Sometimes, differentiating between a mountain range and a huge crack can be difficult, said planetary scientist Alex Patthoff. At least, it’s difficult when you’re trying to identify features on a tiny moon that’s nearly 600 million kilometers away.
This is just one of the obstacles Patthoff, a researcher at the Planetary Science Institute in Tucson, Ariz., and a team of scientists faced as they spent weeks poring over images of Jupiter’s moon Europa to create the first global geological map of its surface. Patthoff and his colleague Erin Leonard will discuss the research over two presentations next week at the Geological Society of America’s annual meeting in Seattle, Wash.
At 3,100 kilometers in diameter, Europa is the smallest of the Galilean moons, which also include Callisto, Io, and Ganymede. It’s one of the few moons in the solar system suspected to have a global ocean underneath an ice shell—a boon for scientists looking for life beyond Earth. Europa may even host geyser-like plumes similar to those on Saturn’s moon Enceladus.
Europa’s surface also seems relatively young: Although the other moons are pockmarked with craters, scientists see barely any on Europa’s surface. This absence of craters could mean that its surface continuously forms anew, thus making Europa a geologically active world.
The Voyager spacecraft first revealed Europa’s strange, red-streaked surface in 1979; then the Galileo mission discovered the internal ocean. Since then, scientists have wanted to return. NASA already has plans: In the 2020s, they intend to send an orbiter called Europa Clipper to the icy moon. And someday, they’d really like to send a lander, but that dream is entirely hypothetical at this point.
But before we can send an orbiter or a lander, scientists need to know where to point the spacecraft to collect data, which means they need a map. Now they have one.
To create the map, the researchers stitched together more than 100 images from the Voyager and Galileo missions to form a mosaic and then spent weeks identifying and categorizing surface features. These features include cracks, ridges, impact craters, regions called “chaos” where the icy surface seemed turbulently disrupted and uneven, and more.
Some problems arose—like trying to differentiate between a ridge and a crack, Patthoff said. Sometimes light plays tricks on the brain—a ridge can cast a dark enough shadow that it looks like a crack, for instance.
The finished map “really shows how the tectonics of the band structures and the chaos regions interact with each other at a global scale,” said David Senske, the deputy project scientist for the Europa Clipper mission.
The global view allowed the mappers to start formulating more and more questions: Why are some features smooth lines while others are jagged? What creates the chaos terrain? How does the internal ocean interact with the icy crust?
“Most of the weirdness, for me at least, rises from the complexity of the surface,” said Leonard, who is a graduate student at the University of California in Los Angeles and coauthor on the research. “I had only studied one region of Europa in depth, so the wide variety of features and the intense complexity of the surface was a bit shocking once I started looking at Europa’s entire surface in detail.”
There’s one question Europa scientists hope to answer, among many. How thick is its icy crust? A 20-kilometer-thick crust of ice could imply that convection occurs beneath the surface and no direct interaction takes place between the internal ocean and the surface. If the crust is just a few kilometers thick, however, the internal ocean could be interacting directly with the surface, creating the features spotted by Voyager and Galileo.
And now, armed with a shiny new map, scientists can target locations for future study to start answering these questions.
—JoAnna Wendel (@JoAnnaScience), Staff Writer
Correction, 20 October 2017: This article has been updated to reflect the true distance to Europa.