评论
打印
There are a lot of ways to search for life on other planets, and over the years, we've talked about plenty of them, from looking for certain gases to unusual types of light.
有很多方法可以搜寻其他行星上的生命。过去这些年间，我们已经讨论过很多方法了：从寻找特定的气体到寻找独特的光，不一而足。
But for the most part, all those methods are governed by one guiding principle: Follow the water.
但基本上，所有这些方法都有一个指导原则：寻找水存在的证据。
That's because, on Earth, we don't know of any species that can survive without it.
因为在地球上没有什么生物可以脱离水存在。
Water is so important that, by definition, the habitable zone around a star is the area where the conditions are right for liquid water to exist.
水非常重要，甚至于可以说：某颗恒星附近的宜居区一定是液态水可以存在的区域。
But as some researchers have pointed out, there's a chance that we're just being a teeny bit Earth-centric.
但是，正如一些科学家所指出的那样：宇宙的运行方式与地球极为不同。
After all, there's an awful lot of chemistry out there, so maybe there's some type of life that uses an alternative to water.
毕竟这里面涉及许多化学进程，所以或许存在某种生物不以水为必需品。
It sounds a little ridiculous, but researchers have begun to look into it.
这听起来或许有些不可思议，但科学家已经开始关注这件事情了。
And they've found that it might not be impossible.
不过，他们发现这可能是子虚乌有的说法。
At first, the idea of life without water sounds so silly because water is straight-up amazing.
首先，生命可以不依赖水而存在，这个想法听上去就荒诞至极，因为水就是神奇的存在。
I mean, look at its structure. Water is a simple molecule of just two hydrogen atoms and an oxygen.
为什么这么说呢，看看水的结构就知道啦。水分子结构很简单——只有2个氢原子和1个氧原子。
But because of the way that those atoms are arranged, one side of that molecule has a slightly positive charge, and the other is slightly negative.
但这几个原子的组成方式使得水分子的一面略带正电荷，而另一面则略带负电荷。
That makes water a polar molecule. And while there are plenty of others like it, water is special because the difference between its charges is relatively large.
这就让水分子成了极性有极分子。虽然有很多分子与水分子结构相像，但水还是特殊的存在，因为其电荷之间的差异相对较大。
Among other reasons, that makes H2O really good at dissolving things.
这也是水容易溶解其他东西的原因之一。
Its charges allow water to interact with other substances so strongly that it disrupts the chemical bonds between their molecules or atoms, causing the other substance to dissolve.
水分子的电荷让其可以与其他物质发生强烈的作用，从而扰乱分子或者原子之间的化学键，进而溶解其他物质。
And water can do that with a huge range of compounds. It's what scientists call a universal solvent.
水能溶解很多化合物，因此，科学家称它为万能溶剂。
A powerful solvent is essential for life, since biological processes require lots of different molecules all in one place.
强力溶剂对于生命而言是必需的，因为很多生物进程的必要条件是许多分子共处在同一空间。
And solvents allow them to mingle and interact. That isn't the only good thing about water, though.
有了溶剂，这些不同的分子就能混合并发生反应。不过，水还有其他有点。
On Earth, its properties may also have allowed life to evolve in the first place.
地球上水的特质可能是让生命出现的一个必要条件。
That's because some molecules can be generally attracted to or repelled by water.
这是因为一些分子可以为水所吸引或者排斥。
And certain substances have both water-loving and water-hating parts in the same package.
而且有些物质既有易溶于水的一面，也有不溶于水的一面。
Phospholipids are one of those substances. In an effort to get close to and away from the water at the same time, they'll organize themselves into nice little bubbles, called vesicles.
磷脂质就是这样一种物质。为了与水保持若即若离的距离，它们会以小泡的形式组织自己，这些泡也称水泡。
That creates tiny pockets of chemistry, where reactions can happen without being disturbed.
这样就能产生一些小的化学反应空间，这样一些化学反应发生的时候就可以不受到干扰。
And that may have allowed life to emerge. As a whole, water's structure makes it a powerful medium for biochemistry,
这可能是生命形成的原因。总体来说，水的结构让水成为生化反应的强大介质。
and that's not to mention all of its other qualities, like the fact that it can keep temperatures on a planet more stable.
这还没提水的其他特质呢，比如水可以保持行星温度的温度。
But just because water is important here doesn't mean it has to be important everywhere.
不过，水对地球重要并不意味着水在其他地方也同等重要。
There are other molecules that can fill some of water's roles.
有些分子也具备水的一些特征。
Take ammonia, which is one nitrogen atom bonded to three hydrogens.
以氨为例，氨是1个氮原子与3个原子通过化学键组合在一起。
Ammonia is abundant in Jupiter's clouds, and it's also been detected in the plumes emitted from Saturn's moon Enceladus.
木星的云层中含有大量氨，从土卫二释放的喷射流中可以探测到氨的存在。
Like water, it has a charge difference among its atoms, making it polar.
与水的相似之处是：氨的原子之间存在电荷差异，所以也就有了电极效应。
That means it can dissolve a lot of substances, including many used in biochemical reactions.
这就意味着氨可以溶解许多物质，包括很多用于生化反应的物质。
But because its structure isn't exactly the same as water, it isn't as good at dissolving the same things.
不过，因为氨的结构不与水完全相同，所以氨擅长溶解的物质与水不完全相同。
That means life on Earth couldn't use it as a direct substitute, but scientists think it's possible that a different kind of life could have evolved to take advantage of it.
