I'm glad you're here,

我真的很高興能來這裡。

because that would be a little weird.

我也很高興你們能來這裡，

I'm glad we're all here.

要不然會有點奇怪。

And by "here," I don't mean here.

我很高興我們都在這裡。

Or here.

我說的「這裡」，不是現場這裡。

But here.

或這裡。

I mean Earth.

卻是這裡。

And by "we," I don't mean those of us in this auditorium,

我指的是地球。

but life,

而且我說的「我們」， 不是這座大禮堂裡的各位，

all life on Earth --

而是生命，

(Laughter)

地球上所有的生命──

from complex to single-celled,

（笑聲）

from mold to mushrooms

從複合體到單細胞，

to flying bears.

從黴菌到蘑菇，

(Laughter)

到飛天熊都是。

The interesting thing is,

（笑聲）

Earth is the only place we know of that has life --

有趣的是，

8.7 million species.

地球是我們知道 唯一有生命的地方──

We've looked other places,

八百七十萬種生物。

maybe not as hard as we should or we could,

我們找過其它地方，

but we've looked and haven't found any;

可能找得還不夠勤快，

Earth is the only place we know of with life.

但是我們找過， 沒有發現任何生物。

Is Earth special?

地球是我們所知唯一有生命的地方。

This is a question I've wanted to know the answer to

地球得天獨厚嗎？

since I was a small child,

這個問題的答案

and I suspect 80 percent of this auditorium

我從小時候就想知道，

has thought the same thing and also wanted to know the answer.

而且我猜禮堂裡八成的人

To understand whether there are any planets --

都想過同樣的事，也想知道答案。

out there in our solar system or beyond --

要知道是否有別的行星──

that can support life,

無論是在我們的太陽系 還是其它地方──

the first step is to understand what life here requires.

能支持生命，

It turns out, of all of those 8.7 million species,

第一步要了解在這裡 生命的要素是什麼。

life only needs three things.

事實證明， 這八百七十萬種物種

On one side, all life on Earth needs energy.

只需要三件事就能維持生命。

Complex life like us derives our energy from the sun,

那邊那張照片， 地球上所有的生命都要能量，

but life deep underground can get its energy

像我們這樣的複合生物體 要從太陽取得能量，

from things like chemical reactions.

地下深處的生物

There are a number of different energy sources

則從化學反應取得能量。

available on all planets.

地球上有很多種能量來源

On the other side,

可供大家使用。

all life needs food or nourishment.

另外那一邊照片，

And this seems like a tall order, especially if you want a succulent tomato.

所有的生命都需要食物或營養。

(Laughter)

這聽起來像是無理的要求， 尤其在你想要一顆多汁的番茄時。

However, all life on Earth derives its nourishment

（笑聲） （譯註：一般超市的番茄都很乾）

from only six chemical elements,

然而，地球上所有的生命

and these elements can be found on any planetary body

只要六種化學元素就能取得營養，

in our solar system.

而這些元素可在

So that leaves the thing in the middle as the tall pole,

我們的太陽系中 每一個行星體上找到。

the thing that's hardest to achieve.

所以這讓中間這項成了高不可及、

Not moose, but water.

很難拿到的東西。

(Laughter)

不是麋鹿，是水。

Although moose would be pretty cool.

（笑聲）

(Laughter)

雖然麋鹿也滿酷的。

And not frozen water, and not water in a gaseous state, but liquid water.

（笑聲）

This is what life needs to survive, all life.

不是冰凍的水，也不是氣狀的水， 而是液態水。

And many solar system bodies don't have liquid water,

這是生命要生存都需要的東西。 所有的生命。

and so we don't look there.

許多太陽系沒有液態水，

Other solar system bodies might have abundant liquid water,

所以我們不去那裡找。

even more than Earth,

有些太陽系可能有大量液態水，

but it's trapped beneath an icy shell,

甚至可能多過地球，

and so it's hard to access, it's hard to get to,

但是都困在冰層底下，

it's hard to even find out if there's any life there.

所以很難拿到，很難接近，

So that leaves a few bodies that we should think about.

甚至很難發現那裡到底有沒有生命。

So let's make the problem simpler for ourselves.

這樣只剩下幾座星體我們應該考慮。

Let's think only about liquid water on the surface of a planet.

