A paper published in Nature Geoscience on September 7 may hold the answer to a long-standing mystery regarding Europa’s icy shell.
When Voyager 2 flew by Europa on July 9, 1979, scientists were surprised to find the moon had a nearly crater-free surface, streaked with numerous cracks. Compared to the more heavily cratered Ganymede and Callisto, Europa’s surface was clearly fresh. As early as 1980, researchers were invoking a familiar model of resurfacing a planet – plate tectonics.
Plate tectonics is the main driver of geological activity here on Earth. The Earth’s crust is broken up into plates, which are pushed around by convection in the mantle. Hot rock wells up along mid-ocean ridges and pushes the plates on either side of the ridge apart. Eventually, as the rock cools and becomes more dense, it gets shoved under less dense rock and back into the mantle, a process known as subduction. Over time, spreading and subduction work to replace ~75% of the Earth’s surface every 150 million years.
The key difference is that on Europa plate tectonics are driven by gravity, not convection. Tidal forces raised by Jupiter, Io, and Ganymede stretch and compress Europa’s icy shell. The movement creates small cracks in the ice that behave much like spreading centers on Earth. When the ice is stretched, the release of pressure allows ice deeper beneath the surface to melt and move along cracks. Over time, the buildup of new ice inside the fracture pushes the plates apart.
While plate tectonics explained the young age of Europa’s icy crust (calculated to be 40-90 million years old), it had one major drawback: researchers weren’t able to identify any areas where older ice was being destroyed. To maintain a constant surface area, old ice must be destroyed at the same rate new ice is created. Given the young age of the moon’s surface, Europa needed to get rid of a lot of old ice, and quickly.
To make the plate tectonics model work, researchers attempted to find a number of explanations to get rid of the old ice. Folding and thickening the icy shell were leading explanations. However, these should have created mountain ranges on Europa, something Voyager 2 and the Galileo orbiter showed didn’t exist. More exotic solutions were put forward, such as remelting along faults where the ice scraped together, but calculations showed that these methods weren’t able to get rid of old ice quickly enough.
Rebuilding a Jigsaw Puzzle
To solve the problem of old ice, Simon Kattenhorn (University of Idaho) and Louis Prockter (Johns Hopkins University Applied Physics Laboratory) took a look at high-resolution imagery taken by the Galileo orbiter. While the images with the highest resolution only covered a very small portion of Europa’s surface, some managed to capture a region of extremely complex surface features. Here, cracks offset ridges and color features, the result of plates shuffling around.
Kattenhorn and Prockter performed what is known as a tectonic reconstruction, essentially reconstructing a jigsaw puzzle by removing the effects of plate movement. When the reconstruction was complete, it revealed a missing band of ice, roughly 100 km wide and covering ~22,000 km^2 (~7,700 mi^2). On Earth, missing areas in reconstructions are usually a sign of subduction. To prove that was also the case on Europa, the two authors took a look at Europa’s topography.
Paralleling the missing band of ice were two heavily crumpled zones of ice dubbed “subsumption bands.” The subsumption bands can’t be zones where old ice is piling up, because the pileup would make the bands into a tall mountain chain. Instead, Kattenhorn and Prockter think that the subsumption bands mark the boundary where old ice is being pushed under new ice. As the two plates are forced together, the leading edge of the overriding plate is compressed, forming tightly folded and faulted zones. Similar crumple zones appear along subduction zones here on Earth.
When the old ice subducts, the plate is gradually reabsorbed into a warmer, slushier region of Europa’s ice shell. The reabsorbtion process might produce small volumes of liquid water that erupt onto the surface. Kattenhorn and Prockter found clusters of small domes on overriding plates that stand about 100m above the surrounding landscape. Volcanic chains on Earth, like the Cascades and Andes, form behind subduction zones as water is squeezed out of the subducting plate.
Explaining how liquid water makes it to the surface from a subducting plate on Europa is difficult. On Earth, water can rise from a subducting plate because it is less dense than the surrounding rock. But on Europa, the water is more dense than the surrounding ice, which means it should sink back into the subsurface ocean. However, the appearance and location of the proposed cryovolcanoes make the possibility that they ultimately originate from a subducting plate too strong to ignore.
If the boundaries identified by Kattenhorn and Prockter are subduction zones, then the missing pieces of the plate tectonics puzzle come together. The discovery puts Europa alongside Earth for the only bodies in the Solar System known to host active plate tectonics. (Note: Europa’s sister moon Ganymede also may have a slower-moving plate tectonics system.)
Looking to the Future
In a NASA press release, Kattenhorn said, “Europa may be more Earth-like than we imagined, if it has a global plate tectonic system. Not only does this discovery make it one of the most geologically interesting bodies in the solar system, it also implies two-way communication between the exterior and interior – a way to move material from the surface into the ocean – a process which has significant implications for Europa’s potential as a habitable world.”
The discovery of plate tectonics is important, because it possibly removes one barrier to life on Europa: the presence of food. Europa’s surface has collected a film of reddish material, probably sulfur compounds spewed out by Io. These sulfur compounds, which have been altered by sunlight, offer a food source for organisms in Europa’s subsurface oceans. Without a way to get the nutrients from the surface into Europa’s oceans, life would instead have to rely on seafloor volcanism.
Seafloor volcanism isn’t a confirmed process on Europa, and in the current configuration of Jupiter’s moons, it might not even be possible. Unlike Io, where tidal forces between Jupiter and Europa create an extremely active volcanic landscape, Europa’s oceans absorb almost all of the tidal energy pumped into it. Unless the tidal forces were orders of magnitude stronger in the past, Europa’s rocky core is solid and has been for a long time. A nutrient-free environment is not one where life can arise and flourish.
Final confirmation of Kattenhorn and Prockter’s discovery may not come until the arrival of NASA’s Europa Clipper (scheduled to arrive in 2025 at the earliest) or the ESA’s Jupiter Icy Moons Explorer (JUICE) in 2030. These missions, which will be able to photograph much larger areas of Europa’s surface in detail, will be able to determine if the subsumption bands are only a local phenomenon or if they occur globally.
Featured image credit: NASA/JPL