The thinking behind the study is that if a saltwater ocean were there, Jupiter’s magnetic
field would create a secondary
magnetic field
in that ocean.
This secondary field would counter Jupiter’s own—and as a result, suppress the rocking of the aurorae.
This suppression indeed happens, and so strongly that it reduces the rocking of the aurorae by some two-thirds, the investigators calculated. As a result, they estimate the ocean is 60 miles (100 kilometers) thick — 10 times deeper than Earth’s oceans — and is buried under a 95-mile (150-kilometer) crust of mostly ice.
Scientists led by Joachim Saur of the University of Cologne in Germany came up with the idea of using Hubble to learn more about the inside of the moon.