Study finds ocean currents may affect rotation of Europa’s icy crust

The research reveals a new explanation for how the icy shell of Jupiter’s moon Europa rotates at a different rate than its interior. NASA’s Europa Clipper will take a closer look.

NASA scientists have strong evidence that Jupiter’s moon Europa has an internal ocean beneath its icy outer shell—a vast body of salty water swirling around the moon’s rocky interior. New computer modeling suggests that water may actually be pushing the ice shell forward, possibly speeding up and slowing the rotation of the moon’s icy shell over time.

Scientists knew that Europa’s shell is probably free, rotating at a different rate than the ocean below and the rocky interior. The new model is the first to show that Europa’s ocean currents could be contributing to the rotation of its icy shell.

A key element of the study involved calculating drag—the horizontal force exerted by the moon’s ocean on the ice above it. The research hints at how the strength of the ocean current and its pull against the ice sheet could even explain some of the geology seen on Europa’s surface. Cracks and ridges could result from the ice shell slowly stretching and collapsing over time as it is pushed and pulled by ocean currents.

“Prior to this, it was known through laboratory experiments and modeling that warming and cooling of Europa’s ocean can lead to currents,” said Hamish Hay, a researcher at the University of Oxford and lead author of the study published in Journal of Geophysical Research: Planets. Hay conducted the research while a postdoctoral research associate at NASA’s Jet Propulsion Laboratory in Southern California. “Now our results highlight a coupling between the ocean and ice shell rotation that was not previously considered.”

It may even be possible, using measurements gathered by NASA’s upcoming Europa Clipper mission, to determine precisely how fast the icy shell is rotating. When scientists compare images collected by the Europa Clipper with those previously taken by NASA’s Galileo and Voyager missions, they will be able to examine locations of ice surface features and potentially determine whether the location of the ice shell of the moon has changed over time.

For decades, planetary scientists have debated whether Europa’s icy shell might be spinning faster than the deep interior. But instead of linking it to ocean movement, scientists focused on an outside force: Jupiter. They thought that as the gas giant’s gravity pulls on Europa, it also pulls on the moon’s shell and causes it to spin slightly faster.

“To me, it was completely unexpected that what’s happening in the ocean circulation could be enough to affect the icy shell. This was a huge surprise,” said co-author and Europa Clipper project scientist Robert Pappalardo of JPL . “And the idea that the cracks and ridges we see on Europa’s surface could be linked to the circulation of the ocean below — geologists don’t usually think, ‘Maybe it’s the ocean that’s doing this.’

Europa Clipper, now in the assembly, test and launch phase at JPL, is scheduled to launch in 2024. The spacecraft will begin orbiting Jupiter in 2030 and will use its array of sophisticated instruments to collect science data as it flies by the moon’s approx. 50 times. The mission aims to determine whether Europa, with its deep interior ocean, has conditions that could be suitable for life.

Like a pot of water

Using techniques developed for studying Earth’s ocean, the paper’s authors relied on NASA supercomputers to build large-scale models of Europa’s ocean. They explored the intricacies of how water moves and how heating and cooling affect that movement.

Scientists believe that Europa’s internal ocean is heating up from below, due to radioactive decay and tidal heating within the moon’s rocky core. Like water heated in a pot on a stove, Europa’s warm water rises to the top of the ocean.

In the simulations, the circulation initially moved vertically, but the rotation of the moon as a whole caused the flowing water to turn in a more horizontal direction—into east-west and west-east currents. By including drag in their simulations, the researchers were able to determine that if the currents are fast enough, there could be enough drag on the ice above to speed up or slow down the shell’s spin speed. The amount of internal heating—and therefore, circulation patterns in the ocean—can change over time, possibly speeding up or slowing down the rotation of the ice shell up.

“The work could be important for understanding how the rotation rates of other ocean worlds may have changed over time,” Hay said. “And now that we know about the possible coupling of internal oceans to the surfaces of these bodies, we may learn more about their geological history as well as about Europa.”

Europa Clipper’s main science goal is to determine if there are places below the surface of Jupiter’s icy moon Europa that could support life. The mission’s three main scientific goals are to understand the nature of the ice shell and the ocean beneath it, along with their composition and geology. The mission’s detailed exploration of Europa will help scientists better understand the astrobiological possibilities for habitable worlds beyond our planet.