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外太空的幽暗真空处哪里有生命?

Where to find life in the blackest vacuum of outer space
外太空的幽暗真空处哪里有生命?

When we think about whether or not aliens exist, we generally imagine them on a vaguely Earth-like planet circling a distant star. We do not normally think of them living out in space itself.

当我们思考外星人是否存在时,我们一般想象他们在围绕遥远的恒星公转的一颗与地球隐约相像的行星上。我们通常不会想他们住在太空中。

But maybe that is not such a ridiculous idea. In April 2016, researchers reported that some of the key building blocks of life can be produced from simple substances under harsh conditions mimicking those of interstellar space.

但也许这并不是一个荒唐的想法。在2016年4月,研究人员报告说,在模拟了星际空间恶劣条件下,可以由简单的物质合成某些构成生命的关键基础构件。
Cornelia Meinert at the University of Nice, France and colleagues showed that a mixture of frozen water, methanol and ammonia – all compounds known to exist in the vast "molecular clouds" from which stars form – can be transformed into a wide range of sugar molecules when exposed to ultraviolet rays, which pervade space. The sugars included ribose, which is a part of the DNA-like molecule RNA.

法国尼斯大学的科妮莉亚·迈纳尔特(Cornelia Meinert)和她的同事表明,冰冻的水、甲醇和氨组成的混合物,将其暴露在充斥外太空的紫外线下,可以转化为多种多样的糖类分子。这些糖类物质包括核糖,是类似脱氧核糖核酸(即DNA)的分子核糖核酸(molecule RNA)的一部分。我们知道水、甲醇和氨等物质都存在于浩瀚的“分子云”(molecular clouds)中,恒星就是从这样的分子云中形成的。

This suggests that the fundamental molecules of life might be formed in outer space, and then delivered to planets like Earth by icy comets and meteorites.

这意味着构成生命的基本分子有可能在外层空间形成,然后再由冰冻状态的彗星(icy comets)和陨石带到类似于地球的行星上。

The finding is actually not surprising. We have known for decades that other building blocks of life can be formed from chemical reactions like this, before being incorporated into comets, asteroids and planets.

这一发现其实并不让人感到惊讶。几十年来我们已经知道,其它构成生命的基本构件,在被带入彗星、小行星和行星之前,可以通过这样的化学反应加以合成。

However, there is a more intriguing possibility. Life itself might not have needed a warm and comfortable planet bathed in sunlight to get going. If the raw ingredients were already out there in interplanetary limbo, might life have started there too?

然而有一个更有趣的可能性。生命本身可能不需要在一个沐浴在阳光下温暖而舒适的星球上产生。如果原材料已经存在于星际空间的边缘,也许生命已经在那里诞生?

Ideas about the origins of life do not often consider this scenario. It is hard enough to figure out how life could have begun on the early Earth, let alone at temperatures close to absolute zero and the near vacuum of interstellar space.

通常关于生命起源的想法并不会考虑这种情况。弄清楚生命如何起源于早期地球已经足够困难,更不用说在接近绝对零度和真空的星际空间。

Making the basic building blocks of life, like sugars and amino acids, is the easy part. There are lots of chemically-plausible ways to do that, starting with the simple molecules found in young solar systems.

生成例如糖类与氨基酸一类的生命基本构件是容易的部分。从早期的太阳系中发现的简单分子入手,有很多种化学上可行的办法可以做到这一点。

The hard bit is persuading these complicated molecules to assemble into something capable of life-sustaining processes like replication and metabolism. Nobody has ever done this, or come up with a completely plausible way it might happen, in the nurturing environment of a warm, rocky planet – let alone in space.

难点在于如何促使这些复杂的分子组合在一起,从而实现一些能够维持生命的过程,比如复制和新陈代谢。在一个温暖的、遍布岩石的星球环境中尚未有人实现,或提出一个完全合理的实现方式,更不用是说在太空里。

Still, there is no fundamental reason why life might not arise far from any star, in what is often regarded as the barren desert of interstellar space. Here is how it might happen.

