NASA's Juno spacecraft has been unraveling the enigma of Jupiter's volcanically active moon, Io, through its close encounters. Io, akin in size to our own moon, is home to approximately 400 volcanoes that persistently emit plumes and lava, coating its surface. Since July 2016, Juno has been in orbit around Jupiter, observing the gas giant and its moons. In December 2023 and February, it executed exceptionally close flybys of Io, coming within a mere 930 miles (1,500 kilometers) of the moon's surface to collect images and data.

These flybys have offered an unprecedented view of Io, including the first-ever observations of its polar regions. The findings from the analysis of this data were presented by researchers at the American Geophysical Union's annual gathering in Washington, DC, and a paper detailing some of these insights was published in the journal Nature.

"Io is truly one of the most captivating celestial bodies in our solar system," remarked Scott Bolton, co-author of the study, Juno's principal investigator, and an associate vice president at the Southwest Research Institute in San Antonio. "We are witnessing a body entirely enveloped by volcanoes across its poles and midsection, which are in a state of perpetual eruption." The new data indicates that each of Io's numerous volcanoes is likely fueled by its own reservoir of molten rock, contradicting the previously held belief of a global magma ocean beneath the surface. This revelation could alter our understanding of moons with extensive subsurface oceans, such as Jupiter's Europa and even planets beyond our solar system.

Galileo Galilei, the progenitor of modern astronomy, first sighted Io on January 8, 1610. However, its extraordinary volcanic activity was not detected until 1979 when Voyager 1 passed by Jupiter and its moons, revealing Io's dynamic and pepperoni pizza-like surface, as described by Bolton. That year, Linda Morabito, an imaging scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, became the first to identify a volcanic plume in an image of Io taken by Voyager 1. This discovery sparked a decades-long curiosity among astronomers about the origins of Io's ceaseless volcanic activity.

"Ever since Morabito's groundbreaking discovery, planetary scientists have been pondering the source of the lava that feeds the volcanoes," Bolton said. "Was there a shallow ocean of scorching magma sustaining the volcanic activity, or was it more localized? We anticipated that data from Juno's two close flybys could shed light on the workings of this tormented moon." Io orbits around Jupiter, the most massive planet in our solar system, which Bolton refers to as a "monster." Io's orbit is not perfect, causing it to sometimes come closer to Jupiter and at other times move farther away. Io completes one orbit around the planet every 42.5 hours. Jupiter's immense gravitational pull squeezes Io as it orbits, akin to a hand compressing a rubber ball, which heats the moon. This phenomenon is known as tidal flexing, the friction from tidal forces that generate internal heat.

"That's what's happening inside Io," Bolton explained. "The squeezing is generating heat, causing Io's interior to literally melt and erupt. The eruptions are relentless. It's like an unending rainstorm. It's just constantly erupting everywhere." The constant flexing from Jupiter's gravitational pull on Io generates immense energy, which could melt a portion of the moon's interior, according to Bolton. If the melting was significant enough, it would create a global magma ocean that Juno could detect with its instruments. During its close flybys, Juno gathered high-precision Doppler data, measuring Io's gravity by tracking how closely passing the moon affected the spacecraft's acceleration. This data was compared with observations from previous missions to Jupiter and its moons, such as NASA's Galileo spacecraft, as well as ground-based telescopes. The combined observations suggest a rigid, mostly solid interior beneath Io's surface, rather than a global magma ocean—solving a 45-year-old mystery that began with Voyager 1's observations.

Instead, the volcanoes are powered by more localized sources, with each having its own magma pocket beneath it. "Juno's discovery that tidal forces do not always result in global magma oceans prompts us to rethink our understanding of Io's interior," said lead study author Ryan Park, a Juno co-investigator and supervisor of the Solar System Dynamics Group at JPL, in a statement. "It has implications for our comprehension of other moons, such as Saturn's Enceladus and Europa, and even exoplanets and super-Earths.

Our new findings offer an opportunity to reconsider what we know about planetary formation and evolution." The mission has also contributed a wealth of imagery that highlights Io's "primordial fantasy land surface," as described by Heidi Becker, a planetary scientist at JPL not involved in the study. The images bring various features on Io into sharp focus, including islands spotted on vast lava lakes, such as Loki Patera, which is so expansive that astronomers liken it to a lava sea on Io's surface. The Juno spacecraft continues to provide new insights about Jupiter and its moons, having recently completed a flyby over Jupiter's swirling cloud tops on November 24. Up next, Juno will pass 2,175 miles (3,500 kilometers) above Jupiter's center on December 27, marking 645.7 million miles (1.04 billion kilometers) since the start of its investigation of Jupiter eight years ago.