by Europa is the smoothest solid object in our solar system, thanks to its thick shell of ice. Yet beneath its smooth exterior, Jupiter’s fourth-largest moon appears to harbor secrets, namely a deep, salty ocean with intriguing potential for extraterrestrial life.
This ocean makes Europa a prime target for scientific studies, including two separate orbital missions slated for launch to Jupiter over the next two years.
And while it will take several years for either probe to arrive, scientists are already shedding light on Europa in other ways, gleaning information from telescope observations, previous flybys of probes, laboratory experiments and computer simulations.
In a new study, researchers from the Jet Propulsion Laboratory (JPL) at the California Institute of Technology in the United States and Hokkaido University in Japan used NASA supercomputers to examine a lesser-known quirk of Europe: why does the shell of ice spin faster than the shell of ice? interior?
According to their research, the desynchronized rotation of the surface could be caused by ocean currents pushing from below. It’s a big eye-opener, says lead author and JPL researcher Hamish Hay, now at the University of Oxford; it’s a revelation that could offer new clues to what’s going on down there.
“Before that, it was known from laboratory experiments and modeling that heating and cooling of the European ocean could drive currents,” says Hay. “Now our results highlight a coupling between the ocean and the rotation of the ice shell that had never been considered before.”
The ice shell floats on the ocean of Europa, so it can rotate independently of the rest of the moon, including ocean, rocky interior and metal core. Scientists have suspected this for a long time, but the forces behind the rotation of the shell are mysterious.
Europa is subject to tidal bending by Jupiter, which deforms the moon thanks to its powerful gravitational pull. This colossal tug of war causes cracks in Europa’s ice shell and likely generates some of the heat from the mantle and core.
With thermal energy released by radioactive decay, this heat from within Europa is thought to rise across the ocean to the frozen surface like a pot of water heating on a stove.
Combined with Europa’s rotation and other factors, this vertical temperature gradient should fuel fairly strong ocean currents.
And according to the study’s estimates, these currents could be strong enough to move the global ice shell above our heads. No one knows exactly how thick the shell is, but estimates vary from around 15 to 25 kilometers (15 miles) thick.
While the scientists knew that Europa’s ice shell was probably spinning on its own, they focused on Jupiter’s gravitational influence as the driving force.
“To me, it was completely unexpected that what happens in the ocean circulation could be enough to affect the ice shell. It was a huge surprise,” said study co-author and scientist Robert Pappalardo. Europa Clipper project, from NASA’s Jet Propulsion Lab.
“And the idea that the fissures and ridges we see on the surface of Europa might be related to ocean circulation below – geologists don’t usually think, ‘Maybe that’s the ocean that does that'”, he adds.
The researchers used NASA supercomputers to build complex simulations of the ocean of Europa, borrowing techniques that have been used to model the oceans on Earth.
These models allow them to drill down into the details of water circulation over Europa, including how these patterns are influenced by ocean warming and cooling.
A key focus of the study was drag, or the horizontal force of the ocean pushing the ice above it. By factoring drag into their simulations, the researchers found that certain faster-moving currents could produce enough drag to speed up or slow down the rotation of Europa’s ice shell.
Although this effect depends on the speed of the currents, the researchers note that Europa’s internal heating can vary over time. This could lead to a corresponding change in the speed of ocean currents, in turn causing the ice shell to spin faster or slower.
As well as helping us understand Europa, this research may also apply to other ocean worlds, the researchers point out, where surface features could offer clues to hidden waters below.
“And now that we know the potential coupling of inland oceans with the surfaces of these bodies, we can learn more about their geological histories as well as those of Europa,” Hay said.
ESA’s Jupiter Icy Moons Explorer (JUICE) is set to launch in April 2023, beginning its journey to study Jupiter’s three large ocean moons: Ganymede, Callisto and Europa.
In late 2024, NASA plans to launch its Europa Clipper orbiter, which will perform nearly 50 close flybys to study the moon’s potential habitability. According to the authors of the new study, it may even be able to accurately measure the rotational speed of Europa’s ice shell.
The study was published in JGR Planets.
