A study by Brown University provides new evidence that the ice sheet of Jupiter's moon, Europa, may have tectonic plates similar to Earth's. The presence of plate tectonic activity could have important implications for the possibility of life in the ocean under the surface of the moon.
The study, published in Journal of Geophysical Research: Planets uses computer models to show that subduction, when one tectonic plate slides under another and sinks deep into a planet, is Physically possible in Europe's ice shell. The findings reinforce previous studies of the surface geology of Europe that found regions where the moon's ice sheet seems to expand in a similar way to the crests of expansion in the middle of the ocean on Earth. The possibility of subduction adds another piece to the tectonic puzzle.
"We have this evidence of extension and spread, so the question is where does that material go?" said Brandon Johnson, an badistant professor in Brown's Terrestrial, Environmental and Planetary Sciences Department and lead author of the study. "On Earth, the answer is subduction zones, what we show is that under reasonable badumptions for conditions in Europe, subduction could be happening there as well, which is really exciting."
Part of the emotion, says Johnson, is that the surface crust is enriched with oxidants and other chemical foods for life. Subduction provides a means for food to come into contact with the subsoil that scientists believe exists under the ice of Europe.
"If there is indeed life in that ocean, subduction offers a way to supply the nutrients it would need," Johnson said. .
Subduction on ice
On Earth, subduction is mainly due to temperature differences between a falling slab and the surrounding mantle. The material of the bark is much colder than the material of the mantle, and therefore more dense. That increased density provides the negative buoyancy necessary to sink a deep slab into the mantle.
Although previous geological studies had hinted that something like subduction could be happening in Europe, it was not clear exactly how that process would work on a world ice. There is evidence, says Johnson, that Europa's ice sheet has two layers: a thin outer shell of very cold ice that sits on a slightly warmer convective ice sheet. If a plate of the outer ice cap is pushed down into the hotter ice, its temperature quickly heats up to that of the surrounding ice. At that time, the slab would have the same density of the surrounding ice and, therefore, would stop descending.
But the model developed by Johnson and his colleagues showed a way in which subduction could occur in Europe, regardless of temperature differences. The model showed that if there were variable amounts of salt in the surface ice layer, it could provide the density differences necessary for a slab to subduct.
"Adding salt to an ice slab would be like adding little weight because salt is denser than ice," Johnson said. "Then, instead of the temperature, we showed that differences in the salt content of the ice could allow subduction in Europe."
And there are good reasons to suspect that there are variations in the salt content in Europe. There is geological evidence of occasional upwelling of water from the subsoil of Europe's ocean, a process similar to the magma outcrop from the Earth's mantle. That upwelling would leave a high salt content in the crust below which it rises. There is also a possibility of cryovolcanism, where the salty contents of the ocean really spray on the surface.
In addition to reinforcing the case for a habitable ocean in Europe, Johnson says, the research also suggests a new place in the solar system to study a process that played a crucial role in the evolution of our own planet.
"It's fascinating to think that we could have tectonic plates somewhere other than Earth," he said. "Thinking from the point of view of comparative planetology, if we can now study plate tectonics in this very different place, it could help us understand how plate tectonics began on Earth."
Johnson's co-authors in the paper Rachel Sheppard, Alyssa Pascuzzo, Elizabeth Fisher and Sean Wiggins are all graduate students at Brown. They took a clbad that Johnson offered called Ocean Worlds, which focused on bodies like Europe that are believed to have oceans beneath icy carapaces.
"This document came up as a clbad project we did together," Johnson said, "and it's exciting that we came up with some interesting results."
The distant moon imitates the tectonics of the Earth
Porosity and salt content determine whether subduction can occur in Europe's ice sheet, Journal of Geophysical Research (2017). DOI: 10.1002 / 2017JE005370, https://onlinelibrary.wiley.com/doi/10.1002/2017JE005370/full