Jupiter’s closest Galilean moon, Io, is the most volcanically active object in the solar system. Kneaded and heated by Jupiter’s rhythmically reversing tidal stresses, the moon hosts hundreds of active volcanoes, some of which spew lava and sulfur dioxide more than 400 kilometers high.
Scientists have long argued over whether Io’s intense volcanic activity is fueled by an underground ocean of magma. Now a new analysis of data from the Galileo spacecraft provides fresh fodder for that debate, suggesting that such an ocean could be absent.
Galileo flew past Io several times in the late 1990s and early 2000s, including a risky pass between Io and Jupiter that exposed the spacecraft to intense radiation and temporarily damaged its computers. Despite the technical glitch, Galileo collected valuable measurements from the hot, ionized gas—or plasma—that escapes from Io into Jupiter’s magnetosphere, feeding a donut-shaped ring of dense plasma around Jupiter called a torus. It also captured interactions between Jupiter’s powerful magnetospheric plasma and Io’s thin atmosphere, which produce brilliant auroras.
On the basis of Galileo’s data, some scientists have concluded that Io’s own magnetic field is driven by a subterranean magma ocean. As Jupiter’s magnetic field sweeps back and forth across the moon, the theory goes, it generates electrical currents within a global conductive ocean of molten rock. This process produces its own magnetic field, which contributes to massive perturbations in the surrounding magnetic field of Io.
But in the new study, Blöcker et al. argue that the measured magnetic perturbations in Io’s environment come instead from asymmetries in Io’s thin atmosphere. Io’s entire atmosphere collapses into frost on a daily basis, whenever it falls into Jupiter’s shadow. The atmosphere is also larger and denser on the side of the moon that faces away from Jupiter. On a local level, massive volcanoes also make Io’s atmosphere irregular. Computer models used in past studies haven’t focused much on these asymmetries, leaving room for error, the team argues.
To remedy that omission, the scientists used the same type of computer model that past groups have employed, called a 3-D magnetohydrodynamic model. Instead of focusing on Io’s interior, they focused solely on the moon’s atmosphere. Their goal was to see if an asymmetrical distribution of gas in Io’s atmosphere alone, independent of any contribution from a global conductive magma ocean in Io’s interior, could produce magnetic perturbations similar to those observed by Galileo.
The model produced the same magnetic field perturbations in Io’s simulated atmosphere as those observed by Galileo. The study doesn’t entirely rule out the possibility that an underground magma ocean could exist on Io, but it suggests that there’s no need for one. Instead, the moon’s unusual atmosphere accounts for Galileo’s observations all on its own. (Journal of Geophysical Research: Space Physics, https://doi.org/10.1029/2018JA025747, 2018)
—Emily Underwood, Freelance Writer