Cryotectonics

Previous posts have looked at the nature of tectonics on the terrestrial planets and how they are very different to Earth. Further out in the solar system, it is not the planets which are of geological interest but the moons, many of these are ice worlds. Cores of rock covered in layers of ice. Some of these have pockets or layers of water in them generated by tidal friction generating heat.

Evidence for this layer of water or at least an icy slush is shown by fountains of ice sprayed into space on both Enceladus and Europa showing that these moons are active places however, there are other telltale signs of a system similar to plate tectonics.

 

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Ice erupting from volcanos on Enceladus (NASA/JPL/Space Science Insitute)

Photos of Europa, an Ice moon of Jupiter, revealed a cracked surface covered in bands. Limited cratering suggests that surface is relatively young (around 40-90 million years). Some of the surface shows repeating bands similar to spreading ridges seen on Earth. Where the surface ice cracks and moves apart and new water rises up and fills the gap with ice.

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Europa as seen by Galileo (Nasa/JPL-Caltech/SETI institute)

These same spreading ridges and associated bands are also clearly seen on Miranda, a moon of Uranus, which has alternating patterns around a central rift, just like is seen on the ocean plates of Earth.

Where tectonics must differ is what happens at the other end; ice is very buoyant making it difficult to find a form of subduction which would allow pulling of the crust down into the water/ slush layers like the plates on Earth. As the surfaces of these moons cannot be ever increasing in diameter, something has to be happening to the ice.

On Europa there no regional scale mountain belts where the crust thickens to accumulate this extra crust. Instead, there is some evidence of complex fault systems. Similar to subduction zones, but which behave differently and have been termed subsumption zones – one section of the ice crust is forced under another, where it rapidly becomes incorporated into the underlying water ice… so rather than continuing to sink like the plates on Earth it rapidly becomes part of the underlying layers, enabling tectonics to continue.

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Miranda as seen my Voyager 2 (Nasa/JPL)

The ice moons of the outer solar system are harsh alien places, but tectonically they may be the most Earth-like of all the bodies in the solar system.