Hubble detects evidence of underground ocean on Ganymede

BBC : Jupiter’s Ganymede is the largest moon in the solar system and is also unusual in that it has a significant magnetic field. Joachim Saur of the University of Cologne in Germany and his colleagues have been using the Hubble Space Telescope to study Ganymede’s magnetic field and aurora. The interactions between the magnetic fields of Jupiter and Ganymede cause wobbles in Ganymede’s field that can be seen in the movements of the aurora. Saur’s team compared their observations with various models of Ganymede’s internal structure. They found that Jupiter’s magnetic field creates a secondary fluctuation in Ganymede’s aurora that would be easily explained by interacting with a body of saltwater. The Hubble data most closely match a model of the moon that includes a subsurface ocean, though that ocean could be anywhere from 100 km deep with a salt content of 5 kg/L to just 10 km deep with 50 kg/L salt concentration.