Source: Journal of Geophysical Research: Planets
Trapped in a gravitational push and pull between Jupiter and other Jovian moons, Io is constantly being stretched and compressed. Heat generated by these contortions has melted pockets of the moon’s interior so much that Io is our solar system’s most volcanically active body.
The James Webb Space Telescope (JWST) recently opened up new opportunities to get to know Io. Using data from its Near Infrared Spectrograph—which sees wavelengths corresponding to different compositions and temperatures—de Pater et al. have made new discoveries about Io’s volcanoes and atmosphere.
The researchers first looked at Io in November 2022 and found an extremely energetic volcanic eruption in the vicinity of the lava flow field Kanehekili Fluctus. These observations revealed, for the first time, that some volcanoes on Io emit an excited form of sulfur monoxide gas, confirming the team’s 2-decade-old hypothesis. JWST also detected an increase in thermal emissions at the massive lava lake in Loki Patera, generated by the lake’s thick, solid surface crust sinking into the molten lava beneath.
Nine months later, in August 2023, the researchers had another chance to peer at the same two regions on Io with JWST. Just as in 2022, Io was in Jupiter’s shadow, making it possible to capture emissions at wavelengths that might otherwise be obscured by sunlight.
The new images captured infrared thermal emissions from the same two regions. However, lava flows from the 2022 Kanehekili region’s eruption had spread to cover more than 4,300 square kilometers—about 4 times the area they covered in 2022. At Loki Patera, a new crust had formed and cooled, in keeping with the lake’s behavior over the past few decades.
The new images also captured sulfur monoxide emissions in Io’s atmosphere above Kanehekili Fluctus—as well as above two other regions without a clear volcanic association, which the researchers attribute to “stealth volcanism.” In another first, the 2023 images revealed sulfur gas emissions at wavelengths never before seen in Io’s atmosphere. Instead of being concentrated in patchy spots like the sulfur monoxide was, the sulfur gas was distributed more evenly across part of the northern hemisphere.
The data suggest that these sulfur emissions did not come from sulfur atoms spewed out of volcanoes but were mainly produced by electrons from Io’s plasma torus—an area around its orbit with high levels of charged particles—penetrating Io’s mostly sulfur dioxide atmosphere and thereby exciting sulfur atoms upon impact. The angle at which JWST observed Io, combined with the northern hemisphere’s location relative to the plasma torus, explained why the detected emissions were concentrated over the northern hemisphere. Alongside data from the Keck Observatory and the Hubble Space Telescope, the new findings suggest this plasma torus–atmosphere system remains quite stable over decades. (Journal of Geophysical Research: Planets, https://doi.org/10.1029/2024JE008850, 2025)
—Sarah Stanley, Science Writer
