Ganymede // Jupiter III - Third Moon of Jupiter

Ganymede /ˈɡænɪmiːd/, a satellite of Jupiter (Jupiter III), is the largest and most massive of the Solar System's moons. The ninth-largest object in the Solar System, it is the largest without a substantial atmosphere. It has a diameter of 5,268 km (3,273 mi), making it 26% larger than the planet Mercury by volume, although it is only 45% as massive.[16] Possessing a metallic core, it has the lowest moment of inertia factor of any solid body in the Solar System and is the only moon known to have a magnetic field. Outward from Jupiter, it is the seventh satellite and the third of the Galilean moons, the first group of objects discovered orbiting another planet.[17] Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.
Ganymede is composed of approximately equal amounts of silicate rock and water. It is a fully differentiated body with an iron-rich, liquid core, and an internal ocean that may contain more water than all of Earth's oceans combined.[18][19][20][21] 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 it. 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 is not fully known, but was likely the result of tectonic activity due to tidal heating.[8]
Ganymede's magnetic field is probably created by convection within its liquid iron core.[22] The meager magnetic field is buried within Jupiter's much larger magnetic field and would show only as a local perturbation of the field lines. Ganymede has a thin oxygen atmosphere that includes O, O2, and possibly O3 (ozone).[15] Atomic hydrogen is a minor atmospheric constituent. Whether Ganymede has an ionosphere associated with its atmosphere is unresolved.[23]
Ganymede's discovery is credited to Galileo Galilei, the first to observe it, on January 7, 1610.[1][g] 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.[25] Beginning with Pioneer 10, several spacecraft have explored Ganymede.[26] The Voyager probes, Voyager 1 and Voyager 2, refined measurements of its size, while Galileo discovered its underground ocean and magnetic field. The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moon Explorer (JUICE), due to launch in 2022. After flybys of all three icy Galilean moons, it is planned to enter orbit around Ganymede.
Orbit and rotation
Laplace resonance of Ganymede, Europa, and Io (conjunctions are highlighted by color changes)
Ganymede orbits Jupiter at a distance of 1,070,400 km, third among the Galilean satellites,[17] and completes a revolution every seven days and three hours. Like most known moons, Ganymede is tidally locked, with one side always facing toward the planet, hence its day is seven days and three hours.[36] Its orbit is very slightly eccentric and 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. The ranges of change are 0.0009-0.0022 and 0.05-0.32°, respectively.[37] These orbital variations cause the axial tilt (the angle between rotational and orbital axes) to vary between 0 and 0.33°.[10]
Ganymede participates in orbital resonances with Europa and Io: for every orbit of Ganymede, Europa orbits twice and Io orbits four times.[37][38] Conjunctions (alignment on the same side of Jupiter) between Io and Europa occur when Io is at periapsis and Europa at apoapsis. Conjunctions between Europa and Ganymede occur when Europa is at periapsis.[37] The longitudes of the Io-Europa and Europa-Ganymede conjunctions change with the same rate, making triple conjunctions impossible. Such a complicated resonance is called the Laplace resonance.[39] The current Laplace resonance is unable to pump the orbital eccentricity of Ganymede to a higher value.[39] The value of about 0.0013 is probably a remnant from a previous epoch, when such pumping was possible.[38] 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.[39] This means that the last episode of the eccentricity excitation happened only several hundred million years ago.[39] Because Ganymede's orbital eccentricity is relatively low-on average 0.0015[38]-tidal heating is negligible now.[39] However, in the past Ganymede may have passed through one or more Laplace-like resonances[h] that were able to pump the orbital eccentricity to a value as high as 0.01-0.02.