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外星海洋里的未知生命会是什么样子的

What life might be like in alien oceans
外星海洋里的未知生命会是什么样子的

Recent discoveries have led astrobiologists to think that moons are the most promising places for alien life to exist in our Solar System. And now several major space missions are being planned over the next decade to search for hints of life there.

最近的发现使天体生物学家认为,卫星是太阳系中最有可能存在外星生命的地方。现在,正计划在未来10年内进行几次重大的太空任务,以寻找那里的生命迹象。

Unlike our neighbouring planets, some of the moons have plenty of liquid water. Jupiter’s moon, Europa, for example, is thought to contain more liquid water than all of Earth's oceans combined. This water – and any life in it – is protected from space radiation and asteroid impacts by a thick layer of kilometers-deep surface ice.

与我们邻近的行星不同,有些卫星拥有大量的液态水。例如,木星的卫星,木卫二(又称欧罗巴,Europa)被认为含有液态水比地球上所有海洋加起来还要多。厚达数千米的表面冰层保护着这片水域,以及其中的所有生命,免受太空辐射和小行星撞击。

The discovery of plumes of water shooting up from Saturn’s moon Enceladus and Europa have suggested they could have warm interiors that can support liquid oceans, heated not by the Sun, but by an internal dynamo powered by radioactive decay in their cores or by tidal heating generated by the gravitational attraction of the planets that they orbit.

从土星的卫星,土卫二(又称恩科拉多斯,Enceladus)和欧罗巴喷出的羽状水柱表明,它们的内部可以支撑液态海洋,不是由太阳加热,而是由其核心的放射性衰变或由它们轨道上行星的引力所产生的潮汐加热。
There is now evidence for water oceans on several moons, including Europa, Enceladus, Callisto and Ganymede. One study published this June estimates that the Enceladus ocean is around one billion years old. Others have suggested it may be billions of years old – plenty of time for life to evolve.

现在有证据表明,在一些卫星上存在有液态的海洋,包括木卫二、土卫二、木卫四(又称卡利斯托,Callisto)和木卫三(又称盖尼米得,Ganymede)。今年6月发表的一项研究估计,土卫二海洋大约有10亿年的历史。还有人认为它可能有数十亿年的历史,有足够的时间让生命进化。

These oceans are thought to be salty, containing sodium chloride, like Earth’s oceans, which is another boost for the prospects of Earth-like life.

这些海洋被认为是咸的,含有氯化钠,就像地球上的海洋一样,这是对发现类似地球生命前景的又一提振。

Also, there is likely to be an interface between the liquid water and the rocky mantle below the oceans – key ingredients for interesting chemistry that scientists think led to the origins of life on Earth. Nasa’s Cassini mission, for example, detected molecules in Enceladus’ water plumes that hint at the existence of hydrothermal vents on the moon’s ocean floor.

此外,液态水和海洋下的岩石地幔之间很可能存在一个界面,这意味着海洋深处存在着有趣的化学反应,科学家们认为这些化学物质是地球上生命起源的关键成分。例如,美国宇航局(NASA)的卡西尼号(Cassini)探测到土卫二的羽流中含有复杂的有机分子,这些分子暗示着卫星海底存在热液喷口。

Similar vents exist in the deep oceans of the Earth, where magma meets salt water and provides heat, chemicals and a substrate helpful for the complex chemistry some scientists think was needed for life to first evolve on our planet. Deep below the surface of Earth’s ocean, there is practically no sunlight, as would be the case for oceans of Jupiter and Saturn’s moons. But that doesn’t mean there is no life. Indeed, on Earth, such vents are teeming with life.

在地球的深海中也存在着类似的喷口,在那里岩浆与咸水交汇,提供热量、化学物质和有助于复杂化学反应的基质。一些科学家认为这些基质是生命最初在地球上进化所需要的。在地球海洋的深处,几乎没有阳光,就像木星和土星的卫星一样。但这并不意味着没有生命。事实上,在地球上,这样的喷口充满了生命。

Some 20 years ago Natural History of an Alien, a BBC documentary, suggested that entire ecosystems could also be based around deep-sea thermal vents on Europa. A team of scientists suggested that bacteria would form the base of the food chain, using chemosynthesis to extract energy from the vents, and building tall tubes of deposits rising many miles above the ocean floor.

大约20年前,英国广播公司(BBC)的纪录片《外星人的自然史》(Natural History of a Alien)指出,整个生态系统也可能以欧罗巴上的深海热泉为基础。一组科学家提出,细菌将形成食物链的基础,利用化学合成从喷口中提取能量,并在海底上方数英里处建造高高的沉淀物管道。

Other creatures, such as fish-like grazers would pierce those tubes to suck in large amounts of bacteria to feed on. They would be territorial, defending their grazing patches against rivals. And, in turn, they would be preyed upon by shark-like animals, streamlined for speed, using echolocation to detect their prey.

