Why Earth may someday look like Mars | Anjali Tripathi

Translator: Joseph Geni Reviewer: Joanna Pietrulewicz

翻译人员: Yijing Li 校对人员: Jiawei Ni

So when you look out at the stars at night,

当你望向夜空中的星星，

it's amazing what you can see.

你会看到美得让人

It's beautiful.

窒息的画面。

But what's more amazing is what you can't see,

但更令人惊叹不已的是 你看不见的部分，

because what we know now

因为我们现在知道，

is that around every star or almost every star,

几乎每一颗恒星周围，

there's a planet,

都有一颗行星，

or probably a few.

或者可能有好几颗。

眼前这幅画面未能展现给你的，

So what this picture isn't showing you

are all the planets that we know about

是我们所知的

out there in space.

宇宙中的所有行星。

But when we think about planets, we tend to think of faraway things

但当我们想到行星， 我们倾向于想到远在天边、

that are very different from our own.

与我们这颗行星 大不相同的那些行星。

But here we are on a planet,

但其实，我们就居住在一颗行星上。

and there are so many things that are amazing about Earth

关于地球，有太多令人惊奇的东西，

以至于我们会不远万里 去寻找类似的星体。

that we're searching far and wide to find things that are like that.

And when we're searching, we're finding amazing things.

而在寻找的过程中， 我们确实发现了惊人的现象。

But I want to tell you about an amazing thing here on Earth.

但我想告诉大家一个 关于地球的不可思议的事实，

那就是每分钟，

And that is that every minute,

400 pounds of hydrogen

有400磅的氢气

and almost seven pounds of helium

和将近七磅的氦气

escape from Earth into space.

从地球上逃逸到太空中。

And this is gas that is going off and never coming back.

而这些气体一旦逃逸， 就不再返回。

So hydrogen, helium and many other things

氢气、氦气和很多其他气体

make up what's known as the Earth's atmosphere.

组成了我们所知的地球大气层。

The atmosphere is just these gases that form a thin blue line

大气层就是由这些气体 组成的一条细细的蓝线，

that's seen here from the International Space Station,

这就是宇航员从国际空间站

a photograph that some astronauts took.

拍摄到的大气层。

And this tenuous veneer around our planet

而正是围绕我们行星的 这个气体薄层

is what allows life to flourish.

让万物得以生生不息。

It protects our planet from too many impacts,

它使我们的行星

免于遭受过多来自陨石 或类似物质的撞击。

from meteorites and the like.

And it's such an amazing phenomenon

而一个非常惊人的现象就是，

that the fact that it's disappearing

这些气体正在消失，

should frighten you, at least a little bit.

这应该令你多少感到有些惊恐。

这就是我研究的内容，

So this process is something that I study

and it's called atmospheric escape.

这一过程被称为“大气逃逸”。

So atmospheric escape is not specific to planet Earth.

大气逃逸不仅仅在地球上发生，

It's part of what it means to be a planet, if you ask me,

在我看来，它正是 行星一词的部分含义，

因为不光是地球， 宇宙中的所有行星

because planets, not just here on Earth but throughout the universe,

can undergo atmospheric escape.

都可能发生大气逃逸。

而这一现象发生的方式， 揭露了行星自身的奥秘。

And the way it happens actually tells us about planets themselves.

Because when you think about the solar system,

因为当你想到太阳系，

you might think about this picture here.

你可能会想到这幅画面——

And you would say, well, there are eight planets, maybe nine.

你会说，嗯，总共有 八颗行星，也可能是九颗。

So for those of you who are stressed by this picture,

被这张图片搞晕的朋友们，

I will add somebody for you.

我再给你们添个东西。

（笑声）

(Laughter)

Courtesy of New Horizons, we're including Pluto.

