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SciShow Space is supported by Brilliant.
《太空科学秀》由Brilliant.org赞助播出。
After studying the solar system for hundreds of years, you'd think we'd at least have the basics figured out, stuff like why planets spin and orbit the way they do.
人类研究太阳系已有数百年的历史，所以一般人都会认为我们已经把基本情况摸清了，比如行星为何会像现在这样自转和公转。
Except, we totally don't.
但根本不是这么回事儿。
And you only have to look at the planet next door to see it.
看看附近的行星，就明白了。
When scientists began observing Venus in detail in the 1950s and ‘60s, they expected it to be pretty unremarkable.
上世纪五六十年代，科学家刚开始观测金星的诸多细节。那时候，他们以为金星的生存条件很棒。
Instead, it turned out to mostly be an inferno of acid rain, one that, of all things, spins backwards!
但最后发现金星经常下酸雨，有如地狱一般。此外还有一点——金星是逆向旋转的。
It's been more than 50 years since then, and while we know a lot more about Venus's climate, we still aren't totally sure why it's rotating the wrong way.
50多年过去了，虽然我们对金星的气候已经知之甚多，但我们依然不是很明白为何金星会逆向旋转。
But we at least have some ideas.
所幸我们不是一点思路都没有。
In astronomy, a backwards spin is called retrograde rotation, and “backwards” is defined relatively.
在天文学里，逆向旋转也称后腿旋转，这个“后退”是相对的后退。
Because the solar system formed from one cloud of spinning gas, the planets all orbit in the same direction: counterclockwise, if you're looking down on the Earth's north pole.
由于太阳系是由一片旋转气体云形成的，所以所有行星都会以相同的方向运行，也就是逆时针运行，前提是从地球北极从上往下看的话。
They also rotate on their axes the same way they orbit, so also counterclockwise.
他们自转的方向与公转一致，也是逆时针方向。
Except for Venus and Uranus.
但金星和天王星除外。
Uranus sits sideways on its axis, probably thanks to a couple of collisions.
天王星有些偏离自转轴，这是因为他们曾经遭受过一些撞击。
Venus, meanwhile, rotates clockwise, and it's much less clear why!
而金星是顺时针旋转的。而我们也愈发搞不明白为何如此了。
One of the earliest hypotheses was that Venus may have been hit so hard by an asteroid that it reversed direction.
起初，我们的假设是：金星可能受到了某颗小行星的强力撞击，从而改变了旋转方向。
The hypothesis seems to have been thrown out there in 1965 by two scientists who worked with the original radar data.
1965年，这个假说似乎受到了两位科学家的强烈否定，这两个人对原始雷达数据进行了研究。
And when I say, “thrown out there,” I mean it.
他们真的是“强烈否定”。
They followed up the suggestion by saying, and I quote, The possibilities are limited mainly by one's imagination; supporting evidence is rather harder to come by.
他们进行了跟踪调查，并表示“其实有很多可能，只是人类没有想到；而且支持性的证据也很难找到而已。”
Which is super convincing.
他们说的实在让人信服。
Either way, when you math it out, the idea kinda falls apart.
无论是哪种情况，但数学计算却无法支撑我们的想象。
It turns out that anything big enough to reverse Venus's rotation would also destroy the planet.
最终的结果表明：要是有什么物体曾改变过金星的旋转情况，也就能在当时直接毁了金星。
The impactor's kinetic energy would be some 10,000 times too high.
这个物体的动能可能近1万倍，这个动能太大了。
So, the impact idea was pretty much shelved.
所以，认为金星旋转情况被改变的可能性基本上就排除了。
Thankfully, it was replaced by a few actual evidence-based hypotheses.
所幸，这个结论被一些以证据为基础的假说打破了。
One of the leading ones, proposed around 1970, is that Venus spins the same way it always has.
其中一个主要的假说是1970年提出的，该假说认为金星一直都是这样旋转的。
Just at some point, much like Will Smith, its life got flipped, turned upside down!
只不过，某个时候，威尔·史密斯发生了天翻地覆的改变！
Not by mom who got scared, though. Just by physics.