其他生物,比如像鱼一样的动物,会刺穿这些管子,吸入大量的细菌作为食物。它们会抵御竞争对手,保卫自己的牧场。反过来,它们也会被鲨鱼一样的动物捕食,它们的身体呈流线型,速度快,利用回声定位来发现猎物。

This is much more advanced than what most scientists expect to find there.

这比大多数科学家期望在那里发现的要先进得多。

Even on Earth, for some 90% of our planet’s history, the only life that existed here was microbial, says Andrew Knoll, a professor of earth and planetary sciences at Harvard University. So, if there is life in space, chances are it’s going to be microbial, Knoll says, and in places like Europa or Enceladus it would have to rely entirely on chemosynthesis for energy, so could probably only support a small biomass.

哈佛大学地球和行星科学教授诺尔(Andrew Knoll)说,即使在地球上,在地球90%的历史中,唯一存在的生命也是微生物。因此,如果太空中存在生命,很可能是微生物,而像木卫二或土卫二这样的地方,必须完全依靠化学合成来获得能量,因此只能维持少量生物。

But such an ecosystem might still be possible, says Dimitar Sasselov, an astronomy professor and director of the Harvard Origins of Life Initiative, a centre supporting multi-disciplinary research to discover whether life is abundant in the universe. Just because Europa’s ocean is cold and lacking in energy, doesn’t necessarily rule out complex ecosystems of a smaller size evolving there.

天文学教授兼哈佛生命起源计划的负责人萨斯洛夫(Dimitar Sasselov)说,但这样的生态系统仍然可能实现。该计划支持多学科研究,旨在发现宇宙中是否存在丰富的生命。虽然木卫二的海洋寒冷且缺乏能量,但并不排除在那里进化出规模较小的复杂生态系统。

“Speculating is fun,” Sasselov says. “My gut feeling is that there is a lot of evolutionary innovation space possible there where you can have something which is small and yet predatory and is a multicellular organism rather than just a single cell.”

萨塞洛夫说:“猜测很有趣。我的直觉是,在那里有很多进化创新空间,在那里,可以拥有一些很小但具有掠夺性的东西,它是一个多细胞生物而不是单细胞。”

Another moon we’re planning to visit presents a whole different puzzle.

我们计划访问的另一个卫星呈现出一个完全不同的谜题。

Saturn’s moon, Titan, is the only world beyond Earth known to have stable bodies of liquid on its surface. When the Huygens probe from the Cassini mission landed there in 2005, it sent back pictures of an Earth-like landscape: river beds and seas.

土星的卫星,土卫六(又称泰坦,Titan),是已知的地球之外唯一在其表面有稳定液体的星球。2005年,卡西尼号飞船的惠更斯号探测器在那里着陆时,传回了类似地球地貌的图片:河床和海洋。

But rather than water, the clouds, rain and seas of Titan are made up of liquid methane and ethane, components of natural gas on Earth. Any water that does exist there is solidified into rocks and mountains because its surface temperature is around -180C (-292F).

但是,泰坦的云、雨和海不是水,而是由液态甲烷和乙烷组成的,它们是地球上天然气的组成部分。任何存在于那里的水都会凝固成岩石和山脉,因为它的表面温度在-180C(-292F)左右。

This means that, while its landscape might look familiar, the actual conditions are totally alien. If there is life, it would rely on methane, not water, and would be exotic – life as we don’t know it. True aliens.

这意味着,虽然它的景观看起来很熟悉,但实际情况却是完全陌生的。如果有生命存在,它将依赖甲烷,而不是水,而且将是奇异的生命,我们所不知道的,真正的外星人。

It is possible and plausible that life exists on Titan, but with a “completely different, independent biochemistry”, says Sasselov, whose long-term goal is to figure out if there is an alternative biochemistry and how to create it in the lab.

萨塞洛夫说,泰坦上存在生命是有可能的,也是有道理的,但也是“完全不同的、独立的生物化学”。萨塞洛夫的长期目标是弄清楚是否存在一种替代生物化学,以及如何在实验室中创造它。

Life on Earth depends on cell membranes made of phospholipids: molecular chains with phosphorus-oxygen heads and carbon-chain tails that bind to each other to form a flexible membrane in water.

地球上的生命依赖于磷脂构成的细胞膜:由磷氧头和碳链尾组成的分子链,这些分子链相互结合,在水中形成一层柔性膜。

Methane-based life would need an alternative way to form cells.