借助“新视野号”太空探测器， 我们将冥王星加入了其中。

我要强调一点，

And the thing here is,

for the purposes of this talk and atmospheric escape,

为了方便演讲及介绍大气逃逸，

我准备讨论一下冥王星，

Pluto is a planet in my mind,

in the same way that planets around other stars that we can't see

不过那些围绕我们看不见的 其他恒星旋转的，

也同样是行星。

are also planets.

So fundamental characteristics of planets

行星的一个基本特点

include the fact that they are bodies

就是它们自身由于重力的

that are bound together by gravity.

束缚而形成一个整体。

So it's a lot of material just stuck together

所以行星仅仅是一大堆物质

在吸引力的作用下聚在一起。

with this attractive force.

And these bodies are so big and have so much gravity.

而这些行星体积庞大，重力也很大，

因此它们是圆的。

That's why they're round.

So when you look at all of these,

你看所有这些行星，

包括冥王星，

including Pluto,

they're round.

都是圆的。

所以你就可以看出， 重力确实在发挥作用。

So you can see that gravity is really at play here.

But another fundamental characteristic about planets

但行星的另一个基本特点，

是我们在这里看不到的。

is what you don't see here,

and that's the star, the Sun,

这是恒星——太阳，

that all of the planets in the solar system are orbiting around.

太阳系的所有行星都围绕它旋转，

And that's fundamentally driving atmospheric escape.

而这从根本上导致了大气逃逸。

The reason that fundamentally stars drive atmospheric escape from planets

恒星从根本上导致 行星大气逃逸的原因在于，

恒星为行星提供粒子、光和热，

is because stars offer planets particles and light and heat

that can cause the atmospheres to go away.

而这些可以导致大气从行星逃逸。

So if you think of a hot-air balloon,

如果你想象一个热气球，

or you look at this picture of lanterns in Thailand at a festival,

或者看看这张图片中， 某个泰国节日的灯笼，

you can see that hot air can propel gasses upward.

你可以看到，热空气会将气体向上推。

如果你有足够的能量和热量，

And if you have enough energy and heating,

which our Sun does,

就像我们的太阳那样，

that gas, which is so light and only bound by gravity,

那么这些仅仅受到 重力束缚的轻盈气体

it can escape into space.

就会逃向太空。

And so this is what's actually causing atmospheric escape

这就是导致大气逃逸的原因，

here on Earth and also on other planets --

无论在地球上，还是在其他行星上——

that interplay between heating from the star

即发生于恒星提供的热量

和克服行星的重力之间的相互作用。

and overcoming the force of gravity on the planet.

我已经讲过，这一过程

So I've told you that it happens

at the rate of 400 pounds a minute for hydrogen

以每分钟400磅氢气

和将近七磅氦气的速度发生。

and almost seven pounds for helium.

但是这个过程到底是什么样呢？

But what does that look like?

Well, even in the '80s,

其实，早在20世纪80年代，

we took pictures of the Earth

我们拍摄的地球照片，

in the ultraviolet

就是透过紫外线，

using NASA's Dynamic Explorer spacecraft.

利用美国国家航天局的 “动力探索者号”太空船完成。

So these two images of the Earth

这两张地球的照片，

show you what that glow of escaping hydrogen looks like,

向我们展示了逃逸的氢气，

shown in red.

即图中红色的光。

And you can also see other features like oxygen and nitrogen

我们还可以看到其他气体的 样貌，如氧气和氮气，

in that white glimmer

即图中白色的微光

in the circle showing you the auroras

所形成的一圈极光，

还有赤道附近的一缕缕光亮。

and also some wisps around the tropics.

这些图片无疑向我们展示了，

So these are pictures that conclusively show us

我们的大气层不只是 被牢牢束缚于地球表面，

that our atmosphere isn't just tightly bound to us here on Earth

but it's actually reaching out far into space,

它实际上还延伸到了遥远的太空中，

and at an alarming rate, I might add.

而且是以一种惊人的速度。

但是地球不是唯一一个 正在经历大气逃逸的星球，

But the Earth is not alone in undergoing atmospheric escape.

