Scientists solve mystery of Europa’s swirling ice shell

Europa is the smoothest solid object in our solar system due to its thick icy crust. Beneath its smooth exterior, Jupiter’s fourth largest moon appears to be hiding secrets—a deep, salty ocean with intriguing potential for alien life. This ocean makes Europa a primary target for scientific research, including two separate orbital missions due to launch to Jupiter over the next two years.

And although it will take a few years for any of the probes to arrive, scientists are shedding light on Europa in other ways by gathering information from telescope observations, previous probe flights, 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 probe a lesser-known quirk of Europa: Why is the icy crust stronger? spinning fast? interior?

According to their research, the desynchronization of surface rotation can be caused by ocean currents pushing from below. Lead author and JPL researcher Hamish Hay, now at Oxford University, explains that this is a major discovery; a discovery that could offer new clues as to what’s going on out there.

“Before that, it was known through laboratory experiments and simulations that warming and cooling of Europa’s ocean could cause currents,” says Hay. “Our results now highlight a never-before-thought-out link between ocean and ice sheet rotation.”

The icy crust floats in Europa’s ocean, so the ocean can rotate independently of the rest of the Moon, including the rocky interior and metallic core. Scientists have long suspected this, but the forces driving the shell’s rotation remain mysterious. Europa is experiencing Jupiter’s tidal bending, which deforms the Moon due to its strong gravity. This massive upheaval is causing cracks in Europa’s icy crust and possibly generating some of the heat in the mantle and core.

This heat from Europa’s interior, along with the thermal energy released by radioactive decay, is thought to rise to the frozen surface across the ocean, like a pot of water on a stove. Combined with Europa’s rotation and other factors, this vertical temperature gradient should feed some pretty strong ocean currents.

According to the study’s predictions, these currents could be strong enough to shift the load of the global ice sheet. No one knows exactly how thick the bullet was, but estimates range from 15 to 25 kilometers (15 miles). Scientists focused on Jupiter’s gravitational effect as the driving force, although they knew that Europa’s icy crust probably rotated on its own.

“It was totally unexpected to me that what was happening in the ocean circulation was enough to affect the ice sheet. It was a big surprise,” says study co-author and Europa Clipper project scientist Robert Pappalardo of NASA’s Jet Propulsion Laboratory.

“The idea that the cracks and ridges we see on Europa’s surface may be related to the circulation of the ocean below — geologists often don’t think ‘Maybe the ocean is doing this,'” he adds.

The researchers used NASA supercomputers to create a complex simulation of Europa’s ocean, borrowing methods used to model Earth’s oceans. These models allow them to delve deeper into the details of Europa’s water circulation, including how these models are affected by the warming and cooling of the ocean. The main focus of the study was drag, or the horizontal force pushing the ice above the ocean. By factoring in friction in their simulations, the researchers found that some of the faster currents could generate enough drag to speed up or slow the rotation of Europa’s icy crust.

While this effect is dependent on the velocity of the flow, the researchers note that Europa’s internal warming may change over time. This can lead to corresponding fluctuations in the speed of ocean currents, which will cause the ice sheet to spin faster or slower. The researchers note that this study not only helps us understand Europa, but may also apply to other ocean worlds where surface features may provide clues about the waters hidden beneath.

“And now that we know the potential connection of the interior oceans to the surface of these objects, we can learn more about their geological history and the history of Europa,” says Hay.

ESA’s Jupiter Icy Moon Explorer (JUICE) is scheduled to launch in April 2023, starting its journey to study Jupiter’s three major oceanic moons, Ganymede, Callisto, and Europa. In late 2024, NASA plans to launch the Europa Clipper orbiter, which will make about 50 close flybys to investigate the moon’s potential habitability. It will even be able to accurately measure how fast Europa’s icy crust is spinning, according to the authors of the new study.