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This is an image of Jupiter’s moon Europa taken by the Galileo spacecraft in 1998.
这是伽利略号航天器在1998年拍摄的木星的卫星木卫二的图像。
You’ve probably noticed that Europa’s surface, which is made of ice, has tons and tons of cracks,
你可能已经注意到木卫二的表面是由冰组成的，有成堆的裂缝，
but I want to direct your attention to this weird repeating arc pattern – each segment of arc is roughly 100km long!
但我想让你注意这个奇怪的重复弧形图案-每段弧形大约有100公里长！
And there are a lot of these arc patterns.
有很多这样的弧形图案。
Most of them are ridges raised up above the surrounding surface, though a few are troughs.
它们中的大多数是隆起于周围表面之上的山脊，但也有少数是凹槽。
After they were discovered, their shape reminded scientists of a mathematical curve called a cycloid ; so the Europa curves are called “cycloid curves”.
在它们被发现后，它们的形状让科学家们想起了一条被称为摆线的数学曲线；因此，木卫二曲线被称为“摆线曲线”。
These curves are weird – geological and astrophysical processes are really good at making round features, or straight features, or wavy features – but what causes repeated arcing cycloids?
这些曲线很奇怪--地质和天体物理过程真的很擅长制造圆形特征，或者是直线特征，或者波浪状特征--但是是什么导致了一圈圈的弧形摆线呢？
Well, we think that the Europan surface is made of frozen water at least several miles thick, which we believe is floating on top of an ocean of liquid water.
嗯，我们认为欧洲大陆的表面是由至少几英里厚的冰冻水构成的，我们相信这些冰冻水漂浮在液态水的海洋之上。
This means its surface kind of works the way tectonic plates do here on earth, spreading apart and generating new ice, crashing together and being subducted, and so on
这意味着它的表面工作方式就像地球上的构造板块一样，散开并产生新的冰，碰撞在一起并被俯冲，等等
– and here on earth, plate tectonics has caused cycloid-esque curves all around the pacific ring of fire.
-在地球上，板块构造在太平洋火环周围形成了摆线状的曲线。
Our best guess for how the pacific arcs form is based on what happens when the ocean plates get pushed under continental plates:
我们对太平洋弧如何形成的最好猜测是基于当海洋板块被推到大陆板块之下时会发生什么：
because the earth’s surface is curved, you get a similar effect to what happens when you dent a ping pong ball – you might get a circle, or if you press harder, multiple circular arcs: cycloid curves!
因为地球表面是弯曲的，你会得到类似于你在乒乓球上凹陷时的效果-你可能会得到一个圆，或者如果你再用力压下去，你可能会得到多个圆弧：摆线曲线！
However, this doesn’t appear to be the answer on Europa, because there are so many cycloid curves and they overlap in tons of places and none of them really show signs of one piece of the surface being pushed under another.
然而，这似乎不是木卫二上的答案，因为有如此多的摆线曲线，它们重叠在很多地方，而且没有一个真正显示出表面的一部分被推到另一部分之下的迹象。
The current best theory for the origin of the Europan cycloids has to do with its weird tides.
目前关于欧罗盘摆线起源的最佳理论与其奇怪的潮汐有关。
Jupiter causes tides on Europa, but they’re not from rotation beneath a tidal bulge (the way tides are here on earth); the same side of Europa always faces Jupiter.
木星引起了木卫二上的潮汐，但它们不是来自潮汐隆起之下的旋转(就像地球上的潮汐一样)；木卫二的同一侧总是面对木星。
No, Europa has tides because its orbit isn’t a perfect circle
不，木卫二之所以有潮汐，是因为它的轨道不是一个完美的圆形
– it’s ever so slightly elliptical, so as Europa moves closer or farther from Jupiter, the nature of Jupiter’s gravitational pull changes.
--它是如此微微的椭圆形，所以当木卫二离木星越来越近或更远时，木星的引力性质就会改变。
On the scale of the whole moon, these tides manifest as a kind of squeezing and stretching,
在整个月球的尺度上，这些潮汐表现为一种挤压和拉伸，
which due to the interaction of geometry and physics results in any given point on the icy surface being pressed together at one time in the orbit, and then stretched at a later point.