但这并不是因为金星的母星受到了惊吓，而是因为物理学原理。
This could've happened because of processes within Venus's interior and atmosphere.
发生这样的变化，可能是因为金星的内部和大气层发生了一些进程。
Venus is differentiated, meaning that it has layers like the Earth does: a core, a mantle, and a crust.
金星也是一颗结构分明的星体，各层结构跟地球有点像：有地核、地幔和地壳。
As the planet rotates, the core and mantle can experience friction where they meet.
在金星旋转的过程中，地核和地幔一旦相遇就会发生摩擦。
Venus also has a really thick atmosphere, which, thanks to the Sun's gravity and heat, experiences tides along with the rest of the planet.
金星的大气层也很厚，而且在太阳引力和热量的影响下，会跟其他部分一样经历潮汐。
This hypothesis says that the core-mantle friction and those atmospheric tides could both put some torque on the planet, and that instability could've flipped Venus over.
该假说认为：无论是地核-地幔间的摩擦，还是大气潮，都会给金星造成一定的压力，就是这种不稳定性导致金星发生了天翻地覆的变化。
Some models suggest this might work only if Venus formed with an initial tilt of about 90 degrees, but others show that it might work with less initial tilt.
一些模型显示：该假说可能是正确的，但前提是金星的初始倾斜度在90°左右。但还有一些模型显示：即便初始倾斜度小一点，也是可能成立的。
Either way, the idea is pretty weird, and thinking about an entire planet flipping is kind of mind-boggling.
无论倾斜度究竟是多少，这个想法可以说是很奇怪的，毕竟要让整个星球天翻地覆是让人难以置信的。
But we have other ideas, too.
不过，我们也有其他的想法。
Another, first suggested in 1964, is that Venus may have gradually slowed down and then reversed direction.
还有一个说法是1964年提出的，该假说认为，金星可能是在逐渐减速后，彻底改变了旋转方向。
This could've been triggered by a few things, including interactions with the Sun's magnetic field, or those atmospheric tides, or a combination of both.
这可能是由几个因素引起的，其中就包括与太阳磁场之间的相互作用、大气潮，或者两者兼有。
Venus's atmosphere would have been the first part of the planet to start rotating retrograde after that spin-down.
金星的大气层可能是减速发生后第一个开始逆向旋转的部分。
Then, that may have provided the rest of the force necessary to get the whole planet going backward.
其旋转产生的作用力可能是最后一根稻草，让整个金星得以逆向旋转。
As a bonus, this idea would also explain why Venus's days are so long.
这个假说还有一个优势就是能解释金星白天如此之长的原因。
But there's no clear winner between these two hypotheses yet.
但这两种假说谁胜谁负还不一定。
To figure out which idea is most likely, we have to know more about Venus's early dynamics, specifically its rotation rate and axial tilt.
为了弄清楚哪种假说更有可能性，我们就必须对金星早期的力学情况有更多的了解，尤其是早期金星的转速和轴倾角。
According to a 2001 paper published in Nature, the axis-flip mechanism is most likely if Venus had a rapid initial rotation rate.
《自然》期刊上2001年曾发表过一篇论文，轴倾角改变的说法只有当金星初始转速很高的时候才最有可能成立。
But if it rotated slower than once every four Earth days and had a relatively small tilt, like less than 70 degrees, then slowing down and reversing is the most probable mechanism.
倘若金星的转速比4个地球日还要慢，并且轴倾角相对较小（比如小于70°）的话，那么减速以及改变方向是最有可能的机制。
Unfortunately, it's kind of hard to get evidence about Venus from four billion years ago.
不过，我们很难获得40亿年前金星情况的证据。
So until we build a time machine, or at least some really good models, the jury is out.
所以，只有有了时光机或者非常棒的模型之后，才能盖棺定论。
Of course, that's not the whole discussion, either.
当然了，这也并不是人们关于金星的所有讨论内容。
Because just to throw a wrench in things, that 1960s impact hypothesis is actually making a comeback. Or at least, a version of it.
因为也有人认为是有东西与金星发生了纠缠，就像上世纪60年代那个影响假说一样，仿佛又绕回了最初的假设，至少是曾经某个版本的假设。
In 2008, one researcher suggested that Venus may have gotten its weird spin back when it was a wee li'l planetesimal.
