NASA released explosive new views of the solar system’s most volcanically active world this week, exciting astronomers and prompting discoveries. Among other standout features, the images show a possible double-plumed eruption, which has never been seen in such detail before.
Io, Jupiter’s third-largest moon, reflects a rust-like, orange-yellow hue. In analyzing the images, captured Saturday by NASA’s Juno spacecraft, several scientists have already identified lava flows, volcanic plumes, mountains and the most vivid depictions yet of some of the moon’s estimated 400 surface volcanoes.
These observations add to astronomers’ steadily growing understanding of Io, which was first photographed up close during a 1979 flyby of Voyager 1, which revealed the world was volcanic. Voyager 2 made its approach just months later, and the Galileo probe followed, visiting the moon throughout the 1990s and 2000s. Now, the Juno mission, which arrived at Jupiter in 2016, is contributing to a clearer understanding of the supervolcanic moon’s tectonic and ever-changing geography.
“That’s the beauty of Io,” Jani Radebaugh, a planetary scientist at Brigham Young University who is not directly part of the Juno mission, tells the New York Times’ Katrina Miller. “Io changes every day, every minute, every second.”
On Feb. 3, the #JunoMission captured two volcanic plumes rising above Jupiter's moon Io – either two vents from one giant volcano, or two volcanoes near each other. The team will be analyzing this against additional data from Juno and other missions to get a better… pic.twitter.com/CEgZMG2rSQ— NASA Solar System (@NASASolarSystem) February 6, 2024
The Juno spacecraft completed its first flyby of Io, capturing stunning imagery, in late December 2023—and this week, it repeated the historic feat. Both times, Juno came within 930 miles of the moon’s surface, marking the closest encounters with Io achieved by any spacecraft in more than 20 years.
But Juno’s images tell just one part of the story. Other data collected by scientific instruments during the flight, which will take weeks or months for scientists to analyze, will divulge greater secrets about the moon’s environment.
As scientists press forward with studying Io, their most lofty goal is to understand “what’s really behind the engine that’s driving all the volcanoes,” Scott Bolton, a physicist at the Southwest Research Institute, which leads the Juno mission, tells the New York Times. “Because they’re all over the place.”
Unlike volcanoes on Earth, those on Io don’t resemble mountains, Julie Rathbun, a planetary scientist at Cornell University not involved with the Juno mission, tells Scientific American’s Meghan Bartels. Instead, they look more like lakes of lava.
Io is constantly under stress, influenced not only by the gravitational pull of Jupiter, but also by its moons Europa and Ganymede. These forces stretch and compress the moon, generating heat that can spur its eruptions. But how this heat is stored and transported within the moon remains unknown.
Two prevailing theories currently seek to explain Io’s volcanic activity: The moon contains either a global ocean of liquid magma or a hot metal core, scientists say. Infrared and ultraviolet cameras on the Juno spacecraft will continue to gather data from the moon, even as it travels away from it, in the hopes of moving the needle on one of the two hypotheses—or, perhaps, revealing a surprising third explanation.
By looking for changes on Io between the two recent flybys, astronomers could also uncover new clues about its volcanic activity over time.
“We’ll get a very good storyboard of how the volcanoes are varying on Io, when are new ones erupting, how big are they,” Bolton tells Scientific American.
The Juno spacecraft has now spent more than seven years orbiting Jupiter, and most of that journey was at safer distances, as its instruments measured the gas giant’s magnetic field. But recently, its so-called “extended mission”—during which the probe has shortened the circumference of its orbit around Jupiter—has allowed it to pass closer to and photograph the moons Ganymede in 2021 and Europa in 2022.
With its orbital period now cut to 33 days, these stunning flybys represent the twilight of Juno’s journey. As it spirals ever nearer to Jupiter, the spacecraft is growing more likely to be destroyed by radiation or crash into one of the planet’s interior moons, writes Sky & Telescope’s David Dickinson. Funding for the mission is set to end in September 2025.
Regardless of Juno’s fate, astronomers won’t have to wait very long until the Jovian system is explored again. In April 2023, the European Space Agency launched its Jupiter Icy Moons Explorer (JUICE), which will reach Jupiter in 2031 and explore its moons Ganymede, Callisto and Europa. And NASA’s Europa Clipper, set to launch in eight months, will get to its namesake moon even sooner, in April 2030.