Until the year 2014, scientists assumed Earth was the only planetary body with active plate tectonics. There was no evidence to suggest otherwise. Plate tectonics are responsible for the shifting continents on Earth, the rising mid-Pacific ridge, the widening Atlantic, volcanic activity on Earth, and earthquakes along faults such as the San Andreas fault in California (United States). But is Earth alone?
Planetary scientist Simon Kattenhorn of the University of Idaho in Moscow thinks the answer is no, as reported in “Plate Tectonics Spotted on Europa” (Sumner 2014). “Earth is not unique—we’ve found another body in the solar system with plate tectonics,” Kattenhorn says of Jupiter’s moon Europa. Scientists have already observed surface-reshaping on other planetary bodies, but Europa is the first found with a patchwork of drifting tectonic plates.
Europa is a curious moon. It formed over 4 billion years ago with the rest of the solar system, but its surface is only 40 million to 90 million years old, as evidenced by its small number of impact craters. We know how the new ice gets to the surface; surface fractures, appearing as dark bands, crisscross the moon where warm, fresh ice wells up to the frigid surface. But what happens to the old ice? Where does it go?
Two years ago, Kattenhorn and coauthor Louise Prockter of Johns Hopkins University Applied Physics Laboratory in Laurel, Md. scoured a Luisiana-sized portion of Europa mapped by NASA’s Galileo spacecraft in 1998. Oddly, the crisscrossing surface fractures didn’t properly line up. It appeared that a 20,000-square-kilometer hunk of landscape was missing from the northern hemisphere.
How did this happen? Kattenhorn and Prockter propose that here, two massive ice slabs smashed together. One sank under the other, blending into the moon’s warmer interior ice. The process is similar to a subduction zone on Earth, where one slab of crust—or tectonic plate—slides beneath another. The plate that slides underneath is melted and becomes part of the molten mantle. Similarly, on Europa the plate that slides underneath is melted and becomes part of the melted ocean below the ice. Kattenhorn and Prockter further suggest that Europa’s entire surface is broken into a network of rigid segments analogous to Earth’s tectonic plates.
This explains why as the fresh ice wells up to the surface through the crisscrossing fractures, older ice also sinks down below and disappears. “The subduction rate on Europa could be very similar to what we have on Earth,” Kattenhorn says. Europa is small enough that with such a network of tectonic plates, its surface could recycle 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.”
The consequence plate tectonics could have on the moon’s ability to support life is interesting. According to Kattenhorn, the rising and sinking slabs could ferry nutrients from the surface down to the subsurface ocean and thus raise the likelihood that this ocean hosts life. Astrobiologist Britney Schmidt of Georgia Tech in Atlanta says the mechanism is “very exciting for Europa’s chances for supporting life.”
The finding has come at an opportune time, according to Schmidt. 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 go back soon.”
- Sumner, Thomas. “Plate Tectonics Spotted on Europa.” Science News 4 Oct. 2014: 10.