Never mind planets. The solar system’s icymoons may well be the best places to hunt for aliens

 別管其他行星了。太陽系中寒冷的衛(wèi)星有可能是尋找外星人的最佳場所

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.

幾十年來，在太陽系之外找尋其他生命體的學(xué)者們都遵循著這樣一條簡單的規(guī)則：“追隨水”。水對地球上的生命體是必不可少的物質(zhì)，其特殊的性質(zhì)讓尋找外星人的學(xué)者認(rèn)為水在其他地方也是不可或缺的。這也是火星受到如此多關(guān)注的一個(gè)原因?；鹦堑拇髿庵杏兴魵猓浔睒O圈有水凍冰塊。組成生命所需的流動(dòng)水可能存在于地下，可能有時(shí)仍能在表層看見，表層中干涸的河流湖泊表明流動(dòng)水曾經(jīng)很普遍。軌道航天器曾發(fā)現(xiàn)坑壁上出現(xiàn)了神秘的黑色溝渠；正在探索火星的兩架美國漫游者中較新的一架“好奇號”最近的觀測結(jié)果表明這些溝渠有可能是滴鹽水，由于其高鹽分含量得以在如此冰冷的氣候下保持液態(tài)。

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.

盡管如此，火星充其量是一個(gè)打了幾塊濕補(bǔ)丁的沙漠。在遠(yuǎn)離太陽的地方有許多冰球體，他們的固體表面下似乎有著一整片海洋的液態(tài)水。即便在太空探索的標(biāo)準(zhǔn)中，這些幾十億公里以外且不能被直接觀測到的水對于這門課題的研究也是一大挑戰(zhàn)。但應(yīng)對這個(gè)挑戰(zhàn)已經(jīng)有了一些計(jì)劃。5月26日，美國航天宇航局（NASA)宣布為“木衛(wèi)二快船“選定了科學(xué)儀器套件，計(jì)劃在2020年初發(fā)射，這項(xiàng)任務(wù)耗資20億的。盡管其名字很航海，但它實(shí)際上不會(huì)在木衛(wèi)二的海洋中航行，木衛(wèi)二是火星的一顆冰冷的衛(wèi)星。然而它可能可以發(fā)現(xiàn)它們的一些化學(xué)屬性，以及他們的生物化學(xué)屬性，僅僅是可能而已。

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.

火星沒能夠提供組成生命體的分子結(jié)構(gòu)的生物化學(xué)屬性。40年前，兩架海盜號探測器在火星著陸，并對表面的物質(zhì)進(jìn)行取樣，做了許多化學(xué)測試以此來尋找生命體。結(jié)果讓人沮喪；并不是所預(yù)期的沒有生命，而是看起來無法兼容生命。此后，所有用于研究火星的衛(wèi)星和探測器就連細(xì)菌都沒有發(fā)現(xiàn)。

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.

所以科學(xué)家對于其他含水衛(wèi)星的興趣大增也不足為奇了。據(jù)證實(shí)，土星的一顆小衛(wèi)星，土衛(wèi)二以及木衛(wèi)二有充足的水：土衛(wèi)二的水量可能有蘇必利爾湖那么多。而木衛(wèi)二的水量可能比整個(gè)地球加起來還多。

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.

這聽起來可能很奇怪。木星和土星距離太陽的距離比火星遠(yuǎn)得多。木衛(wèi)二的平均溫度為-171°C，非常寒冷。但太陽光不是讓一個(gè)天體熱起來的唯一途徑，重力也可以。對大多數(shù)地球上的生物來說，由于衛(wèi)星和太陽的重力作用產(chǎn)生的潮汐只會(huì)對海平面產(chǎn)生作用。但像木星或土星這種大衛(wèi)星的重力作用對其軌道上的衛(wèi)星產(chǎn)生的潮汐的力量大到能揉捏衛(wèi)星核心的巖石，通過摩擦使其升溫。這種從下而來的加熱使土衛(wèi)二和木衛(wèi)二都能在其冰涼寒冷的表面下蘊(yùn)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.

就土衛(wèi)二而言，已經(jīng)觀測到其部分海洋已流失到太空中。一架名為卡西尼的觀測器發(fā)現(xiàn)有幾柱水流從靠近衛(wèi)星南極的地方噴射而出，這架觀測器已經(jīng)沿著土星軌道運(yùn)行了10幾年。木衛(wèi)二也同樣時(shí)不時(shí)的有水流噴涌而出。2013年，哈勃太空望遠(yuǎn)鏡觀測到衛(wèi)星表面有看上去像羽毛狀的物體噴射而出。

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.

生命體不能光靠水而存活。其他物質(zhì)，尤其是碳和氮也是必須的。因此這也是土衛(wèi)二和木衛(wèi)二如此誘人的原因。有跡象表明這兩顆衛(wèi)星的海洋都與一個(gè)巖石海底相接觸，這個(gè)海底能提供生命體所需的元素。木衛(wèi)二的這種跡象不是直接而來的，而是從其內(nèi)部的電腦模型而來并分析了穿越其表面的褐色條紋，這些褐色條紋被認(rèn)為是由于太陽輻射而形成的退了色的鹽沉積物。

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，土衛(wèi)二的這種跡象非?？煽?。當(dāng)卡西尼觀測器觀察到衛(wèi)星表面噴射而出的羽毛狀物質(zhì)后不久，其控制員定向讓其陷入其中一處。盡管卡西尼并不是用來完成這類任務(wù)的，但其飛行器還是成功捕獲了一些噴霧并對它進(jìn)行了分析。結(jié)果表明這些物質(zhì)恰恰就是生命體所需的那類復(fù)雜的碳基分子。

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.

