Icy scars (Image: NASA)

Jupiter’s moon Europa sports a chilly version of plate tectonics. The icy shell that encases the moon has sections that slide beneath each other in a similar way to Earth’s continents and ocean floors. The finding strengthens the notion that Europa could harbour life.

Europa has long been considered one of the best sites in the solar system to search for extraterrestrial life, because it has more water than even Earth does. Its global ocean is encased in a shell of ice 20 to 30 kilometres thick. Rust-coloured lines in the shell are thought to be areas where new ice forms. But this creates a puzzle.

“Nobody thinks Europa is getting bigger,” says Francis Nimmo at the University of Southern California, Santa Cruz. So where was all the ice going?


Now, analysis of images from the Galileo spacecraft suggest that large chunks of ice may be returning to the ocean beneath, in the first known instance of tectonics on another world.

Simon Kattenhorn at the University of Idaho and his colleagues treated the Galileo images like a massive jigsaw puzzle. In the same way geologists studying Earth’s continents use the matching shapes of geological features to reconstruct past mega-continents, Kattenhorn and his colleagues spun and shifted chunks of ice around to match up truncated lines and ridges on Europa’s surface.

Missing piece

They found that a piece of the puzzle roughly the size of Massachusetts was missing. The best explanation is that it was forced under the ice crust, in a “subsumption” zone similar to Earth’s subduction zones where rocky crust slides beneath rocky crust.

“This paper is important because it has been very hard to identify areas where crustal destruction is taking place on Europa,” says Nimmo, who did not take part in the study.

On Earth, subduction is driven by our planet’s hot core, which heats the mantle, causing it to rise up to the surface and move the crustal plates. Despite Europa’s cold temperatures, something similar could be happening there.

The surface of Europa is -173 °C, but deep down the water is closer to 0 °C. That temperature difference could make the deepest layers of ice move around slowly, a bit like glaciers here on Earth. That could produce enough pressure to push slush up through weak points in the brittle surface ice, driving the movement of the plates.

This could be good news for life under the surface, as tectonics would deliver new material – including organic molecules deposited by comets – to this environment, which is otherwise completely sealed off from space.

All this remains tantalising speculation. But it is neat to note that Earth appears to have an inside-out twin, where tectonics driven by ice instead of fire could create ideal conditions for life inside its crust, instead of on its surface.

Nature Geoscience, DOI: 10.1038/ngeo2245