Until now, Earth was the only world in the solar system known to be shaped by giant plates of rock moving on top of the warmer, viscous layer underneath -- this is why we have mountains and volcanoes. Jupiter’s icy moon Europa may also have an active system of plate tectonics, except with ice instead of crust, researchers report in Nature Geoscience this week.
With Earth’s surface recycling process, as new surface material forms at mid-ocean ridges (an activity called sea seafloor spreading), old material is destroyed at subduction zones, where tectonic plates converge and ultimately overlap when one’s forced under the other.
The relatively young surface of Europa is riddled with cracks and ridges above what’s believed to be an ocean of liquid saltwater. Surface blocks are known to have shifted, like the ground on either side of the San Andreas fault in California, and many parts show evidence of extension -- wide bands that form as the surface is ripped apart and fresh icy material from the underlying shell moves into the newly created gap. However, no one knows how the surface accommodates the new material. Where’d all the old ice go?
Using images from NASA’s Galileo spacecraft -- which orbited Jupiter and its moons for about a decade until 2003 -- Simon Kattenhorn from the University of Idaho and Louise Prockter from Johns Hopkins looked for evidence of Europa’s icy crust expanding. They reconstructed the geological history of a portion of the surface that’s criss-crossed by ridges and fractures, piecing together what the surface looked like before the disruption occurred.
They discovered some unusual geological boundaries: Nearly 20,000 square kilometers of terrain were missing between two plates of ice from Europa’s high northern latitudes. The missing plate of ice, they suggest, sank beneath a second surface plate and into the warmer ice layers beneath. (The surface of Europa is minus 173 degrees Celsius, but deep down it’s closer to 0 degrees Celsius.) The similarity with seafloor spreading on Earth suggests that a plate tectonic system is operating on Europa.
The duo also saw ice volcanoes on the overriding plate, possibly formed through melting and absorption into Europa’s ice shell as it dove below the surface. The lack of mountains at the subduction zone implies the material was pushed into the interior rather than crumpled up, like two plates smashed together.
This illustration of subduction shows how a cold, brittle, outer portion of Europa's 20- to 30-kilometer-thick ice shell moved into the warmer shell interior, and was ultimately subsumed. A low-relief subsumption band was created at the surface of the overriding plate.
"Europa may be more Earth-like than we imagined, if it has a global plate tectonic system," Kattenhorn says in a news release. "It also implies two-way communication between the exterior and interior -- a way to move material from the surface into the ocean -- a process which has significant implications for Europa's potential as a habitable world."
Images: NASA/JPL/University of Arizona (top), Noah Kroese, I.NK (middle)