也就是说，地球上的生命无法直接用氨来替代水，不过，一些科学家认为有可能另一种生命可以通过氨来实现进化。
There are even ammonia-loving and ammonia-hating molecules out there, which could give rise to those vesicles for chemical reactions.
或许太空里有亲氨和厌氨的分子也说不定呢，而这些分子可能可以产生囊泡，从而实现化学反应。
So ammonia is a promising alternative to water, but it's not perfect.
所以，氨有望代替水，但并非无懈可击的替代物。
Its weaker charges and interactions make chemical reactions just a bit more tough.
因为氨的电荷更弱，反应也更弱，所以化学反应发生的条件更为苛刻。
Plus, while water is liquid over a range of 100 degrees Celsius, ammonia's liquid range is just 44 degrees at Earth-like pressures.
此外，由于液态水温度范围最高可达100摄氏度，而液态氨在地球这样的气压下最多可达44摄氏度。
That would give life less wiggle room if a planet's climate fluctuated.
这样的话，如果行星气候有变，那么生命可存活的可能性就更小了。
It can stay liquid at higher temperatures if there's higher pressure, like in a gas giant, but right now we just don't understand how life could get started suspended in the clouds of a gassy planet.
如果压强更高，那么液态氨可达到的温度也会更高，比如气体巨星的压强就更高。但目前来看，我们还无法解释生命在气体巨星的云层中是如何存续的。
So another option researchers are considering is hydrocarbons, like methane and ethane.
科学家考虑的另一个替代物是碳氢化合物，比如甲烷和乙烷。
Broadly speaking, these are strings of carbon and hydrogen, and on Earth, they make up natural gas.
广义而言，碳氢化合物就是碳原子和氢原子联结而成的物质，在地球上，它们可以产生自然的气体。
But elsewhere in the solar system, where it's much colder, these hydrocarbons exist as liquids.
但在太阳系的其他地方，由于温度更低，所以这些碳氢化合物是以液态形式存在的。
We discovered the most famous example of this in 2004, when the Cassini-Huygens mission revealed that the surface of Saturn's moon Titan is covered with lakes and oceans of liquid hydrocarbons.
其中最著名的一个例子是2004年发现的，当时卡西尼号发现土星六表面上布满了湖泊和海洋，都是由液态碳氢化合物组成的。
At first, things like methane might seem like a weird substitute for water, because they aren't polar, so they don't have the same power to dissolve things.
乍一看，甲烷等碳氢化合物用来代替水可能看起来有些奇怪，因为它们并不具备极性，所以他们溶解物质的能力就与水不同。
But it's not like hydrocarbons can't dissolve anything.
但碳氢化合物还是可以溶解物质的：
They can dissolve all kinds of oils and fats, so some reactions would be possible.
包括各类油脂、脂肪，所以有些反应是可以发生的。
Also, some molecules, like ones used in DNA, might be even more stable in hydrocarbons than in water.
此外，一些分子，比如脱氧核糖核酸里的一些分子，在碳氢化合物里的稳定性高于在水中的稳定性。
So maybe it would be even easier for life to evolve there.
所以或许有些生命在碳氢化合物中更容易存续。
Scientists have also suggested that something like vesicles could form in places like Titan, although they would likely use a different molecule, since there aren't really phospholipids there.
一些科学家表示，囊泡这样的东西可以在土卫六上形成，虽然形成时所用的分子很有可能不同，毕竟土卫六上没有磷脂质。
So again, while hydrocarbon-based life might look a bit different, it's not totally out of the question.
那么，说回来，以碳氢化合物为存续基础的生命可能看起来有些不同，但也并不是不可能。
Ammonia and hydrocarbons aren't the only water alternatives, either.
氨和碳氢化合物也并不是水唯二的替代品。
Scientists have also been studying everything from liquid nitrogen to sulphuric acid, and each seems to have some potential.
一些科学家已经在研究各类物质了，从液态氮到硫酸不一而足。很多物质都有可能可以替代。
So, at the end of the day, maybe water isn't necessary for life.
所以，或许我们最后会发现，或许水并不是生命的必要条件。
Before we can say that for sure, though, scientists will need to learn more about biochemistry, and more about water itself.
但在有实锤之前，科学家还是需要更多的了解生物化学反应以及水本身的信息。
Because some researchers think we don't even understand all of the ways that water is necessary yet.
因为一些科学家认为我们不了解水必要性的所有原因。
Either way, as we start to understand this better, scientists might have to change what they consider a habitable planet.
无论是哪种情况，我们都已经在了解更多信息的路上了。科学家可能必须要改变他们眼中宜居的星球。
And if they do, that will literally open up a whole new world of study.
如果科学家能做到，那么他们就打开了新世界研究的大门。
Thanks for watching this episode of SciShow Space!
感谢收看本期的《太空科学秀》！
There's lots to explore in the universe, from chemistry to astrodynamics, and we're thankful for the chance to unpack so much of it.
宇宙有很多可以探索的东西，从化学到天体动力学不一而足。我们感谢有机会揭开大量的谜题。
So to our patrons on Patreon, thank you!
所以我们要感谢忠实粉丝们！
And if you want to support the show and help us make more episodes like this, you can go to patreon.com/scishow.
如果大家想要表达对节目的支持，并帮助我们制作类似本期的视频的话，可以访问patreon.com/scishow。
