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 under its icy outer shell – an enormous body of salty water swirling around the moon’s rocky interior. New supercomputer modeling suggests the water may be pushing the ice shell along, possibly speeding up and slowing down the rotation of the moon’s icy shell over time.

Scientists have known that Europa’s shell is probably free-floating, rotating at a different rate than the ocean below and the rocky interior. The new modeling 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 that the moon’s ocean exerts on the ice above it. The research hints at how the power of the ocean flow and its drag against the ice layer could even account for some of the geology seen on Europa’s surface. Cracks and ridges could result from the icy shell slowly stretching and collapsing over time as it is pushed and tugged by the ocean currents.

“Before this, it was known through laboratory experiments and modeling that heating and cooling of Europa’s ocean may drive currents,” said Hamish Hay, a researcher at the University of Oxford and lead author of the study published in JGR: Planets. Hay performed 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 the rotation of the icy shell that was never previously considered.”

It might even be possible, using measurements gathered by NASA’s upcoming Europa Clipper mission, to determine with precision how fast the icy shell rotates. When scientists compare images gathered by Europa Clipper with those captured in the past by NASA’s Galileo and Voyager missions, they will be able to examine locations of ice surface features and potentially determine if the position of the moon’s icy shell has changed over time.

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

“To me, it was completely unexpected that what happens in the ocean’s circulation could be enough to affect the icy shell. That 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 tied to the circulation of the ocean below – geologists don’t usually think, ‘Maybe it’s the ocean doing that.’”

Europa Clipper, now in its assembly, test, and launch operations phase at JPL, is set to launch in 2024. The spacecraft will begin orbiting Jupiter in 2030 and will use its suite of sophisticated instruments to gather science data as it flies by the moon about 50 times. The mission aims to determine if Europa, with its deep internal ocean, has conditions that could be suitable for life.

Like a Pot of Water

Using techniques developed to study Earth’s oceans, the paper’s authors relied on NASA supercomputers to make large-scale models of Europa’s oceans. They explored the complexities of how the water circulates, and how heating and cooling affect that movement.

Scientists believe that Europa’s internal ocean is heated from below, due to radioactive decay and tidal heating within the moon’s rocky core. Like water heating 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 veer in a more horizontal direction – in east-west and west-east currents. The researchers, by including drag in their simulations, were able to determine that if the currents are fast enough, there could be an adequate drag on the ice above to speed up or slow down the shell’s rotation speed. The amount of interior heating – and thus, circulation patterns in the ocean – may change over time, potentially speeding up or slowing the rotation of the icy shell above.

“The work could be important in understanding how other ocean worlds’ rotation speeds may have changed over time,” Hay said. “And now that we know about the potential coupling of interior oceans with the surfaces of these bodies, we may learn more about their geological histories as well as Europa’s.”

How can we combat data theft, which is a real issue for society? Quantum physics has the solution. Its theories make it possible to encode information (a qubit) in single particles of light (a photon) and to circulate them in an optical fiber in a highly secure way. However, the widespread use of this telecommunications technology is hampered in particular by the performance of the single-photon detectors. A team from the University of Geneva (UNIGE), together with the company ID Quantique, has succeeded in increasing their speed by a factor of twenty. This innovation makes it possible to achieve unprecedented performances in quantum key distribution. 

Buying a train ticket, booking a taxi, getting a meal delivered: these are all transactions carried out daily via mobile applications. These are based on payment systems involving an exchange of secret information between the user and the bank. To do this, the bank generates a public key, which is transmitted to their customer, and a private key, which it keeps secret. With the public key, the user can modify the information, make it unreadable and send it to the bank. With the private key, the bank can decipher it.

This system is now threatened by the power of quantum supercomputers. To resolve this, quantum cryptography - or quantum key distribution (QKD) - is the best option. It allows two parties to generate shared secret keys and transmit them via optical fibers in a highly secure way. This is because the laws of quantum mechanics state that measurement affects the state of the system being measured. Thus, if a spy tries to measure the photons to steal the key, the information will be instantly altered and the interception revealed.

Current limitations

One limitation to the application of this system is the speed of the single-photon detectors used to receive the information. In fact, after each detection, the detectors must recover for about 30 nanoseconds, which limits the throughput of the secret keys to about 10 megabits per second. A UNIGE team led by Hugo Zbinden, associate professor in the Department of Applied Physics at the UNIGE Faculty of Science, has succeeded in overcoming this limit by developing a detector with better performance. This work was carried out in collaboration with the team of Félix Bussières from the company ID Quantique, a spin-off of the university.

‘‘Currently, the fastest detectors for our application are superconducting nanowire single-photon detectors,’’ explains Fadri Grünenfelder, a former doctoral student in the Department of Applied Physics at the UNIGE Faculty of Science and first author of the study. ‘‘These devices contain a tiny superconducting wire cooled to -272°C. If a single photon hits it, it heats up and ceases to be superconducting for a short time, thus generating a detectable electrical signal. When the wire becomes cold again, another photon can be detected.’’

Record performance

By integrating not one but fourteen nanowires into their detector, the researchers were able to achieve record detection rates. ‘‘Our detectors can count twenty times faster than a single-wire device,’’ explains Hugo Zbinden. ‘‘If two photons arrive within a short time in these new detectors, they can touch different wires and both be detected. With a single wire, this is impossible’’. The nanowires used are also shorter, which helps to decrease their recovery time.

Using these sensors, scientists were able to generate a secret key at a rate of 64 megabits per second over 10 km of fiber optic cable. This rate is high enough to secure, for example, a videoconference with several participants. This is five times the performance of current technology over this distance. As a bonus, these new detectors are no more complex to produce than the current devices available on the market.

These results open up new perspectives for ultra-secure data transfer, which is crucial for banks, healthcare systems, governments, and the military. They can also be applied in many other fields where light detection is a key element, such as astronomy and medical imaging.