了解宇宙是如何运行的（2010）

How the Universe Works S06E09

Part 5 Dec. 22, 2021, 7:23 p.m.

to eventually form second-generation planets,
金星和地球
Venus and Earth.
然而有一个星球却站在一边旁观
Yet one planet stood back and watched from the sidelines.
那颗行星就是火星，这就是它与地球相比
That planet was Mars, and that is the secret to its old age,
年龄大的秘密
compared to Earth.
如果火星确实比地球更古老
If Mars is indeed older than the Earth,
那就意味着它是太阳系
that would imply that it's one of the original planetary
最原始的行星胚胎之一
embryos of the solar system.
火星基本上是早期完成的
Mars is essentially done early.
它在这整个过程的外♥围♥
It is on the outside of this whole process,
坐落在外面，不再吸积更多的质量
sitting out not accreting any more mass
看着地球和金星
and watching while the Earth and Venus
从其他已建成的大型星体中形成
form out of the other big bodies that have been built.
那么，是什么阻止了
So what prevented Mars from colliding
火星与其他行星胚胎相撞呢？
with the rest of the planetary embryos?
答案是……木星
The answer... Jupiter.
在大迁徙过程中，木星移♥动♥到了我们今天
During the grand tack, Jupiter moved to the same distance
发现火星距离太阳同样的位置
from the Sun that we find Mars today,
并在这个过程中吃掉了这颗红色行星的午餐
and in the process ate the red planet's lunch.
木星清除了
Jupiter removes all of the material
火星在接下来的数千万年里
that Mars otherwise would have been building on
赖以生存的所有物质
for the next tens of millions of years,
基本上清除了太阳系的一大块
essentially clears out a big chunk of the solar system
让火星挨饿
and starves Mars.
如果木星不在那里
If Jupiter were not there,
我们就会认为火星已经形成了
than we would have expected Mars
一个成熟的超级类地球行星
to have formed a fully-fledged super-Earth planet.
地球是由这一堆残骸形成的
Earth formed from the wreckage of this pileup,
但每次我们仰望夜空时
but a reminder of this population lives on,
都会看到一个天体……月亮
every time we look to the night sky... The Moon.
我们确信我们知道月球是如何形成的
We were convinced we knew how the Moon formed.
然而事实证明，我们完全错了
Turns out, we were completely wrong.
一个研究太阳系的远距离观测者
A distance observer studying our solar system
会立刻注意到一些奇怪的事情……
would notice something strange right away...
地球的月亮那么大
The size of Earth's moon.
相比之下，大多数行星的卫星都很小
Most planet's moons are tiny by comparison.
我们是怎么得到这么大的卫星的？
How did we get a moon so big?
有一段时间我们意识到
For a while we've realized it couldn't have formed
它不可能和地球同时形成
at the same time as the Earth.
这根本说不通
It just doesn't make sense.
关于月球形成的标准观点是
The standard idea of the Moon's formation
一个火星大小的天体
is that an object about the size of Mars
与早期的地球相撞
collided with the early Earth.
很多碎片被抛进绕地球的轨道
A lot of the debris was thrown into orbit around the Earth,
然后它们结合在一起形成了月球
and it coalesced to form the Moon.
我们称这个火星大小的天体为“泰雅(Thea)”
<译者注：第四季第七集为Theia，Thea音译为西娅>
<但为了与之前一至，所以还是译为泰雅>
We call this Mars-sized object Thea,
但月球的确切形成时间和方式
but exactly when and how the Moon formed
仍然是一个谜
remains a mystery.
自从阿波罗任务以来
Ever since the Apollo missions,
我们一直在寻找一块月球岩石
we've been searching for a piece of lunar rock
来解开这个秘密
that can unlock this secret.
加州大学洛杉矶分校Melanie Barboni的团队是少数几个
Melanie Barboni's team at UCLA is one of the few groups
被授权分♥析♥这些珍贵的月球样本的团队之一
authorized to analyze these precious lunar samples.
但有个问题……
But there's a problem...
大多数月球岩石
Most moon rock is contaminated
在较近的过去被暴♥力♥事件污染和破坏
and damaged by violent events in the more recent past.
小行星撞击月球，在这个过程中
Asteroids hit the Moon, and there are geological
会进行大量的混合
processes that do a lot of mixing,
很难从月球的构造中
and it's very difficult to find a pristine sample
找到一个原始的样本
from the Moon's very formation.
梅勒妮和她的团队想出了一个新奇的方法
Melanie and her team have come up with a novel
来解决这个问题
answer to that problem.
他们没有确定整个岩石的年代
Rather than date the entire rock,
