Scientists used Hubble to observe the aurorae caused by Ganymede’s electric field as it interacted with that of Jupiter. The evidence derived from that suggests there must be a large amount of saltwater beneath Ganymede, probably 100 kilometres deep.
Identifying liquid water is crucial in the search for habitable worlds beyond Earth and for the search for life as we know it.
“This discovery marks a significant milestone, highlighting what only Hubble can accomplish,” said John Grunsfeld, assistant administrator of NASA’s Science Mission Directorate. “A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth.”
The innovative way of observing Ganymede was the brainchild of Joachim Saur of the University of Cologne in Germany.
“I was always brainstorming how we could use a telescope in other ways,” said Saur. “Is there a way you could use a telescope to look inside a planetary body? Then I thought, the aurorae! Because aurorae are controlled by the magnetic field, if you observe the aurorae in an appropriate way, you learn something about the magnetic field. If you know the magnetic field, then you know something about the moon’s interior.”
If a saltwater ocean were present, Jupiter’s magnetic field would create a secondary magnetic field in the ocean that would counter Jupiter’s field. This “magnetic friction” would suppress the rocking of the aurorae. This ocean fights Jupiter’s magnetic field so strongly that it reduces the rocking of the aurorae to 2 degrees, instead of 6 degrees if the ocean were not present.
Scientists estimate the is buried under a 150-kilometre crust, consisting mostly of ice.
Scientists first suspected an ocean in Ganymede in the 1970s.
A related story from Cosmos: The Habitable Zone