Europa’s shell likely floats freely, spinning at a different speed than the water below and the rocky surface of its interior. Beneath its icy exterior, Jupiter’s moon Europa has a vast ocean of salt water that flows around its rocky interior.

New computer models suggest that the water may push the ice shell along, possibly accelerating and slowing the rotation of the moon’s icy shell over time. The model is the first to show that Europa’s ocean currents could contribute to icy shell rotation.

Calculating drag — the horizontal force the moon’s ocean exerts on the ice above — was a critical part of the study. The study suggests that some of the geology observed on Europa’s surface could be explained by the strength of ocean currents and resistance to the ice cover. The frozen shell can develop ridges and cracks due to the ocean currents pushing and pulling on it over time.

Hamish Hay, a researcher at the University of Oxford and lead author of the study, said: “Previously it was known through laboratory experiments and modeling that warming and cooling of Europa’s ocean currents can cause. Now our results highlight a coupling between the ocean and icy shell rotation that had never been considered before.

It’s possible to estimate how fast the icy shell is spinning using data collected by NASA’s planned Europa Clipper mission. Scientists will be able to evaluate the locations of features of the ice surface and perhaps determine whether the moon’s icy crust has moved over time by comparing the photos from Europa Clipper with photos taken in the past by NASA’s Galileo and Voyager missions.

Co-author and Europa Clipper Project Scientist Robert Pappalardo of JPL said: “It was completely unexpected to me that what happens in the ocean’s circulation could be enough to affect the icy shell. That was a huge surprise. And the idea that the cracks and ridges we see on Europa’s surface could be related to the circulation of the ocean below – geologists don’t usually think, ‘Maybe it’s the ocean doing that.’”

Scientists used techniques — developed to study Earth’s oceans — to create large-scale models of Europa’s ocean on NASA supercomputers. They examined the subtleties of water circulation, including how heating and cooling affect it.

In the simulations, the circulation started to move vertically. Still, the moon’s overall rotation caused the water to diverge into east-west and west-east currents, which are more horizontal. The researchers concluded that if the wind is fast enough, there may be enough drag on the ice above to speed up or slow down the shell’s rotational speed by incorporating drag into their calculations. The degree of inner heat, and consequently the ocean’s circulation patterns, can change over time, potentially causing the frozen shell above to spin faster or slower.

Hamish Hay, a researcher at the University of Oxford and lead author of the study published in JGR: Planets, said: “The work could be important for understanding how the rotational speeds of other ocean worlds may have changed over time. And now that we know about the possible coupling of inland seas to the surfaces of these bodies, we can learn more about their geological history and that of Europe.”

Magazine reference:

  1. HCFC Hay, I. Fenty, R. T. Pappalardo, Y. Nakayama. Turbulent drag at the ice-ocean interface of Europa in rotational convection simulations: implications for non-synchronous rotation of the ice shell. JGR planets. DOI: 10.1029/2022JE007648