Water May Not Be the Only Sign of Alien Life水可能不是外星生命的唯一标志

蔡宙

When it comes to looking for alien life， scientists mostly focus on where there is water. Now researchers suggest that looking at “bioessential” elements such as phosphorus and molybdenum could help judge a world’s potential for life.

There is life virtually wherever there is water on Earth， from clouds high above the surface to the deepest layer of Earth’s crust. As such， the search for life outside Earth typically concentrates on worlds that are “habitable，” possessing temperatures conducive to hosting liquid water on its surface.

For example， although the surface of Venus is currently hot enough to melt lead， a 2016 study suggested it may have been habitable until as recently as 715 million years ago. Scientists have even conjectured that if life once existed on Venus， it still might survive within its clouds.

However， “there are of course other ingredients needed for life as we know it，” said study senior author Avi Loeb， chair of astronomy at Harvard University in Cambridge. For example， on Earth， elements that are key to how much life oceans hold may include nitrogen and phosphorus. Nitrogen is needed to create proteins， and both nitrogen and phosphorus are key ingredients of DNA and RNA. Several recent studies suggest the increased availability of phosphorus in the oceans about 635 million to 800 million years ago may have even helped support the evolution of animals on Earth， the researchers noted.

To see what roles such bioessential elements might play in the evolution of alien life， the researchers focused on how accessible they might be on worlds with liquid oceans underneath their frozen surfaces， much like Jupiter’s moon1 Europa2 and Saturn’s moon Enceladus3. “People suspect there may be life in liquid water under the ice in Europa and Enceladus， and both NASA and ESA [the European Space Agency] have plans to visit them with missions such as Europa Clipper 1，” Loeb said.

On Earth， a major source of phosphorus in oceans is via the weathering of so-called felsic rock by mildly acidic rainwater. Phosphorus is in turn removed from Earth’s seas by hydrothermal activity; prior work suggests hydrothermal activity exists on Enceladus and presumably Europa as well， the researchers said.

Radiation from Jupiter constantly bathes Europa’s surface， generating molecules known as oxidants， and as Europa’s icy surface churns， these oxidants can enter Europa’s hidden seas， where they can react with sulfides and make the water highly acidic. As such， Europa may possess enough phosphorus to support life， although highly acidic oceans might stifle chances for life， the researchers said.

In contrast， previous research suggested the underground oceans of Enceladus may be strongly alkaline. In the new study， the scientists calculated that if a world’s oceans are either neutral or alkaline and possess hydrothermal activity， “phosphorus may be entirely removed from the subsurface ocean world in a very short timescale compared to the age of the solar system—millions of years，” said study lead author Manasvi Lingam， an astrophysicist at Harvard.

The scientists also suggested trace metals such as molybdenum， manganese and cobalt might prove bioessential as well. “Molybdenum plays a crucial role in several enzymes， most notably in fixing nitrogen”—that is， breaking apart the powerful chemical bonds that hold nitrogen atoms in pairs in the atmosphere and “fixing” the resulting single nitrogen atoms into vital organic molecules， Lingam said. In addition， molybdenum “affects protein synthesis as well as metabolism and growth in many organisms，” he explained.

Furthermore， “manganese plays an important role in the context of generating oxygen via photosynthesis in chloroplasts，” Lingam said. “Cobalt has a variety of biological roles in metabolism—most notably， it forms a component of the vitamin B-12.”

“The idea of a habitable zone goes back to the 1950s， and since that time we’ve learned a lot， such as the existence of subsurface oceans， so it’s important to move our thinking about habitability being just about water to specific elements and chemicals that might be essential for life，” said astrophysicist Adam Frank at the University of Rochester in New York， who did not participate in this study.

One way to remotely see if alien worlds outside of our solar system might have bioessential elements is to look at their stars， which might shed light on the compositions of their planets and moons. The presence of an element in a star would generate a unique spectrum of colors visible in its starlight， and “could thereby provide us with some information regarding the habitability of any planets orbiting them，” Lingam said.

If a world’s levels of bioessential elements are low， that may limit their potential for life as we know it. Although the new study suggested that future missions to Europa and Enceladus only had a slim chance of detecting life， they are “an excellent opportunity to falsify our model， and therefore we are in favor of such missions，” Lingam said.

Planetary scientist Jonathan Lunine at Cornell University in Ithaca， New York， who did not take part in this research， cautioned “these are calculations based on simple assumptions， and we must always remember that planets and moons are more complex than we expect—that is one of the lessons of the discoveries made by planetary exploration. So we should not take the results as definitive， but as a way of pointing toward some of the observations that ought to be made by future missions.”

The researchers cautioned that although bioessential elements might prove rare on average across a world， patches could exist on a world where levels of these elements， and the chances for life， are higher， Lingam said. And of course， the researchers only accounted for life as we know it—“ life as we do not know it might follow different chemical routes than on Earth， which would be a more exciting finding than finding life as we do know it，” Loeb said.

