Jupiter's moon Europa features a warm subterranean ocean covered in ice. For years, scientists have wondered if certain surface features are the result of plate tectonics, which, if true, would make Europa the only known place in the Solar System other than Earth to experience large, subduction-driven quakes. What's more, the presence of tectonic activity would bolster the moon's ability to harbour primitive life.
Europa. (Image: NASA)
Europa has what it takes to support plate tectonics, according to new research published today in Journal of Geophysical Research: Planets. Using computer models, a team lead by Brown University planetary scientist Brandon Johnson was able to demonstrate the physical feasibility of icy plates driving deep into the icy interior in a processes similar to what's seen on Earth. Excitingly, this same process could be delivering important minerals to the ocean below, heightening the moon's status a potentially habitable world.
Europa has surface features reminiscent of Earth's mid-ocean ridges. For astronomers, this hinted at geological processes akin to subduction zones, where, on Earth, tectonic plates slide underneath one another, sinking deep into the planet's interior. Several years ago, researchers Simon Kattenhorn and Louise Prockter posited this explanation when they noticed that a 20,000km2 chunk of ice had mysteriously disappeared from Europa's surface. Their explanation was that Europa's surface, like a gigantic jigsaw puzzle, is composed of tectonic plates, and that occasionally a plate of ice will sink beneath the other into warmer layers below.
But this observational evidence of extension and spreading needed to be supported by geophysical reality. To that end, Johnson's team ran a computer simulation to see if it was possible for ice to sink in this way.
On our planet, subduction is primarily driven by differences in temperature between a descending slab and the surrounding mantle. Dense crustal material features a negative buoyancy that drives it down into the mantle. The Brown University scientists figured a similar thing happens on Europa, but with ice. In the case of Europa, the researchers surmised that the moon has two frozen layers - an outer lid of very cold ice that sits above a layer of slightly warmer convecting ice. Their models showed that subduction is indeed possible in this alien environment, but only if the outer shell contains varying amounts of salt. This added ingredient provides the necessary density differences for a slab to conduct.
"Adding salt to an ice slab would be like adding little weights to it because salt is denser than ice," said Johnson in a statement. "So rather than temperature, we show that differences in the salt content of the ice could enable subduction to happen on Europa."
Encouragingly, we have reason to believe that Europa features these variabilities in salt content. Every once in a while some water upwells from the interior in a process similar to magma pouring out of the Earth's mantle. This phenomenon should leave high salt content in the crust under which it rises.
"This supports the idea that something like plate tectonics is occurring on Europa and may tell us about the composition of Europa's ice shell," write the authors in the new study. "Our work also implies that the plates will sink all the way to Europa's subsurface ocean. This is important because material from the surface of Europa could act as food for life that may exist in Europa's ocean."
Europa's surface crust is likely packed with oxidants and other chemicals friendly to life. The subduction process could be acting as a conveyor built, delivering these nutrient-laden minerals to the subsurface ocean beneath the ice. Relatedly, recent research into Saturn's moon Enceladus, another snowball moon with a subsurface ocean, shows that it has the chemical ingredients for life owing to hydrothermal processes near volcanic vents. So when it comes to searching for signs of life in the Solar System, these distant moons are clearly where our attention should be directed.