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We probably know more about Mars than any other planet in the solar system, besides Earth.
在太阳系除了地球之外，我们可能最熟悉火星了。
For decades we've been sending probes and landers and rovers, and we're finally planning to send crewed missions, too.
几十年来，我们一直在派遣探测器、登陆器和火星探测车，也终于计划发射载人飞船了。
We've learned a lot about the planet over the years,
多年来，我们学到了很多有关行星的知识，
but as we find out more, we keep uncovering new mysteries, important, fundamental aspects of Mars that we just can't explain.
随着火星的新秘密不断被发掘出来，我们对它重要而基本的方面却无法解释。
Here are three of them.
下面介绍的就是其中的三个方面。
When it comes to the search for life, methane is pretty important.
在探寻生命的过程中，甲烷非常重要。
On Earth, almost all the methane in the atmosphere comes from life forms: whether termites or microbes or cow farts.
在地球上，大气中几乎所有的甲烷都来自生命形态：无论是白蚁、微生物还是奶牛放的屁。
So when the Curiosity rover detected a spike in methane in Mars' atmosphere, it was an exciting moment.
因此，“好奇号”探测车探测到火星大气层中甲烷的激增时，这时刻真令人激动！
The spike was announced in late 2014, and stretched across a two-month period a year earlier.
这一激增是在2014年年底公布的，并在一年前的两个月里持续增长。
It's surprising there's any methane on Mars at all, much less a big spike.
火星上有甲烷就令人惊讶了，更不用说还有一个大的激增。
The sun's radiation breaks it down so quickly that it shouldn't last more than a few hundred years.
太阳的辐射使甲烷快速分解，所以它不应该持续几百年。
But scientists have come up with a few possible explanations.
但科学家想出了几种可能的解释。
If this was a one-time thing, it could have come from an underground lake suddenly releasing a bunch of old, trapped methane.
如果只是偶然激增，那可能是地下蓄水层释放了远古甲烷气体。
If the spike is seasonal, though, it's possible that compounds in the Martian soil release methane during certain times of year.
如果激增是季节性的，可能是火星土壤中的化合物在一年的某些时间释放出甲烷引起的。
Or, that life on Mars releases methane during certain seasons.
或者是火星上的生命在某些季节释放出甲烷造成的。
Methane production on Earth is the same way, increasing at times of year when plants and bacteria are more active.
地球上甲烷的产生也是同样的道理，它在植物和细菌更活跃的时候数量增加。
Any sign of Martian life is kind of a huge deal, so researchers started looking for another methane increase to see if there's a seasonal pattern.
火星上出现任何生命迹象都是一件大事，所以，研究人员开始寻找另一种甲烷的增量，以确定是否存在季节性模式。
So far, Curiosity hasn't found another example of a methane increase, so maybe it really was just a subsurface lake.
到目前为止，“好奇号”还没有发现另一种甲烷增加的例子，所以也许它真的只是一个地下湖引起的。
For now, we don't know for sure.
现在我们也不确定。
Even if it was a lake, life could still have produced that methane in the first place.
即使它是一个湖，仍有可能生命在一开始就产生了那种甲烷。
It's just more likely that it was produced through active geology instead.
它更有可能是由活跃的地质活动产生的。
Here's the thing, though: only 5% of the methane on Earth is produced by geologic processes,
不过还有一件事，地球仅有5%的甲烷是由地质活动产生的，
so Mars would have to be pretty darn active to explain away the methane.
所以如果那样解释甲烷的来源的话，火星必须非常活跃。
So, no matter what the answer to this mystery is, it'll be an important discovery.
那么不管这个秘密的答案是什么，它都是一项重要的发现。
Methane isn't the only thing we didn't expect to find on Mars.
甲烷并不是唯一一个我们想不到会在火星上发现的东西。
We also found gullies, areas of the Martian surface that look like old rivers, and in one case, a huge ocean.
我们还发现了沟壑，它们所在的火星表面看起来像古老的河流，有一个像巨大的海洋。
If liquid water once did carve out those features, there doesn't seem to be much of it left now. That wouldn't be surprising.
如果液态水曾经雕刻出这些地貌，那么现在似乎没剩多少了。这不奇怪。
Water on Mars would probably have been lost over time because of its weaker gravity and thinner atmosphere.
因为火星上引力更弱，大气层更薄，上面的水可能随着时间消失了。
But water isn't necessarily what created the gullies and lakes.
但是水不一定是创造沟壑和湖泊的必要条件。
It's actually pretty hard for scientists to come up with a scenario where Mars would be warm enough to support a huge ocean full of water.
科学家很难想出那种情景：火星足够温暖，足以支撑满是水的大洋。
Which is why they've been trying to figure out what else could have caused those dark markings on the surface.
他们一直试图弄清楚还有什么东西会在火星表面留下这些黑色的斑纹，这就是原因。
Some of them could have been created by a process that doesn't exist on Earth at all: the sublimation of natural dry ice.
