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：阿塔卡马沙漠是地球上除两极外最干旱的地域，研讨发现这里也有生命存在。而作爲地球上最接近火星外表的中央，对天体生物学家的火星研讨有更普遍的意义。

位于南美洲的阿塔卡马沙漠简直从不下雨。一条沿着智利海岸线延伸的1000公里长的地带，就是地球上除两极之外最枯燥的沙漠。某些地域的年均匀降雨量可低至每年一毫米或两毫米，一些阿塔卡马的气候站自从设立以来甚至从未见过降水。

IT DOES not rain much in the Atacama desert. A 1,000km strip of land running along the Chilean coast, it is Earth’s driest desert outside its poles. Average annual rainfall in certain parts can be as low as a millimetre or two a year, and some Atacaman weather stations have never seen a drop of water.

但这里偶然也会下雨。正如柏林科技大学的天体生物学家德克舒尔茨-马库奇和他的同事在《美国国度迷信院院刊》上宣布的报告一样：2015年的一场沙漠暴雨提供了一个完满的自然实验，次要用于探测生命所能接受的各种形态的极限。

Yet it does rain occasionally. And as Dirk Schulze-Makuch, an astrobiologist at the Technical University of Berlin, and his colleagues report in Proceedings of the National Academy of Sciences, a desert downpour in 2015 offered the perfect natural experiment for probing the limits of what sorts of conditions life can tolerate

但是阿塔卡马并不是完全没有生命的。一些特殊的动动物在不是那麼干旱的地域勉强生活。迷信家们甚至在最爲干旱的地域发现了微生物生命的证据。但是，令人不太清楚的是，这些微生物能否由于休眠而可以忍耐这种极端干旱，还是说它们仅仅是被风吹来却无法在恶劣的新环境中生活的闯入者的死亡遗骸。

The Atacama is not quite lifeless. A few specialized animals and plants scrape a living in the less arid parts. And scientists have found evidence of microbial life even in the very driest areas. What is less clear, though, is whether those microbes are natives able to endure such extreme aridity, perhaps by becoming dormant, or whether they are merely the dead remains of interlopers, blown in on the wind but unable to survive in their harsh new environment.

这次降雨提供了一个检验以上两种假说的时机。假如这些微生物还活着（知识处于休眠形态），那麼这场雨就会迎来一个长久的黄金时代，由于生态零碎中最稀缺的资源，水，暂时变得充足了。但是，假如沙漠只是分布着尸体，那麼降雨对该地域生活的活动性就没有什麼区别了。

The rains offered a chance to check. If the microbes were alive, or dormant, then the rain should have ushered in a brief golden age of growth, as the scarcest resource in the ecosystem—water—became briefly abundant. If the desert was merely strewn with corpses, though, then it would have made no difference.

因而，在2015年的降雨后不久，舒尔茨-马库奇博士和他的同事们在沙漠中停止发掘，并从不同深度的土壤中采集样本。他们在2016年和2017年前往以获取更多样本。2015年的数据显示了微生物代谢的证据，包括三磷酸腺苷（ATP）分子（ATP分子被一切细胞用来贮存能量），以及酶、脂肪酸和生化进程的副商品。最重要的是，当他们检验降雨当时两年的样品时，一切这些化学物质都变得十分稀少。2017年的沙漠土壤中ATP分子的程度约爲2015年的千分之一，而其它生物分子的程度也有相似的下降。这些研讨后果失掉了其它证据的支持，标明不同品种的细菌和真菌存在于不同层次的沙漠土壤中。这标明了这个沙漠中存在有组织的生态零碎，而不只仅是细菌墓地。

Shortly after the rains in 2015, therefore, Dr Schulze-Makuch and his colleagues dug trenches in the desert and took samples from the soils at various depths. They returned in 2016 and 2017 to get more samples. The ones from 2015 showed evidence of microbial metabolism, including molecules like adenosine triphosphate, or ATP, which is used by all cells to store energy, as well as enzymes, fatty acids and the by-products of biochemical processes. Crucially, when they turned to the later samples, all those chemicals had become dramatically scarcer. Levels of ATP in 2017 were around a thousandth of their value in 2015. There were similar falls for other biological molecules, too. Those results were bolstered by others showing that different kinds of bacteria and fungi were present in different layers of desert soil. That suggests organized ecosystems rather than mere bacterial boneyards.

换句话说，地球上最枯燥的土壤似乎的确可以养活微生物生态零碎，当雨水最终到来时，这些生态零碎会蓬勃开展。作爲生命韧性的证明，这些后果很风趣。但它能够有更普遍的意义。诸如舒尔茨-马库奇博士等天体生物学家以为的，阿塔卡马是地球上最接近火星外表的中央。有少量的地质证据标明，数十亿年前的火星比明天要潮湿的多。来自轨道探测器的后果标明，偶然呈现在古代行星外表的水滴，能够是由地下冰的间歇性消融形成的。假如火星上的确呈现了生命，那麼它能够依然在坚持，在岩层中休眠，等候着下一个消融的水带来的长久脉动。

The driest soils on Earth, in other words, do indeed seem to play host to microbial ecosystems that can flourish when the rains eventually come. As a demonstration of life’s resilience, that is interesting enough. But it may have broader implications. Astrobiologists such as Dr Schulze-Makuch see the Atacama as the closest Earthly analogue for the surface of Mars. There is plenty of geological evidence to suggest that the Mars of billions of years ago was much wetter than it is today. And results from orbiting probes suggest that dribbles of water occasionally appear on the modern planet’s surface, possibly caused by the intermittent melting of subterranean ice. If life did arise on Mars, then it may still be clinging on, lying dormant in the regolith and waiting for the next brief pulse of meltwater.

无可否认，火星比阿塔卡马愈加气候恶劣。除了干旱之外，火星外表十分冰冷，并遭到太阳辐射的冲击。这种辐射会发生弱小的氧化剂，能够会毁坏任何活细胞。假如有什麼生物可以生活上去，它将会被埋藏在火星土壤深处。这就是为何由Roscosmos和欧洲航天局建造并将于2020年发射的ExoMars遨游车，携带可以在地表以下两米处停止发掘的钻机的缘由之一。

Admittedly, Mars is even less hospitable than the Atacama. Besides the aridity, the Martian surface is bitterly cold and blasted by solar radiation. That radiation creates powerful oxidants that would likely destroy any living cells. If anything does survive, it will be deeply buried. That is one reason why the ExoMars rover—built by Roscosmos and the European Space Agency, and due to blast off in 2020—carries a drill capable of digging almost two meters below the surface.

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