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Extraterrestrial Cycloids – Why Are They On Europa?

This is an image of Jupiter’s moon Europa
taken by the Galileo spacecraft in 1998. You’ve 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 called
“cycloid curves”. 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? 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 on – and 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 ball – you 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 they’re
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 circle – it’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, it’ll 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 called “tailcracking” 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.
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