With just a glance at Jupiter’s small, icy moon Europa, the James Webb Space Telescope (JWST) has bolstered the case that Earth is not our solar system’s sole habitable “ocean world.”
Europa is one of the most tantalizing places in the solar system: a wealth of observational evidence shows it hides an ocean beneath its icy crust that may possess conditions suitable for life. Now a circumstantial study by JWST—which was designed to test an instrument’s capacity more than anything else—suggests that Europa’s ocean contains carbon dioxide, a key factor in understanding the world’s potential for fostering biology.
“What we got was much, much more than we ever expected,” says Gerónimo Villanueva, a planetary scientist at NASA’s Goddard Space Flight Center in Maryland and co-author of one of two new papers reporting the JWST findings.
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The brief observations used JWST’s Near-Infrared Spectrograph (NIRSpec) instrument, which is designed to capture spectra—barcodelike measurements that quantify the amount of light at different wavelengths, which scientists can translate into information about chemical composition, temperature and other characteristics of the light’s source. The scientists were particularly interested in CO2 ice, which NASA’s Galileo mission to Jupiter had detected on Europa’s surface during flybys of the moon before the spacecraft’s demise in 2003.
Because carbon is one of six elements vital to life—along with hydrogen, nitrogen, oxygen, phosphorus and sulfur—knowing where CO2 exists on Europa and how it got there is key to understanding whether the moon might support life. “It was a big question whether this was originating from within Europa or potentially from outside of Europa,” says Samantha Trumbo, a planetary scientist at Cornell University and co-author of one of the two new papers on JWST’s observations.
Trumbo, Villanueva and their colleagues especially wanted to understand whether Europa’s surface CO2 deposits come from reservoirs of the compound dissolved in the dark waters of the subsurface ocean, which is sandwiched between overlying ice and the moon’s rocky center. Such a scenario would mark another similarity between Europa’s ocean and Earth’s deep-sea hydrothermal vent systems, where some scientists believe our planet’s life arose.
“Look at our own ocean—it’s got lots of CO2 dissolved in it,” says Bonnie Buratti, a planetary astronomer at NASA’s Jet Propulsion Laboratory. Buratti is deputy project scientist on NASA’s Europa Clipper mission, which is scheduled to explore the icy moon during the 2030s, and she was not involved in either piece of new research.
Both papers, in which separate teams independently analyze the same JWST data, found particularly strong spectral signals of CO2 at Tara Regio, a region of Europa strewn with a jumbled jigsaw mix of landscape features that scientists have dubbed “chaos terrain.”
“It’s exactly what it sounds like. It’s chaotic,” says Emily Martin, a planetary geologist at the National Air and Space Museum, who was not involved in either of the new papers.
“What we think is happening, when it comes to chaos terrain, is that at some point, the surface got warm enough to break up into these little ice rafts—in some cases, large ice rafts—so that the entirety of Tara Regio is these broken-up puzzle pieces floating around in this now-solidified icy, slushy matrix,” she says. Chaos terrain, she notes, is “a really good indicator of a location that you want to spend some more time looking at.”
Chaos terrain is especially intriguing for astrobiologists because its inferred breaking-up-and-refreezing formation process could pull water from within Europa up to the surface, allowing for easier access and study. And Tara Regio’s chaos terrain is doubly compelling: Hubble Space Telescope observations from 2017 revealed that the region also harbors sodium chloride—the compound in common table salt—which presumably comes from upwellings of subsurface water as well.
“[The information from JWST is] exciting because it tells us that the carbon we’re seeing on the surface of Europa that we can actually study is coming from the interior,” Trumbo says. “It’s very strong evidence, at least, that there’s carbon in the ocean.”
That logic makes sense to Martin, too. “It’s really a compelling argument to say the unique geologic thing that we observe here is probably related to the unique compositional thing that we see here,” she says.
The new findings do include one item that might come as a disappointment. Some observations in recent years have suggested that, much like Saturn’s icy moon Enceladus, Europa may shoot plumes of salty water out into space, which would allow nearby spacecraft to directly sample it. But the evidence for such outbursts at Europa has been much scarcer than at Enceladus, where NASA’s Cassini spacecraft flew right through a plume in 2008.
But in JWST’s glance at Europa, there’s no clear evidence for the presence of any plumes. That null result doesn’t necessarily mean that no plumes exist—they may well be sporadic or smaller than expected—but it’s far from the clinching detection scientists might have hoped for. “We cannot say the plumes exist or not on this object, that’s for sure, because we only have one specific measurement done now—and at that particular moment, we didn’t see the plume,” Villanueva says.
The new findings are only the beginning of a resurgent focus on Europa, however. Trumbo and her colleagues have already won observing time on JWST to revisit the icy moon, and this time they’ll let the telescope’s gaze linger long enough to gain a view of Europa’s entire surface.
And no matter how spectacular JWST’s observations prove to be, they will soon be superseded by others far closer and more intimate. The European Space Agency’s Jupiter Icy Moons Explorer (JUICE) mission launched this spring and will arrive in the Jupiter system in 2031; it will make two close passes by Europa during its mission. And NASA’s Europa Clipper, which Buratti works on, is due to launch in October 2024; beginning in 2031, it will make nearly 50 flybys of Jupiter’s mysterious icy moon. Buratti says that although Clipper’s basic trajectory is set, there’s still time to fine-tune its observing plan to squeeze as much science as possible from any close passes of Tara Regio.
“The scientists are busily working away on trying to optimize our observations, so this message from JWST is a gift,” Buratti says. “It really is a gift.”