而是在月球样本中分离出一颗微小的、原始的晶体
they isolate a tiny, pristine crystal within a lunar sample,
这种晶体被称为锆石
known as a zircon.
现在我们不想要整块岩石
Now we don't want the whole rock,
我们只想要那些岩石里面的锆石
we want only tiny zircon that are inside those rocks.
这些锆石
These zircons formed
是在泰雅与地球碰撞后形成的
just after Thea's collision with Earth.
月球的地壳曾经是熔融的
Once the molten crust of the Moon,
后来冷却并凝固下来
it cooled and solidified.
这比你在海滩上找到的沙粒
This is much smaller than the grain of sand
还要小得多
you find on the beach.
锆石是大自然赋予我们的
Zircon is the most perfect clock
用来确定月球年代的最完美的时钟
that nature gave us to date the Moon
因为它具有很强的稳定性
because it's very resistant.
这里你可以看到它的表面非常光滑
Here you can see its surface is very smooth.
上面没有裂痕
There is no fractures on it.
锆石像时钟一样滴答作响
Zircons tick off time like clocks.
它们含有大的放射性元素
They contain large radioactive elements
然后衰变成小的放射性元素
that decay into smaller ones.
科学家可以通过测量放射性衰变
Scientists can tell how old the crystal
来判断晶体的年龄
is by measuring the radioactive decay.
梅兰妮发现的锆石改写了月球的历史
The zircons Melanie found rewrite the history of the Moon.
月球比我们想象的
The Moon is around 140 million years older
要早1.4亿年
than what we thought.
这意味着月球的形成
This means the Moon formed
不晚于太阳诞生后
no later than 60 million years
6000万年
after the birth of the Sun.
这就把月球的形成放在了
This places the formation of the Moon right in the middle
30颗行星叠加破坏的中间位置
of the destruction of the 30-planet pileup.
泰雅完全有可能曾经是
It's entirely possible that Thea was once a member
这群火星大小物体中的一员
of this colony of Mars-sized objects.
它不仅仅是某个
It wasn't just some Mars object
在太阳系的外层火星大小的天体
that was out in the outer solar system
然后突然冲进并撞向我们
and came careening in and smashed into us.
正是这些不复存在的星子撞上了我们
It was one of these no-longer-existing planetesimals
形成了月球
that slammed into us and formed the Moon.
但科学家们在月球上寻找泰雅的踪迹却一无所获
But scientists looking for traces of Thea
却一无所获
on the Moon draw blanks.
月球岩石有趣的一点是
One of the intriguing things about moon rocks
它们在化学上与地球上的岩石非常相似
is how similar they are, chemically, to rocks on Earth.
它有相同的地球化学指纹
It has the same geochemical fingerprints,
地球的氧同位素
the oxygen isotopes, of the Earth,
以及地球上所有其他的化学同位素
and all the other chemical isotopes of the Earth.
它看起来就像地球上的石头
It looks just like Earth rock.
如果月球真的是一次巨大碰撞的产物
If the Moon really is the product of one giant collision,
那么，无论撞击地球的是什么
well, whatever hit the Earth,
地球和月球的撞击比例应该是不同的
there should be different proportions of that on the Earth
但我们没有发现
as opposed to the Moon, but we don't find that.
月球上的物质与地球上的物质是一样的
The Moon is identical to material from the Earth,
除了它缺少重元素，铁和镍
except it's missing heavy elements, iron and nickel,
这些元素在地核中发现
found in the Earth's core.
相反，它主要包含在地壳和地幔中发现的
Instead, it mainly contains lighter rocky elements
较轻的岩石元素
found in the Earth's crust and mantle.
为什么？
Why?
这不是正面碰撞
It wasn't a head-on collision.
这是一次擦草碰撞
It was a grazing collision.
现在，这很重要，因为较重的物质
Now, that's important because the heavy material
开始下沉到地球的中心
was starting to sink into the center of the Earth,
而较轻的物质漂浮到顶部
and the lighter stuff was floating to the top.
如果这是擦掠碰撞
And if this were a grazing collision,
那么较轻的物质就会溅落出来
then that lighter material would have been splashed out,
这就是月球的形成
and that's what would have formed the Moon.
事实上，月球的密度比地球小
And the Moon is, in fact, less dense than the Earth,
如果它是由靠近顶部的较轻的物质形成的
which makes sense if it formed from this lighter material
这是有道理的
that was near the top.
它看起来就像你从地球的地幔中取了一小块
It looks like you took a blob of the Earth's mantle
然后把它放到地球周围的太空中
and just put it into space around the Earth.
一次正面碰撞会在月球上