行星（2019）

And the reason is because they are young.
比土星本身年轻了近45亿年
Nearly 4.5 billion years younger than Saturn itself.
土星环起源之迷是一个活跃的研究领域
The mystery of the origin of the rings is an active area of research.
“卡西尼”号♥所提供的证据表明
And the evidence that Cassini delivered hints that the solution
迷题的答案不在于土星本身
lies not with the planet itself,
而在于土星轨道之上的
but with the world's tracked in orbit around it -
土星的卫星
Saturn's moons.
土星有62颗大卫星和无数的小卫星
Saturn has 62 large moons and countless smaller ones -
至今仍在不断发现
we're still discovering them.
所以土星系统本身上就像是一个迷你的太阳系
So it's like a mini solar system in itself.
这些卫星大小各异 有行星大小的土卫六
And those moons range in size from planetary-size objects,
也有形状不规则的小石块
like Titan, to small irregular-shaped lumps of rock.
而且它们在大小 形状和表面特征上
And there's so much diversity in size and form
各有不同 这使整个系统就像一本历史书
and surface features, that the whole system is like a history book.
如果我们能读懂它 我们就会发现
And if we learn to read it then we see that Saturn
土星极其动荡而多变的过去
has had a very violent and changeable past.
看这个 这就是土卫一 它被称为死星的原因
Look at this, this is Mimas, which is often called the Death Star moon
显而易见
for obvious reasons.
它有一个巨大的撞击坑 这说明很久以前
It has this huge impact crater, which tells you that once, long ago,
土卫一
曾经有过一次剧烈的撞击
there was a very violent impact
险些就彻底摧毁了这颗卫星
that almost - but not quite - destroyed the moon.
这是土卫八
And this is Iapetus.
这是一个非常大的卫星 几乎有行星大小
It's a very large moon, almost planetary-size.
直径约1500公里
It's about 1,500 kilometres across.
它有一半是白色 一半是黑色
And half of it is white, and half of it is dark.
土卫八还有另一个独特的特征
Iapetus also has another distinctive feature
那就是赤道附近的山脊
which is this ridge all the way around the equator, and you can see
它在这张照片上也很清晰
the scale of it from this photograph.
可见它的规模有多大
And this is clearly visible.
它是太阳系中最高的山脉之一
It's one of the highest mountain ranges in the Solar System,
高度落差达到了20公里
20km from peak to the ground below.
人们认为它的形成 是因为这颗卫星曾经有一道星环
And it's thought that this formed because a ring that was once
就像土星环一样 后来它掉落到了土卫八的表面
around the moon, just like Saturn's rings, collapsed onto its surface.
“卡西尼”号♥继续飞行 它让我们清晰地看到了
As Cassini continued its journey, it revealed in detail multiple moons
几颗几乎完全由冰构成的卫星
made almost entirely of ice.
许多位于土星环内或是周围的卫星
And many of them had taken on extraordinary forms
造型都很奇特
in and around the rings.
土卫十八 直径28公里
土卫七 直径266公里
土卫五 直径1528公里
“卡西尼”号♥探测器对冰质卫星有了更为细微的分♥析♥
As Cassini analysed the ice moons in ever greater detail,
人们发现它们中的许多都是由与土星环
it became apparent that many of them were made of exactly
完全相同的物质构成的
the same material as the rings themselves.
土星环和土星的卫星有着千丝万缕的联♥系♥
The rings and moons were profoundly linked.
“卡西尼”号♥看到了这么多的土星卫星
But for all the moons that Cassini saw,
可是似乎却少了一颗
it now seemed likely that one was missing.
1亿年前
在恐龙统治地球的时代
When dinosaurs roamed the Earth,
我们怀疑土星的轨道上有一颗卫星不见了
we now suspect Saturn had a moon in orbit that no longer exists.
一颗直径或许约有400公里的卫星
A moon perhaps 400km across
几乎完全由冰构成
and formed almost entirely of ice.
可是它的命运已经注定了
But this world was doomed.
它的轨道距离土星太近了 无法抵抗土星
It found itself orbiting too close to resist the immense forces
巨大的引力
of Saturn's gravity.
引力塑造了土星系统
Gravity is the sculptor of the Saturnian system
也是其内在变化的动因
but also the instigator of change within it.
我们住往认为引力是将物质聚集在一起的力量
We tend to think of gravity as a force that pulls things together,
但它也能造成事物的毁灭
but it can also act to rip things apart.
想想地球上的潮汐 它们是由月球
Think of the tides here on Earth, they're caused by the difference
对于地球两侧的引力差
in gravitational pull of the moon from one side of the Earth
引起的
to the other.
这种引力差可以很小 但也很强大
And that difference which can be quite subtle is also powerful,
它能移♥动♥整个海洋
it can move entire oceans.
这叫作潮汐引力 但地球对月球也有潮汐效应
It's called tidal gravity, but the Earth also has a tidal effect
因为地球是个完整的行星
on the moon, and because the Earth is an entire planet
所以这种效应会更强
that effect is much more powerful.
地球的引力足以导致月球表面变形
The pull of the Earth is enough to deform the moon's surface.
