惑星科学：木星の衛星イオに浅いマグマの海はない

Io does not have a shallow global magma ocean beneath its surface, counter to previous claims, suggests a paper published in Nature this week. Observations from NASA’s Juno spacecraft, combined with all-available historic data, indicate that volcanic activity on Jupiter’s second smallest moon is unlikely to be sourced from a magma ocean just below the surface. The findings may prompt a rethink about Io’s interior as well as have implications for what is understood about planetary formation and evolution.

Io is the most volcanically active body in the solar system. The volcanic activity is thought to be powered by tidal deformations caused by variations in Jupiter’s gravitational pull, because of Io’s elliptical orbit. 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. Such measurements were taken by the Juno spacecraft in two recent flybys, and combining these observations with historic data, Ryan Park and colleagues calculate 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, the authors report. Whether there may be some regions of magma deep inside the moon remains to be determined, they add.

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, the authors conclude.