The smallest of the Galilean moons, Jupiter's moon Europa is covered pole to pole in a sheet of water ice 10 miles thick. Below the rime of ice that glazes the planet, scientists are all but certain that Europa has a hidden ocean of liquid water, so deep that it makes the Mariana Trench look like a surface scratch. NASA's forthcoming Europa Clipper probe is planned to visit the Jovian moon in 2030 or thereabouts, and that data would be invaluable. But between now and then, scientists have a secret weapon: the James Webb Space Telescope. While the JWST excels at deep-field images, it can also pull its focus inward to look at targets within our solar system, and do fine-grained spectral analysis of its target.
This week, two papers appeared in Science from two independent teams that used data from their own Webb telescope time to arrive at the same conclusion. Both teams zeroed in on a bright spot on Europa called Tara Regio, where the ice is relatively very young and the crust so thoroughly disrupted that scientists call it "chaos terrain." There, they found evidence of carbon—specifically, carbon dioxide ice, otherwise scarce on Europa's surface. This suggests that it was dredged up from below. It could be from the churning ice, shattered and thrown around by the titanic tidal forces of Jupiter itself. Or, it could have been uncovered when an impact smashed the crust, like when an asteroid hits Jupiter and leaves a telltale "splash" of subsurface chemicals. But in either case, it's strong evidence for the hidden ocean below Europa's obscuring ice.
"Previous observations from the Hubble Space Telescope show evidence for ocean-derived salt in Tara Regio," explained Samantha Trumbo of Cornell University, lead author of one of the two papers. "Now we're seeing that carbon dioxide is heavily concentrated there as well. We think this implies that the carbon probably has its ultimate origin in the internal ocean."
"That's not a trivial thing," added Trumbo. "Carbon is a biologically essential element."
"Scientists are debating to what extent Europa’s ocean connects to its surface. I think that question has been a big driver of Europa exploration," said Geronimo Villanueva of NASA’s Goddard Space Flight Center, lead author of the second paper. "This suggests that we may be able to learn some basic things about the ocean's composition even before we drill through the ice to get the full picture."
Both teams identified the CO₂ using data from Webb’s Near-Infrared Spectrograph (NIRSpec). NIRSpec has a spatial resolution of 200 x 200 miles (320 x 320 kilometers) per pixel on the surface of Europa, which is comparable in size to our own Moon.
“These observations only took a few minutes of the observatory’s time,” said Heidi Hammel of the Association of Universities for Research in Astronomy, a Webb interdisciplinary scientist leading Webb’s Cycle 1 Guaranteed Time Observations of the solar system. “Even with this short period of time, we were able to do really big science. This work gives a first hint of all the amazing solar system science we’ll be able to do with Webb.”
