With more than 400 active volcanoes, Io, a small moon of Jupiter, is recognized as the most volcanic body in the solar system. Its spectacular eruptions, so powerful that they throw plumes of lava up to kilometers into space, can even be observed from Earth using large telescopes. However, a major question remained unanswered: what is the source of this excessive volcanic activity? Thanks to recent data from NASA’s Juno probe, this 44-year-old mystery has finally been solved.
A discovery that dates back to 1979
In 1979, thanks to the Voyager 1 mission, scientist Linda Morabito made a revolutionary discovery that would mark the history of space exploration. By analyzing the images transmitted by the probe, it observes a huge plume of lava rising from the surface of Io, a moon of Jupiter.
This striking spectacle, with volcanic jets projected kilometers into space, profoundly surprised the scientific community. Until then, no one imagined that such a small star (barely larger than our own Moon) could be the seat of such intense and violent volcanic activity. This discovery challenged our knowledge of the solar system’s moons and raised a fascinating question: what colossal force could fuel such disproportionate volcanism on such a tiny world?
Since that time, researchers have wondered about the origin of this phenomenal energy. Does Io hide a global ocean of magma beneath its surface that would be capable of fueling its volcanoes? Or do the latter draw their lava from localized pockets of magma? The mystery remained unsolved…until today.
The Juno mission to the rescue
To answer this question, NASA sent the Juno probe in 2011 to study Jupiter and its moons. In 2023 and 2024, the spacecraft made two close flybys of Io, approaching only 1,500 km from its infernal surface, which made it possible to measure the gravity of the moon with unprecedented precision. This data provided an opportunity for researchers to study the moon’s interior based on how it responds to tidal forces.
The results, recently published in the journal Nature, profoundly change what we thought we knew about Io. For years, scientists tended to favor presence of global ocean of magma extending beneath the moon’s crust, somewhat like a “burning layer” beneath its entire surface. This hypothesis seemed logical, as the heat generated by tidal bending could have melted much of Io’s interior.
However, new analyzes from Juno contradict this idea. Indeed, if a global ocean existed, Io would present a gravitational deformation much more marked than that observed. Instead, the researchers discovered that Io’s interior is mostly solid. Volcanoes therefore draw their lava from localized magma chambers located beneath the surface of the moon.
To simplify: imagine a network of small pockets of magma scattered everywhere under Io’s crust, each serving as a reservoir for one or more volcanoes.
But then, why is Io so volcanic?
The answer lies in tidal bending. Io orbits Jupiter in a slightly elliptical path, meaning its distance from the planet constantly varies. At each orbit, Io is compressed then releaseda bit like an anti-stress ball that you constantly squeeze and release. This deformation continues then produces heat inside the moon which is enough to melt rock and create local magma chambers, ultimately giving rise to the 400 active volcanoes observed today.
This discovery does not only concern Io. It has major implications for the study of other moons and planets in the solar system and beyond. In particular, we know that Europa, another moon of Jupiter, and Enceladus, a moon of Saturn, probably harbor underground oceans of liquid water. Understanding the internal dynamics of Io helps refine models for these potentially habitable worlds. Exoplanets, located outside our solar system, also sometimes experience similar gravitational forces. The results obtained on Juno can therefore help to understand their formation and evolution.
