The findings were detailed in a study published on Dec. 12 in Nature and discussed during a media briefing at the American Geophysical Union's annual meeting in Washington. This event is the largest Earth and space science gathering in the United States.

Io, roughly the size of Earth's Moon, is the most volcanically active body in the solar system, hosting around 400 volcanoes that erupt almost constantly, covering its surface with lava and volcanic deposits. The moon's volcanic activity was first observed in 1979, when Linda Morabito, an imaging scientist at NASA's Jet Propulsion Laboratory (JPL), identified a volcanic plume in images from the Voyager 1 spacecraft.

"Since Morabito's discovery, planetary scientists have wondered how the volcanoes were fed from the lava underneath the surface," said Scott Bolton, Juno principal investigator from the Southwest Research Institute in San Antonio. "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."

During Juno's close flybys in December 2023 and February 2024, the spacecraft came within 930 miles (1,500 kilometers) of Io's surface. Using NASA's Deep Space Network, the mission collected precise Doppler data, measuring Io's gravitational influence on the spacecraft's acceleration. These measurements revealed details about tidal flexing - a process in which gravitational forces from Jupiter generate internal heat within Io.

Io's orbit, which brings it close to Jupiter every 42.5 hours, results in relentless gravitational squeezing. "This constant flexing creates immense energy, which literally melts portions of Io's interior," Bolton explained. The Juno team's analysis found that Io's tidal deformation did not align with the presence of a global magma ocean, suggesting a more rigid interior with localized magma reservoirs instead.

"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, lead author of the study and supervisor of the Solar System Dynamics Group at JPL. "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."

As Juno continues its mission, it recently completed its 66th science flyby over Jupiter on Nov. 24, and its next approach to the gas giant will occur on Dec. 27. At that time, the spacecraft will fly about 2,175 miles (3,500 kilometers) above Jupiter's cloud tops, bringing its total mileage to over 645.7 million miles (1.039 billion kilometers) since entering orbit in 2016.

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