然而,没有根本性的理由来解释为何生命不会在出现在任何远离恒星的地方,这种地方通常被视为星际空间中的贫瘠沙漠地带。以下是这个过程如何发生。

First, we had better agree on what counts as "life". It does not necessarily have to look like anything familiar.

首先,我们最好就什么可以被看作“生命”达成一致。它不一定要像任何我们熟悉的东西。

As an extreme case, we can imagine something like the Black Cloud in astronomer Fred Hoyle's classic 1959 science-fiction novel of that name: a kind of sentient gas that floats around in interstellar space, and is surprised to discover life on a planet.

作为一个极端的例子,我们可以想象某种类似于天文学家弗莱德·霍伊尔(Fred Hoyle)1959年的同名经典科幻小说中所描绘的黑云(Black Cloud)一样的物质——某种漂浮于星际空间的智能气体,很惊讶地发现一个星球上有生命。

But Hoyle could not offer a plausible explanation for how a gas, with an unspecified chemical make-up, could become intelligent. We probably need to imagine something literally a bit more solid.

但霍伊尔未能提供关于此事的合理解释:即一种未明确阐述其化学成分的气体如何变得有智能。我们可能需要想象一些更实在的东西。

While we cannot be sure that all life is carbon-based, as it is on Earth, there are good reasons to think that it is likely. Carbon is much more versatile as a building block for complex molecules than, say, silicon, the favourite element for speculations about alternative alien biochemistries.

虽然我们不能肯定所有的生命都如同地球上一样是碳基的,但是有充分的理由认为这是可能的。将硅元素替代碳元素作为外星生物生化基础的猜想备受欢迎,然而与硅相比,碳在构成复杂分子方面用途更广。

Astrobiologist Charles Cockell at the University of Edinburgh in the UK thinks that the broad basis of life on Earth – that it is carbon-based and requires water – "reflects a universal norm". He concedes that "I have a quite conservative view, which science generally proves is misguided." But still, for now let's stick with carbon-based life. How could it be generated in outer space?

英国爱丁堡大学的天体生物学家查尔斯·科克尔(Charles Cockell)认为地球上生命的广泛基础是基于碳,并且需要水,这“反映了一种普遍的准则”。他承认,“我有一种相当保守的观点,而科学往往证明这是错误的。”但是,现在让我们坚持碳基生命论。那么它又是如何外层空间产生呢?

The basic chemistry is not a problem. As well as sugars, life on Earth needed amino acids, the building blocks of proteins. But we know that these can be formed in outer space too, because they have been found in "primitive" meteorites that have never seen a planetary surface.

基本的化学不是问题。除了糖,地球上的生命还需要氨基酸,即蛋白质的基础构件。但是我们也知道,这些物质可以在外太空中合成,因为在“原始”的陨石中发现了任何一个行星表面都未发现的氨基酸。

They might be made on icy grains from some variation of a chemical reaction called the Strecker synthesis, after the 19th-Century German chemist who discovered it. The reaction involves simple organic molecules called ketones or aldehydes, which combine with hydrogen cyanide and ammonia. Alternatively, light-driven chemistry triggered by ultraviolet light will do the job.

他们可能是在冰粒上通过某种称为斯特克勒尔合成(Strecker synthesis)的化学反应的变体加以合成的,这种合成反应以19世纪发现该反应的德国化学家命名。该反应涉及酮(ketones)或乙醛(aldehydes)的简单有机分子与氰化氢(hydrogen cyanide)和氨化合。或者通过紫外光照射触发光化学反应加以实现。

It looks at first as though these reactions should not take place in deepest space, without a source of heat or light to drive them. Molecules encountering one another in frigid, dark conditions do not have enough energy to get a chemical reaction started. It is as if they run into a barrier that is too high for them to jump over.