一些科学家创造出了地球上根本没有的演变过程：天然干冰的升华。
Dry ice is frozen carbon dioxide, and there's a lot of CO2 on Mars.
干冰是冻结的二氧化碳，火星上有许多二氧化碳。
Seasonally, CO2 forms frost on the ground in certain places.
二氧化碳会季节性地在某些地面形成霜。
Sunlight could shine through the dry ice, heating the base and vaporizing it into CO2 gas.
阳光可以穿透干冰加热，把它蒸发成二氧化碳气体。
That process would release trapped martian soil with enough force to creat gullies.
这个过程会让火星土壤释放出足够的力量，创造出沟壑。
But dry ice can't explain all the gullies on Mars'surface.
但干冰不能解释火星表面所有沟壑的形成。
There are some in places where dry ice wouldn't have vaporized that way.
一些地方的干冰不能那样蒸发。
Some scientists argue that this process wouldn't produce the gullies from scratch, it would just make them bigger.
一些科学家认为，干冰不会直接形成沟壑，只是让它们变大了而已。
So there could be a few different processes at work.
所以可能还有不同的程序在起作用。
Another possibility is that the gullies are formed by gases or winds that blow dust and rocks down steep hills.
另一种可能性是，沟壑是由气体或风活动形成的，它们从陡峭的山上吹落尘土和岩石。
Some of the gullies near the equator even look like they could be volcanic.
一些靠近赤道的岩石甚至看起来像火山。
But recent evidence from a 4.5 billion year-old Martian meteorite seems to suggest that Mars has indeed lost a lot of water.
但最近一块有着4.5亿年历史的火星陨石证据表明，火星似乎确实失去过很多水。
Plus, we've seen some meandering gullies that really look like they could only have been formed by water.
另外，我们看到了一些弯曲沟壑，它们看起来只能是由水形成的。
Those two observations have reignited the idea that there was a lot of water on Mars in the past, including a whole ocean.
这两个观察结果重燃了过去火星上有很多水甚至有整个海洋的想法。
But for now, it's still controversial.
但就目前而言，它仍有争议。
There's a lot of conflicting evidence, so we're a long way from knowing for sure.
我们有很多有争议的证据，所以距离确定还有很长的路要走。
Speaking of conflicts: the northern and southern halves of Mars are totally different, and we have no idea why.
说到争议：火星的南北半球完全不同，我们也不知道为什么。
The northern half of Mars is super flat, while the south has a bunch of huge mountains and volcanoes.
火星的北半球超级平坦，而南半球有一群巨大的山脉和火山。
The most popular explanation is that a huge impact 4 billion years ago caused the split, around the same time as the impact that created our moon.
最流行的解释是，40亿年前一次巨大撞击导致了火星分裂，同时创造了月球。
Back then, the solar system was a chaotic mess, with regular, almost constant collisions.
那时，太阳系一片混乱，几乎经常发生碰撞。
An impact in the northern hemisphere would have caused the volcanic activity that created the mountains in the southern hemisphere, sending magma shooting to the surface.
对北半球的撞击导致了火山活动，火山向地表喷发岩浆，在南半球形成了山脉。
It is possible that that's what happened, but if it did, it would be the biggest known impact in the history of the solar system.
这种情况有可能发生，如果发生的话，它会是已知的太阳系史上最大的撞击。
Plus, if a giant rock smashed into Mars, it should have left a pretty clear mark on its surface.
而且，如果一块巨大岩石撞击火星的话，它会在其表面留下相当清晰的印记。
But there's no obvious crater rim in the northern hemisphere.
但在北半球没有明显的火山口。
Some researchers have proposed that a series of smaller impacts caused the divide instead,
一些研究人员提出，一系列较小的影响导致了这种分裂。
but one enormous impact seems to fit our simulations better.
但一次巨大的撞击似乎更符合我们的假设。
Another problem is that if early Mars was that volcanically violent,
另一个问题是，如果早期的火星火山频发的话，
it would be even less likely to have a liquid water ocean, since the planet would have lost much of its water very early in life.
它将更不可能有液态水的海洋，因为它在生命的早期就会失去很多水。
So it might turn out that the leading explanations for these last two Mars mysteries can't both be right.
因此，最后这两个火星秘密的主要解释不可能都是对的。
But one thing's for sure: there's still a lot we have left to learn about our next-door planet.
但有一件事是确定的，火星还有许多地方等待我们了解。
Thanks for watching this episode of SciShow Space, and thanks especially to our patrons on Patreon who help make this show possible.
感谢您收看本期的太空科学秀，要特别感谢“Patreon”赞助者的大力支持。
If you want to help us keep making episodes like this, you can go to patreon.com/scishow to learn more.
如果你想帮助我们继续制作这样的节目，可以登录patreon.com/scishow 了解更多信息。
And don't forget to go to youtube.com/scishowspace and subscribe!
也不要忘了登录youtube.com/scishowspace，点击订阅哦！