2008年，一位研究人员表示，金星还是一颗小行星体的时候就已经逆向旋转了。
They argue that, billions of years ago, another object about the same size slammed into it and sent it spinning like a backward top.
他们认为，几十亿年前，另一个跟金星差不多大的物体与其发生撞击，让金星开始逆向旋转。
But instead of destroying baby Venus, those two pieces came together to form a full-sized planet.
但是撞击并未导致金星毁灭，反而将他们两个结合在一起，形成了一颗完整的行星。
Unlike in the ‘60s, there's actually some potential evidence for this now.
这次的假说跟上世纪60年代的不同，因为是有证据的。
Based on Venus's topography, we don't think there's a lot of water in the planet's interior compared with Earth's.
基于金星的地形，我们认为，金星内部跟地球不一样，含水量不高。
And a huge impact could have provided the energy to get rid of it.
所以大型的冲击会摆脱它的充足能量。
This hypothesis would explain why Venus is so dry, but there are other competing models, too.
该假说可以解释金星含水量低的原因，但也有其他能够自圆其说的假说。
Like, it's possible Venus lost its water through evaporation instead.
比如，有个假说认为，金星有可能是通过蒸发作用来散发水分的。
To learn more, it would help to do a mineralogical survey of the planet to determine if there are any water-containing compounds there.
要了解更多的话，就要对金星进行矿物学调查，从而判定金星里是否有含水物质。
If there are, it means Venus couldn't have lost all its water so early in that big impact.
如果有含水物质的话，就意味着金星不可能在早期那次大型撞击中失掉了所有的水分。
But we haven't done one of those yet, because Venus is a place where robots go to die.
但我们还没有进行实地调查，毕竟金星是一个机器人都难以存活的地方。
The good news is, it's arguably easier to build a Venus-resistant rover than a time machine.
有个好消息是：建造能抵御进行环境的探测器要比时光机容易多了。
So hopefully we can get some evidence for at least one of those hypotheses soon.
所以，我们还是有希望在不久的未来得到证据，进一步验证这些假说的。
And speaking of making hypotheses and gathering evidence, thanks to Brilliant.org for sponsoring this episode and introducing me to this Science Essentials quiz on Observations, Questions, and Hypotheses.
说到假说和收集证据的事儿，这里我要感谢Brilliant.org赞助了本期的节目，同时也让我了解到有关观测的科学必要知识测验，即提问与假说。
In this quiz, you're a scientist studying the fictional exoplanet “Xlyem,”
在这项测验中，测试者会扮演科学家的身份，对外星行星Xlyem进行研究。
and making observations and hypotheses based on what you experience on the planet, as well as what you already know from earth.
期间，测试者要根据自己对该行星此前的认知来进行观测并提出假说。
The questions and explanations walk you through the scientific process, but what's so special about it is that in the quiz format, it reminds (and even requires) you to check your earthling biases before moving on to the next question.
其中的问题和解释会带大家走一遍整个科学流程，而关于这个测试最特别的一点是：它会提醒（甚至要求）大家对自己既有的想法多加思考，然后才会进入下一题。
When I'm not making SciShow Space videos, I'm teaching kids how to learn and operate in the world by using their imaginations, so the creativity and complex thinking required in this lesson are right up my alley.
我不做《科学太空秀》节目的时候，就会教小孩子动用自己的想象力进行学习和实操，所以，本节课中所需的创造性和复杂思考就是我擅长的领域咯。
If you want to check it out, head to brilliant.org/scishowspace to sign up for free.
如果大家想一探究竟的话，就免费注册brilliant.org/scishowspace吧。
Brilliant is also offering 20% off the annual premium subscription to the first 200 SciShow Space viewers to sign up.
brilliant.org现在正在为前200位注册的《太空科学秀》观众提供20%的订阅费减免。
o click on the link in the description to check it out and let me know in the comments how you do on this quiz!
大家可以点击描述中的链接来一探究竟。有任何意见也可以评论在里面哦，我会看到的！
Thanks for watching and supporting SciShow Space.
感谢收看本期的《太空科学秀》。