初看起来,没有热源或光源的触发,这样的反应似乎不应在太空深处发生。在寒冷黑暗的环境中,当一个分子遇到了另一种分子,并没有足够的能量来促使化学反应的发生。就好像它们遇到了一个难以逾越的高大障碍。

However, in the 1970s the Soviet chemist Vitali Goldanski showed otherwise. Some chemicals could react even when chilled to just four degrees above absolute zero, which is about as cold as space gets. They just needed a bit of help from high-energy radiation such as gamma-rays or electron beams – like the cosmic rays that whizz through all of space.

然而,在上世纪70年代,苏联化学家维塔利·戈尔丹斯基(Vitali Goldanski)表明恰恰相反。一些化学物质即便冷冻至绝对零度之上四度也会发生化学反应,这一温度与太空环境的温度相近。他们只是需要一点伽玛射线(gamma-rays)或电子束(electron beams)助一臂之力——比如嗖嗖穿过太空的那些宇宙射线。

Under these conditions, Goldanski found that the carbon-based molecule formaldehyde, which is common in molecular clouds, could link up into polymer chains several hundred molecules long.

在这些条件下,戈尔丹斯基发现分子云中常见的碳基分子甲醛(formaldehyde)可以形成长达几百个分子的高分子聚合链(polymer chains)。

Goldanski believed that such space-based reactions might have helped the molecular building blocks of life assemble from simple ingredients like hydrogen cyanide, ammonia and water.

戈尔丹斯基相信这种太空中的反应可能有助于如氰化氢、氨和水等简单成分组装成生命的基础构件。

But it is far more difficult to coax such molecules to combine into more complex forms. The high-energy radiation that might help get the first reactions started then becomes a problem.

但诱使这类分子组合成更复杂的形态要困难得多。高能射线可能会促使你最初的反应发生,之后就成了问题。

Ultraviolet and other forms of radiation can induce reactions like those Meinert demonstrated. But Cockell says they are just as likely to smash molecules as they are to form them. Potential biomolecules – progenitors of proteins and RNA, say – would be broken apart faster than they were being produced.

紫外线和其它形式的射线可以诱导那些迈纳尔特证明了的化学反应的发生。但科克尔说它们打碎分子的几率和形成分子的几率一样高。潜在的生物分子——比如蛋白质和核糖核酸的祖细胞——被分解比被制造出来要快得多。

"Ultimately the question is whether other completely alien environments would give rise to self-replicating chemical systems that can evolve," says Cockell. "I don't see why that wouldn't happen in very cold environments or on the surfaces of ice grains, but generally I think these environments aren't very conducive to very complex molecules."

“最终,问题就是另一个完全陌生的环境会产生能够进化的自我复制的化学系统吗,”科克尔说,“我看不出这种情况在非常寒冷的环境中或冰粒表面为何不会发生,但一般来说,我认为这些环境并不十分有利于复杂的分子。

Planets offer two much gentler energy sources: heat and light. Life on Earth is largely powered by sunlight, and it is a fair bet that life on "exoplanets" around other stars would draw on the energy reserves of their own suns.

行星提供两种更温和的能源:光和热。地球上的生命在很大程度上依赖太阳光赋予能量,因此我们可以猜测围绕其它恒星运转的“系外行星”(exoplanets)上的生命将汲取自己恒星的能量储备。

Vital heat can also come from elsewhere. Some scientists believe that the first life on Earth was not powered by sunlight, but by volcanic energy released from the planet's interior at hot vents in the deep sea. Even today, these vents still spew out a warm, mineral-rich brew.

重要的热量也来自其他地方。一些科学家认为,地球上最初的生命并不是由太阳提供能量,而是来自于深海热喷口(hot vent)释放出来的地球内部的火山能量。即使在今天,这些通风口还喷出温暖的,富含矿物的热液。

There is also heat in Jupiter's major moons. This comes from the huge tidal forces exerted by the giant planet, which squeeze the interiors of the moons and heat them up through friction. This tidal energy keeps the sub-surfaces of the icy moons Europa and Ganymede melted into oceans, and makes Io's surface fiery and volcanic.

