Europa, long one of the Solar System’s most fascinating worlds, might be subject to forces very similar to those that form the surface of Earth: plate tectonics.
One of the four largest moons of Jupiter, Europa is covered by water ice, and likely has a free-flowing water ocean below. The water is kept liquid by Jupiter’s immense gravitational force, which creates tides in Europa’s core — keeping the rock hot enough. Additionally, Europa’s surface, one of the smoothest in the solar system, shows few signs of impact craters. This has long suggested to experts that the moon’s ice crust is very young on a planetary age scale and that something has been causing the ice to disappear and re-form over time.
Now two scientists have made a convincing argument that Europa’s surface has been formed by icy plate tectonics. Studying high-resolution images collected over a decade ago by the Galileo spacecraft, geologist Simon Kattenhorn and planetary scientist Louise Prockter have found evidence that Europa’s surface ice mantle is composed of plate-like sections moving over a layer of somewhat warmer ice, similarly to how the giant rocky plates on Earth’s surface move about on a layer of weaker mineral formations. Their findings have just been published in the journal Nature Geoscience.
Europa’s ice surface is “crisscrossed by this network of features, fractures and ridges and features we call bands, in all different orientations and in a pattern that’s very distinctive,” says Kattenhorn. “If you took a photo and printed it out, and cut it into chunks and threw it in a pile like a jigsaw puzzle, you’d be able to put them back together. We took the pieces and moved them backwards in time, to create the original picture” of Europa’s surface.
“Like a jigsaw puzzle, you’d be able to put them back together.”
On Earth, spreading seafloors have created gaps between once-unified land masses; on Europa there are similar features called dilational bands. “You can close the bands, just like you’d take away the ocean basin on Earth to move the continents back together,” says Kattenhorn, and in some places piece together matching edges. Some of the Europa ice plates also show evidence of moving laterally, creating transform faults like California’s San Andreas Fault.
But in some locations, “when we [virtually] moved some of those plates back to create the original pattern, there was a big gap in the picture,” says Kattenhorn, “as if two of the pieces fell on the floor and you couldn’t find them.” This, he says, indicated that some of what was once on Europa’s surface had disappeared.
On Earth this would be evidence of a subduction zone: an area where one continental plate is sliding downward beneath another, to be reincorporated back into the planet’s rocky mantle. On Europa, Kattenhorn and Prockter argue, it suggests that a plate of super-cold surface ice is moving downward to be reincorporated into the moon’s ice mantle, which is slightly warmer. They call these areas subsumption bands.
“We’re not suggesting [that] the plates subduct and then melt,” says Kattenhorn. “We’re saying it’s subsuming” – or being recombined with the ice below by the forces of heat, pressure, and possibly chemical reactions as well. On the edges of these missing gaps were very distinctive surface features — tabular zones termed subsumption bands, broad flat areas of ice a few tens of kilometers wide, where deformation occurred. Similar land deformations happen along Earth’s subduction zones.
On the topmost ice plates, the researchers also found signs of “cryolavas”: slush-like substances that erupted onto the surface and then froze. But the analogies with Earthly lava pretty much end there. “Even on Europa, water is denser than ice,” says Kattenhorn. “To get liquid water to the surface, you have to force it through material that is less dense.” So while subduction pressure may be forcing water to the surface of Europa, the origins of these cryolavas remain a mystery.
“Maybe there are already pockets of fluid within the ice shelf – a paper was published on that a few years ago – similar to subglacial lakes in Antarctica,” he says. “And if they’re squeezed in some way, those fluids are being brought up to the surface.”
Kattenhorn and Prockter will continue to use data from Galileo to study other sites on Europa for signs of plate tectonics. But “these images are already over a decade old,” he says. “There is still plenty to rediscover [in them], but we are starting to scrape the bottom of the barrel. We need new data.”
Kattenhorn, who has been studying the Jovian moon for nearly two decades, hopes that that the evidence of tectonic activity on Europa — the only body in the Solar System besides Earth to show signs of them — will help increase the momentum behind NASA’s Europa Clipper proposal for a new scientific mission to Jupiter’s moon.
“I just see this as the culmination of 16 years of work, a labor of love really,” he says, “to unravel the mysteries of what happened on Europa.”
Correction: An earlier version of this article incorrectly described some of the surface features along subsumption bands on Europa as compared to those along subduction zones on Earth. We regret the error.