所以我們來為自己簡化一下問題。

There are only three bodies to think about in our solar system,

我們只要想想 在星球表面的液態水就好。

with regard to liquid water on the surface of a planet,

這樣我們的太陽系裡 只剩三座星體可想，

and in order of distance from the sun, it's: Venus, Earth and Mars.

考慮行星表面是否有液態水，

You want to have an atmosphere for water to be liquid.

與太陽的距離依次是 金星、地球及火星。

You have to be very careful with that atmosphere.

你要有大氣層使水能維持液態。

You can't have too much atmosphere, too thick or too warm an atmosphere,

你必須非常小心考慮大氣層。

because then you end up too hot like Venus,

你不能有太大的大氣層， 太厚或太溫暖都不行，

and you can't have liquid water.

因為這樣會像金星一樣太熱，

But if you have too little atmosphere and it's too thin and too cold,

而且沒有液態水。

you end up like Mars, too cold.

但是如果你的大氣層太少， 而且太薄太冷，

So Venus is too hot, Mars is too cold,

結果就會像火星一樣太冷。

and Earth is just right.

所以金星太熱，火星太冷，

You can look at these images behind me and you can see automatically

地球則是剛剛好。

where life can survive in our solar system.

看看我後面的照片，你自然會看到

It's a Goldilocks-type problem,

我們的太陽系 哪裡有生命可以存活。

and it's so simple that a child could understand it.

這是典型的 《金髮女孩與三隻熊》問題，

However,

而且這太簡單了，小孩都能理解。

I'd like to remind you of two things

然而，

from the Goldilocks story that we may not think about so often

我要提醒各位兩件事，

but that I think are really relevant here.

是《金髮女孩與三隻熊》故事中， 常常被我們忽略

Number one:

但我認為非常切題的事。

if Mama Bear's bowl is too cold

第一：

when Goldilocks walks into the room,

金髮女孩走進屋時， 如果熊媽媽碗裡的粥太冷，

does that mean it's always been too cold?

那是指粥早就冷了？

Or could it have been just right at some other time?

還是曾有剛剛好的時間？

When Goldilocks walks into the room determines the answer

金髮女孩走進屋的時間決定了

that we get in the story.

故事的答案。

And the same is true with planets.

行星也一樣。

They're not static things. They change.

行星不是靜態的東西。 它們一直在改變。

They vary. They evolve.

它們變化，它們演化。

And atmospheres do the same.

大氣層也一樣。

So let me give you an example.

我舉一個例子。

Here's one of my favorite pictures of Mars.

這是我很愛的一張火星照片。

It's not the highest resolution image, it's not the sexiest image,

這既非最高解析度，也並非最美，

it's not the most recent image,

更不是最新的照片，

but it's an image that shows riverbeds cut into the surface of the planet;

但這張照片顯示河床切過行星表面；

riverbeds carved by flowing, liquid water;

被流動、液態水切割出的河床；

riverbeds that take hundreds or thousands or tens of thousands of years to form.

河床要上百、上千 甚至上萬年才能形成。

This can't happen on Mars today.

現在的火星不可能發生這件事。

The atmosphere of Mars today is too thin and too cold

現在的火星大氣層太薄太冷，

for water to be stable as a liquid.

水因而不能維持液態。

This one image tells you that the atmosphere of Mars changed,

這一張照片告訴我們 火星的大氣改變了，

and it changed in big ways.

而且改變很大。

And it changed from a state that we would define as habitable,

它從我們定義的適居狀態 變成現在的樣子，

because the three requirements for life were present long ago.

因為生命三大要素 在很久以前曾經出現過。

Where did that atmosphere go

本來能讓水在地表 維持液態的大氣去了哪裡？

that allowed water to be liquid at the surface?

嗯，有人說它逃逸到宇宙。

Well, one idea is it escaped away to space.

大氣粒子有足夠的能量

Atmospheric particles got enough energy to break free

逃脫此行星的重力，

from the gravity of the planet,

散逸到太空，一去不返。

escaping away to space, never to return.

這種現象在有大氣的 星體上都會發生。

And this happens with all bodies with atmospheres.

彗星的尾巴

Comets have tails

正是大氣逃逸絕佳的視覺提醒。

that are incredibly visible reminders of atmospheric escape.

但是金星的大氣也會隨時間逃逸，

But Venus also has an atmosphere that escapes with time,

火星及地球也一樣。

and Mars and Earth as well.