木星的主要卫星也有热源。这是由这颗巨型行星所施加的巨大潮汐力挤压了卫星的内部,并通过摩擦加热。这种潮汐能量使冰冷的木卫二和木卫三(Ganymede)的表面融化成海洋,使木卫一(Io)的表面炽热,并遍布火山。

It is hard to see how molecules clinging to icy grains in interstellar space could find any such nurturing energy. But that might not be the only option out there.

很难想象在星际空间中粘附在冰粒上的分子会找到任何培育性的能量。但这可能不是唯一的选择。

In 1999, planetary scientist David Stevenson of the California Institute of Technology proposed that galaxies might be full of "rogue planets" floating beyond the outermost reaches of a stellar neighbourhood, too far from their "parent" star to feel its gravity, heat or light.

1999年,美国加州理工学院的行星科学家戴维·史蒂文森(David Stevenson)提出,星系中可能充满了漂浮在一个恒星系最外层之外的“流浪行星”,这些行星距离“母星”太远,不受到其引力、热或光的影响。

These worlds, Stevenson said, could have formed like any other regular planet, close to a nascent star and within its surrounding nebula of gas and dust.

史蒂文森说,这些世界可以像其他任何普通的行星那样,在靠近一颗新生恒星,并在其周围的气体和尘埃星云中形成。

But then the gravitational tug of large planets, like our own Jupiter and Saturn, could sling some planets on "escape trajectories", propelling them off beyond their solar system into the empty space between stars.

木星和土星等巨行星的引力拖曳,可把某些行星抛掷到“逃逸轨迹上”,把它们推出其自身的太阳系外,进入空旷的星际空间。

That might seem to consign them to a cold and barren future. Yet Stevenson argued that, on the contrary, these rogue planets might be "the most common sites of life in the Universe" – because they might stay warm enough to support liquid water under, as it were, their own steam.

这看起来似乎是把它们交付给了一个寒冷而荒芜的未来。然而,史蒂文森认为,相反这些流浪行星可能是“宇宙中生命最常见的场所”,因为它们能够保持足够的温暖以维持液态水。

All of the rocky planets in the inner solar system come with two internal heat sources.

内太阳系(inner solar system)所有的岩石行星都有两种内部热源。

First, each planet has a fiery core still hot from the primordial fury of its formation. On top of that, they contain radioactive elements. These warm up the interior of the planet with their decay, just as a lump of uranium is warm to the touch. On Earth, radioactive decay inside the mantle contributes about half of the total heating.

首先,每一颗行星都有从最初形成至今依然的炙热核心。此外,它们含有放射性元素。这些放射性元素的衰变保持了星球内部的温度,正如一块铀摸上去是暖的。在地球上,地幔(mantle)的放射性衰变贡献了总热量的一半。

Primordial heat and radioactive decay inside rocky rogue planets could warm them for billions of years – perhaps enough to keep the planets volcanically active and provide the energy for life to start.

在流浪的岩石行星中原始热量和放射性衰变能使他们保持温暖达数十亿年——也许足以让行星保持火山的活跃,并为生命的起源提供能量。

Rogue planets could also have dense, heat-retaining atmospheres. Compared with gas giants like Jupiter and Saturn, Earth's atmosphere is thin and tenuous, because the Sun's heat and light have stripped away lighter gases like hydrogen. Mercury is so close to the Sun that it barely has any atmosphere at all.

流浪行星可能也有高密度能够维持热量的大气。与气体组成的巨行星木星和土星相比,地球的大气层是稀薄的,因为太阳的光照和热度将较轻的气体如氢气剥离出去。水星离太阳如此之近,已经几乎没有大气层。

Yet on an Earth-sized rogue planet, far beyond its parent star's influence, much of the original atmosphere might remain in place. Stevenson estimated that the resulting temperature and pressure could be enough to sustain liquid water at the surface, even without any sunlight.