由于几何和物理的相互作用，导致冰面上的任何给定点在轨道上一次性被挤压在一起，然后在稍后的一点被拉伸。
And the aspect of tides key to understanding the cycloid curves is that the direction of the compression and stretching changes over the course of each orbit, rotating around and around like a hand on a clock.
而潮汐对理解摆线曲线的关键是，压缩和拉伸的方向在每个轨道的过程中都会发生变化，就像时钟上的指针一样旋转。
Specifically, the compression/tension direction rotates clockwise in the southern hemisphere and counterclockwise in the northern hemisphere, and it takes one orbit to complete a full rotation.
具体地说，压缩/拉伸方向在南半球是顺时针旋转，在北半球是逆时针旋转，需要一个轨道才能完成完整的旋转。
So, when there’s enough stretching tension to form a crack in the ice, the crack will start propagating perpendicular to the tension.
因此，当有足够的拉伸张力在冰中形成裂缝时，裂缝将开始垂直于张力扩展。
But remember, the direction of the tension is changing.
但请记住，张力的方向正在改变。
If, say the crack is growing to the east in the northern hemisphere, the counterclockwise-changing tension will curve it up away from the equator
如果裂缝在北半球向东扩展，逆时针变化的张力将使其从赤道向上弯曲
(and if it’s going west, it’ll curve down towards the equator).
(如果向西，则向赤道向下弯曲)。
As Europa continues orbiting, the tension will eventually turn to compression, so the crack will stop growing.
随着木卫二继续运行，张力最终会转变为压缩，因此裂缝将停止增长。
The compression angle will continue turning, though, and by the time the compression turns back to tension, the direction will have rotated back around enough that the crack will make a sharp turn when it starts cracking again.
不过，压缩角度将继续旋转，当压缩恢复为拉伸时，方向将旋转回足够大的位置，当裂缝再次开始开裂时，它将急转弯。
At which point it resumes its upwards-curving trajectory.
在这一点上，它恢复了向上弯曲的轨迹。
There’s probably a little more subtlety due to a stress-strain process called “tailcracking” that helps the sharp corners form for each new segment,
可能会有一些微妙的原因，这是由于一种被称为“尾裂”的应力应变过程，它帮助每一个新的部分形成了尖锐的角落，
but this is basically the best current theory explaining the Europan cycloids: cracks grow because the tides from Jupiter create tension in the ice, and that tension direction changes over time, curving the crack.
但这基本上是解释欧洲摆线的最好的当前理论：裂缝的形成是因为来自木星的潮汐在冰中产生了张力，而张力的方向随着时间的推移而改变，弯曲了裂缝。
Then the process repeats again, starting the crack off in the original direction, curving again, and so on.
然后，该过程再次重复，沿着原来的方向开始裂缝，再次弯曲，依此类推。
And that’s how waves form on a frozen world.
这就是波浪在冰冻世界中形成的方式。
This video was supported by NASA’s James Webb Space Telescope Project at the Space Telescope Science Institute.
这段视频得到了美国宇航局在太空望远镜科学研究所的詹姆斯·韦伯太空望远镜项目的支持。
There’s still a ton we don’t know about Europa - the Hubble telescope has detected what we think are plumes jetting from Europa’s surface
关于木卫二还有很多我们不知道的-哈勃望远镜探测到了我们认为是从木卫二表面喷出的羽流
- perhaps water spouting up through cracks in Europa’s icy surface, and if so, the plumes could give us insights into the oceans beneath.
-可能是水从木卫二冰层表面的裂缝中喷出，如果是这样的话，这些羽流可以让我们深入了解木卫二下面的海洋。
The James Webb telescope will use its powerful thermal imaging and spectroscopy to investigate Europa’s plumes
詹姆斯·韦伯望远镜将利用其强大的热成像和光谱技术来调查木卫二的羽流，
and to study the geologic activity, tides, and tectonics of Europa and other outer solar system planets and moons,
并研究木卫二和其他太阳系外行星和卫星的地质活动、潮汐和构造，
hopefully answering questions about how they formed, how they continue to behave, and whether they have conditions amenable to life.
希望能回答有关它们是如何形成的、它们如何继续活动以及它们是否具有适合生命生存的条件的问题。