Mars, our nearest neighbor, is much smaller than Earth,

我们最近的邻居火星，比地球小得多，

so it has much less gravity with which to hold on to its atmosphere.

所以它借以束缚大气的重量也轻得多。

And so even though Mars has an atmosphere,

因此， 虽然火星有大气层，

we can see it's much thinner than the Earth's.

但我们可以看到， 它比地球的大气层薄得多。

Just look at the surface.

看看火星的表面，

我们看到了火山口， 这表明火星未曾有过可以阻止

You see craters indicating that it didn't have an atmosphere

that could stop those impacts.

这些撞击的大气层。

Also, we see that it's the "red planet,"

另外我们还看到， 火星是一颗“红色行星”，

and atmospheric escape plays a role

而大气逃逸扮演的一个角色

in Mars being red.

就是使火星成为红色。

That's because we think Mars used to have a wetter past,

这是因为我们认为， 火星曾经比现在湿润，

当水获得足够的能量， 它就分解为氢气和氧气，

and when water had enough energy, it broke up into hydrogen and oxygen,

and hydrogen being so light, it escaped into space,

而氢气很轻，会逃逸到太空中，

and the oxygen that was left

而留下来的氧气

oxidized or rusted the ground,

则氧化使地表生锈，

making that familiar rusty red color that we see.

使地表变成了这种我们熟悉的锈红色。

So it's fine to look at pictures of Mars

所以当我们观察火星的图片，

and say that atmospheric escape probably happened,

我们有理由说， 大气逃逸可能曾经发生过。

but NASA has a probe that's currently at Mars called the MAVEN satellite,

不过，美国国家航天局 正在利用太空探测器MAVEN探测火星，

and its actual job is to study atmospheric escape.

它的实际任务就是研究大气逃逸。

MAVEN即“火星大气与 挥发物演化”太空船。

It's the Mars Atmosphere and Volatile Evolution spacecraft.

And results from it have already shown pictures very similar

探测传回的图片非常类似于

to what you've seen here on Earth.

我们在地球上看到的景象。

We've long known that Mars was losing its atmosphere,

我们早已知道，火星一直在失去大气，

but we have some stunning pictures.

但我们这里有一些令人震惊的图片。

Here, for example, you can see in the red circle

比如在这张图中， 我们可以看到，红色圆圈

is the size of Mars,

就是火星的轮廓；

and in blue you can see the hydrogen escaping away from the planet.

而蓝色的部分， 就是从火星上逃逸的氢气，

So it's reaching out more than 10 times the size of the planet,

它延伸到了火星自身尺寸 十多倍的距离之外，

这一距离，足以使氢气 不再受到火星引力的束缚，

far enough away that it's no longer bound to that planet.

It's escaping off into space.

于是它逃逸到了太空中。

And this helps us confirm ideas,

这帮助我们验证了我们的观点，

即通过氢气的逃逸来解释 为什么火星是红色的。

like why Mars is red, from that lost hydrogen.

But hydrogen isn't the only gas that's lost.

但是氢气不是火星丧失的唯一气体。

我提到了地球上的氦气， 以及一些氧气和氮气，

I mentioned helium on Earth and some oxygen and nitrogen,

and from MAVEN we can also look at the oxygen being lost from Mars.

而通过MAVEN我们也可以 观察从火星逃逸的氧气。

我们可以看到，由于氧气更重，

And you can see that because oxygen is heavier,

it can't get as far as the hydrogen,

它不会逃逸到像氢气那么远的距离，

but it's still escaping away from the planet.

但它仍然向火星之外逃逸。

You don't see it all confined into that red circle.

可以看到氧气并不是 全部集中在红色圆圈里。

So the fact that we not only see atmospheric escape on our own planet

我们不仅在自己的行星上 能看到大气逃逸，

but we can study it elsewhere and send spacecraft

还可以发射太空船， 在其他地方研究这一现象，

allows us to learn about the past of planets

这让我们得以了解行星的过去，

but also about planets in general

乃至行星的概况，

and Earth's future.

