The Galilean moons are the four major satellites of Jupiter—Io, Europa, Ganymede, and Callisto—first recorded in telescopic observations by Galileo Galilei in January 1610 and announced in his treatise Sidereus Nuncius on March 13, 1610. Galileo initially called them the “Medicean stars,” while the modern names, proposed by Simon Marius in 1614, became standard centuries later. NASA summarizes that these bodies provided early, decisive support for heliocentrism by demonstrating that not everything orbits Earth. Library of Congress; NASA Science – Jupiter’s Moons. (loc.gov)

Overview and orbital architecture

• –Ordered by distance from Jupiter, the sequence is Io, Europa, Ganymede, Callisto. Mean orbital distances are about 422,000 km (Io), 671,000 km (Europa), 1,070,000 km (Ganymede), and 1,883,000 km (Callisto), and the corresponding orbital periods are approximately 1.77, 3.55, 7.15, and 16.69 Earth days. NASA lists their diameters as roughly 3,630 km (Io), 3,138–3,160 km (Europa), 5,262–5,268 km (Ganymede), and 4,800 km (Callisto). NASA Science – Ganymede (In Depth); NASA Science – Voyager Fact Sheet; NASA Science – Europa Facts; NASA Science – Callisto Facts; NASA Science – Natural and False Color Views of Europa. (solarsystem.nasa.gov)
• –Io, Europa, and Ganymede participate in the 4:2:1 Laplace resonance, which maintains small orbital eccentricities and powers tidal stressing; Callisto lies outside the resonance. NASA and classic analyses show that the resonance continuously exchanges angular momentum among the three inner satellites, maintaining eccentricity and driving internal heating. NASA Science – Europa Facts; NASA Science (NTRS) – The Laplacian resonance and tidal dissipation. (science.nasa.gov)
• –All four moons are tidally locked, always keeping the same face to Jupiter. NASA Science – Ganymede (In Depth). (solarsystem.nasa.gov)

Io

• –Io is a dense, rocky world and the most volcanically active body in the Solar System, with plumes and lava fountains driven by extreme Tidal heating from Jupiter’s gravity and the resonant tugs of Europa and Ganymede. NASA reports hundreds of volcanoes and tides in Io’s solid crust of up to ~100 m. NASA Science – Io Facts. (science.nasa.gov)
• –Active volcanism on Io was first recognized in Voyager 1 images from March 1979; a navigation engineer, Linda A. Morabito, identified a towering plume off Io’s limb. JPL’s image catalog documents eruption plumes and calderas in the Voyager data, while NASA’s Juno mission pages highlight Morabito’s 1979 discovery. NASA JPL – Io Volcanic Eruption (Voyager 1); NASA – Juno: “Io’s volcanic rage”. (jpl.nasa.gov)

Europa

• –Europa’s surface is dominated by water ice crisscrossed by ridges and bands. Galileo spacecraft magnetometer measurements revealed a time-varying, induced magnetic signature consistent with a global subsurface, electrically conducting ocean; subsequent analyses strengthened the case. Science (Kivelson et al., 2000); Nature (Khurana et al., 1998); NASA Science – Europa Facts. (ovid.com)
• –Typical working estimates place the ice shell at roughly 15–25 km over an ocean perhaps 60–150 km deep, making the total water inventory comparable to or exceeding Earth’s. NASA summarizes the evidence and ranges based on Galileo and telescopic data. NASA Science – Europa Facts. (science.nasa.gov)

Ganymede

• –Ganymede is the largest moon in the Solar System (diameter ~5,262–5,268 km), larger than the planet Mercury, and uniquely possesses an intrinsic magnetic field. The Galileo spacecraft discovered Ganymede’s internal dipole in 1996. NASA Science – Ganymede Facts; Nature (Kivelson et al., 1996). (science.nasa.gov)
• –Observations and modeling indicate a differentiated interior (rock/metal plus thick ice) and strong evidence for a deep, likely salty ocean beneath the ice, consistent with Hubble auroral constraints and Galileo data. NASA Science – Ganymede and Jupiter (HST/aurorae); NASA Science – Ganymede Facts. (science.nasa.gov)

Callisto

• –Callisto is a dark, heavily cratered icy world with the Solar System’s most ancient, densely cratered surface. NASA notes possible evidence for a deep subsurface ocean inferred from Galileo-era magnetometer data and modeling, though its existence and depth remain under study. NASA Science – Callisto Facts; NASA Solar System – Callisto (In Depth); Britannica – Callisto. (science.nasa.gov)

Discovery, naming, and historical significance

• –Galileo’s winter 1610 observations (beginning Jan. 7–8) showed “four planets” circling Jupiter, published promptly in Sidereus Nuncius; this evidence undercut Ptolemaic geocentrism and bolstered heliocentrism. A modern facsimile and translation detail the publication history. Library of Congress; Cabinet Oxford – Sidereus Nuncius, 1610. (loc.gov)
• –The familiar names—Io, Europa, Ganymede, Callisto—derive from Simon Marius’s 1614 Mundus Iovialis, reflecting a suggestion by Johannes Kepler; NASA and Britannica trace the eventual adoption of these names alongside Galileo’s original numerical scheme. NASA Science – Ganymede: Exploration; Britannica – Jupiter: The Galilean satellites. (science.nasa.gov)

Exploration

• –Early flybys by Pioneer 10/11 and Voyager 1 and 2 revealed global diversity, followed by the [Galileo (spacecraft)] orbiter (1995–2003), which transformed understanding of interior structures, volcanism, and possible oceans. NASA’s mission archives summarize these milestones. NASA Science – Ganymede: Exploration. (science.nasa.gov)
• –Europa-focused missions are now a central theme. NASA’s Europa Clipper launched on Oct. 14, 2024, aboard a Falcon Heavy and is en route to begin 49 Europa flybys after arrival in 2030, investigating habitability with a comprehensive instrument suite. NASA News Release; NASA Science – Europa Clipper. (nasa.gov)
• –The European Space Agency’s JUICE (Jupiter Icy Moons Explorer) launched on Apr. 14, 2023, to perform an extensive tour of the Jovian system and ultimately orbit Ganymede, arriving in 2031. ESA Press Release. (esa.int)

Scientific themes

• –Resonant forcing and tidal dissipation underpin the moons’ geologic and interior evolution, explaining Io’s volcanism and likely sustaining liquid water layers within Europa (and possibly Ganymede and Callisto). Foundational spacecraft magnetometer results and subsequent modeling provide the key evidence base. Nature (Khurana et al., 1998); Science (Kivelson et al., 2000); NASA Science – Europa Facts. (pubmed.ncbi.nlm.nih.gov)