Ganymede: Jupiter’s Beautiful Ice Sandwich Moon
published during a waxing crescent moon.

converted PNM file

Credit: USGS Astrogeology Science Center/Wheaton/NASA/JPL-Caltech

The first postcard from Juno, the humble spacecraft on its way to Jupiter, was taken nearly 7 million miles away, but the sight was a cozy, familiar one. Its biggest compatriots, also known as the Galilean moons, are all there — including the most massive moon in our solar system, Ganymede.


Credit: NASA/JPL-Caltech/SwRI/MSSS

Ganymede’s unkempt surface has less pizzazz than more famously dramatic moons, like Saturn’s atmospheric Titan or Jupiter’s volcanic Io. But recent studies point to a potential saltwater ocean that looms under Ganymede’s crusty dirt-colored ground.


Credit: NASA/ESA

The moon’s regal size isn’t its only claim to a galactic title: Besides surpassing our own moon and the planet Mercury in size, Ganymede is also the only moon in our solar system with a magnetic field, and a global magnetic field suggests a hot core.

To figure out its interior, a team of scientists turned to the Hubble telescope, which at first seems like an odd choice, given the telescope doesn’t exactly have penetrative capabilities. But Hubble is able to observe an aurora — and auroras trace a celestial body’s magnetic field lines, which can provide even more clues to its insides, lead study author Joachim Saur said in a statement.


Credit: NASA, ESA, and A. Felid (STScl)

Basically, the auroras’ movements point to a huge amount of saltwater under its surface. Because Ganymede is locked in Jupiter’s magnetic field, the moon’s auroral ovals rock back and forth when the planet’s magnetic fields change. The friction between the two fields can suppress just how much those auroras shift. The observed oscillations only averaged 2 degrees, but the team’s calculations that don’t incorporate an ocean point to about a 6-degree oval shift, meaning an ocean’s forces likely combats Jupiter’s field and reduces the auroral rocking.


Credit: NASA/JPL-Caltech

The ice beneath the surface may even be a sandwich layered with ice and liquid water, according to another study, The salt in the water can make it so dense, it sinks below the Ice IV layer as depicted above — the deepest and most dense ice on Ganymede. A rocky seafloor that meets salty water is prime real estate for emerging life, putting Ganymede in an exclusive cosmic water-and-potential-life club. So next time you see the moon using your binoculars, you can think of it as a massive space sub possibly crawling with growths.

The ocean is estimated to be under a nearly 100-mile crust and can have up to 25 times more water than Earth. NASA’s Galileo mission first confirmed the presence of an internal ocean in the mid-1990s.