Europa’s ice shell could be quaking

Even before astronomers noticed last year that Europa was spouting jets of water vapor from its icy surface, they thought there was something shifty about Jupiter’s moon. While the 66 other Jovian moons are pitted with craters, Europa sports some unusual blemishes: an abundant crisscrossing of ridges tens of kilometres long. Many are abruptly interrupted by smooth ice patches.

Two geologists think they can explain why. Backed by photos taken by the Galileo space probe, they suggest Europa’s thick shell isn’t continuous but is made up of distinct plates of ice. These plates move away from each other in some places, exposing gaps which are then filled by deeper ice rising upward. In other places they slide over each other and push surface ice downward and form ridges.

“We knew that stuff has been moving over the surface, and up from beneath and breaking through, but we weren’t able to figure where all the older stuff was going,” said study coauthor Dr. Louise Prockter, a planetary scientist at Johns Hopkins. “We’ve found for the first time evidence that material is going back into the interior.” The study was published last month in Nature Geoscience.

On Earth, this kind of tectonic activity replenishes compounds necessary for life, such as carbon dioxide, by letting them move up from the interior through fissures to the surface. Now, scientists say a similar mechanism could apply to Europa. Astronomers think the moon harbors a subsurface ocean of liquid water that feeds the vapor plumes, and could be habitable.

“It’s certainly significant to find another solid body in the solar system that undergoes some kind of surface recycling,” said Peter Driscoll, a planetary scientist at the University of Washington who was not involved in the study.

Prockter, together with Simon Kattenhorn, a geologist at the University of Idaho, Moscow, worked with photographs of a part of Europa’s surface covering 20,000 km2. The pictures were shot by Galileo when it orbited Jupiter from 1995 to 2003.

“We go in using something like Photoshop and start cutting the image up,” Dr. Prockter explained. They then pieced them back together so that the crisscrossing ridges lined up end-to-end, and compared what they had to the surface as it is today.

“Once we started doing the reconstruction, we ended up with a big gap right in the middle,” she said.

The researchers concluded the missing bit had dived beneath another plate.

Although only some of Galileo’s photographs were at a resolution high enough to be useful for the study, Dr. Prockter said it was unlikely that their finding was a one-off because signs of displacement were visible all over Europa’s surface.

Nevertheless, Dr. Driscoll cautioned against using Earth’s tectonic activity as a model for Europa’s. “There are a number of missing features” that define tectonics on Earth, he said, such as arc volcanos and continents. “And many of the properties of Earth’s features may not be expected for an icy shell like Europa, where the materials are extremely different.”

A better gauge of these disparities might be a probe to the Jovian moon that NASA has planned for the mid-2020s.

“I think the timing right now is very important,” said Candice Hansen, a member of NASA’s Planetary Science Subcommittee. She says the Europa study will help scientists working on the probe secure the requisite funding and commitment from Congress.

“I am very enthusiastic about a mission to Europa, and this exciting result is one more reason to go,” she said.

Artist's concept of the Europa Clipper mission investigating Jupiter's icy moon Europa.
Artist’s concept of the Europa Clipper mission investigating Jupiter’s icy moon Europa. Image credit: NASA/JPL-Caltech