而一个远离母星影响的地球大小的流浪行星,大部分原始的大气可能会被保留下来。史蒂文森估计,由此产生的温度和压力足以在表面维持液态水,即使没有任何阳光。

What's more, rogue planets would not be plagued by giant meteorite impacts, as Earth has been. They might even be ejected from their native solar system with moons in tow, giving them the benefit of some heating by tidal forces.

此外,流浪行星不会像地球所遭受的那样被巨大的陨石撞击所困扰。他们甚至可能会被抛射出他们的太阳系,后面紧跟着卫星,这还能给他们带来一些潮汐力加热的好处。

Even if a rogue planet did not have a thick atmosphere, it could still be habitable.

即使流浪行星没有浓厚的大气,它仍然可以居住。

In 2011, planetary scientist Dorian Abbot and astrophysicist Eric Switzer at the University of Chicago calculated that planets about three and a half times the size of the Earth could become covered over with a thick layer of ice. This would insulate an ocean of liquid water many kilometres below the surface, heated by its interior.

2011年,芝加哥大学的行星学家多利安·阿博特(Dorian Abbot)和天体物理学家埃里克·施伟策(Eric Switzer)计算出三个半地球大小的行星表面会覆盖一层厚厚的冰。这将使其表面以下数公里深处的液态海洋被隔离开来,通过其内部加热。

"The total biological activity would be lower than on a planet like Earth, but you should still be able to have something," says Abbot.

“生物的总体活性比在地球上的要低,但是你仍然可以拥有一些东西,”阿博特说。

He hopes that when space probes investigate the subsurface oceans of Jupiter's icy moons in the coming decades, we will learn more about the possibilities of life on iced-over rogue planets.

他希望在未来的几十年里,当太空探测器在木星的卫星探测冰层下的海洋时,关于被冰层覆盖的流浪行星上出现生命的可能性我们会了解更多。

Abbot and Switzer called these orphaned worlds "Steppenwolf planets", because, they say, "any life in this strange habitat would exist like a lone wolf wandering the galactic steppe". The habitable lifetime of such a planet could be up to ten billion years or so, similar to that of Earth, says Abbot.

阿博特和施伟策称这些孤儿世界为“荒原狼星球”(Steppenwolf planets),他们说,因为“在这个奇怪的栖息地里,任何生命都会像一只在银河草原上游荡的孤独的狼”。这样一个行星的可居住年限可以达到一百亿年左右,类似于地球。

If these ideas are right, then outside our solar system rogue planets in interstellar space could be the closest places where extraterrestrial life exists.

如果这些想法是正确的,那么我们的太阳系外星际空间中的流浪行星可能是距我们最近的外星生命存在的地方。

They would be very hard to spot at such a distance, being dark and relatively tiny.

由于暗淡而且相对较小,他们会很难在这样一个距离上被发现。

But with luck, say Abbot and Switzer, such a planet passing within about a thousand times the Earth-Sun distance could just about be discerned from the small amount of sunlight it would reflect and the infrared radiation of its own warmth. We might hope to see it with the telescopes currently used to look for exoplanets around other stars.

但幸运的是,阿博特和施伟策说,这样的一颗行星在大约日地距离(Earth-Sun distance)的一千倍远处通过时,可以通过它反射的少量阳光和它自身热量散发的红外辐射而观测到。我们希望能够使用近来用于寻找环绕其它恒星运行的地外行星的专门望远镜观测到它。

If life can originate and survive on an interstellar Steppenwolf planet, say Abbot and Switzer, there is a profound implication: life "must be truly ubiquitous in the Universe".

如果生命能够在星际荒原狼星球上起源和生存,阿博特和施伟策说,这将有着深刻的启示意义:“生命在宇宙中一定无处不在”。

It would be a strange kind of life on these interstellar worlds. Imagine bathing in warm volcanic springs under perpetual night, like a winter vacation in Iceland. But if that is all you had ever known, it would seem like home.

在这些星际世界中的生命一定非常奇特。想象一下在永恒的黑夜中在温暖的火山温泉中沐浴,好像在冰岛过冬天假期。但如果这是你所知道的全部,那就好像在家里一样。
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