There Is No Liquid Magma Ocean Beneath Io’s Surface

The north polar region of Io captured by NASA’s Juno space probe (Image: NASA/JPL-Caltech/SwRI/MSSS; image processing Ted Stryk)

Contrary to previous assumptions, there is no liquid magma ocean beneath the surface of Io, the third largest of Jupiter’s moons. Instead, recent discoveries point to an almost solid mantle. These findings, published in Nature, result from collaborative research involving the University of Bologna, NASA’s Jet Propulsion Laboratory and other institutions participating in the Juno mission.

The analysis of Io draws on historical data from the Galileo mission, recent close flybys by Juno, and a series of astrometric observations.

Marco Zannoni, professor at the Department of Industrial Engineering of the University of Bologna, explained: “The two close flybys of Io were specifically designed as part of Juno’s mission to determine whether a global magma ocean actually existed on this moon. The results show that this is not the case. This finding not only revolutionises our understanding of Io’s internal structure but also provides new insights into the geological evolution of similar celestial bodies.”

Similar in size and mass to Earth’s Moon, Io stands out in Jupiter’s system due to its intense volcanic activity, which makes it the most geologically active body in the solar system. This phenomenon is fostered by Jupiter’s immense gravitational pull along Io’s eccentric orbit.

For decades, scientists had believed that Jupiter’s gravitational influence could “melt” Io’s interior, thus creating a magma ocean beneath its surface. Observations from NASA’s Galileo mission (1995 – 2003) seemed to support this theory, with magnetic induction measurements suggesting the presence of a magma ocean on Io.

However, this scenario has now been overturned by new observations made by Juno, NASA’s spacecraft that replaced Galileo and has been studying Jupiter and its moons since 2016. Juno performed two close flybys of Io at an altitude of around 1,500 kilometres, collecting data with precision higher than Galileo’s. The data collected indicates that the theorised global magma ocean does not exist. Instead, the new estimates align with the presence of a nearly solid mantle beneath Io’s surface.

Luis Gomez Casajus, researcher at the Department of Industrial Engineering of the University of Bologna, stated: “Nearly 20 years after the conclusion of the Galileo mission, with no further in-situ data, the space probe Juno has provided conclusive evidence on this matter. We cannot entirely rule out the presence of a magma ocean, but it would lie around 500 kilometres below the surface and resemble Earth’s hypothesized basal magma ocean rather than a shallow one.”

The study was published in Nature under the title “Io’s tidal response precludes a shallow magma ocean”. The research team from the University of Bologna included Luis Gomez Casajus, Marco Zannoni, Andrea Magnanini and Paolo Tortora from the Laboratory of Radio Science and Planetary Exploration. Their work, funded by the Italian Space Agency, was carried out at the Forlì Technopole, home to the Aerospace CIRI and the Department of Industrial Engineering.