以及地球的未来。

So one way we actually can learn about the future

因此，我们了解未来的一个途径

is by planets so far away that we can't see.

就是去研究不可见的遥远行星。

And I should just note though, before I go on to that,

在我继续讲下去之前， 我应该强调一点，

I'm not going to show you photos like this of Pluto,

我不打算给你们展示 像这样的冥王星图片，

这可能让你们有点失望，

which might be disappointing,

but that's because we don't have them yet.

不过这是因为我们还没有图片。

But the New Horizons mission is currently studying atmospheric escape

但是“新地平线”任务正在研究大气逃逸，

being lost from the planet.

也就是冥王星失去的大气。

所以继续关注，等待结果吧。

So stay tuned and look out for that.

But the planets that I did want to talk about

我原本想谈论的行星

are known as transiting exoplanets.

被称为凌日系外行星。

任何一个围绕太阳以外的 恒星旋转的行星

So any planet orbiting a star that's not our Sun

is called an exoplanet, or extrasolar planet.

都被称为外部行星，或系外行星。

And these planets that we call transiting

而我们称之为“凌日”的这些行星

have the special feature

有一个特点，

that if you look at that star in the middle,

那就是如果你观察这颗恒星的中央，

you'll see that actually it's blinking.

你会看到它其实在闪烁。

而它闪烁的原因，

And the reason that it's blinking

is because there are planets that are going past it all the time,

就是一直都有行星从那里经过，

而且遵从特定的轨道，

and it's that special orientation

这样行星就挡住了来自恒星的光，

where the planets are blocking the light from the star

that allows us to see that light blinking.

于是我们就看到了闪烁的光。

And by surveying the stars in the night sky

通过在夜空中观测这些恒星的

for this blinking motion,

闪烁运动，

we are able to find planets.

我们就可以找到行星。

This is how we've now been able to detect over 5,000 planets

我们就是用这种方法， 在银河系中探测到了

5000多颗行星。

in our own Milky Way,

and we know there are many more out there, like I mentioned.

而且就像我讲过的， 在银河系外还有很多行星。

So when we look at the light from these stars,

所以当我们观察这些行星发出的光，

what we see, like I said, is not the planet itself,

就像我说的，我们看到的不是行星本身，

but you actually see a dimming of the light

而是光亮的明暗，

that we can record in time.

我们可以记录它随时间的变化。

So the light drops as the planet decreases in front of the star,

当行星绕到恒星前方，光亮就会变暗，

and that's that blinking that you saw before.

这就是你们之前看到的闪烁。

So not only do we detect the planets

所以我们不仅探测行星，

还透过不同的波长来观察星光。

but we can look at this light in different wavelengths.

我已经讲过，可以透过 紫外线来观察地球和火星。

So I mentioned looking at the Earth and Mars in ultraviolet light.

如果利用哈勃太空望远镜 观察凌日系外行星，

If we look at transiting exoplanets with the Hubble Space Telescope,

we find that in the ultraviolet,

我们会发现，透过紫外线，

you see much bigger blinking, much less light from the star,

当行星经过恒星前方时， 闪烁变得更明显，

when the planet is passing in front.

星光也更暗。

And we think this is because you have an extended atmosphere of hydrogen

我们认为，因为有一层厚厚的氧气

all around the planet

围绕着整个行星，

所以行星看上去更膨胀，

that's making it look puffier

and thus blocking more of the light that you see.

于是挡住了更多光。

So using this technique, we've actually been able to discover

运用这一技术，我们已经发现了

a few transiting exoplanets that are undergoing atmospheric escape.

好几颗正在发生 大气逃逸的凌日系外行星。

在我们已发现的行星中，

And these planets can be called hot Jupiters,

for some of the ones we've found.

有一些可称为热木星。

And that's because they're gas planets like Jupiter,

这是因为这些行星 和木星一样，是气体行星，

但它们和自己的恒星距离相当近，

but they're so close to their star,

about a hundred times closer than Jupiter.

