《Science Advances》：太陽系中極有可能有地外生命存在
導讀：自古以來，人類最想回答的問題之一就是：地球外有生命嗎？而現在科學家比以往任何時候都更接近於回答這個問題，這得益於大量的技術進步和每個新的航天器的發射 - 有時甚至要感謝證據直接落在我們的身邊。
Introduction: It』s one of the most compelling questions humanity has tried to answer: Is there life beyond Earth? Scientists are closer than ever to answering that question, thanks to a host of technological advances and each new spacecraft that launches—and sometimes even thanks to evidence falling right onto our laps.
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證據就包括1998年撞到地球上的兩顆隕石。根據上星期三最新發表在《科學進展》的研究報告，在這些太空岩石中鑲嵌著微小的明亮的藍色鹽晶體，它們是一小塊充滿有機化合物的外星水。
Evidence like two meteorites that crashed into Earth in 1998. Nestled within those space rocks were tiny bright blue salt crystals, and inside of those crystals were tiny pockets of extraterrestrial water filled with organic compounds, the building blocks of life, according to new research published on Wednesday.
論文的第一作者、英國開放大學的行星科學家陳女士告訴《新聞周刊》說：「我認為太陽系中還可能存在著其他生命。而且我們正在研究可能最終找到這些生命的科學。這就是為什麼我們喜歡搜尋任務，不是嗎？
「I think it』s likely」 that there is other life within our solar system, first author on that work, Queenie Chan, a planetary scientist at Open University in the U.K., told Newsweek. And even better, she says, we』re doing the science that might finally find it. 「That』s why we like searching missions, isn』t it?」
這意味著像美國宇航局的黎明號飛船這樣的任務，一直在研究小行星帶最大的目標：矮行星穀神星。陳博士和她的合著者認為，他們研究的鹽晶體可能首先在穀神星或類似上形成，因此他們已經密切關注了黎明飛船所搜集到的結果。這包括確定氯化鈉（食鹽）和二氧化硫。她說：「當我看到這些化合物的時候，我意識到在我們的樣本中也看到它們。」
That means missions like NASA』s Dawn spacecraft, which has been studying the largest object in the asteroid belt, the dwarf planet Ceres. Chan and her co-authors believe the salt crystals they studied may have first been formed on Ceres or a similar object, so they have paid close attention to the results the Dawn spacecraft has gathered. That includes identifying sodium chloride, better known as table salt, and sulfur dioxide. 「When I saw them I was like, 『Hey, I saw that in our sample too,』」 she said.
儘管「黎明號」任務將在今年晚些時候結束，但是在途中還有很多後續任務，比如美國宇航局的「歐洲快船」計劃在20世紀20年代發射之後，環繞土星冰冷的衛星飛行。科學家們懷疑該土星的衛星可以主宰生命，這要歸功於衛星在冰殼下隱藏著巨大的鹹海和火山活動的理論。無論如何，生活 - 就像我們所知道的那樣 - 需要水，科學家認為可能形成在衛星「歐羅巴」冰殼上的常見化學物質，如氧氣和二氧化碳，以及能源（地球上，通常是太陽，但在歐羅巴可能是地質活動）。
Although the Dawn mission will end later this year, there are plenty of successors on the way, like Europa Clipper, NASA』s plan to fly by Saturn』s icy moon after a 2020s launch. Scientists suspect that it could host life, thanks to the theory that the moon is hiding a giant salty ocean and volcanic activity under its icy shell. Life—as we know it, anyway—requires water, common chemicals like oxygen and carbon dioxide that scientists think could be forming on Europa"s icy shell, and an energy source (on Earth, that"s usually the sun, but on Europa it could be geological processes).
