In 1610, Galileo Galilei looked up at the night sky through a telescope of his own design. Spotting Jupiter, he noted the presence of several “luminous objects” surrounding it, which he initially took for stars. In time, he would notice that these “stars” were orbiting the planet, and realized that they were in fact Jupiter’s moons – which would come to be named Io, Europa, Ganymede and Callisto.
Of these, Ganymede is the largest, and boasts many fascinating characteristics. In addition to being the largest moon in the Solar System, it is also larger than even the planet Mercury. It is the only satellite in the Solar System known to possess a magnetosphere, boasts a thin oxygen atmosphere, and (much like its cousins Europa and Callisto) is believed to have an interior ocean.
Discovery and Naming:
Though Chinese astronomical records claim that astronomer Gan De may have spotted a moon of Jupiter (probably Ganymede) with the naked eye as early as 365 BCE, Galileo Galilei is credited with making the first recorded observation of Ganymede on January 7th, 1610 using his telescope. At the time, he named them the “Medicean Stars” – after his patron, the Grand Duke of Tuscany, Cosimo de’ Medici.
Simon Marius, a German astronomer and contemporary of Galileo’s who claimed to have independently discovered Ganymede, suggested alternative names at the behest of Johannes Kepler. However, the names of Io, Europa, Ganymede and Callisto – which were all taken from classical mythology – would come to be adopted by the 20th century.
Prior to this, the Galilean Moons were named Jupiter I through IV based on their proximity to the planet (with Ganymede designated as Jupiter III). Following the discovery of moons of Saturn, a naming system based on that of Kepler and Marius was used for Jupiter’s moons. In Greek mythology, Ganymede was the son of King Tros (aka. Ilion), the namesake of the city of Troy (Ilium).
Size, Mass and Orbit:
With a mean radius of 2634.1 ± 0.3 kilometers (the equivalent of 0.413 Earths), Ganymede is the largest moon in the Solar System and is even larger than the planet Mercury. However, with a mass of 1.4819 x 10²³ kg (the equivalent of 0.025 Earths), it is only half as massive. This is due to Ganymede’s composition, which consists of water ice and silicate rock (see below).
Ganymede’s orbit has a minor eccentricity of 0.0013, with an average distance (semi-major axis) of 1,070,400 km – varying from 1,069,200 km at periapsis to at 1,071,600 km apoapsis. Ganymede takes seven days and three hours to completes a revolution. Like most known moons, Ganymede is tidally locked, with one side always facing toward the planet.
Its orbit is inclined to the Jovian equator, with the eccentricity and inclination changing quasi-periodically due to solar and planetary gravitational perturbations on a timescale of centuries. These orbital variations cause the axial tilt to vary between 0 and 0.33°. Ganymede has a 4:1 orbital resonance with Io and a 2:1 resonance with Europa.
Essentially, this means that Io orbits Jupiter four times (and Europa twice) for every orbit made by Ganymede. The superior conjunction between Io and Europa occurs when Io is at periapsis and Europa is at apoapsis, and the superior conjunction between Europa and Ganymede occurs when Europa is at periapsis. Such a complicated resonance (a 4:2:1 resonance) is called the Laplace Resonance.
Composition and Surface Structure:
With an average density of 1.936 g/cm3, Ganymede is most likely composed of equal parts rocky material and water, mainly in the form of ice. The mass fraction of ices is between 46–50%, slightly lower than that in Callisto, with the possibility of some additional volatile ices such as ammonia being present. Ganymede’s surface has an albedo of about 43%, which suggests that water ice makes up a mass fraction of 50-90%.
Near-infrared and ultra-violet surveys have also revealed the presence of carbon dioxide, sulfur dioxide, and possibly cyanogen, hydrogen sulfate and various organic compounds. More recent data has shown evidence of salts such as magnesium sulfate and possibly sodium sulfate, which may have originated from the subterranean ocean (see below).
Ganymede’s interior appears to be fully differentiated, consisting of a solid inner core made of iron, a liquid iron and iron-sulfide outer core, a silicate mantle, and a a spherical shell of mostly ice surrounding the rock shell and the core. The core is believed to measure 500 km in radius, and has a temperature of about 1500 – 1700 K and pressure of up to 10 GPa.
The most compelling evidence for the existence of a liquid, iron-nickel-rich core is Ganymede’s intrinsic magnetic field. The convection in the liquid iron, which has high electrical conductivity, is the most reasonable model of magnetic field generation.The density of the core is believed to be 5.5 – 6 g/cm³, while the silicate mantle has an estimated density of 3.4 – 3.6 g/cm³.
This mantle is composed of silicate materials, most likely chondrites and iron. The outer ice shell is the largest layer of all, measuring an estimated 800 km (497 miles) thick. The precise thicknesses of this and other layers in the interior of Ganymede depends on the assumed composition of silicates and amount of sulfur in the core.
Scientists also believe that Ganymede has a thick ocean nestled between two layers of ice – a tetragonal layer between it and the core and a hexagonal layer above it. The presence of this ocean has been confirmed by readings taken by orbiters and through studies of how Ganymede’s aurora behaves. In short, its behavior is affected by Ganymede’s magnetic field, which in turn is affected by the presence of a large, subsurface salt-water ocean.
Ganymede’s surface is asymmetric, with the leading hemisphere is brighter than the trailing one, which is similar to Europa.
There is some speculation on the potential habitability of Ganymede’s ocean.
An analysis published in 2014, taking into account the realistic thermodynamics for water and effects of salt, suggests that Ganymede might have a stack of several ocean layers separated by different phases of ice, with the lowest liquid layer adjacent to the rocky mantle below. Water–rock contact may be an important factor in the origin of life. 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.
Universe Today has whole series of podcasts about the Solar System at Astronomy Cast.