Ganymede (moon)

Ganymede, or Jupiter III, is the <a href="/facts/List_of_Solar_System_objects_by_size/Aob1rVgh">largest and most massive</a> <a href="/facts/Natural_satellite/AE4KQKbq">natural satellite</a> of <a href="/facts/Jupiter/95m7XjMT">Jupiter</a>, and in the <a href="/facts/Solar_System/QIcLSazQ">Solar System</a>. Despite being the only moon in the Solar System with a substantial <a href="/facts/Magnetic_field/Ta8Ev588">magnetic field</a>, it is the largest Solar System object without a substantial atmosphere. Like <a href="/facts/Saturn/D54LcGJ9">Saturn</a>'s largest moon <a href="/facts/Titan_(moon)/lg0l9PCH">Titan</a>, it is larger than the planet <a href="/facts/Mercury_(planet)/lVa1GIyX">Mercury</a>, but has somewhat less <a href="/facts/Surface_gravity/Q5QVbA7e">surface gravity</a> than Mercury, <a href="/facts/Io_(moon)/p3Uc8FmK">Io</a>, or the <a href="/facts/Moon/zKHqEMPb">Moon</a> due to its lower density compared to the three. Ganymede <a href="/facts/Orbit/nD2TPySG">orbits</a> Jupiter in roughly seven days and is in a 1:2:4 <a href="/facts/Orbital_resonance/eQLMfBWu">orbital resonance</a> with the moons <a href="/facts/Europa_(moon)/2A9nleK3">Europa</a> and <a href="/facts/Io_(moon)/p3Uc8FmK">Io</a>, respectively.
Ganymede is composed of <a href="/facts/Silicate/7NdthSRM">silicate rock</a> and <a href="/facts/Water_substance/FFsl65l1">water</a> in approximately equal proportions. It is a fully <a href="/facts/Planetary_differentiation/fvjC1107">differentiated</a> body with an iron-rich, <a href="/facts/Planetary_core/LyQ7DbAl">liquid metallic core</a>, giving it the lowest <a href="/facts/Moment_of_inertia_factor/yUEhx8o0">moment of inertia factor</a> of any solid body in the Solar System. Its <a href="/facts/Planetary_oceanography/uckThwLf">internal ocean</a> potentially contains more water than all of Earth's oceans combined. 
Ganymede's <a href="/facts/Magnetic_field/Ta8Ev588">magnetic field</a> is probably created by <a href="/facts/Convection/E8vsCPhu">convection</a> within its core, and influenced by tidal forces from Jupiter's far greater magnetic field. Ganymede has a thin <a href="/facts/Oxygen/acyn6o8r">oxygen</a> <a href="/facts/Atmosphere/oUpLaGRF">atmosphere</a> that includes O, O2, and possibly O3 (<a href="/facts/Ozone/o0E5gn2p">ozone</a>). <a href="/facts/Atomic_hydrogen/ncVpPbuE">Atomic hydrogen</a> is a minor atmospheric constituent. Whether Ganymede has an <a href="/facts/Ionosphere/mT9tTBE5">ionosphere</a> associated with its atmosphere is unresolved.
Ganymede's surface is composed of two main types of terrain, the first of which are lighter regions, generally crosscut by extensive grooves and ridges, dating from slightly less than 4 billion years ago, covering two-thirds of Ganymede. The cause of the light terrain's disrupted geology is not fully known, but may be the result of <a href="/facts/Tectonics/jPcFacFP">tectonic</a> activity due to <a href="/facts/Tidal_heating/Ntl6sXJ0">tidal heating</a>. The second terrain type are darker regions saturated with <a href="/facts/Impact_crater/nWntooh4">impact craters</a>, which are dated to four billion years ago.
Ganymede's discovery is credited to <a href="/facts/Simon_Marius/yb54huCF">Simon Marius</a> and <a href="/facts/Galileo_Galilei/98VT2z3v">Galileo Galilei</a>, who both observed it in 1610, as the third of the <a href="/facts/Galilean_moon/aN6tLUFc">Galilean moons</a>, the first group of objects discovered orbiting another planet. Its name was soon suggested by astronomer Simon Marius, after the <a href="/facts/Greek_mythology/M7k4FvW8">mythological</a> <a href="/facts/Ganymede_(mythology)/a4zUDN0C">Ganymede</a>, a <a href="/facts/Troy/RYze9Kpu">Trojan</a> prince desired by <a href="/facts/Zeus/9wTwkEDt">Zeus</a> (the Greek counterpart of <a href="/facts/Jupiter_(mythology)/JPetM2m9">Jupiter</a>), who carried him off to be the cupbearer of the gods.
Beginning with <a href="/facts/Pioneer_10/om14fT0h">Pioneer 10</a>, several spacecraft have explored Ganymede. The <a href="/facts/Voyager_program/cPm6qnNx">Voyager</a> probes, <a href="/facts/Voyager_1/39jCxR9I">Voyager 1</a> and <a href="/facts/Voyager_2/9FAEaHUr">Voyager 2</a>, refined measurements of its size, while <a href="/facts/Galileo_(spacecraft)/H57agAgz">Galileo</a> discovered its underground ocean and magnetic field. The next planned mission to the <a href="/facts/Moons_of_Jupiter/Rq3GMt28">Jovian system</a> is the <a href="/facts/European_Space_Agency/L8YRNV0C">European Space Agency</a>'s <a href="/facts/Jupiter_Icy_Moons_Explorer/XcRkV5uS">Jupiter Icy Moons Explorer</a> (JUICE), which was launched in 2023. After flybys of all three icy Galilean moons, it is planned to enter orbit around Ganymede.

Ganymede (moon) open-in-new

Ganymede (moon)