However, Gan De reported the color of the companion as reddish, which is puzzling since the moons are too faint for their color to be perceived with the naked eye. Size comparison of Earth, the Moon (top left), and Ganymede (bottom left) History Ĭhinese astronomical records report that in 365 BC, Gan De detected what might have been a moon of Jupiter, probably Ganymede, with the naked eye. After flybys of all three icy Galilean moons, it is planned to enter orbit around Ganymede. The next planned mission to the Jovian system is the European Space Agency's Jupiter Icy Moon Explorer (JUICE), which was launched in 2023. The Voyager probes, Voyager 1 and Voyager 2, refined measurements of its size, while Galileo discovered its underground ocean and magnetic field. Beginning with Pioneer 10, several spacecraft have explored Ganymede. 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. Ganymede's discovery is credited to Simon Marius and Galileo Galilei, who both observed it in 1610, as the third of the Galilean moons, the first group of objects discovered orbiting another planet. Whether Ganymede has an ionosphere associated with its atmosphere is unresolved.

Atomic hydrogen is a minor atmospheric constituent. Ganymede has a thin oxygen atmosphere that includes O, O 2, and possibly O 3 ( ozone). The meager magnetic field is buried within Jupiter's far larger magnetic field and would show only as a local perturbation of the field lines. Ganymede's magnetic field is probably created by convection within its liquid iron core, also created by Jupiter's tidal forces. Possessing a metallic core, it has the lowest moment of inertia factor of any solid body in the Solar System.

Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively. 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. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of it. Its surface is composed of two main types of terrain. 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. Ganymede is composed of approximately equal amounts of silicate rock and water. Like Titan, it is larger than the planet Mercury, but has somewhat less surface gravity than Mercury, Io or the Moon. It is the largest Solar System object without an atmosphere, despite being the only moon of the Solar System with a magnetic field. Ganymede, or Jupiter III, is the largest and most massive natural satellite of Jupiter as well as in the Solar System, being a planetary-mass moon.

Io moon mass