒冷的卫星有可能是寻找外星人的最佳场所
FOR decades, researchers looking for life elsewhere in the solar system have stuck to a simple rule: “follow the water”.The stuff is essential to life on Earth, and its peculiar properties mean those hunting aliens think the same will be true elsewhere. This is one reason that Mars has received so much of their attention. Mars has water vapour in its atmosphere, and water ice in the shining cap at its north pole. Liquid water—which is the form life needs—may well exist underground, and may still sometimes be seen at the surface, where dried-up rivers and lakes suggest itwas once commonplace. Orbiting spacecraft have seen mysterious dark channels appear on crater walls; recent results from Curiosity, the newer of the two American rovers now exploring Mars, suggest these could be trickles of brine,kept liquid in the bitter cold by their high salt content.
几十年来，在太阳系之外找寻其他生命体的学者们都遵循着这样一条简单的规则：“追随水”。水对地球上的生命体是必不可少的物质，其特殊的性质让寻找外星人的学者认为水在其他地方也是不可或缺的。这也是火星受到如此多关注的一个原因。火星的大气中有水蒸气，其北极圈有水冻冰块。组成生命所需的流动水可能存在于地下，可能有时仍能在表层看见，表层中干涸的河流湖泊表明流动水曾经很普遍。轨道航天器曾发现坑壁上出现了神秘的黑色沟渠；正在探索火星的两架美国漫游者中较新的一架“好奇号”最近的观测结果表明这些沟渠有可能是滴盐水，由于其高盐分含量得以在如此冰冷的气候下保持液态。
But for all that, Mars is at best a desert with a few wet patches. Farther from the sun there are a number of icy bodies that seem to boast whole oceans of liquid water beneath their solid surfaces.Even by the standards of space exploration, this water—billions of kilometres away and hidden from direct inspection—is a challenging subject for study. But there are plans afoot to meet that challenge. On May 26th NASA announced the suite of scientific instruments it has selected for Europa Clipper, a $2 billion mission which could be launched in the early 2020s. Despite its nicely nautical name, it will not actually sail the seas of Europa, an icy moon of Jupiter. But it might be able to find out something of their chemistry—and,just possibly, their biochemistry.
尽管如此，火星充其量是一个打了几块湿补丁的沙漠。在远离太阳的地方有许多冰球体，他们的固体表面下似乎有着一整片海洋的液态水。即便在太空探索的标准中，这些几十亿公里以外且不能被直接观测到的水对于这门课题的研究也是一大挑战。但应对这个挑战已经有了一些计划。5月26日，美国航天宇航局（NASA)宣布为“木卫二快船“选定了科学仪器套件，计划在2020年初发射，这项任务耗资20亿的。尽管其名字很航海，但它实际上不会在木卫二的海洋中航行，木卫二是火星的一颗冰冷的卫星。然而它可能可以发现它们的一些化学属性，以及他们的生物化学属性，仅仅是可能而已。
It is biochemistry—evidence of molecules made by life—that Mars has failed to provide. Four decades ago two Viking probes landed and subjected samples of its surface to various chemical tests as a way of looking for life. The results were frustrating; not quite what was expected from lifelessness, but not seemingly compatible with life. For all the satellites and rovers that have studied Mars since, there has been no sign of so much as a bacterium.
火星没能够提供组成生命体的分子结构的生物化学属性。40年前，两架海盗号探测器在火星着陆，并对表面的物质进行取样，做了许多化学测试以此来寻找生命体。结果让人沮丧；并不是所预期的没有生命，而是看起来无法兼容生命。此后，所有用于研究火星的卫星和探测器就连细菌都没有发现。
It is thus hardly surprising that interest in water elsewhere has risen. And it has turned out that Enceladus, a small moon of Saturn and Europa have it inabundance: Enceladus may have as much as Lake Superior. Europa may have more than the whole Earth.
所以科学家对于其他含水卫星的兴趣大增也不足为奇了。据证实，土星的一颗小卫星，土卫二以及木卫二有充足的水：土卫二的水量可能有苏必利尔湖那么多。而木卫二的水量可能比整个地球加起来还多。
That may sound odd. Jupiter and Saturn are much farther from the sun than Mars is. The average temperature on Europa is achilly -171°C. But sunlight is not the only way to heat an astronomical body.Gravity can do it, too. To most Earthlings, the tides produced by the gravity of the moon and sun matter only to sea levels. But the tides that a huge planet like Jupiter or Saturn can produce in an orbiting moon are powerful enough to knead the rocks of the moon’s core, warming them up via friction. Such heatingfrom below allows both Enceladus and Europa to harbour oceans under their cold,icy surfaces.
