Jupiter's moon Ganymede ("GAN uh meed") is the largest moon in our solar system and the only moon with its own magnetic field. The magnetic field causes auroras, which are ribbons of glowing, electrified gas, in regions circling the moon’s north and south poles. When Jupiter’s magnetic field changes, the aurorae on Ganymede also change, “rocking” back and forth.
Ganymede also has large, bright regions of ridges and grooves that slice across older, darker terrains. These grooved regions are a clue that the moon experienced dramatic upheavals in the distant past.
In 2015, NASA's Hubble Space Telescope found the best evidence to date for an underground saltwater ocean on Ganymede. The subterranean ocean is thought to have more water than all the water on Earth's surface.
Ganymede has three main layers. A sphere of metallic iron at the center (the core, which generates a magnetic field), a spherical shell of rock (mantle) surrounding the core, and a spherical shell of mostly ice surrounding the rock shell and the core. The ice shell on the outside is very thick, and about 500 miles (800 kilometers) thick. The surface is the very top of the ice shell. Though it is mostly ice, the ice shell might contain some rock mixed in. Scientists believe there must be a fair amount of rock in the ice near the surface. Ganymede's magnetic field is embedded inside Jupiter's massive magnetosphere.
Astronomers using Hubble found evidence of a thin oxygen atmosphere on Ganymede in 1996. The atmosphere is far too thin to support life as we know it.
In 2004, scientists discovered irregular lumps beneath the icy surface of Ganymede. The irregular masses may be rock formations, supported by Ganymede's icy shell for billions of years. This tells scientists that the ice is probably strong enough, at least near the surface, to support these possible rock masses from sinking to the bottom of the ice. However, this anomaly could also be caused by piles of rock at the bottom of the ice.
Spacecraft images of Ganymede show the moon has a complex geological history. Ganymede's surface is a mixture of two types of terrain. Forty percent of the surface of Ganymede is covered by highly cratered dark regions, and the remaining sixty percent is covered by a light grooved terrain, which forms intricate patterns across Ganymede.
On June 7, 2021, NASA's Juno spacecraft made a flyby of Ganymede. The photos – one from the Jupiter orbiter’s JunoCam imager and the other from its Stellar Reference Unit star camera – show the surface in remarkable detail, including craters, clearly distinct dark and bright terrain, and long structural features possibly linked to tectonic faults.
“This is the closest any spacecraft has come to this mammoth moon in a generation,” said Juno Principal Investigator Scott Bolton of the Southwest Research Institute in San Antonio. “We are going to take our time before we draw any scientific conclusions, but until then we can simply marvel at this celestial wonder – the only moon in our solar system bigger than the planet Mercury.”
Ganymede's grooved terrain probably is the result of tensional faulting or the release of water from beneath the surface. Groove ridges as high as 2,000 feet (700 meters) have been observed and the grooves run for thousands of miles across Ganymede's surface. The grooves have relatively few craters and probably developed at the expense of the darker crust. The dark regions on Ganymede are old and rough, and the dark cratered terrain is believed to be the original crust of the satellite. Lighter regions are young and smooth (unlike Earth's Moon). The largest area on Ganymede is called Galileo Regio.
The large craters on Ganymede have almost no vertical relief and are quite flat. They lack central depressions common to craters often seen on the rocky surface of the Moon. This is probably due to a slow and gradual adjustment to the soft icy surface. These large phantom craters are called palimpsests, a term originally applied to reused ancient writing materials on which older writing was still visible underneath newer writing. Palimpsests range from 30 to 250 miles (50 to 400 kilometers) in diameter. Both bright and dark rays of ejecta exist around Ganymede's craters – rays tend to be bright from craters in the grooved terrain and dark from the dark cratered terrain.
Ganymede was discovered by Galileo Galilei on Jan. 7, 1610. The discovery, along with three other Jovian moons, was the first time a moon was discovered orbiting a planet other than Earth. The discovery of the four Galilean satellites eventually led to the understanding that planets in our solar system orbit the Sun, instead of our solar system revolving around Earth.
Simon Marius probably made an independent discovery of the moons at about the same time that Galileo did, and he may have unwittingly sighted them up to a month earlier, but Galileo was the first to publish his discovery.
How Ganymede Got its Name
Ganymede is named after a boy who was made cupbearer for the ancient Greek gods by Zeus – Jupiter to the Romans.
Galileo originally called Jupiter's moons the Medicean planets, after his patrons, the Medici family. He referred to the individual moons numerically as I, II, III, and IV. Galileo's naming system was used for a couple of centuries.
It wouldn't be until the mid-1800's that the names of the Galilean moons, Io, Europa, Ganymede, and Callisto, would be officially adopted, and only after it became apparent that naming moons by number would be very confusing as new additional moons were being discovered.