5 fascinating facts about the solar system's largest moon that sound too strange to be true

Jupiter's moon Ganymede, with a diameter of about 3,270 miles (5,268 kilometers), is the largest moon in our solar system. Italian astronomer Galileo Galilei discovered Ganymede on January 7, 1610, alongside three other large moons of Jupiter. This marked the first time a moon was observed orbiting a planet other than Earth, a revelation that ultimately helped prove that Earth revolves around the Sun. Here are five extraordinary facts about this massive Jovian satellite.

NASA launched its Galileo mission on October 18, 1989, and deliberately crashed the spacecraft into Jupiter on September 21, 2003, to protect the planet's moons from potential Earthly contamination. During its 14-year operational life, the Galileo mission fundamentally transformed our understanding of the Jovian system. In 1996, the spacecraft discovered that Ganymede is the only moon in our solar system to have a personal magnetic field. However, because the moon orbits so close to its parent planet, Ganymede’s magnetic field remains fully embedded within the massive magnetic field of Jupiter.

The photo is an enhanced image of Ganymede taken by NASA's Juno spacecraft during a June 7, 2021, flyby on the probe's 34th close pass of Jupiter.

Thanks to its unique magnetic field, Ganymede experiences stunning auroras near its north and south poles. These auroras appear as glowing ribbons of hot, electrically charged gas. Because Ganymede's magnetic field lies within Jupiter's, whenever the gas giant's magnetic field changes, Ganymede's auroras "rock" back and forth. A team of scientists led by Joachim Saur of the University of Cologne in Germany studied this rocking motion using the Hubble Space Telescope to probe the interior of the moon.

“I was always brainstorming how we could use a telescope in other ways. Is there a way you could use a telescope to look inside a planetary body? Then I thought, the aurorae! Because aurorae are controlled by the magnetic field, if you observe the aurorae in an appropriate way, you learn something about the magnetic field. If you know the magnetic field, then you know something about the moon's interior,” Saur reflected.

The photo shows a Hubble image of Ganymede's auroral belts overlaid on a Galileo orbiter image of the moon.

NASA’s Hubble Space Telescope has found evidence suggesting Ganymede harbors an underground saltwater ocean. Remarkably, observations reveal this subsurface ocean might contain more water than all the water on Earth's surface combined. As scientists continue their search for habitable worlds, this discovery is undeniably crucial. “This discovery marks a significant milestone… In its 25 years in orbit, Hubble has made many scientific discoveries in our own solar system. A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth,” stated John Grunsfeld, associate administrator of NASA's Science Mission Directorate at NASA Headquarters in Washington, D.C.

The photo shows an artist's concept of the moon Ganymede with auroral ovals over its northern and southern mid-latitudes as it orbits the giant planet Jupiter.

NASA's Juno spacecraft launched on August 5, 2011, and reached Jupiter in 2016. During a close flyby of Ganymede, the probe's Jovian InfraRed Auroral Mapper (JIRAM) spectrometer collected unprecedented data. Analyzing this infrared data, scientists observed mineral salts and organic compounds on the surface of the moon. Juno captured detailed infrared images of Ganymede, revealing surface features smaller than 1 kilometer (0.62 miles) across. Based on those images, scientists identified several chemicals on Ganymede's surface, including hydrated sodium chloride, ammonium chloride, sodium bicarbonate, and possibly aliphatic aldehydes.

Juno co-investigator from Italy’s National Institute for Astrophysics in Rome, Federico Tosi, stated, “The presence of ammoniated salts suggests that Ganymede may have accumulated materials cold enough to condense ammonia during its formation. The carbonate salts could be remnants of carbon dioxide-rich ices.”

The photo shows an image of Ganymede taken by NASA's Galileo spacecraft.

With a diameter of about 3,270 miles (5,268 kilometers), Ganymede is physically larger than the planets Mercury and Pluto. Jupiter is approximately 484 million miles (778 million kilometers) from the Sun, and Ganymede orbits the gas giant at a distance of about 665,000 miles (1.07 million kilometers). Because of its great distance from the Sun, sunlight takes about 43 minutes to travel to Jupiter and its moons, including Ganymede.