这听起来可能很奇怪。木星和土星距离太阳的距离比火星远得多。木卫二的平均温度为-171°C，非常寒冷。但太阳光不是让一个天体热起来的唯一途径，重力也可以。对大多数地球上的生物来说，由于卫星和太阳的重力作用产生的潮汐只会对海平面产生作用。但像木星或土星这种大卫星的重力作用对其轨道上的卫星产生的潮汐的力量大到能揉捏卫星核心的岩石，通过摩擦使其升温。这种从下而来的加热使土卫二和木卫二都能在其冰凉寒冷的表面下蕴藏海洋。
In the case of Enceladus bits of this ocean have been observed escaping into space. A probe called Cassini, which has been orbiting Saturn for more than a decade, has spotted jets of water shooting up from near the moon’s south pole. Europa, too, may spew out water on occasion.In 2013 the Hubble space telescope spotted what seemed to be plumes erupting from the moon’s surface.
就土卫二而言，已经观测到其部分海洋已流失到太空中。一架名为卡西尼的观测器发现有几柱水流从靠近卫星南极的地方喷射而出，这架观测器已经沿着土星轨道运行了10几年。木卫二也同样时不时的有水流喷涌而出。2013年，哈勃太空望远镜观测到卫星表面有看上去像羽毛状的物体喷射而出。
Life cannot live by water alone. Other elements are needed—notably carbon and nitrogen. And that is what makes Enceladus and Europa so tantalising. There is evidence that the oceans on both are in contact with a rocky seafloor that can provide the elements life needs.For Europa, that evidence comes indirectly, from computer modelling of its interior and analysis of the brown streaks that cross its surface, which are thought to be salt deposits discoloured by solar radiation.
生命体不能光靠水而存活。其他物质，尤其是碳和氮也是必须的。因此这也是土卫二和木卫二如此诱人的原因。有迹象表明这两颗卫星的海洋都与一个岩石海底相接触，这个海底能提供生命体所需的元素。木卫二的这种迹象不是直接而来的，而是从其内部的电脑模型而来并分析了穿越其表面的褐色条纹，这些褐色条纹被认为是由于太阳辐射而形成的退了色的盐沉积物。
For Enceladus, by contrast, the evidence issolid. Soon after Cassini spotted the plumes coming from the moon’s surface,its controllers directed it to plunge through one of them. Although it had not been designed for such a mission, the craft was able to capture some of thespray and analyse it. It turned out to be full of exactly the sorts of complicated carbon-based molecules that might serve as precursors to life.
相反，土卫二的这种迹象非常可靠。当卡西尼观测器观察到卫星表面喷射而出的羽毛状物质后不久，其控制员定向让其陷入其中一处。尽管卡西尼并不是用来完成这类任务的，但其飞行器还是成功捕获了一些喷雾并对它进行了分析。结果表明这些物质恰恰就是生命体所需的那类复杂的碳基分子。
Whether they have actually given rise tolife is, of course, another question. But the more researchers look, the more intriguing the Saturnian moon becomes. On March 12th, Sean Hsu of the University of Colorado and his colleagues published a paper in Nature analysing tiny silicate grains spewed out by the plumes. They concluded these were almost certainly produced at the bottom of a salty, alkaline ocean in hydrothermal vents—structures that form when mineral-laden water circulates through hot rocks on an ocean’s floor. Such hydrothermal vents are found in Earth’s oceanstoo, and are a strong candidate for being the site of the origin of life.Laboratory experiments have shown that tiny pockets inside them can act like primitive cells, concentrating organic chemicals inside and maintaining avoltage difference across the “cell” wall that could drive chemical reactions.These things are both, literally, vital features of living cells.
至于是否它们真的是带来生命的理由就另当别论了。但是研究者越是仔细研究土星，就发现它变得越发有趣。3月12日，科罗拉多大学的肖恩.许和他的同事在《自然》杂志上发表了一篇论文，分析了从羽毛状物质中喷射出的微小硅酸盐颗粒。他们的结论是这些颗粒肯定是来自热液喷口处含大量盐的碱性海洋底部，海洋的碱性成分是由富含矿物质的水循环通过海洋河床上的结构。这种热泉喷口在地球上的海洋中也存在，并且作为生命起源最有说服力的场所。实验表明它们内部的小口袋就好像原始细胞，将内部的有机化学物质聚集并在整个“细胞”壁内保持一个电压差以此来产生化学反应。这些物质，理论上来说是活细胞非常重要的特征。
Europa and Enceladus are not the only solar-system bodies likely to have oceans. In March researchers led by Joachim Saur of Cologne University, in Germany, announced that Ganymede, Jupiter’s biggest moon, has a subterranean ocean; so may a number of other satellites of Jupiter, Saturn and even distant Neptune. Indeed so could Ceres, a dwarf planetin the asteroid belt, and Pluto, a dwarf planet even farther off than Neptune.But it is on Europa and Enceladus that, for the moment, attention isfocused.
