Markings on Europa’s surface suggest that the ice crust is at the mercy of the waters below. Most importantly, a recent visit by Juno revealed what could be plume activity that, if real, would allow future missions to collect a sample of the inner ocean without having to land.
It has been almost two years since Juno came closest to Europe, but its observations are still being analyzed. Notably, five images taken by Juno on September 29, 2022 were the first close-up images of Europa since the Galileo spacecraft last visited in 2000, even though it has been orbiting Jupiter since 2016.
That’s arguably a shocking neglect of one of the solar system’s most fascinating worlds, but it could also have provided a long baseline for seeing what’s changed.
Europa is the smoothest object in the solar system, thanks to constant surface renewal by its inner ocean. Still, it is far from featureless, and Juno discovered some steep depressions 20 to 50 kilometers wide and fracture patterns that suggest “true polar wander.”
“True polar wander occurs when Europa’s icy shell becomes decoupled from its rocky interior, causing high stresses on the shell that lead to predictable fracture patterns,” said Dr. Candy Hansen of the Planetary Science Institute in a statement.
The idea behind true polar wander is that the shell that rests on Europa’s inner ocean rotates at a different speed than the rest of the moon. The water underneath is thought to move and pull the shell with it, with currents in the ocean influencing the shell’s movements. The currents, in turn, are thought to be driven by warming in Europa’s rocky core, as the gravity of Jupiter and its larger moons turns Europa into a giant stress ball.
As this happens, interactions between the ocean and ice could expand and compress regions, creating cracks and ridges that have been seen since Voyager 2’s visit.
Hansen is part of a team studying Juno’s images of Europe’s southern hemisphere. “This is the first time that these fracture patterns have been mapped in the Southern Hemisphere, suggesting that the impact of true polar wander on the surface geology of Europe is more extensive than previously thought,” the scientist said.
Not all changes to the maps of Europe are the result of internal ocean currents. It seems that even NASA is falling for optical illusions. “Crater Gwern no longer exists,” Hansen said. “What was once thought to be a 13-mile-wide impact crater – one of Europe’s few documented impact craters – Gwern, was revealed in JunoCam data to be a series of intersecting ridges that created an oval shadow.”
However, Juno gives more than she takes. The team is excited about something they’re calling “Platypus” because of its shape, not because it has a bunch of features that shouldn’t fit together. Ridge formations on its edge appear to be collapsing into it, and the team thinks this process could be caused by pockets of saltwater that have partially penetrated the ice shell.
Named by planetary scientists and apparently never seen by a real platypus, the feature is outlined in yellow and has an area of blue ridges.
Image credit: NASA/JPL-Caltech/SwRI
Such pockets would be exciting indirect targets for study by the Europa Clipper, but even more interesting are dark patches that may have been deposited by cryovolcanic activity.
“These features indicate present-day surface activity and the presence of underground liquid water on Europa,” said 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 survey the ocean’s interior for signs of life by simply flying through a cloud and collecting some ice flakes, without having to land, let alone drill.
Currently, polar wander could cause quite modest changes to the positions of structures on Europa’s surface, but there is evidence that a shift of more than 70 degrees occurred for unknown reasons millions of years ago.