"A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth,” says NASA’S Science Mission Directorate, John Grunsfeld.
Ganymede is the largest moon in our solar system. In fact, it is larger than the planet Mercury. It is the only one to have a magnetic field, which is indicative of a magnetic core. The tidal forces of Jupiter and its other moons heat Ganymede’s interior, which causes enough melting of the core to generate a magnetic field.
These magnetic fields cause aurorae, belts of electrified gas that circles the poles. Due to changes in Jupiter’s magnetic field as it rotates, Ganymede’s auroral belts should shift by six degrees every 10 hours (a day on Jupiter).
Joachim Saur, of the University of Cologne in Germany, led a team of scientists to investigate the aurorae of Ganymede. “If you know the magnetic field,” said Saur, “then you know something about the moon’s interior.”
After pointing the Hubble telescope at Ganymede for 5 hours, the researchers found the aurorae barely moved. The aurorae experienced only one-third of the expected shift, which meant something was dampening the effect of Jupiter’s magnetic field.
The best answer for what causes this dampening effect is the presence of a saltwater ocean on Ganymede. The charged particles in the ocean produce a secondary magnetic field that opposes Jupiter’s.
Ganymede has been suspected to have a sub-surface ocean since the 1970’s, but it wasn’t until 2002, when NASA’S Galileo mission measured the moon’s magnetic field, that there was any supporting evidence.
With NASA’s Europa Clipper and the European Space Agency’s JUICE set to launch in the early 2020’s, to investigate the Jovian moons, it is an exciting time to be an Earthling.