這只是程度及規模的問題。

It's just a matter of degree and a matter of scale.

所以我們要找出逃逸的速率，

So we'd like to figure out how much escaped over time

才能解釋這種轉變。

so we can explain this transition.

大氣怎麼得到逃逸所需的能量？

How do atmospheres get their energy for escape?

粒子怎麼得到足以逃逸的能量？

How do particles get enough energy to escape?

如果我們簡化一下來看， 有兩種方式。

There are two ways, if we're going to reduce things a little bit.

第一，陽光。

Number one, sunlight.

從太陽射出的光被大氣粒子吸收，

Light emitted from the sun can be absorbed by atmospheric particles

使粒子增溫。

and warm the particles.

是，我是在跳舞，不過它們──

Yes, I'm dancing, but they --

（笑聲）

(Laughter)

天啊，我在自己的婚禮上都沒跳。

Oh my God, not even at my wedding.

（笑聲）

(Laughter)

它們得到足夠的能量逃逸

They get enough energy to escape and break free

並掙脫重力， 這一切只要增溫就夠了。

from the gravity of the planet just by warming.

得到能量的第二種方法是太陽風。

A second way they can get energy is from the solar wind.

這些是從太陽表面噴出的 粒子、團塊及物質，

These are particles, mass, material, spit out from the surface of the sun,

而且它們在太陽系裡

and they go screaming through the solar system

以每秒 400 公里的速率呼嘯而過，

at 400 kilometers per second,

在太陽風暴期間有時候還更快，

sometimes faster during solar storms,

而且它們飛馳通過行星際空間，

and they go hurtling through interplanetary space

朝著行星及其大氣飛去，

towards planets and their atmospheres,

因而它們也可以提供能量

and they may provide energy

讓大氣粒子逃逸。

for atmospheric particles to escape as well.

這就是我感興趣的東西，

This is something that I'm interested in,

因為這跟適居性有關。

because it relates to habitability.

我曾說過金髮女孩的故事中有兩件事

I mentioned that there were two things about the Goldilocks story

我要你們注意並提醒你們一下，

that I wanted to bring to your attention and remind you about,

第二件事更微妙一點。

and the second one is a little bit more subtle.

如果熊爸爸碗裡的粥太燙了，

If Papa Bear's bowl is too hot,

而熊媽媽的粥太冷了，

and Mama Bear's bowl is too cold,

那熊寶寶的粥不就應該更冷了，

shouldn't Baby Bear's bowl be even colder

如果我們順著趨勢想？

if we're following the trend?

你一輩子都沒懷疑過的這件事，

This thing that you've accepted your entire life,

仔細一想後可能沒那麼簡單。

when you think about it a little bit more, may not be so simple.

當然，行星的溫度取決於 它與太陽的距離。

And of course, distance of a planet from the sun determines its temperature.

這就與適居性有關。

This has to play into habitability.

但是我們可能還必須考慮其它事。

But maybe there are other things we should be thinking about.

可能碗本身

Maybe it's the bowls themselves

也會決定這個故事的結局，

that are also helping to determine the outcome in the story,

也就是「剛剛好」的問題。

what is just right.

我可以跟大家講很多

I could talk to you about a lot of different characteristics

這三個行星不同的特性，

of these three planets

這些特性可能也會影響適居性，

that may influence habitability,

但是我有私心， 因為那跟我的研究有關係，

but for selfish reasons related to my own research

而且站在這裡拿著遙控器的是我， 不是你們──

and the fact that I'm standing up here holding the clicker and you're not --

（笑聲）

(Laughter)

所以我只跟你們講一兩分鐘就好，

I would like to talk for just a minute or two

來談一下磁場。

about magnetic fields.

地球有磁場；金星及火星沒有。

Earth has one; Venus and Mars do not.

磁場在行星內部深處產生，

Magnetic fields are generated in the deep interior of a planet

由導電的液態物質環繞流動

by electrically conducting churning fluid material

而產生這個環繞地球的 巨大古老磁場。

that creates this big old magnetic field that surrounds Earth.

如果你有羅盤， 你會知道哪邊是北邊。

If you have a compass, you know which way north is.

金星與火星沒有這個。

Venus and Mars don't have that.

如果你在金星及火星上用羅盤，

If you have a compass on Venus and Mars,

恭喜喔！你迷路了。

congratulations, you're lost.

