The Solar System is ancient. Many of the bodies in it show their age with impacts that date back to the violent early days of the Late Heavy Bombardment and craters embedded in craters. Earth is different in that plate tectonics and other geological processes constantly remake its surface. But even the Earth looks pretty old compared to Jupiter's moon Europa. Based on the number of impacts present, Europa looks to be less than 100 million years old.

A variety of evidence indicates that Europa's dynamic surface comes from the fact that the moon has a thin crust of ice above a large sub-surface ocean. Geysers and other features also suggest that the moon is geologically active. But the precise mechanism that drive the surface remodeling have remained uncertain. Now, two researchers are proposing that the mechanism is the same as it is on Earth: plate tectonics.

The proposal is put forth by the University of Idaho's Simon Kattenhorn and Johns Hopkins' Louise Prockter, and it was released by Nature Geoscience yesterday. The authors note the clear evidence of remodeling and point out that we've already identified a source of new ice reaching the surface: some features on the moon's surface appear to be sites of spreading, analogous to a mid-ocean ridge. But assuming the moon hasn't been growing larger, there must be some process that removes old ice from the surface.

The authors suggest that there's also a Europan analog of subduction zones on Earth, where old crust gets drawn back into the interior of the moon. They support their idea with a reanalysis of images from the Galileo orbiter, which photographed Europa's surface with a resolution where each pixel represents about 200 meters.

Europa's surface has a variety of features, from dark, highly visible bands to more subtle ridges. But a number of these extend for great distances across the moon. Others terminate suddenly, giving the surface a patchwork appearance. The authors note that in some cases, continuations of the features can be found at a different location on the moon's surface, suggesting that the two ends have slid separate from each other. This, the researchers say, is the result of faults where two plates slide past each other.

In other areas, the features disappear entirely. At the sites of disappearance, the authors identify areas that they call "subsumption zones," where they think the surface ice is being subsumed into the interior. At the area of the site where the ice may enter the interior, the surface appears to consist of a series of small ridges. Beyond the ridges, on the plate that remains on the surface, the authors identify smooth areas that may represent what they call "cryolavas"—areas where a liquidy slush flowed onto the surface and froze. In one case, a patch of this ice is clearly associated with a volcano-like feature.

Having identified the sites where things are moving, the authors then worked backwards to identify the original configuration of the ice in this region. Doing this, many features that are now discontiguous line up again—ridges that have been separated can be rejoined—and the authors end up with a 100 km wide area that's simply vanished. They suggest that the whole region (20,000 square kilometers) has been subsumed back below the crust.

It's a compelling visual case that plate tectonics is active on the surface of Europa, shifting enormous blocks of ice around. There's just the small problem of the physics.

On Earth, hotter, molten rock is less dense than cooler and/or solid rock. This is what drives the convection in the mantle, which in turn drives the motion of the plates above it. It's also what forces molten rock to the surface in sometimes spectacular volcanic eruptions.

The exact converse is true on Europa. The water of the interior is denser than the ice that floats above it. There's no obvious reason for the ice to sink towards the interior or dense water to be driven to the surface in cryovolcanism, which creates a bit of a problem for the idea of plate tectonics.

It's not an insurmountable problem, though. The authors say that the force generated by spreading centers has to go somewhere, and it could be capable of driving ice into a slushier interior. Local forces at the site of subsumption can generate pressure that powers geysers and cryovolcanism. And the whole system is operating against a backdrop of intense tidal forces from Jupiter's gravity—something with no analog on Earth. So the whole thing is plausible, but it needs a lot more study.

Right now, however, all we have are images from Galileo, which didn't carry an instrument that registered the altitude of any surface features. (In some cases, altitude can be inferred if a feature is tall enough to cast a shadow.) Ideally, we'd like to go back with a laser altimeter and something to measure the local density of the icy crust. Right now, however, the only probes that could potentially carry these instruments are still just proposals that haven't yet reached the formal design stage.

Nature Geoscience, 2014. DOI: 10.1038/NGEO2245 (About DOIs).