[9] Ganymede's discovery is credited to Simon Marius and Galileo Galilei, who both observed it in 1610,[2][g] as the third of the Galilean moons, the first group of objects discovered orbiting another planet.

[26] Its name was soon suggested by astronomer Simon Marius, after the mythological Ganymede, a Trojan prince desired by Zeus (the Greek counterpart of Jupiter), who carried him off to be the cupbearer of the gods.

The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moons Explorer (JUICE), which was launched in 2023.

[30] Chinese astronomical records report that in 365 BC, Gan De detected what might have been a moon of Jupiter, probably Ganymede, with the naked eye.

Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell...

I think, therefore, that I shall not have done amiss if the First is called by me Io, the Second Europa, the Third, on account of its majesty of light, Ganymede, the Fourth Callisto...[36][37]This name and those of the other Galilean satellites fell into disfavor for a considerable time, and were not in common use until the mid-20th century.

[42] The Ganymedian orbital eccentricity is somewhat puzzling; if it is not pumped now it should have decayed long ago due to the tidal dissipation in the interior of Ganymede.

[43] With a diameter of about 5,270 kilometres (3,270 mi) and a mass of 1.48×1020 tonnes (1.48×1023 kg; 3.26×1023 lb), Ganymede is the largest and most massive moon in the Solar System.

The average density of Ganymede, 1.936 g/cm3 (a bit greater than Callisto's), suggests a composition of about equal parts rocky material and mostly water ices.

The dark terrain, which comprises about one-third of the surface,[57] contains clays and organic materials that could indicate the composition of the impactors from which Jovian satellites accreted.

[9][59] The tidal flexing of the ice may have heated the interior and strained the lithosphere, leading to the development of cracks and horst and graben faulting, which erased the old, dark terrain on 70 percent of the surface.

[64] One significant feature on Ganymede is a dark plain named Galileo Regio, which contains a series of concentric grooves, or furrows, likely created during a period of geologic activity.

[9][70] [71] The precise thicknesses of the different layers in the interior of Ganymede depend on the assumed composition of silicates (fraction of olivine and pyroxene) and amount of sulfur in the core.

[20] The analysis also notes that the extreme depths involved (~800 km to the rocky "seafloor") mean that temperatures at the bottom of a convective (adiabatic) ocean can be up to 40 K higher than those at the ice–water interface.

In March 2015, scientists reported that measurements with the Hubble Space Telescope of how the aurorae moved confirmed that Ganymede has a subsurface ocean.

[70][82] In 1972, a team of Indian, British and American astronomers working in Java, Indonesia and Kavalur, India claimed that they had detected a thin atmosphere during an occultation, when it and Jupiter passed in front of a star.

Such an airglow is excited when molecular oxygen is dissociated by electron impacts,[17] which is evidence of a significant neutral atmosphere composed predominantly of O2 molecules.

HST observed two bright spots located in the northern and southern hemispheres, near ± 50° latitude, which is exactly the boundary between the open and closed field lines of the Ganymedian magnetosphere (see below).

Sodium is at least 13 times less abundant around Ganymede than around Europa, possibly because of a relative deficiency at the surface or because the magnetosphere fends off energetic particles.

[23] The permanent magnetic moment carves a part of space around Ganymede, creating a tiny magnetosphere embedded inside that of Jupiter; it is the only moon in the Solar System known to possess the feature.

[96] The Ganymedian magnetosphere has a region of closed field lines located below 30° latitude, where charged particles (electrons and ions) are trapped, creating a kind of radiation belt.

In the polar cap regions, at latitudes higher than 30°, magnetic field lines are open, connecting Ganymede with Jupiter's ionosphere.

[23] Given that Ganymede is completely differentiated and has a metallic core,[9][82] its intrinsic magnetic field is probably generated in a similar fashion to the Earth's: as a result of conducting material moving in the interior.

[9] Some research has suggested that, given its relatively small size, the core ought to have sufficiently cooled to the point where fluid motions, hence a magnetic field would not be sustained.

[71] In this respect, Ganymede is different from Callisto, which apparently failed to melt and differentiate early due to loss of the accretional heat during its slower formation.

[74][102] Alternative theories explain Ganymede's greater internal heating on the basis of tidal flexing[103] or more intense pummeling by impactors during the Late Heavy Bombardment.

[71] By comparison, the radioactive heating of undifferentiated Callisto caused convection in its icy interior, which effectively cooled it and prevented large-scale melting of ice and rapid differentiation.

[102] A study from 2020 by Hirata, Suetsugu and Ohtsuki suggests that Ganymede probably was hit by a massive asteroid 4 billion years ago; an impact so violent that may have shifted the moon's axis.

Data from those flybys were used to refine the size of Ganymede, revealing it was larger than Saturn's moon Titan, which was previously thought to have been bigger.

[133] The Russian Space Research Institute proposed a Ganymede lander astrobiology mission called Laplace-P,[134] possibly in partnership with JUICE.