James Webb Space Telescope Discovery

JWST sees Jupiter moons in a new light

With its sensitive infrared cameras and high-resolution spectrometer, the JWST is revealing new secrets of Jupiter’s Galilean satellites, in particular Ganymede, the largest moon, and Io, the most volcanically active. A spectroscopic map of Ganymede (left) derived from JWST measurements shows light absorption around the poles characteristic of the molecule hydrogen peroxide. A JWST infrared image of Io (right) shows hot volcanic eruptions at Kanehekili Fluctus (center) and Loki Patera (right). The circles outline the surfaces of the two moons. Image credit: Ganymede: Samantha Trumbo, Cornell; Io: Imke de Pater, UC Berkeley In two separate publications, astronomers who are part of JWST’s Early Release Science program report the first detection of hydrogen peroxide on Ganymede and sulfurous fumes on Io, both the result of Jupiter’s domineering influence. “This shows that we can do incredible science with the JWST on solar system objects, even if the object is really very bright, like Jupiter, but also when you look at very faint things next to Jupiter,” said Imke de Pater, professor emerita of astronomy. Samantha Trumbo, a 51 Pegasi b postdoctoral fellow at Cornell University, led the study of Ganymede, which was published July 21 in the journal Science Advances. Using measurements captured by the near infrared spectrometer (NIRSpec) on JWST, the team detected the absorption of light by hydrogen peroxide — H2O2 — around the north and south poles of the moon, a result of charged particles around Jupiter and Ganymede impacting the ice that blankets the moon. “JWST revealing the presence of hydrogen peroxide at Ganymede’s poles shows for the first time that charged particles funneled along Ganymede’s magnetic field are preferentially altering the surface chemistry of its polar caps,” Trumbo said. The astronomers argue that the peroxide is produced by charged particles hitting the frozen water ice around the poles and breaking the water molecules into fragments — a process called radiolysis — which then recombine to form H2O2. They suspected that radiolysis would occur primarily around the poles on Ganymede because, unlike all other moons in our solar system, it has a magnetic field that directs charged particles toward the poles. “Just like how Earth’s magnetic field directs charged particles from the sun to the highest latitudes, causing the aurora, Ganymede’s magnetic field does the same thing to charged particles from Jupiter’s magnetosphere,” she added. “Not only do these particles result in aurorae at Ganymede, as well, but they also impact the icy surface.” On Europa, however, the peroxide was detectable over much of the surface, perhaps, in part, because it has no magnetic field to protect the surface from the fast-moving particles zipping around Jupiter. “This is likely a really important and widespread process,” Trumbo said. “These observations of Ganymede provide a key window to understand how such water radiolysis might drive chemistry on icy bodies throughout the outer solar system, including on neighboring Europa and Callisto.” Io’s sulfurous environment In a second paper, de Pater and her colleagues report new Webb observations of Io that show several ongoing eruptions, including a brightening at a volcanic complex called Loki Patera and an exceptionally bright eruption at Kanehekili Fluctus. Because Io is the only volcanically active moon in the solar system — Jupiter’s gravitational push and pull heats it up — studies like this give planetary scientists a different perspective than can be obtained by studying volcanos on Earth. JWST measurements obtained in November 2022 overlaid on a map of Io’s surface. Thermal infrared measurements (right) show a brightening of Kanekehili Fluctus, a large and, during the observation period, very active volcanic area on Io. Spectral measurements (left) show forbidden infrared emissions from sulfur monoxide centered on the volcanic area. The coincidence confirms a theory that SO is produced in volcanic vents and, in the very thin atmosphere of Io, remain around long enough to emit the forbidden line that would normally be suppressed by collisions with other molecules in the atmosphere. Image credit: Chris Moeckel and Imke de Pater, UC Berkeley; Io map courtesy of USGS For the first time, the researchers were able to link a volcanic eruption — at Kanehekili Fluctus — to a specific emission feature produced by so-called “forbidden” transitions of the gas sulfur monoxide (SO). Sulfur dioxide (SO2) is the main component of Io’s atmosphere, coming from sublimation of SO2 ice, as well as ongoing volcanic eruptions, similar to the production of SO2 by volcanos on Earth. The volcanos also produce SO, which is much harder to detect than SO2. In particular, the forbidden SO emission line is very weak because SO is in such low concentrations and produced for only a short time after being excited. Moreover, the observations can only be made when Io is in Jupiter’s shadow, when it is easier to see the glowing SO gases. When Io is in Jupiter’s shadow, the SO2 gas in Io’s atmosphere freezes out onto its surface, leaving only SO and newly emitted volcanic SO2 gas in the atmosphere. “These observations with Webb show for the first time that this excited SO actually did come from a volcano,” de Pater said. Webb will observe Io again in August with NIRSpec. The upcoming observation and the earlier one, which took place on Nov. 15, 2022, were taken when Io was in the shadow of Jupiter so that light reflected from the planet did not overwhelm the light coming from Io. De Pater noted, too, that the brightening of Loki Patera was consistent with the observed period of eruptions at the volcano, which brighten, on average, about every 500 Earth days, with the brightening lasting for a couple of months. She determined this because it was not bright when she observed the moon with Keck in August and September 2022, nor was it bright when another astronomer observed it from April through July 2022. Only the JWST captured the event. “The Webb observations showed that actually eruptions had started, and that it was much brighter than what we had seen in September,” she said. Credit: Berkeley