至于是否它們真的是帶來生命的理由就另當(dāng)別論了。但是研究者越是仔細(xì)研究土星，就發(fā)現(xiàn)它變得越發(fā)有趣。3月12日，科羅拉多大學(xué)的肖恩.許和他的同事在《自然》雜志上發(fā)表了一篇論文，分析了從羽毛狀物質(zhì)中噴射出的微小硅酸鹽顆粒。他們的結(jié)論是這些顆?？隙ㄊ莵碜詿嵋簢娍谔幒罅葵}的堿性海洋底部，海洋的堿性成分是由富含礦物質(zhì)的水循環(huán)通過海洋河床上的結(jié)構(gòu)。這種熱泉噴口在地球上的海洋中也存在，并且作為生命起源最有說服力的場所。實(shí)驗(yàn)表明它們內(nèi)部的小口袋就好像原始細(xì)胞，將內(nèi)部的有機(jī)化學(xué)物質(zhì)聚集并在整個(gè)“細(xì)胞”壁內(nèi)保持一個(gè)電壓差以此來產(chǎn)生化學(xué)反應(yīng)。這些物質(zhì)，理論上來說是活細(xì)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.  

太陽系天體可能有海洋的天體不單單只有木衛(wèi)二和土衛(wèi)二。今年三月，德國科隆大學(xué)的研究者由喬基姆.索爾帶領(lǐng)，宣布木星最大的衛(wèi)星木衛(wèi)三在地表下面有海洋；因此木星，土星甚至遙遠(yuǎn)的海王星的其他衛(wèi)星也可能有類似的海洋。谷神星，小衛(wèi)星帶上的一顆矮衛(wèi)星，以及比海王星更加遠(yuǎn)的矮衛(wèi)星——冥王星，也可能擁有海洋。但目前為止，大家的注意力都集中在木衛(wèi)二和土衛(wèi)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推進(jìn)木衛(wèi)二快船，這架剛被宣布使用的儀器就能更多的解析木衛(wèi)二，當(dāng)它環(huán)繞木星時(shí)會(huì)時(shí)不時(shí)略過木衛(wèi)二和其異常稀薄的大氣層。幸運(yùn)的話，如果發(fā)現(xiàn)像2013年哈勃觀測到的一縷水，那么它就能夠直接飛越其中了解海洋。如果沒那么幸運(yùn)，那么海洋水樣的直接檢測就需要被推遲數(shù)十年，耗費(fèi)更多資金嘗試著陸和鉆井方式。

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.

木衛(wèi)二比土衛(wèi)二遠(yuǎn)得多，但在很多方面木衛(wèi)二更具希望，也是更切實(shí)際的目標(biāo)。NASA位于加利福尼亞州艾姆斯研究中心的衛(wèi)星科學(xué)家克里斯.麥凱指出，正如卡西尼號已經(jīng)證明的那樣，水射流能夠讓任務(wù)變得簡單。只需要飛越水層然后將采集到的樣本帶回地球。樣本不需要一定是有機(jī)物。甚至是在有些情況下，只采集到組成蛋白質(zhì)的原材料氨基酸也非常具有價(jià)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.

麥凱博士希望能從木衛(wèi)二那里輕易采樣回來，也許可以證明這個(gè)觀點(diǎn)的救星。日本宇宙航空研究開發(fā)機(jī)構(gòu)已經(jīng)與NASA有所接觸，期望能夠聯(lián)手進(jìn)行這個(gè)任務(wù)。日本科學(xué)家和美國科學(xué)家一樣，掌握相似的返回樣本，曾在2010年利用探測儀隼鳥號從一顆彗星羽中檢測到灰塵。如果日本方面能夠承擔(dān)一半的花費(fèi)，那么木衛(wèi)二之旅連同土衛(wèi)二之旅就都有可能進(jìn)入NASA的預(yù)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.

麥凱博士稱，去到木衛(wèi)二的好處是這個(gè)單一的空間任務(wù)能夠解決是否衛(wèi)星能夠居住，而不僅僅是適宜居住的問題。生物和非生物化學(xué)物質(zhì)都能產(chǎn)生氨基酸原材料。但它們中有許多具有“手性”，也就是說同一物質(zhì)可以有兩種排列方式，就好像四個(gè)手指和一個(gè)大拇指可以組成左手或右手一樣。簡單的化學(xué)反應(yīng)傾向于使化學(xué)物質(zhì)同時(shí)具有左手和右手的性質(zhì)。地球上的生物化學(xué)成分，看起來在其他地方也適用，傾向于產(chǎn)生左手性質(zhì)的化學(xué)產(chǎn)物。如果飛行器飛越水層獲取了許多左手性質(zhì)的氨基酸，我們基本可以斷定在木衛(wèi)二上是否存在外星人。