陳博士說，她的新研究支持這些「歐羅巴」有生命的猜測，他們特別認為，隕石中發現的晶體是從一個特別富含水分的小行星上、通過一種叫做「冷噴發」（一種能夠噴出水或者冰的地質活動，而不是熔岩噴發）的過程產生的。
Chan says her new research supports these suspicions of life on Europa because the story she and her colleagues put together for the salt crystals they studied relies on conditions also believed to exist on this moon. In particular, they think the crystals found within the meteorite were shot off a particularly water-rich asteroid by cryovolcanism, a type of geologic activity that spews water or ice instead of lava.
科學家懷疑歐羅巴也可能發生冷噴發這種現象。而不像傳統的地質活動，這些噴發不會產生高溫，如果溫度太高，這些能夠支持生命產生的化學物質將會被毀掉。 「我們的研究證明，這種情況與我們在隕石上發現的情況類似，」陳談到科學家關於在「歐羅巴」發生的過程的假設。 「在這個海洋世界裡，可能有很多種化合物。」
Scientists suspect the cryovolcanism may occur on Europa as well. And unlike traditional geologic activity, these eruptions don"t reach high temperatures that would irreparably toast these life-supporting chemicals. 「Our study proves that, ok, this kind of scenario is similar to what we found on the meteorite,」 Chan said of scientists』 hypotheses about what』s happening on Europa. 「It is likely that inside this ocean world there is a whole wide range of chemical compounds.」
或者考慮火星，目前非凡的科考機器人的遊樂場：「好奇號」。我們在地球上有幾塊珍貴的火星樣本（以火星隕石的形式存在），可以用來分析陳博士在小行星出生的樣本中尋找的幾種基本化合物。科學家尋找的一個關鍵化合物類型是氨基酸 - 生命的基石 - 陳和她的同事們找到了。
Or consider Mars, currently the playground of science robot extraordinaire, Curiosity. We have a few precious pieces of Mars here on Earth in the form of Martian meteorites, which can be analyzed for the same basic compounds Chan went looking for in her asteroid-born samples. One of the key types of compounds scientists look for is amino acids—the building blocks of life—which Chan and her colleagues did find.
但更令人興奮的是氨基酸的一個特定子集。每種類型的氨基酸都有兩種「口味」，科學家稱其為左手和右手形式。生物只使用左手版本，稱為「同手性」 - 這是科學家希望作為外星生命跡象的氨基酸。
But even more exciting would be a particular subset of amino acids. Each type of amino acid comes in two flavors, which scientists call left- and right-hand forms. Living things only use the left-handed versions, called homochirality—those are the amino acids scientists hope to find as an indication of alien life.
問題是，在地球上污染隕石是非常容易發生的，所以科學家對這樣的分析結果並不感到自信。在火星上，這是一個不同的故事。 「如果他們發現氨基酸的同手性，這將是驚人的，」陳說到「好奇號」的任務。
The problem is, it』s ridiculously easy to contaminate meteorites here on Earth, so scientists don』t feel confident about the results they would get from such an analysis. On Mars, it』s a different story. 「If they find homochirality of amino acids, that would be phenomenal,」 Chan said of the Curiosity mission.
陳博士告訴所有人，她認為這可能是在太陽系等待被發現的地方隱藏的生命。如果你把範圍擴大到太陽系以外，出現生命的可能性就會變得更大。她說：「這樣說起來更容易，畢竟宇宙如此之大。」
Chan says that all told, she thinks it』s likely there』s life tucked away somewhere else in the solar system waiting to be found. And if you expand the scope beyond our solar system, the odds of life existing get even better. 「It』s easier to say that, the universe is so big,」 she said.
無論是在我們自己的社區還是在其它地方，科學家都在尋找相同的標準。可能是罕見的，但這些標準應該就在那裡。陳博士說：「你必須有很多合適的條件，很多條件都必須打勾才能做到這一點。」
Whether in our own neighborhood or beyond, scientists are looking for the same criteria. They may be rare, but they should be out there somewhere. 「You have to have the right conditions,」 Chan said. 「A lot of boxes have to be ticked to make that feasible.」
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Organic matter in extraterrestrial water-bearing salt crystals
DOI: 10.1126/sciadv.aao3521
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