在45亿年前 月球到地球的距离比现在近了17倍
The effect was particularly strong 4.5 billion years ago
这种效应尤其强烈
when the moon was nearly 17 times closer.
当时地球的拉动力导致了月球表面坚固岩石的
Back then, the pull from the Earth caused a tide of solid rock
升降起伏
to rise and fall.
如果月亮再近一点 就会超过所谓的
If the moon had been any nearer it would have crossed what we call
洛希极限 潮汐引力就会增强
the Roche limit, a place where tidal gravitational forces are so strong,
导致卫星四分五裂
moons can get ripped apart.
古罗马人以他们的农神萨图努斯来命名土星
The Romans named the planet Saturn after their god of time and harvest.
在古代神话中还有一个更可怕的传说
And in one of the more gruesome tales from classical mythology,
萨图努斯吃掉了自己刚出生的孩子 为了不让他们
Saturn actually ate his newborn babies in order to prevent them
夺走他的权力
from taking his power.
有意思的是 “卡西尼”号♥探测器让我们了解到
What's sort of interesting is, that what we've learned from Cassini
至少从比喻的角度来说 这名字挺合适
is that, at least metaphorically speaking, they weren't far wrong.
土星 1000万-1亿年前
如今的主流理论认为 当时在土星大气层之外
Just beyond Saturn's atmosphere, our leading theory suggests
一定有一颗冰质卫星靠近过
an icy moon must have approached close to
甚至是进入了土星的洛希极限之内
or even just inside the planet's Roche limit.
当土星巨大的潮汐引力
As Saturn's immense tidal gravitational forces
作用于这颗卫星时 它开始分♥裂♥
acted across the moon, it began to rupture.
土星开始吞噬它的“孩子”
Saturn began to devour its child.
在土星周围的轨道上
Up to 15,000 trillion tonnes of ice
多达1.5亿亿吨♥的冰体支离破碎
broke apart in orbit around Saturn.
由于移♥动♥速度很快
Because of the speeds the ice fragments were travelling,
碎冰很可能在几天之内就扩散开来
it's likely that in just a few days they spread out
形成了这颗巨大行星之外的环
to encircle the great giant.
标志性的土星环就此诞生了
Saturn's iconic ring had been born.
如今巨大的土星环也发生了一些变化
Today, the giant ring has evolved.
土星强大的引力场使得它保持了
Saturn's powerful gravitational field has helped keep
近乎完美的圆形
its near perfect circular shape.
不过内部的碰撞导致它扁平扩散开来
But collisions within have caused it to flatten out.
碎片形成了一个比木星还要宽的圆环
Now this debris forms a disc wider than Jupiter,
而平均厚度却只有10米
yet, on average, just ten meters thick.
在土星环内部 一些月球大小的冰块环绕着土星运动
Within, moon-sized chunks of ice orbit the structure
清除出开阔的空隙带
clearing great voids.
这就把原来的一道环
Turning one ring...
变成了许多道环
into many.
在一些地方 卫星把冰颗粒向上拉起
In places, moons have pulled particles of ice upwards
形成超过两公里高的尖峰
to create strange peaks over 2km high.
在合适的条件下
And in just the right conditions,
这些冰峰就会在土星环上投下壮观的阴影
they cast spectacular shadows across the rings.
这个曾经不起眼的由岩石和冰构成的世界
This once tiny world of rock and ice...
历经了众多天翻地覆的剧变
that has undergone so many great transformations...
如今的它成为了太阳系中的一颗瑰宝
has today become the Solar System's jewel.
“卡西尼”号♥探测器加深了我们对土星
Cassini had deepened our understanding of the origin
起源和演化的认识
and evolution of Saturn.
但它的使命还远没有结束
But its mission was far from over.
因为就在土星环之外 它又发现了另一个宝藏
Because just beyond the rings it discovered another treasure.
有可能解答我们对于太阳系及其以外
A place that may hold answers to some of our deepest questions
生命存在可能性的一些最深层问题
about the possibility of life in the Solar System and beyond.
土卫二位于土星环的外缘
Over a billion kilometres from the warmth of the sun,
距离温暖的太阳
just on the outer edge of Saturn's rings,
有十亿公里
lies the icy moon Enceladus.
土卫二是一颗很小的卫星
Enceladus is quite a small moon,
只相当于冰岛那么大
it's only about the size of Iceland actually.
但这并不意味着它乏善可陈
But that does not make it dull.
它是太阳系中反光率最高的天体
It is the most reflective object in the solar system -
超过90%的光线都被反射了回去
over 90% of the light that hits it bounces back,
这是因为它的表面覆盖着纯净的冰
and that's because its surface is covered in pure ice.
同时也遍布着裂缝
It's also crisscrossed by crevasses
就像我们在冰川上看到的那样
like this one you see in this glacier,
只不过规模更为庞大
but on a much grander scale.
最大的一条被称为虎纹
The largest, known as the tiger stripes,
有100多公里长
are over 100km long.
尽管土卫二的地表很美
But for all its surface beauty,
但它真正与众不同之处
it's what's going on below the ice
还在于地表之下