Jupiter’s moon Ganymede is, with a diameter of 5,268 km the largest moon in the Solar System, even larger than the planet Mercury. It also has the highest mass of all moons, with 2.025 times the mass of the Earth’s Moon. It is the seventh moon outward from Jupiter and one of the four so called Galilean moons (the other three are: Io, Europa and Callisto – all discovered by Galileo Galilei in the year 1610.)
Ganymede orbits Jupiter at a distance of 1,070,400 km and completes a revolution every seven days and three hours. Like most known moons, Ganymede is tidally locked, with one side of the moon always facing toward the planet. Ganymede participates in orbital resonances with Europa and Io: for every orbit of Ganymede, Europa orbits twice and Io orbits four times.
Its surface is composed of two main types of terrain. Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of the satellite. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. The cause of the light terrain’s disrupted geology was likely the result of tectonic activity brought about by tidal heating.
Ganymede is composed of approximately equal amounts of silicate rock and water ice, with additional volatile ices such as ammonia. Water ice seems to be ubiquitous on the surface. The brighter, grooved regions have a more icy composition than the dark regions. Besides water, analysis has revealed several other materials on Ganymede’s surface, like carbon- and sulfur dioxide and various organic compounds. Especially the dark regions contain clays with organic materials.
The large craters on Ganymede have almost no vertical relief and are quite flat. They lack central depressions common to craters. This is probably due to slow and gradual adjustment to the soft icy surface. These large phantom craters range from 50 to 400 km in diameter. Both bright and dark rays of ejecta exist around Ganymede’s craters — rays tend to be bright from craters in the light, grooved terrain and dark from the dark cratered.
A saltwater ocean, which could potentially also host life, is believed to exist nearly 200 km below Ganymede’s surface, sandwiched between layers of ice. The magnesium- and sodium sulfates that were also found, likely originate from this subsurface ocean. Just how deep this ocean is, and whether it exists in pockets or as a continuous band around the moon, are questions the JUICE- team hopes to answer. (JUICE is the next planned mission to the Jovian moons.)
Ganymede appears to be fully differentiated, consisting of an iron-rich, liquid core, silicate mantle and an outer, very thick (maybe 800 km thick) ice mantle which might contain some rock mixed in. The existence of the iron-rich core provides a natural explanation for the intrinsic magnetic field of Ganymede.
The convection in the liquid iron, which has high electrical conductivity, is the most reasonable model of magnetic field generation. Ganymede is the only moon in the Solar System known to possess a magnetosphere, although the meager magnetosphere is buried within Jupiter’s much larger magnetic field. But, this field is powerful enough to generate an aurora, like Earth’s.
Ganymede also has polar caps which extend to 40° latitude, likely composed of water frost. The caps’ formation is due to the migration of water to higher latitudes and bombardment of the ice by plasma. The presence of a magnetic field on Ganymede results in more intense charged particle bombardment of its surface in the unprotected polar regions; sputtering then leads to redistribution of water molecules, with frost migrating to locally colder areas within the polar terrain.
Ganymede has a tenuous oxygen atmosphere that includes O, O2, and possibly O3 (ozone). But, the atmosphere is far too thin to support life as we know it. Another minor constituent of the Ganymedian atmosphere is atomic hydrogen.
Ganymede probably formed by an accretion in Jupiter’s disk of gas and dust surrounding Jupiter after its formation. The accretion of Ganymede probably took about 10,000 years, much shorter than the 100,000 years estimated for Callisto. Ganymede formed closer to Jupiter then Callisto, where the disk was denser, which explains its shorter formation timescale.
Several probes (Pioneer 10 and 11, Voyager 1 and 2, and Galileo) flying by or orbiting Jupiter have explored Ganymede. The next planned mission to the Jovian system is the Jupiter Icy Moon Explorer (JUICE), due to launch in 2022. After flybys of the other three Galilean moons, the probe is planned to enter orbit around Ganymede in 2032.
Image Credit: NASA/JPL