Landmarks on Europa’s surface suggest that the ice crust is at the mercy of the waters below. Most importantly, a recent visit from Juno reveals what could be plume activity, which could allow future missions to sample the internal ocean without having to land.
It’s been almost two years since Juno made its closest approach to Europe, but its observations are still being analyzed. Remarkably, despite orbiting Jupiter since 2016, five images taken by Juno on September 29, 2022 were the first close-ups of Europa since the Galileo spacecraft’s last visit in 2000.
This undoubtedly represents a shocking neglect of one of the solar system’s most intriguing worlds, but it could also have provided a long basis for seeing what had changed.
Europa is the smoothest object in the solar system, thanks to its constant resurfacing driven by the internal ocean. Nevertheless, it is far from characteristic, and Juno saw some depressions with steep walls 20 to 50 kilometers wide and fault patterns thought to be indicative of “true polar wander”.
“True polar wander occurs when Europa’s icy shell becomes disconnected from its rocky interior, resulting in high stress levels on the shell, leading to predictable fracture patterns,” says Dr. Candy Hansen of the Planetary Science Institute said in a statement.
The idea behind true polar walks is that the shell resting on top of Europa’s internal ocean spins at a different speed than the rest of the moon. It is thought that the water underneath moves and drags the shell with it, with currents in the ocean influencing the shell’s movements. The currents, in turn, are likely driven by heating in Europa’s rocky core, as the gravity of Jupiter and its larger moons turn Europa into a giant stress ball.
In doing so, the interactions between the ocean and the ice can stretch and compress areas, creating the cracks and ridges we’ve seen since Voyager 2’s visit.
Hansen is part of a team examining Juno’s images of Europe’s southern hemisphere. “This is the first time these fault patterns have been mapped in the Southern Hemisphere, suggesting that the effect of true polar wandering on the surface geology of Europe is greater than previously identified,” he said.
Not all changes on the maps of Europe are the result of internal ocean currents. It seems even NASA is falling for optical illusions. “Crater Gwern no longer exists,” Hansen said. “What was once thought to be a 21-kilometer-wide impact crater – one of Europe’s few documented impact craters – Gwern, was revealed by JunoCam data to be a series of intersecting ridges creating an oval shadow.”
However, Juno gives more than it takes away. The team is excited about something they call the Platypus because of its shape, and not because it has some mismatched features. It appears that mountain formations at the edge are collapsing into it, and the team thinks this process could be caused by pockets of salt water that have partially penetrated the icy shell.
The feature was named by planetary scientists who have apparently never seen a real platypus, outlined in yellow, with an area edged in blue.
Image credit: NASA/JPL-Caltech/SwRI
Such spots would be exciting indirect targets for exploration by the Europa Clipper, but even more interesting are the dark spots that may have been deposited by cryovolcanic activity.
“These features indicate current surface activity and the presence of subsurface liquid water on Europa,” says Heidi Becker of the Jet Propulsion Laboratory. Such activity has been confirmed in the geysers of Enceladus, but there is conflicting evidence as to whether it is currently occurring on Europa.
Such an activity would make it possible to sample the inland sea for signs of life just by flying through a plume and collecting some ice flakes, without having to land, let alone drill.
Currently, polar wandering may cause fairly modest adjustments to the locations of features on the European surface, but there is evidence that a shift of more than 70 degrees occurred millions of years ago, for unknown reasons.