以甲烷为基础的生命需要另一种方式来形成细胞。

A Cornell University team led by chemical engineer Paulette Clancy showed in 2015 that small molecules made from nitrogen, carbon and hydrogen could build cells fit to survive in Titan’s conditions.

2015年,由化学工程师克兰西(Paulette Clancy)领导的康奈尔大学(Cornell University)研究小组发现,由氮、碳和氢组成的小分子可以构建适合泰坦生存条件的细胞。

Since then, Nasa researchers have confirmed the presence of vinyl cyanide in Titan’s atmosphere, an organic compound that could provide such cellular membranes. So, at least in theory, cells that could form a very different life in Titan’s vast methane oceans could physically exist there.

从那时起,美国宇航局的研究人员已经证实,泰坦的大气层中存在乙烯基氰化物,这种有机化合物可以提供这样的细胞膜。因此,至少在理论上,泰坦巨大的甲烷海洋中,可能存在着能够形成截然不同生命的细胞。

“In some respects, what we see here on Earth is a matter of chance,” says Theresa Fisher, astrobiologist at Arizona State University, US. There’s an “enormous amount of potential variety” that we could see in life on other worlds, she says.

美国亚利桑那州立大学的天体生物学家费舍尔(Theresa Fisher)说:“在某些方面,我们在地球上看到的是一个偶然的问题。在其他世界的生活中,我们可以看到巨大的潜在的多样性。”

"There might emerge a fluorescence of new and very diverse organisms occupying a range of new niches,” adds Sarah Blaffer Hrdy, a professor emerita in anthropology at the University of California, Davis. "Assuming any of these creatures evolve to be as social, intelligent and communicative as say cetaceans or elephants, and as manipulative, dexterous and clever as chimpanzees or orangutans, I see no reason why they could not eventually evolve more sophisticated technological and cultural capacities.”

加州大学戴维斯分校(University of California, Davis)人类学荣誉教授赫迪(Sarah Blaffer Hrdy)补充道:“可能会出现一种荧光现象,显示出新的、非常多样化的生物体,它们占据了一系列新的生态位。假设这些生物中的任何一种进化到像鲸目动物或大象那样具有社会性、智慧和沟通能力,以及像黑猩猩或猩猩那样善于操纵、灵巧和聪明,它们最终为什么不能进化出更先进的技术和文化能力。”

Lauren Sallan, a palaeontologist at the University of Pennsylvania, thinks alien life will be microbial – and there’re only so many ways to be a microbe.

宾夕法尼亚大学的古生物学家萨兰(Lauren Sallan)认为,外星生命将是微生物,而微生物的存在途径会有很多。

As far as multicellular aliens are concerned, she says, things may get more complicated. “We would recognise that they’re doing the same kind of jobs because everything is focused on either taking in energy or consuming things to get energy,” she says. “But the way that they go about it would be pretty unpredictable.”

她表示:“就多细胞外星人而言,事情可能变得更加复杂。我们会意识到他们在从事相同的工作,因为一切都集中在吸收能量或消耗东西来获取能源上。但他们采取的方式将是不可预测的。”

“We really don’t know what are the limits of life,” says David Charbonneau, professor of astronomy at Harvard University, who adds that this is why we need to send more probes to examine the moons.

哈佛大学天文学教授夏博诺(David Charbonneau)说:“我们真的不知道生命的极限是什么。这就是为什么我们需要发射更多的探测器来检测卫星。”

So, it’s good news that there are plans to do just that.

所以,这是一个好消息,我们正计划这么做。

Nasa announced this summer that its Dragonfly mission will launch in 2026 and arrive on Titan in 2034. It will land a drone-like craft to explore dozens of promising locations and look for signs of life.

美国宇航局今年夏天宣布,它的蜻蜓计划(Dragonfly mission)将于2026年发射,2034年到达泰坦。并将着陆一架类似无人机的飞行器,探索数10个有希望的地点,寻找生命迹象。

Nasa is also exploring the possibility of sending an autonomous submarine to study Titan’s largest northern sea, Kraken Mare, which is some 1,000 km (621 miles) wide, with depths estimated at 300m (1,000ft), similar in size to North America’s Great Lakes. This would be the first opportunity to explore a sea on another world, and it could inform the design of future submarines to explore the subsurface waters of Europa and other moons. The mission is still in its conceptual stage, some 20 years away, with scientists and engineers starting to investigate how to even build such a submarine.

美国宇航局还在探索能否派遣一艘自主潜艇去研究泰坦最大的克拉肯海(Kraken Mare)的可能性。该海域宽度约1000公里(621英里),深度估计为300米(1000英尺),大小与北美五大湖相仿。这将是探索另一个世界海洋的第一次机会,也将为未来探索木卫二和其他卫星地下水域的潜艇设计提供参考。该任务仍处于概念阶段,大约20年后,科学家和工程师开始研究如何建造这样一艘潜艇。

Intriguingly, Titan is also thought to have a liquid ocean of water deep beneath its icy outer layer, which would mean that in addition to its exotic surface life based on liquid methane, there could exist more Earth-like life under its surface.

