It's quite an exciting discovery that Jupiter's Ganymede has water and that the 60-mile deep ocean beneath the moon's surface was bigger than all of the water on Earth's surface. Researchers found out about the presence of water on Ganymede thanks to auroras on the moon.
Aside from being the biggest moon in the solar system, Ganymede is remarkable because it is the only one to have a magnetic field of its own. This lets it produce auroras or radiant strips of electrified gas that end up circling the moon's poles.
Since Ganymede is located so close to Jupiter, changes in the planet's magnetic field affects the moon's own magnetic field directly. Should the magnetic field on Jupiter shift when the planet rotates, Ganymede's would as well, causing the moon's auroras to shift in a related movement.
Using the Hubble Space Telescope, researchers observed Ganymede for seven hours, scrutinizing any movement. The telescope can't peer below the moon's surface but it was able to readily observe the auroras shimmering over it. Jupiter rotates every 10 hours and researchers saw the planet's magnetic field shift, which in turn made the auroras on Ganymede sway.
According to a computer model the researchers had, auroras will sway by 6 degrees if the moon were frozen. If an ocean was present under all of that ice, the salts in the water would counteract the magnetic field, leading auroras to sway just by 2 degrees.
Ganymede's auroras swayed by just 2 degrees and that was how researchers found that the moon was harboring an ocean under its frozen crust.
Researchers first thought that Ganymede was home to an ocean back in the 1970s, but it was only in 2002 that magnetic fields on the moon were measured, confirming suspicions. However, these measurements involved observation periods that were too brief, so the secondary magnetic field produced by the ocean was not detected.
The research that led to the discovery that Ganymede has an ocean has been detailed in the journal Space Physics.
Researchers include Joachim Saur, Oliver Hartkorn, Stefan Duling, Sven Simon, Lorenz Roth, Fritz Neubauer, Xianzhe Jia, Alexandre Wennmacher, Darrell Strobel, Melissa McGrath, Paul Feldman, Kurt Retherford, Fabrizio Musacchio and Ulrich Christensen.
Water is crucial to life so the discovery of Ganymede's ocean bolsters belief that other places in the universe may be able to support life other than the planet Earth.