Volcanic activity on Io — the innermost Galilean moon of Jupiter and the most volcanically active body in the Solar System — is unlikely to be sourced from a global magma ocean just below the surface, according to a new analysis of Juno and Galileo Doppler data from the Deep Space Network and astrometric observations.
The internal structure of Io as revealed by the present study. Image credit: Sofia Shen / NASA / JPL / Caltech.
Slightly larger than Earth’s Moon, Io is the most volcanically active body in the Solar System.
It is the innermost of Jupiter’s Galilean moons, which in addition to Io include Europa, Ganymede and Callisto.
Locked in a gravitational tug of war among Jupiter, Europa and Ganymede, Io is constantly being squeezed, leading to frictional heat buildup in its interior — believed to be the cause for its sustained and widespread volcanic activity.
The moon’s volcanic activity was first discovered in 1979, when Linda Morabito, an engineer on NASA’s Voyager mission, spotted an eruption plume in one of the images taken by the spacecraft during its famous Grand Tour of the outer planets.
Since then, countless observations have been made that document Io’s restless nature, from both space and Earth-based telescopes.
“Io is the innermost Galilean moon, orbiting Jupiter every 42.5 hours,” said Juno co-investigator Dr. Ryan Park from NASA’s Jet Propulsion Laboratory and colleagues.
“It has a mean diameter of 3,643 km and a bulk density of 3,528 kg/m3, making it about 5% larger in both diameter and density than the Moon.”
“Due to Io’s eccentric orbit, its distance from Jupiter varies by about 3,500 km, leading to variations in Jupiter’s gravitational pull.”
“Similar to tides on the Moon raised by Earth, these gravitational variations cause tidal deformation on Io, which is theorized to serve as the primary energy source for the intense volcanic activity and infrared emission observed on Io’s surface.”
The amount of tidal energy may be sufficient to cause melting of Io’s interior, potentially forming a subsurface magma ocean, but this theory is debated.
Measuring the extent of Io’s tidal deformation could help to determine whether the shallow magma ocean theory is plausible.
“Since Morabito’s discovery, planetary scientists have wondered how the volcanoes were fed from the lava underneath the surface,” said Juno principal investigator Dr. Scott Bolton, a researcher at the Southwest Research Institute.
“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.”
The north polar region of Io was captured by NASA’s Juno during the spacecraft’s 57th close pass of the gas giant on December 30, 2023. Image credit: NASA / JPL-Caltech / SwRI / MSSS / Gerald Eichstädt.
NASA’s Juno spacecraft made extremely close flybys of Io in December 2023 and February 2024, getting within about 1,500 km of its surface.
During the close approaches, Juno communicated with NASA’s Deep Space Network, acquiring high-precision, dual-frequency Doppler data, which was used to measure Io’s gravity by tracking how it affected the spacecraft’s acceleration.
Combining these observations with the archival Doppler data from NASA’s Galileo mission and from ground-based telescopes, the researchers calculated the extent to which Io is deformed by tidal forces.
The results are not consistent with what would be expected if a shallow global magma ocean was present, which suggests that Io has a mostly solid mantle.
Whether there may be some regions of magma deep inside the moon remains to be determined.
The findings indicate that tidal forces do not always create global magma oceans, which may have implications for our understanding of other moons, such as Enceladus or Europa.
“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,” Dr. Park said.
“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.”
The team’s paper was published this week in the journal Nature.
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R.S. Park et al. Io’s tidal response precludes a shallow magma ocean. Nature, published online December 12, 2024; doi: 10.1038/s41586-024-08442-5
