SAN FRANCISCO – Scientists may have spotted the first evidence for active plate tectonics on another world. Jupiter's moon Europa is covered in an ice crust bearing scars that may reveal movement similar to that of Earth's rocky plates.

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Europa’s Oceans Might Taste Like Earth’s OceansEuropa was already considered to be among the most scientifically intriguing bodies in the solar system and one of the most promising places to hunt for life in the solar system because of the liquid ocean that resides beneath its crust. If the latest findings turn out to be true, it could be another point in favor of the moon's potential habitability by providing a way to get nutrients from the surface down into the ocean.

"What’s exciting is that this would be the only other place outside of Earth where a plate-tectonic-style system is occurring," said planetary scientist Alyssa Rhoden, a NASA postdoctoral program fellow who studies Europa, but was not involved in the new research.

Europa's icy surface has been estimated to be between 40 million and 90 million years old, making it one of the youngest surfaces in the solar system, and far younger than the moon itself, which is more than 4 billion years old. This means that somehow the crust is being refreshed either by resurfacing or recycling of old crust.

Scientists believe new ice is being formed on Europa along linear features called dilational bands. There are thousands of kilometers of these bands on the planet, potentially creating significant amounts of new ice crust. The problem is that nobody knows where all the old crust is disappearing.

"Unless Europa has been expanding within the last 40 to 90 million years, there has to be some process on this icy moon that’s able to accommodate a large amount of new surface area being created at dilational bands," planetary scientist Simon Kattenhorn of the University of Idaho said during a presentation about the new research Dec. 13 at the American Geophysical Union meeting.

On Earth new crust is created along seams on plates that underlie the oceans, known as mid-ocean ridges. Here, the two sides of the plate move away from each other as molten material rises from the mantle below the crust to fill the gap, cool and form new crust. A similar process is thought to create new ice along Europa's dilational bands.

All the new crust formed on Earth is balanced out primarily by the destruction of older crust in a process known as subduction. When an oceanic plate runs into a plate bearing a continent, the ocean crust is usually pushed beneath the continent where it gradually melts and is recycled into the mantle.

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Now Kattenhorn and his colleague Louise Prockter of Johns Hopkins University think they have found something similar to Earth's subduction zones on Europa. By mapping the criss-crossing fractures and other linear features on a 106,000-square-kilometer section of Europa's surface using imagery from the Galileo mission, the scientists identified potential zones where the ice crust is being pushed down below the surface.

If Europa's crust is being recycled by being pushed down along subsumption bands and recreated along dilational bands, it could provide a way to get potential nutrients from the surface into the water ocean below where they could be critical for harboring life.

"Having that mixing seems to be pretty important for establishing life," said Rhoden, who cofounded Destination Europa to push for a mission to the moon.

Kattenhorn and Prockter located the subsumption bands by trying to unravel geologic time to reconstruct the original geology before the missing crust disappeared. They found features that had been separated and shifted by movement of the crust, and when they matched those features all up again, there was a large area of crust missing.

“Once the reconstruction is done, we have this big area here that is missing, 92 kilometers wide, that is alongside one of these zones that we refer to using the term subsumption bands,” Kattenhorn said in his presentation.

They say the crust in the area they mapped was pushed down underneath an overriding section of crust along a 23-kilometer-wide subsumption band, which would be analogous to a subduction zone on Earth.

"It’s an important thing to look for," planetary scientist Michael Manga of the University of California, Berkeley told WIRED. "We see expansion at the surface so there must be contraction somewhere."

But Manga's not yet convinced the subsumption bands are the answer, and says there are some potential problems with the idea. For one, if a buoyant plate is pushed down into denser liquid, it should bow upward to compensate for the buoyancy. "If there is contraction or compression things should be lifted upwards," he said.

Uplift occurs along subduction zones on Earth, for example, but the subsumption bands are basically topographically flat. Though Kattenhorn suggested in his presentation that this could be explained by the ice being pushed down at a relatively shallow angle.

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Rhoden thinks Kattenhorn and Prockter's reconstructions do seem to show that there's no way to recreate the original geology without the surface having been subsumed. But, she says, it's hard to extrapolate from the small study area to a global system. The problem is that only about 10 percent of Europa's surface has imagery good enough to do these kinds of detailed reconstructions.

"I think we are going to be hard pressed to make any global map of tectonics with our current data set," Rhoden told WIRED. This is a good argument for a new mission to Europa, which could also help us understand Earth better, she says. There is still a lot of disagreement among scientists about what drives plate tectonics on Earth, and even if the driving force on Europa is different, it could be enlightening.

And while Europa is far away, it does have some advantages over Earth as a study subject. "On Europa, there's a lot less vegetation, a lot less cities, a lot less obscuring the evidence of what has gone on in Europa's past," Rhoden said.