有趣的是,土卫六还被认为在它冰冷的外层下有一个液态的海洋,这意味着除了它以液态甲烷为基础的奇异表面生命外,它的表面下还可能存在更多类似地球的生命。

Another possibility for layers of different types of life on a single world is Ganymede, Jupiter’s moon. Some scientists think this moon has several different layers of ocean, separated by different types of ice that form at different depths and pressures. If this is the case, each layer could, in theory, host different lifeforms adapted to local conditions at that depth.

在同一个星球上存在不同类型生命的另一种可能性是木卫三,木星的卫星。一些科学家认为这颗卫星有几层不同的海洋,被不同深度和压力下形成的不同类型的冰隔开。如果是这样的话,理论上讲,每一层都可以容纳适合该深度和局部条件的不同生命形式。

Ganymede is set for a visit by the European Space Agency’s 2022 Juice mission, which will also visit two of Jupiter’s other moons – Callisto and Europa – to study their habitability and look for signatures of life.

欧洲航天局(European Space Agency)将于2022年进行果汁计划(Juice mission),访问木卫三,该计划还将访问木星的另外两颗卫星——木卫四和木卫二,研究它们的宜居性,并寻找生命的迹象。

Meanwhile, Nasa’s Europa Clipper is planning to orbit Jupiter and fly past Europa multiple times to investigate whether it could harbour conditions suitable for life, with a take-off date of 2023. Nasa is also discussing sending a lander to Europa, as early as 2025.

与此同时,美国宇航局的“木卫二快船”(Europa Clipper)计划绕木星轨道运行,并多次飞越木卫二,以考察它是否具备适合生命生存的条件。起飞日期为2023年。美国宇航局也在讨论最早在2025年向木卫二发射着陆器。

And, there is a private, Nasa-backed plan for a mission to Enceladus to look for life there that could take off in 2025 if it gets the green light later this year.

美国宇航局支持的一项私人计划是前往土卫二寻找生命,如果能在今年晚些时候获得批准,该计划有望在2025年启动。

But to really figure out what life might exist in these alien oceans, we will need to send a submersible, which will be tricky as such a vehicle would have to drill through several kilometres of ice to even reach the ocean. Nasa is funding some conceptual studies on how to do that.

但是,要真正弄清楚这些外星海洋中存在什么生命,我们需要派遣一艘潜水器,这将是一项很棘手的任务,因为这种潜水器必须能钻过几公里厚的冰层到达海洋。美国宇航局正在资助一些关于如何做到这一点的概念性研究。

One concept, for a nuclear-powered “tunnelbot” to search for life on Europa, was presented at a 2018 meeting of the American Geophysical Union in Washington DC, by scientists at the University of Illinois at Chicago and Nasa. Their bot would sample ice and water as it descended, sending information back to the surface through a fibre optic cable.

2018年在华盛顿举行的美国地球物理联盟(American Geophysical Union)会议上,伊利诺斯大学芝加哥分校(University of Illinois at Chicago)和美国宇航局的科学家们提出了一个概念,那就是用核动力“隧道机器人”在木卫二上寻找生命。他们的机器人会在冰层和水面的下降过程中采样,通过光缆将信息传回地面。

But, if lifeforms there turn out to be truly alien, we might struggle to detect them. It’s also possible there simply isn’t any life there yet.

但是,如果那里的生命形式被证明是真正的外星人,我们可能很难发现它们。也有可能那里根本没有生命存在。

In the distant future, though, some five billion years from now, when our Sun runs out of hydrogen fuel and starts expanding into a red giant phase before it eventually dies, it will melt the ice on these moons and turn them into much more Earth-like places. There should be liquid water on their surface and more temperate climates, perhaps opening up the possibility of life evolving there then – or at least harbouring refugees from the scorched Earth.

然而,在遥远的未来,大约50亿年后,当我们的太阳耗尽氢燃料,并开始膨胀为红巨星时期,在太阳最终死亡之前,将融化这些卫星上的冰,并使它们变成更像地球的地方。它们的表面应该有液态水,气候也应该更温和,或许会开启那里生命存活的可能性,或者至少为逃离焦土的难民提供避难所。

In the distant future, if we are to survive, we will all have to become migrants and hope these newly habitable worlds welcome us as our own world gets too hot for life.

在遥远的未来,如果想要生存,我们都必须成为移民,并希望这些新的居住世界欢迎我们,因为我们自己的世界变得太热,不适合生存。
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