Water May Not Be the Only Sign of Alien Life水可能不是外星生命的唯一標志

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.

說到尋找外星生命，科學家關注的大多集中在該地是否有水存在。現在，研究者認為，查看諸如磷、鉬等“生物必需”元素可能有助于判斷一個星球到底有沒有生命存在。

在地球上，無論是高空的云層還是地殼最深層，幾乎哪里有水，哪里就有生命。因此，尋找外太空生命時，人們通常會把視線投向溫度易于表面儲存液態水的“宜居”星球。

比如，盡管現在的金星表面已經熱得足以融化鉛，2016年的一項研究卻顯示，這顆行星可能一直到7.15億年前都是宜居的。科學家們甚至推測，如果金星上曾經有生命存在，那么這種生命至今可能還存在于金星的云層中。

然而，哈佛大學天文系主任、研究報告的主要作者阿維·勒布說，“當然，就我們所熟悉的生命而言，還有其他必要元素”。比如，在地球上，決定海洋可以孕育多少生命的關鍵元素可能包括氮和磷。氮是制造蛋白質不可缺少的，而氮和磷又都是DNA和RNA的關鍵組分。研究人員提到，近期的多項研究表明，大約6.35億到8億年前海洋中增加的磷元素甚至可能促進了地球上的動物進化。

為了弄清這些生物必需元素在外星生命的進化中可能起到怎樣的作用，研究人員集中研究了在冰凍表面下有著液態海洋的星球上——類似木衛二及土衛二——這些元素有多容易獲取。勒布說：“人們猜測，在木衛二和土衛二冰面下的液態水中可能存在生命。美國航空航天局與歐洲航天局也都計劃探索這兩顆星球，如‘歐羅巴快帆一號’探索任務。”

在地球上，海洋中的磷元素主要是通過微酸雨水侵蝕所謂的長英質巖石獲取。而這些磷元素又隨著熱液活動離開海洋。研究人員表示，之前的研究表明土衛二上有熱液活動，并且很可能木衛二上也存在。

木星的輻射一直照著木衛二的表面，生成名叫氧化劑的微粒。而當木衛二的冰封表面發生劇烈攪動時，這些氧化劑就會進入表層下隱藏的海洋中，與硫化物反應，使海水呈強酸性。這樣一來，木衛二就可能擁有足夠維持生命體存在的磷，但是研究者也指出，強酸性的海水可能扼殺生命體存活的機會。

與此相反的是，此前的研究表明，土衛二的地下海洋可能是強堿性的。在新的研究中，科學家們推測，如果一顆星球上的海水呈中性或堿性并且有熱液活動，那么“磷可能在短短幾百萬年間就從地下海洋世界完全消失——說這幾百萬年短，是相對整個太陽系的壽命而言的”，該研究報告領銜作者、哈佛天體物理學家馬納斯維·林格姆如是說。

科學家們還表示，鉬、錳和鈷這樣的痕量金屬也可能是生物必需的。林格姆說，“鉬在好幾種酶中起著決定性作用，尤其在固氮方面”——也就是說，分解使氮原子在大氣中成對存在的強大的化學鍵，并將分解而來的單氮原子“固定”到極為重要的有機分子中。另外，他解釋說，鉬“對很多有機體的蛋白質合成、新陳代謝和生長有影響”。

此外，林格姆指出：“錳在葉綠體通過光合作用產生氧氣的過程中起著重要作用。鈷則在新陳代謝中具有多種生物學作用——尤為重要的是，它是維生素B-12的一種成分。”

紐約羅切斯特大學的天體物理學家亞當·弗蘭克說：“宜居區的概念可以追溯到20世紀50年代。自那之后我們又了解了許多新東西，比如地下海洋的存在，因此，考慮宜居條件時不能只考慮水，還要考慮生命必需的具體元素和化學物質，這一點很重要。”弗蘭克沒有參與這項研究。

如果要遠程判斷太陽系外的世界是否具有生物必需的元素，有一個辦法就是觀察恒星，恒星可能會揭示其行星及衛星的組成成分。恒星中一種元素的存在會使其產生一種星光中可見的獨特色譜，“從而可能為我們提供其行星是否宜居的信息”，林格姆說。

如果一顆星球上的生物必需元素含量很低，那或許會限制有潛在生命的可能性。盡管新的研究表明，未來探索木衛二和土衛二能真正發現生命的可能性很低，但這樣的探索“可以提供證偽我們模型的良機，因此我們支持這樣的探索任務”，林格姆說。

位于紐約州伊薩卡的康奈爾大學的行星科學家喬納森·盧寧沒有參與這項研究，他提醒說：“這些都是基于簡單假設的計算，我們必須始終記住，行星和衛星比我們想的要復雜得多——這正是行星探索的發現給我們的一個教訓。所以，我們不應該把這些結果當作定論，而應將其視為未來探索任務進行某些所需觀察的一種方法。”

林格姆說，研究者們提醒，雖然生物必需元素在一顆星球上或許普遍非常稀少，但也可能存在一些區塊，這些元素的含量及生命存在的可能性更高些。當然，研究者這里所說的生命僅指我們所熟悉的生命——勒布說：“我們不熟悉的生命遵循的化學路徑可能不同于地球上的，這會比發現我們熟悉的生命更令人激動。”

（譯者單位：上海外國語大學）