The search for life on other planets is focused on planets beyond our solar system; however, life could exist much closer than we imagine.
Since the discovery of Europa, an icy moon of Jupiter, astrobiologists have speculated that under its thick layer of ice, with an orography reminiscent of the ice blocks of our own planet, there could be activity of tectonic plates that sustain alien life.
Under the assumption that Europa is a frozen body and that it is too far from the Sun for it to melt the ice, it is necessary that there is another force capable of causing internal heat in Europa.
But there is a viable option: that Europa has tectonic plates similar to Earth and that its internal heat, agitated by the great gravitational mass of Jupiter, causes these movements; which would have important implications for the possibility of life under the ice.
Now a study published in the Journal of Geophysical Research: Planets has shown that subduction , the process by which one tectonic plate slides under another and sinks deep inside a planet, is physically possible in Europa.
The findings reinforce previous studies of Europa’s surface geology that found regions where the moon’s ice sheet appears to expand in a manner similar to expansion ridges in the middle of the ocean on Earth.
What do subduction movements imply for life?
The surface crust is enriched with chemical elements. Subduction provides a means for this food to come into contact with the subsoil ocean layers that probably exist under the ice of Europa.
So if there is life in that ocean, subduction offers a way to supply the nutrients it would need.
According to Brandon Johnson, assistant professor in the Department of Earth, Environmental and Planetary Sciences at Brown University and lead author of the study: “What we show is that, under reasonable assumptions for conditions in Europe, subduction could be occurring there. also”.
On Earth, subduction is mainly due to temperature differences between a falling slab and the surrounding mantle. The crust material is much colder than the mantle material and therefore denser. That density provides the buoyancy necessary to sink a slab deep into the mantle.
Although previous geological studies had hinted that some subduction-like process might be going on in Europa, it was unclear exactly how that process would work in an icy world.
There is evidence, according to the researchers, that Europa’s ice has two layers : a thin outer layer of very cold ice that sits on top of a slightly warmer convective ice layer.
If an outer ice cap plate is pushed down into the warmer ice, its temperature rapidly warms up to that of the surrounding ice. At this time, the slab would have the same density as the surrounding ice and thus stop descending.
How could subduction happen in Europe?
But the model developed by Johnson and his colleagues showed one way that subduction could occur in Europa, regardless of temperature differences. The model showed that if there were varying amounts of salt in the surface ice sheet, it could provide the density differences necessary for a slab to subduct.
“Adding salt to a slab of ice would be like adding little weight to it because salt is denser than ice,” Johnson said. “So instead of temperature, we showed that differences in the salt content of the ice could allow subduction in Europa.”
References: ‘Porosity and salt content determine if subduction can occur in Europa’s ice shell’. Brandon C. Johnson1 *, Rachel Y. Sheppard1, Alyssa C. Pascuzzo1, Elizabeth A. Fisher1, and Sean E. Wiggins.
Image: NASA / JPL-Caltech / SETI Institute