大约是木星的百分之一。

And because there's all this lightweight gas that's ready to escape,

而且由于有这些会逃逸的轻质气体，

and all this heating from the star,

以及来自恒星的热量，

you have completely catastrophic rates of atmospheric escape.

大气逃逸的发生是毫无疑问的。

So unlike our 400 pounds per minute of hydrogen being lost on Earth,

与地球每分钟失去400磅氢气不同，

for these planets,

这些行星

每分钟失去13亿磅氢气。

you're losing 1.3 billion pounds of hydrogen every minute.

So you might think, well, does this make the planet cease to exist?

你可能会想，呃， 这会不会让行星不复存在？

And this is a question that people wondered

而这正是人们观测

when they looked at our solar system,

太阳系时的疑问，

因为距离太阳较近的行星为岩态，

because planets closer to the Sun are rocky,

and planets further away are bigger and more gaseous.

而距太阳远一些的行星体积更大， 气态程度更高。

Could you have started with something like Jupiter

如果一个行星一开始像木星这样，

离太阳又很近，

that was actually close to the Sun,

它的气体会不会全跑光？

and get rid of all the gas in it?

We now think that if you start with something like a hot Jupiter,

我们目前认为，如果一个行星 一开始像热木星一样，

you actually can't end up with Mercury or the Earth.

那么它最终不会变得像水星或者地球。

But if you started with something smaller,

但是一个体积更小的行星，

it's possible that enough gas would have gotten away

是有可能因为气体逃逸太多

that it would have significantly impacted it

而受到明显影响的，

and left you with something very different than what you started with.

最终，这个行星将变得 和一开始截然不同。

以上的内容都是泛泛而谈，

So all of this sounds sort of general,

我们想到的可能只是太阳系，

and we might think about the solar system,

but what does this have to do with us here on Earth?

但这和地球上的我们有什么关系？

Well, in the far future,

其实，在遥远的未来，

太阳会变得更亮。

the Sun is going to get brighter.

And as that happens,

如此一来，

the heating that we find from the Sun is going to become very intense.

太阳给我们带来的热量 就会变得非常强烈。

In the same way that you see gas streaming off from a hot Jupiter,

与我们所讲的热木星 流失气体的方式一样，

gas is going to stream off from the Earth.

地球也会流失气体。

And so what we can look forward to,

所以我们可以预料，

or at least prepare for,

或者至少有所准备的，

is the fact that in the far future,

就是在遥远的未来，

the Earth is going to look more like Mars.

地球将会看上去更像火星。

Our hydrogen, from water that is broken down,

水分解产生的氢气

将以更快的速度逃逸到太空中，

is going to escape into space more rapidly,

and we're going to be left with this dry, reddish planet.

留给我们一个干燥的红色星球。

So don't fear, it's not for a few billion years,

不过别害怕， 最近几十亿年内还不会发生，

so there's some time to prepare.

我们还有时间做准备。

（笑声）

(Laughter)

但是我想让大家意识到 正在发生的事情：

But I wanted you to be aware of what's going on,

大气逃逸不只是在未来，

not just in the future,

but atmospheric escape is happening as we speak.

而是此时此刻就在发生。

So there's a lot of amazing science that you hear about happening in space

关于太空和那些遥远的行星， 大家听说过很多

and planets that are far away,

惊人的科学发现。

我们正在研究这些行星， 以便对那里的世界有更多了解。

and we are studying these planets to learn about these worlds.

But as we learn about Mars or exoplanets like hot Jupiters,

但在我们探索火星，或是 像热木星这样的系外行星的同时，

we find things like atmospheric escape

我们发现了诸如大气逃逸这样的现象，

从而大大增进了我们对地球的了解。

that tell us a lot more about our planet here on Earth.

So consider that the next time you think that space is far away.

如果你下次再觉得太空遥不可及， 不妨想想我今天的演讲。

Thank you.

谢谢大家。

（掌声）

(Applause)