Jupiter hasbut one in particular reigns supreme: . The biggest Jovian moon is larger than both Mercury and Pluto and has its own magnetic field. It's an unusual world, with an internal ocean, incredibly thin atmosphere and an icy shell and it has fascinated astronomers since Galileo first discovered it in 1610.
Galileo's discovery was monumental, but he didn't quite have the tools at his disposal to really examine the moon. But 410 years later, NASA does. On On Dec. 26, 2019, the Jovian Infrared Auroral Mapper (JIRAM) in snapped the first images of the moon's icy north pole, mapping the region for the first time.
The images show an unusual form of ice exists at the pole, a type that we don't encounter on Earth, because the magnetic field filters particles from the sun -- plasma -- toward it. Without a decent atmosphere, it's basically raining plasma down on Ganymede's ice.
"The JIRAM data show the ice at and surrounding Ganymede's north pole has been modified by the precipitation of plasma," said Alessandro Mura, a co-investigator on Juno from the National Institute for Astrophysics in Rome, in a NASA release.
"It is a phenomenon that we have been able to learn about for the first time with Juno because we are able to see the north pole in its entirety."
The plasma prevents the ice from taking on the structure we are used to seeing on Earth. When water freezes here it forms a crystalline structure -- layer upon layer of water molecules form a lattice of hexagonal rings. At Ganymede's poles, the ice takes on an amorphous form. Its molecular structure is disordered; there's no lattice, no rings. Analyzing and understanding these structures will provide further clues to the formation of Jupiter's moons and the forces at play during their evolution.
And Juno should receive some help in the next decade. The European Space Agency will seek to explore Ganymede when it launches JUICE -- the Jupiter Icy Moons Explorer spacecraft in 2022. By 2029, it will reach Jupiter and it should start performing close up science at Ganymede around 2032. NASA will explore another interesting Jupiter moon around the same time with, an orbiting spacecraft set to .