NASA's Juno spacecraft, on its mission to orbit and observe Jupiter and its moons since July 2016, has made significant flybys of Io, Jupiter's fiery moon, that are unraveling the enduring enigma of why this small satellite is the most volcanically active body in our solar system. Io, similar in size to Earth's moon, is home to an estimated 400 volcanoes that continuously release plumes and lava, coating its surface in a fiery embrace.

In December 2023 and February, Juno ventured within 930 miles (1,500 kilometers) of Io's surface, capturing unprecedented images and data that are shedding new light on the moon's volcanic nature. These close encounters have allowed scientists to observe Io's poles for the first time, offering a comprehensive view of its smoldering landscape. The results from the flyby data were presented at the American Geophysical Union’s annual meeting in Washington, DC, with some findings published in the journal Nature.

"Io is one of the most intriguing objects in the whole solar system," said study coauthor Scott Bolton, Juno principal investigator at the Southwest Research Institute in San Antonio. "We can see that this body is completely covered with volcanoes over both poles and all across its middle, constantly going off." The new data suggests that Io's numerous volcanoes are each likely powered by their own chamber of hot magma, rather than being fed by a global ocean of magma beneath the surface, a hypothesis long held by astronomers.

This revelation could alter our understanding of moons dominated by subsurface global oceans in our solar system, such as Jupiter’s moon Europa, and even planets beyond our solar system. Io's discovery by Italian astronomer Galileo Galilei, the father of modern astronomy, on January 8, 1610, was followed much later by the detection of its wild volcanic activity in 1979 when Voyager 1 flew by Jupiter and its moons. That year, Linda Morabito, an imaging scientist at NASA’s Jet Propulsion Laboratory, became the first person to identify a volcanic plume on Io, sparking a decades-long mystery about the origins of its constant volcanic activity.

"Since Morabito’s discovery, planetary scientists have wondered how the volcanoes were fed from the lava underneath the surface," Bolton said. "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."

Io's orbit around Jupiter, the largest planet in our solar system, is imperfect, bringing it closer to and farther from the gas giant during its orbit. This gravitational seesaw, known as tidal flexing, generates internal heat due to friction from tidal forces, warming Io and causing its interior to melt and erupt.

"The constant flexing of Jupiter’s tug on Io generates immense energy, which would melt part of the moon’s interior," Bolton said. "And if the melting was substantial enough, it would create a global magma ocean that Juno could use its instruments to detect."

During its close flybys, Juno captured high-precision Doppler data, measuring Io’s gravity by tracking how closely zipping past the moon affected the spacecraft’s acceleration. The data, compared with observations from previous missions and ground-based telescopes, points to a rigid, mostly solid interior beneath Io’s surface, rather than a global magma ocean, solving a 45-year mystery started by Voyager 1’s observations.

Instead, the volcanoes are powered by more local sources, with each having its own pocket of magma beneath it. "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 lead study author Ryan Park, a Juno co-investigator at JPL. "It has implications for our understanding of other moons, such as (Saturn’s) 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."

The mission has also helped capture an array of imagery that showcases Io’s "primordial fantasy land surface," bringing different features on Io into focus like never before. The Juno spacecraft continues to contribute new insights about Jupiter and its moons, with future flybys planned to further our understanding of these celestial bodies.