太阳系天体可能有海洋的天体不单单只有木卫二和土卫二。今年三月，德国科隆大学的研究者由乔基姆.索尔带领，宣布木星最大的卫星木卫三在地表下面有海洋；因此木星，土星甚至遥远的海王星的其他卫星也可能有类似的海洋。谷神星，小卫星带上的一颗矮卫星，以及比海王星更加远的矮卫星——冥王星，也可能拥有海洋。但目前为止，大家的注意力都集中在木卫二和土卫二上。
If NASA moves forward with Europa Clipper,the suite of instruments just announced will allow it to learn a lot about Europa—which it will repeatedly skim past as it orbits Jupiter—and its remarkably tenuous atmosphere. If it is lucky, and a plume of water like those seen by the Hubble in 2013 appears, it will be able to fly right through it and learn about the ocean itself. If it is not, though, direct examination of the ocean’s water will have to be deferred to some far more ambitious landing-and-drilling effort decades hence.
如果NASA推进木卫二快船，这架刚被宣布使用的仪器就能更多的解析木卫二，当它环绕木星时会时不时略过木卫二和其异常稀薄的大气层。幸运的话，如果发现像2013年哈勃观测到的一缕水，那么它就能够直接飞越其中了解海洋。如果没那么幸运，那么海洋水样的直接检测就需要被推迟数十年，耗费更多资金尝试着陆和钻井方式。
Enceladus is a lot farther away than Europa, but in many ways it is both a more promising and a more realistic target. Chris McKay, a planetary scientist at NASA’s Ames research centre, in California, points out that, as Cassini has already demonstrated, the water jets make a mission easy. All you have to do is fly through one and bring whatever you capture back to Earth. That might not necessarily be an organism.Proteins, or even, under some circumstances, just the amino-acid building blocks from which they are made, would be greatly suggestive by themselves.
木卫二比土卫二远得多，但在很多方面木卫二更具希望，也是更切实际的目标。NASA位于加利福尼亚州艾姆斯研究中心的卫星科学家克里斯.麦凯指出，正如卡西尼号已经证明的那样，水射流能够让任务变得简单。只需要飞越水层然后将采集到的样本带回地球。样本不需要一定是有机物。甚至是在有些情况下，只采集到组成蛋白质的原材料氨基酸也非常具有价值。
Dr McKay hopes that the very ease of bringing back a sample from Enceladus may prove the idea’s saviour. NASA has been approached by JAXA, Japan’s space agency, with a view to a joint mission.The Japanese, like the Americans, have experience with sample-return of this sort, having flown a probe called Hayabusa that retrieved dust from the plumeof a comet in 2010. And if the Japanese picked up half the cost, then a trip to Enceladus could just about sneak into NASA’s budget alongside the one toEuropa.
麦凯博士希望能从木卫二那里轻易采样回来，也许可以证明这个观点的救星。日本宇宙航空研究开发机构已经与NASA有所接触，期望能够联手进行这个任务。日本科学家和美国科学家一样，掌握相似的返回样本，曾在2010年利用探测仪隼鸟号从一颗彗星羽中检测到灰尘。如果日本方面能够承担一半的花费，那么木卫二之旅连同土卫二之旅就都有可能进入NASA的预算中。
The advantage of going to Enceladus, says Dr McKay, is that a single space mission could address the question of whether the moon is actually inhabited, rather than merely habitable. Amino-acid building blocks can be produced by both living and non-living chemistry. But many of them are “chiral”, which means the same components can be arranged in two different ways, just as four fingers and thumb can be arranged to make a left hand or a right hand. Straightforward chemistry tends to make things in both right- and left-handed flavours. Biochemistry on Earth—and, it seems very likely, anywhere else—has a very marked preference for the production of left-handed ones. If a probe flying through a watery plume picked up a cargo of left-handed amino acids, that would pretty much close the case on whether there are aliens on Enceladus.
麦凯博士称，去到木卫二的好处是这个单一的空间任务能够解决是否卫星能够居住，而不仅仅是适宜居住的问题。生物和非生物化学物质都能产生氨基酸原材料。但它们中有许多具有“手性”，也就是说同一物质可以有两种排列方式，就好像四个手指和一个大拇指可以组成左手或右手一样。简单的化学反应倾向于使化学物质同时具有左手和右手的性质。地球上的生物化学成分，看起来在其他地方也适用，倾向于产生左手性质的化学产物。如果飞行器飞越水层获取了许多左手性质的氨基酸，我们基本可以断定在木卫二上是否存在外星人。 |
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