（笑聲）

(Laughter)

這會影響適居性嗎？

Does this influence habitability?

又怎麼影響？

Well, how might it?

許多科學家認為行星的磁場

Many scientists think that a magnetic field of a planet

就像大氣層的防護罩，

serves as a shield for the atmosphere,

使在行星四周的太陽風粒子偏轉，

deflecting solar wind particles around the planet

有點像（星際爭霸戰） 力場防護罩效應，

in a bit of a force field-type effect

因為跟這些粒子的電荷有關。

having to do with electric charge of those particles.

我喜歡把它比喻為沙拉吧上面 擋噴嚏的防護罩。

I like to think of it instead as a salad bar sneeze guard for planets.

（笑聲）

(Laughter)

是的，以後看這片的同僚會發現，

And yes, my colleagues who watch this later will realize

這是史上頭一遭有人在我們這行

this is the first time in the history of our community

把太陽風等同為黏液。

that the solar wind has been equated with mucus.

（笑聲）

(Laughter)

好吧，所以這個效應， 就是地球可以受到保護

OK, so the effect, then, is that Earth may have been protected

幾十億年，

for billions of years,

因為我們有磁場。

because we've had a magnetic field.

大氣至今無法逃脫。

Atmosphere hasn't been able to escape.

另一方面，火星沒有受到保護，

Mars, on the other hand, has been unprotected

因為它沒有磁場，

because of its lack of magnetic field,

所以幾十億年來，

and over billions of years,

可能已經揭掉了夠多的大氣，

maybe enough atmosphere has been stripped away

使這個原本適居的行星轉變為

to account for a transition from a habitable planet

今天我們看到的樣子。

to the planet that we see today.

其他科學家認為磁場的

Other scientists think that magnetic fields

作用可能更像船上的帆，

may act more like the sails on a ship,

讓行星能與太陽風的能量 產生更多交互作用，

enabling the planet to interact with more energy from the solar wind

比自身的交互作用還要更多。

than the planet would have been able to interact with by itself.

帆可以聚集風的能量。

The sails may gather energy from the solar wind.

磁場可以聚集太陽風的能量，

The magnetic field may gather energy from the solar wind

讓更多大氣逃逸。

that allows even more atmospheric escape to happen.

這個理論還需要證明，

It's an idea that has to be tested,

但是其影響及作用方式

but the effect and how it works

看起來很明顯。

seems apparent.

那是因為我們知道

That's because we know

太陽風的能量會注入

energy from the solar wind is being deposited into our atmosphere

地球上的大氣層。

here on Earth.

那股能量順著磁力線傳導，

That energy is conducted along magnetic field lines

向下導入兩極，

down into the polar regions,

產生炫麗無比的極光。

resulting in incredibly beautiful aurora.

如果你曾看過極光，那真是壯麗！

If you've ever experienced them, it's magnificent.

我們知道能量進來了。

We know the energy is getting in.

我們試著測量有多少粒子跑出去，

We're trying to measure how many particles are getting out

還有磁場是否 以任何方式影響這一點。

and if the magnetic field is influencing this in any way.

所以我丟了一個問題給各位，

So I've posed a problem for you here,

但是我還沒有答案。

but I don't have a solution yet.

我們沒有解答。

We don't have a solution.

但是我們正在找。 怎麼找呢？

But we're working on it. How are we working on it?

嗯，我們送了幾艘太空船 到這三個行星上。

Well, we've sent spacecraft to all three planets.

有些已經在軌道上環繞了，

Some of them are orbiting now,

包括目前正在環繞火星的 MAVEN （火星大氣與揮發物演化任務）太空船，

including the MAVEN spacecraft which is currently orbiting Mars,

我有參與這項計畫，

which I'm involved with and which is led here,

而且由此地的科羅拉多大學領導。

out of the University of Colorado.

我們用它測量大氣逃逸。

It's designed to measure atmospheric escape.

我們對金星及地球也做了 類似的測量。

We have similar measurements from Venus and Earth.

一旦我們收及到所有的測量數據，

Once we have all our measurements,

我們就能把這些綜合起來， 我們就能了解

we can combine all these together, and we can understand

這三個行星如何與太空環境，

how all three planets interact with their space environment,

與它們周邊的環境交互作用，

with the surroundings.

我們就能決定磁場 對適居性的重要與否。