了解宇宙是如何运行的（2010）

How the Universe Works S01E06

Part 2 April 18, 2021, 7:57 p.m.

This process is called accretion.
这些东西陆续增大, 变成岩石
As these things got bigger,
they became basically rocks.
之后岩石相撞, 形成巨石
Then rocks slam into other
rocks and form boulders-
巨石相撞, 形成更大的巨石
Boulders smash together
to form bigger boulders.
最后, 结聚至相当体积
Eventually, you've got
something big enough
本身已有足够引力把物质牵引过来
that its gravity was strong enough,
that it could start drawing material in.
于是再不用以相撞来增加质量
So instead of just slamming into
material and gaining mass that way,
而是主动把物质牵引过来
it was actually actively
pulling material in.
在我们的太阳系里
In our own solar system,
初时有许多初生行星, 也许上百颗
there were many growing infant
planets at first, maybe a hundred.
但大部分长不大
Most of them didn't make it.
要是看看小行星带里的灶神星
If you go to the asteroid belt
and look at the asteroid 4 Vesta,
就可知道岩质行星要有多大
that is a good indicator of
how big a rocky planet has to be
才能把自己塑成球体
before it can pull itself
into a spherical shape.
灶神星宽才329哩
Vesta is only 329 miles across
不足以形成球体
not quite big enough to become a sphere.
行星要增长至500哩宽才得成球体
For a growing planet to become round,
it has to reach 500 miles across.
如此才有足够引力把自己压成球状
Then it has enough gravity
to crush it into a sphere.
500哩以下只能维持不规则形状
Any smaller, and it
stays an irregular shape.
圆形的初生行星不断吞并物质
As round infant planets
keep eating up stuff,
每次撞击都进一步加热
each collision makes
them hotter and hotter
直至开始融化
until they start to melt.
现在引力开始分隔轻、重物质
Now gravity begins to separate
the heavy stuff from the light
较轻的物质浮起, 形成一层硬壳
Lighter materials tend to
float up into crusty film,
较重的物质, 许多是金属
and the heavier materials,
many of them metals,
下沉形成高密度的行星核心
falling down and forming a much
denser core at the centre of the planet
年轻行星终于现出行星雏型
The young planets are finally
beginning to look like planets.
但接着又要经历连番的猛烈冲击
But now they have to survive a
period of violence and destruction,
这个残酷阶段将决定哪个行星生存
a brutal phase that determines
which planets will live
哪个行星灭亡
and which planets will die.
太阳诞生之后
After the birth of the sun,
同一团气尘云衍生我们的八颗行星
our eight planets all evolved from
the same cloud of dust and gas,
但最后结果又截然不同
and yet they ended up
completely different
新生行星并没有甚么常规可依
There was no real blueprint
for each of the newborn planets.
诚然要受物理定律和化学定律规范
They did obey the laws
of physics and chemistry,
但要紧关头又纯粹是机缘巧合
but the most important things
happened by pure chance.
45亿年前
4.5 billion years ago,
大约一百颗初生行星围绕太阳
around 100 baby planets circled our sun.
期后上演一场碰碰车大赛
It turned into a demolition derby.
行星撞行星, 大部分被毁
Planet hit planet. Most were destroyed.
这些行星在新生期要承受千锤百炼
The early history of these planets
would have been very, very violent,
每颗行星来到最后成长阶段
with lots of these impacts taking place
都要承受许多撞击
in the final stages of
the growth of each planet.
撞击起来, 物体互相碰撞
As these impacts took place,
as objects ran into each other,
芸芸星子之中
certain objects began to grow
at the expense of all the others
部分会不断并吞而壮大
in this swarm of planetesimals.
这些行星, 这些准行星
And these planets, these things
that would become planets,
不断增长, 越来越大
grew and grew, and as they got bigger,
它们横扫周围比自己小的星子
they swept up all the smaller
planetesimals around them,
于是那颗原行星的表面
the consequence on the
surface of that protoplanet
受大量的太空瓦砾矗炸
being an enormous amount of
bombardment by debris from space.
轰炸过后
When it was over,
只剩下四颗各不相似的岩质行星
all that was left were four
very different rocky planets.
每颗行星都留下饱受轰炸的印记
Each planet's impact
history left its stamp,
所以它们各不相同
and that's why they're all
so different from each other.
火星是一片冰雪荒原
Mars is a frozen wasteland.
地球有液态水流动
Earth flows with liquid water.
金星是一片火山地狱
Venus is a volcanic hellhole.
水星细小、苍凉、热不可耐
And Mercury is tiny,
bleak and super hot,
那是经受一次惊天动地撞击的结果
the result of a monster collision.
以水星为例, 水星密度高而地壳薄
Mercury, for example, is extremely
dense and has a very thin crust.
原本可能是一个较大的行星
So it's possible it started
off as a bigger planet.
后来受外物侧面撞击
And then something hit it at an angle,
撞去轻身外壳, 余下高密度的核心
and it sheared off the lighter-weight
crust, leaving only the dense core.
成长中的地球也受过猛烈冲击
The young Earth also took a big hit.
发育阶段晚期, 地球与外物碰撞
Sometime late in its development, the
Earth was impacted by another object
地球地幔里的瓦砾也飞了出来
that ripped debris out
of the Earth's mantle
落入围绕地球的轨道
which then went into
orbit around the Earth
再逐步积聚而成今天的月球
and re-accumulated to
form what is now the moon.
也有证据显示火星受过撞击
There's also evidence that
something crashed into Mars.
火星北半球的地壳比南半球薄
The Northern Hemisphere has a
thinner crust than the Southern.
有一派理论解释说
A theory that has emerged
for how this happened
火星形成之初
is that early in the planet's history,
有物件撞击火星的北半球
the Northern Hemisphere of
Mars was whacked by some object
大量地壳碎片弹出
that blasted a lot of
the crust off of it.
碎片在火星的南半球重新积聚
And that crust re-accumulated
on the southern half of Mars.
这类撞击带来两个后果:
All these collisions did two things.
生存下来的初生行星数目减少
They cut down the number
of surviving infant planets.
同时为生还行星带来更多材料
And they brought more
ingredients to the survivors.
要是与富含金属的物件相撞
If you had a collision with
something that was metal-rich,
金属碎片会往下沉, 形成核心
those chunks would tend to descend
down into what was becoming the core
要是与轻身物质或冰块撞碰
where if you collided with
something light or icy,
它们会向上浮, 形成外壳
they would tend to just float about
and form part of the crust instead.
接近太阳的四颗岩质行星将近完成
The four rocky planets close
to the sun were almost complete.
核心是高热的固态铁
They had a solid, hot-iron core
外罩一层液态铁
surrounded by a layer of liquid iron,
上面再包一层熔岩
all wrapped in a jacket of molten rock.
熔岩之上就是地壳
Above that, an outer surface crust.
岩质行星的基本结构相同
These rocky planets all
formed in the same basic way,
基本素材也相同
from the same basic stuff.
但它们又各有差异
But each of them was very different.
大小不一, 命运更有天壤之别
Different sizes and
very different destinies.
太空看似空虚, 其实不然
Space may look empty, but it's not
内里充满太阳喷出来的物质
It's full of stuff blown out of the sun.
太阳产生强盛磁场
The sun generates
powerful magnetic fields
磁场如一个一个巨环由表面喷出
that rise above the
surface in giant loops.
碰撞时
When they clash,
触发一道超热的带电粒子流
it triggers a storm of super
hot, highly-charged particles