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  • This is an image of Jupiter’s moon Europa taken by the Galileo spacecraft in 1998.

  • Youve probably noticed that Europa’s surface, which is made of ice, has tons and

  • tons of cracks, but I want to direct your attention to this weird repeating arc pattern

  • each segment of arc is roughly 100km long!

  • And there are a lot of these arc patterns.

  • Most of them are ridges raised up above the surrounding surface, though a few are troughs.

  • After they were discovered, their shape reminded scientists of a mathematical curve called

  • a cycloid ; so the Europa curves are calledcycloid curves”.

  • These curves are weirdgeological and astrophysical processes are really good at

  • making round features, or straight features, or wavy featuresbut what causes repeated

  • arcing cycloids?

  • 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.

  • 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 onand here

  • on earth, plate tectonics has caused cycloid-esque curves all around the pacific ring of fire.

  • Our best guess for how the pacific arcs form is based on what happens when the ocean plates

  • get pushed under continental plates: because the earth’s surface is curved, you get a

  • similar effect to what happens when you dent a ping pong ballyou might get a circle,

  • or if you press harder, multiple circular arcs: cycloid curves!

  • 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.

  • The current best theory for the origin of the Europan cycloids has to do with its weird

  • tides.

  • Jupiter causes tides on Europa, but theyre not from rotation beneath a tidal bulge (the

  • way tides are here on earth); the same side of Europa always faces Jupiter.

  • No, Europa has tides because its orbit isn’t a perfect circleit’s ever so slightly

  • elliptical, so as Europa moves closer or farther from Jupiter, the nature of Jupiter’s gravitational

  • pull changes.

  • On the scale of the whole moon, these tides manifest as a kind of squeezing and stretching,

  • 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.

  • 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.

  • 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.

  • So, when there’s enough stretching tension to form a crack in the ice, the crack will

  • start propagating perpendicular to the tension.

  • But remember, the direction of the tension is changing.

  • 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 (and if it’s going west, itll curve down

  • towards the equator).

  • As Europa continues orbiting, the tension will eventually turn to compression, so the

  • crack will stop growing.

  • 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.

  • At which point it resumes its upwards-curving trajectory.

  • There’s probably a little more subtlety due to a stress-strain process calledtailcracking

  • that helps the sharp corners form for each new segment, 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.

  • Then the process repeats again, starting the crack off in the original direction, curving

  • again, and so on.

  • And that’s how waves form on a frozen world.

  • This video was supported by NASA’s James Webb Space Telescope Project at the Space

  • Telescope Science Institute.

  • 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 - perhaps water spouting up through

  • cracks in Europa’s icy surface, and if so, the plumes could give us insights into the

  • oceans beneath.

  • The James Webb telescope will use its powerful thermal imaging and spectroscopy to investigate

  • Europa’s plumes and to study the geologic activity, tides, and tectonics of Europa and

  • other outer solar system planets and moons, hopefully answering questions about how they

  • formed, how they continue to behave, and whether they have conditions amenable to life.

This is an image of Jupiter’s moon Europa taken by the Galileo spacecraft in 1998.

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地外圓環--為什麼它們在歐羅巴上? (Extraterrestrial Cycloids - Why Are They on Europa?)

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    林宜悉 發佈於 2021 年 01 月 14 日
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