Researchers from the Juno mission have finally solved a mystery that has puzzled scientists for over four decades.

In 1979, NASA Jet Propulsion Laboratory (JPL) scientist Linda Morabito discovered volcanic activity on Jupiter’s innermost moon, Io. But Io isn’t just volcanically activeâ€"it is the most volcanically active body in the entire solar systemâ€"and scientists now have a better appreciation of the subsurface workings of the Moon’s lava sources.

NASA scientists have determined that Io’s approximately 400 active volcanoes are likely fueled by individual chambers of hot magma, and not a single massive underground shared magma ocean. The discovery, published in Nature on December 12, solves a mystery that has puzzled researchers for over four decades. Their findings push scientists to reconsider not just their understanding of the Jovian moon, but of other celestial bodies, as well.

Io is about the same size as Earth’s Moon, and is marked by seemingly continuous volcanic activity. Astronomer Galileo Galilei discovered it in 1610, in 2011â€"some four centuries laterâ€"NASA launched its Juno mission. The spacecraft has been exploring the Jovian system since 2016, beaming jaw-dropping images back to Earth as well as surprising revelations about our solar system’s largest planet.

“Since Morabito’s discovery, planetary scientists have wondered how the volcanoes were fed from the lava underneath the surface,†Scott Bolton, Juno principal investigator from the Southwest Research Institute in San Antonio, who participated in the study, said in a NASA JPL statement. “Was there a shallow ocean of white-hot magma fueling the volcanoes, or was their source more localized? We knew data from Juno’s two very close flybys could give us some insights on how this tortured moon actually worked.â€

In December 2023 and February 2024, Juno engaged in said flybys of Io, reaching as close as 930 miles (1,500 kilometers) to its volcano-dotted surface. During those maneuvers, Juno gathered data to infer the moon’s gravity based on how it impacted the spacecraft’s acceleration, revealing significant information about Io’s “tidal flexing.â€

Tidal flexing happens when a celestial body is stretched and deformed by the gravitational pull of another nearby body. The friction from the movement generates heat, so the phenomenon is also known as tidal heating. Io, for example, follows an elliptical orbit around Jupiter, and the variation in distances from Jupiter (and consequently of Jupiter’s gravitational pull) along this orbit constantly squeezes the moon, causing extreme tidal flexing.

“This constant flexing creates immense energy, which literally melts portions of Io’s interior,†said Bolton. “If Io has a global magma ocean, we knew the signature of its tidal deformation would be much larger than a more rigid, mostly solid interior. Thus, depending on the results from Juno’s probing of Io’s gravity field, we would be able to tell if a global magma ocean was hiding beneath its surface.†In other words, the greater the tidal deformation, the more likely Io’s volcanoes were fueled by a larger magma source, such as an ocean, and not just individual magma chambers.

Back on Earth, the team compared Juno’s data with results from previous missions and ground telescopes. The researchers conclude that Io’s tidal flexing points to the presence of individual magma chambers rather than a single massive magma ocean.

“Juno’s discovery that tidal forces do not always create global magma oceans does more than prompt us to rethink what we know about Io’s interior,†said Ryan Park, a Juno co-investigator from JPL who co-led the study. “It has implications for our understanding of other moons, such as Enceladus and Europa, and even exoplanets and super-Earths. Our new findings provide an opportunity to rethink what we know about planetary formation and evolution.â€

It remains to be seen what other revealing information Juno will glean from the Jovian system on its next close flyby of Jupiter on December 27.