The first-ever infrared images of Ganymede’s north pole, taken on December 26, 2019 with the JIRAM instrument aboard NASA’s Juno spacecraft, show that the gigantic moon’s polar ice lacks any crystalline structure like the water ice we’re familiar with typically does here on Earth. This is a result of constant bombardment by charged plasma in Jupiter’s powerful magnetosphere being funneled down onto Ganymede’s poles by the moon’s own internally-generated magnetic field, creating what’s known as amorphous ice.
Ganymede is the largest moon in the Solar System followed by Saturn’s Titan and then Earth’s Moon. It’s one of the four Galilean satellites easily visible from Earth in binoculars or small telescopes as small points of light arranged in a line next to Jupiter, along with Callisto, Io, and Europa.
News from NASA on July 22, 2020:
The only moon in the solar system that is larger than the planet Mercury, Ganymede consists primarily of water ice. Its composition contains fundamental clues for understanding the evolution of the 79 Jovian moons from the time of their formation to today.
Ganymede is also the only moon in the solar system with its own magnetic field. On Earth, the magnetic field provides a pathway for plasma (charged particles from the Sun) to enter our atmosphere and create aurora. As Ganymede has no atmosphere to impede their progress, the surface at its poles is constantly being bombarded by plasma from Jupiter’s gigantic magnetosphere. The bombardment has a dramatic effect 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. 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.”
— Alessandro Mura, Juno co-investigator at the National Institute for Astrophysics in Rome
Knowing the top of Ganymede would be within view of Juno on Dec. 26 flyby of Jupiter, the mission team programmed the spacecraft to turn so instruments like JIRAM could see Ganymede’s surface. At the time surrounding its closest approach of Ganymede – at about 62,000 miles (100,000 kilometers) – JIRAM collected 300 infrared images of the surface, with a spatial resolution of 14 miles (23 kilometers) per pixel.