A process called subduction — when a tectonic plate slides underneath another and sinks deep into a planetary body’s interior — is physically possible in the ice shell of Jupiter’s moon Europa, according to new research from Brown University. This is important because material from the surface of Europa could act as food for life that may exist in the moon’s subsurface ocean.
The surface of Europa looms large in this newly-reprocessed color view; image scale is 1.6 km per pixel; north on Europa is at right. Image credit: NASA / JPL-Caltech / SETI Institute.
“We have this evidence of extension and spreading, so the question becomes where does that material go? On Earth, the answer is subduction zones. What we show is that under reasonable assumptions for conditions on Europa, subduction could be happening there as well, which is really exciting,” said Dr. Brandon Johnson, lead author of the study, published in the Journal of Geophysical Research: Planets.
“Part of the excitement is that surface crust is enriched with oxidants and other chemical food for life. Subduction provides a means for that food to come into contact with the subsurface ocean scientists think probably exists under Europa’s ice.”
“If indeed there’s life in that ocean, subduction offers a way to supply the nutrients it would need.”
On Earth, subduction is driven largely by differences in temperature between a descending slab and the surrounding mantle. Crustal material is much cooler than mantle material, and therefore denser. That increased density provides the negative buoyancy needed to sink a slab deep into the mantle.
Though previous geological studies had hinted that something like subduction could be happening on Europa, it wasn’t clear exactly how that process would work on an icy world.
“There’s evidence that Europa’s ice shell has a two layers: a thin outer lid of very cold ice that sits atop a layer of slightly warmer, convecting ice. If a plate from the outer ice lid was pushed down into the warmer ice below, its temperature would quickly warm to that of the surrounding ice. At the point, the slab would have the same density of the surrounding ice and would therefore stop descending,” Dr. Johnson said.
But the model developed by the team showed a way that subduction could happen on Europa, regardless of temperature differences.
The model showed that if there were varying amounts of salt in the surface ice shell, it could provide the necessary density differences for a slab to subduct.
“Adding salt to an ice slab would be like adding little weights to it because salt is denser than ice. So rather than temperature, we show that differences in the salt content of the ice could enable subduction to happen on Europa,” Dr. Johnson said.
“And there’s good reason to suspect that variations in salt content do exist on Europa. There’s geological evidence for occasional water upwelling from Europa’s subsurface ocean — a process similar to the upwelling of magma from Earth’s mantle. That upwelling would leave high salt content in the crust under which it rises. There’s also a possibility of cryovolcanism, where salty ocean contents actually spray out onto the surface.”
“In addition to bolstering the case for a habitable ocean on Europa, our study also suggests a new place in the Solar System to study a process that’s played a crucial role in the evolution of our own planet.”
“It’s fascinating to think that we might have plate tectonics somewhere other than Earth.
“Thinking from the standpoint of comparative planetology, if we can now study plate tectonics in this very different place, it might be able to help us understand how plate tectonics got started on the Earth.”
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Brandon C. Johnson et al. Porosity and salt content determine if subduction can occur in Europa’s ice shell. Journal of Geophysical Research: Planets, published online December 4, 2017; doi: 10.1002/2017JE005370
