Plate tectonics spotted on Europa

Shifting landscape on Jupiter’s frozen moon is first outside Earth

image of Jupiter’s frosty moon Europa

FROZEN WORLD  Drifting ice slabs periodically refresh the surface of Jupiter’s frosty moon Europa. The frigid world is the first spot outside Earth where scientists have discovered active plate tectonics.

Ted Stryk/Galileo Project/JPL/NASA

Plate tectonics churns the icy exterior of Jupiter’s moon Europa, researchers report September 7 in Nature Geoscience. The finding marks the first evidence of plate tectonics elsewhere in the universe.

“Earth is not unique — we’ve found another body in the solar system with plate tectonics,” says planetary scientist Simon Kattenhorn of the University of Idaho in Moscow. “This tells us that this process can happen on more than just rocky planets like Earth.”

While previous observations have seen surface reshaping, such as volcanic activity, on other planetary bodies, such as Saturn’s moon Titan (SN: 1/25/14, p. 14), Kattenhorn says Europa is the first found with a patchwork of drifting tectonic plates.

The rising and sinking ice slabs on Europa’s surface may provide a mechanism for nutrients to move from the moon’s surface to its subsurface ocean, Kattenhorn argues. Such transport would bolster the likelihood that the moon’s ocean hosts life. Astrobiologist Britney Schmidt of Georgia Tech in Atlanta says the mechanism is “very exciting for Europa’s chances for supporting life.”

Despite forming over 4 billion years ago, at the same time as the rest of the solar system, Europa’s icy surface is surprisingly young, an estimated 40 million to 90 million years old.  Only a few dozen impact craters blemish its veneer. Dark bands crisscross the moon where warm, fresh ice wells up to the frigid surface, but a mystery remained: Where is the old material?

Europa plate tectonics graphic
CRUSHED ICE Where two ice slabs, analogous to Earth’s tectonic plates, collide on Europa, one slab dives under the other, forming a subduction zone. The sinking ice fuses with the warmer inner ice shell surrounding the moon’s subsurface ocean. Noah Kroese/I.NK
Two years ago, Kattenhorn and coauthor Louise Prockter of Johns Hopkins University Applied Physics Laboratory in Laurel, Md., spotted something odd as they scoured a Louisiana-sized portion of Europa mapped by NASA’s Galileo spacecraft in 1998. In the moon’s northern hemisphere, a 20,000-square-kilometer hunk of landscape was missing. Like a torn photograph placed so that the pieces overlap, Europa’s crisscrossing surface fractures didn’t properly line up.

The researchers propose that this discrepancy marks where two massive ice slabs smashed together with one sinking under the other and blending into the moon’s warmer interior ice. The action resembles a subduction zone on Earth, where one slab of crust — or tectonic plate — slides beneath another. Studying the maps, Kattenhorn and Prockter trace the boundary between the ice slabs over 1,700 kilometers along Europa’s surface where the missing landscape plunged into the moon’s interior. They further suggest that Europa’s entire surface is broken into a network of rigid segments analogous to Earth’s tectonic plates.

“The subduction rate on Europa could be very similar to what we have on Earth,” Kattenhorn says. Because of Europa’s small size, that rate could mean the whole surface recycles itself in less than 90 million years. “That’s exciting,” he says, “because it means we now have a mechanism that explains the young surface.”

This churning could benefit any possible life in Europa’s subsurface ocean, Kattenhorn says. As radiation from Jupiter bombards the moon’s surface, energy-packed organic nutrients may form from simpler chemicals. The tectonic activity offers a way for these surface nutrients to plunge down into the liquid water tucked under Europa’s thick ice shell.

Schmidt says that the finding comes at an opportune time. Europa Clipper, a proposed NASA mission to the frozen moon, recently entered its early design stages in hopes of a 2022 launch.

“Everything we’ve discovered about Europa makes it more and more Earth-like and exciting for the potential of life beyond our planet,” she says. “This research shows we need to go back to Europa and we should to go back soon.”

Editor’s Note: This story was updated September 9, 2014 to correct the main image credit.

More Stories from Science News on Planetary Science