The water worlds in our Solar System are really starting to add up! New data from NASA’s Hubble Space Telescope has returned the best evidence yet that there is an underground saltwater ocean on Jupiter’s moon Ganymede.
Ganymede is a fascinating spot. It’s Jupiter’s largest moon and is also the largest moon in our Solar System. It has its own magnetic field as well as an oxygen atmosphere, far too thin to support life as we know it, but thick enough to support aurorae from that magnetic field — ribbons of electrified gas that circle the north and south poles. And because Ganymede orbits close to Jupiter, the moon is also inside the gas giant’s magnetic field. When Jupiter’s magnetic field changes, it causes Ganymede’s aurorae to rock back and forth.
Scientists have known about Ganymede’s magnetic field for decades though the first definitive proof only came in 2002 from NASA’s Galileo satellite. It took a series of brief measurements of the moon’s magnetic field in 20-minute intervals but didn’t capture the rocking aurorae.
More recent observations of Ganymede’s aurorae revealed the extent of their rocking, and this tipped scientists off to the possible existence of a subterranean ocean. Saltwater is a conductive liquid that can generate a secondary magnetic field in large quantities on a moon or Planet. Studying the known magnetic field helped scientists figure out that there might be a subsurface ocean. The secondary field from Ganymede’s saltwater ocean counters Jupiter’s, visibly suppressing the rocking of the moon’s aurorae.
Preliminary observations say that Ganymede’s ocean is 60 miles deep, which is 10 times deeper than the Earth’s oceans, and buried under a 95-mile thick crust made mostly of ice.
Because water is essential for life as we know it on Earth, we tend to follow the water when it comes to looking for life on other planets. In addition to Ganymede, there’s water under the surface of Jupiter’s moon Europa, too. Anyone else want to see a multi-Jovian-moon mission launch?