说到寻找外星生命，科学家关注的大多集中在该地是否有水存在。现在，研究者认为，查看诸如磷、钼等“生物必需”元素可能有助于判断一个星球到底有没有生命存在。

在地球上，无论是高空的云层还是地壳最深层，几乎哪里有水，哪里就有生命。因此，寻找外太空生命时，人们通常会把视线投向温度易于表面储存液态水的“宜居”星球。

比如，尽管现在的金星表面已经热得足以融化铅，2016年的一项研究却显示，这颗行星可能一直到7.15亿年前都是宜居的。科学家们甚至推测，如果金星上曾经有生命存在，那么这种生命至今可能还存在于金星的云层中。

然而，哈佛大学天文系主任、研究报告的主要作者阿维·勒布说，“当然，就我们所熟悉的生命而言，还有其他必要元素”。比如，在地球上，决定海洋可以孕育多少生命的关键元素可能包括氮和磷。氮是制造蛋白质不可缺少的，而氮和磷又都是DNA和RNA的关键组分。研究人员提到，近期的多项研究表明，大约6.35亿到8亿年前海洋中增加的磷元素甚至可能促进了地球上的动物进化。

为了弄清这些生物必需元素在外星生命的进化中可能起到怎样的作用，研究人员集中研究了在冰冻表面下有着液态海洋的星球上——类似木卫二及土卫二——这些元素有多容易获取。勒布说：“人们猜测，在木卫二和土卫二冰面下的液态水中可能存在生命。美国航空航天局与欧洲航天局也都计划探索这两颗星球，如‘欧罗巴快帆一号’探索任务。”

在地球上，海洋中的磷元素主要是通过微酸雨水侵蚀所谓的长英质岩石获取。而这些磷元素又随着热液活动离开海洋。研究人员表示，之前的研究表明土卫二上有热液活动，并且很可能木卫二上也存在。

木星的辐射一直照着木卫二的表面，生成名叫氧化剂的微粒。而当木卫二的冰封表面发生剧烈搅动时，这些氧化剂就会进入表层下隐藏的海洋中，与硫化物反应，使海水呈强酸性。这样一来，木卫二就可能拥有足够维持生命体存在的磷，但是研究者也指出，强酸性的海水可能扼杀生命体存活的机会。

与此相反的是，此前的研究表明，土卫二的地下海洋可能是强碱性的。在新的研究中，科学家们推测，如果一颗星球上的海水呈中性或碱性并且有热液活动，那么“磷可能在短短几百万年间就从地下海洋世界完全消失——说这几百万年短，是相对整个太阳系的寿命而言的”，该研究报告领衔作者、哈佛天体物理学家马纳斯维·林格姆如是说。

科学家们还表示，钼、锰和钴这样的痕量金属也可能是生物必需的。林格姆说，“钼在好几种酶中起着决定性作用，尤其在固氮方面”——也就是说，分解使氮原子在大气中成对存在的强大的化学键，并将分解而来的单氮原子“固定”到极为重要的有机分子中。另外，他解释说，钼“对很多有机体的蛋白质合成、新陈代谢和生长有影响”。

此外，林格姆指出：“锰在叶绿体通过光合作用产生氧气的过程中起着重要作用。钴则在新陈代谢中具有多种生物学作用——尤为重要的是，它是维生素B-12的一种成分。”

纽约罗切斯特大学的天体物理学家亚当·弗兰克说：“宜居区的概念可以追溯到20世纪50年代。自那之后我们又了解了许多新东西，比如地下海洋的存在，因此，考虑宜居条件时不能只考虑水，还要考虑生命必需的具体元素和化学物质，这一点很重要。”弗兰克没有参与这项研究。

如果要远程判断太阳系外的世界是否具有生物必需的元素，有一个办法就是观察恒星，恒星可能会揭示其行星及卫星的组成成分。恒星中一种元素的存在会使其产生一种星光中可见的独特色谱，“从而可能为我们提供其行星是否宜居的信息”，林格姆说。

如果一颗星球上的生物必需元素含量很低，那或许会限制有潜在生命的可能性。尽管新的研究表明，未来探索木卫二和土卫二能真正发現生命的可能性很低，但这样的探索“可以提供证伪我们模型的良机，因此我们支持这样的探索任务”，林格姆说。

位于纽约州伊萨卡的康奈尔大学的行星科学家乔纳森·卢宁没有参与这项研究，他提醒说：“这些都是基于简单假设的计算，我们必须始终记住，行星和卫星比我们想的要复杂得多——这正是行星探索的发现给我们的一个教训。所以，我们不应该把这些结果当作定论，而应将其视为未来探索任务进行某些所需观察的一种方法。”

林格姆说，研究者们提醒，虽然生物必需元素在一颗星球上或许普遍非常稀少，但也可能存在一些区块，这些元素的含量及生命存在的可能性更高些。当然，研究者这里所说的生命仅指我们所熟悉的生命——勒布说：“我们不熟悉的生命遵循的化学路径可能不同于地球上的，这会比发现我们熟悉的生命更令人激动。”

（译者单位：上海外国语大学）