That ocean makes Europa a prime target for scientific investigation, including two separate orbiter missions to be launched toward Jupiter over the next two years.
And while it will take several years for one of the probes to arrive, scientists are already shedding light on Europa in other ways, drawing insights from telescope observations, previous probe flybys, laboratory experiments and computer simulations.
In a new study, researchers from the Jet Propulsion Laboratory (JPL) at the California Institute of Technology in the US and Hokkaido University in Japan used NASA supercomputers to investigate a lesser-known peculiarity of Europa: Why does the ice shell rotate faster than the interior?
According to their research, the unsynchronized rotation of the surface may be caused by ocean currents pushing from below. It’s a big revelation, explains lead author and JPL researcher Hamish Hay, now at the University of Oxford; it’s a revelation that can provide new clues about what’s going on down there.
“Prior to this, it was known through laboratory experiments and modeling that warming and cooling of Europa’s oceans can drive currents,” says Hay. “Now our results highlight a link between the ocean and the rotation of the icy shell that was never previously considered.”
The ice shell floats on Europa’s seas, allowing it to rotate independently of the rest of it the moon, including the ocean, rocky interior and metallic core. Scientists have long suspected this, but the forces driving the shell’s rotation have been mysterious.
Europa is subject to tidal bending by Jupiter, which distorts the moon through its strong gravity. This colossal tug of war is causing cracks in Europa’s ice shell and likely generating some of the mantle and core heat.
Along with thermal energy released from radioactive decay, this heat from Europa’s interior is thought to rise through the ocean toward the frozen surface like a pot of water heated on a stove.
Combined with Europa’s rotation and other factors, the vertical temperature gradient should fuel some pretty strong ocean currents.
And according to estimates in the study, these currents could be powerful enough to move the global ice sheet overhead. No one knows exactly how thick the shell is, but estimates range from about 15 to 25 kilometers (15 miles) thick.
While scientists knew that Europa’s icy shell probably rotates on its own, they had focused on Jupiter’s gravitational influence as the driving force.
“To me, it was completely unexpected that what’s happening in the ocean circulation could be enough to affect the icy shell. It was a huge surprise,” said study co-author and Europa Clipper Project scientist Robert Pappalardo, of NASA’s Jet Propulsion Lab.
“And the idea that the cracks and ridges we see on Europa’s surface could be tied to the circulation of the ocean below—geologists don’t usually think, ‘Maybe the ocean is doing it,'” he adds.
The researchers used NASA supercomputers to build complex simulations of Europa’s oceans, borrowing techniques that have been used to model oceans on Earth.
These models let them delve deeper into the details of water circulation on Europa, including how these patterns are affected by ocean warming and cooling.
A key focus of the study was drag, or the horizontal force from the ocean that pushes the ice across it. By including drag in their simulations, the researchers found that some faster currents could produce enough drag to speed up or slow down the rotation of Europa’s ice shell.
While this effect depends on the flow’s speed, the researchers note that Europa’s internal heating may vary over time. This can lead to corresponding variation in the speed of ocean currents, which in turn causes faster or slower rotation of the ice sheet.
In addition to helping us understand Europa, this research could also apply to other ocean worlds, the researchers point out, where surface features could provide hints about water hidden below.
“And now that we know about the potential coupling of interior oceans with the surfaces of these bodies, we can learn more about their geological history as well as Europa’s,” Hay says.
Scheduled to launch in April 2023, ESA’s Jupiter Icy Moons Explorer (JUICE) will begin its journey to study Jupiter’s three large, ocean-bearing moons: Ganymede, Callisto and Europa.
In late 2024, NASA plans to launch its Europa Clipper orbiter, which will perform nearly 50 close flybys to investigate the moon’s potential habitability. According to the authors of the new study, it may even be able to accurately measure how fast Europa’s ice shell is rotating.
The study was published in JGR Planets.