# Callisto (moon) <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Callisto in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=Callisto&embed=1" data-title="Callisto in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=Callisto`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: set the speed to 1 day/s and follow Callisto on the highlighted outer ring, which takes about seventeen seconds to go round while Ganymede laps it more than twice and Io more than nine times; press space whenever Ganymede and Callisto line up and note that their meetings, unlike those of the inner three moons, fall at no fixed spot; drag the view edge-on and scroll in to see how far outside the tight inner system Callisto travels.* **Callisto** is the outermost of the four [[Galilean_moons|Galilean moons]] of [[Jupiter]] and the second largest of them after [[Ganymede_(moon)|Ganymede]]. With a radius of 2,410.3 km it is the third-largest moon in the [[PORTAL_Solar_System|Solar System]], after Ganymede and [[Titan_(moon)|Titan]], almost as wide as [[Mercury_(planet)|Mercury]] but holding only about 33% as much mass (derived).[^nasa-jsat][^nasa-ssat][^nasa-fs] It orbits 1.88 million km from Jupiter, about 4.9 times the distance of Earth's [[Moon]] from [[Earth]] (derived), once every 16.7 days.[^jpl-sat-elem][^nasa-jsat] Callisto's surface ranks among the oldest and most densely cratered anywhere in the Solar System, and shows almost no trace of internal geological activity: impacts have shaped nearly everything visible.[^nasa-callisto][^greeley2000] The moon is made of rock and ice in roughly equal parts, with a density of about 1,830 kg/m³, and its interior appears to be only partly separated into layers.[^anderson2001][^kuskov2005] The Galileo spacecraft nevertheless found that Callisto responds to Jupiter's magnetic field as if a salty ocean lay beneath its ice at a depth of more than 100 km.[^khurana1998][^zimmer2000] Because it lies outside the resonance that links the three inner Galilean moons, it is not significantly heated by [[Tide|tides]], and because it lies outside Jupiter's main radiation belt its surface receives far less radiation than those of the inner moons.[^musotto2002][^cooper2001] A thin atmosphere of [[Carbon|carbon]] dioxide and [[Oxygen|oxygen]] surrounds it.[^carlson1999][^carberrymogan2023] The explorer at the top of this page opens Jupiter's own frame with the four large moons at their true distances and periods; Callisto's highlighted orbit is the wide ring at the edge of the view, well outside the 1:2:4 chain of [[Io_(moon)|Io]], [[Europa_(moon)|Europa]] and Ganymede.[^jpl-sat-elem] ## History ### Discovery Galileo Galilei found Callisto in January 1610 together with the other three large moons of Jupiter, and reported them in his *Sidereus Nuncius* that March; Simon Marius claimed to have observed them independently at about the same time.[^galileo1610][^rice-satellites] ### Name Like the other Galilean moons, Callisto is named after a figure loved by Zeus in Greek myth: a nymph, or in some versions the daughter of Lycaon, who was a companion of Artemis, the goddess of the hunt.[^rice-satellites] Marius proposed the name soon after the discovery and credited the idea to [[Johannes_Kepler|Johannes Kepler]].[^marius1614][^rice-satellites] The mythological names were little used until the mid-twentieth century; older literature knows the moon as Jupiter IV, the fourth satellite in Galileo's numbering, a label it kept even after E. E. Barnard found a fifth, much closer moon, Amalthea, in 1892.[^barnard1892] English has no settled adjective for the name. "Callistoan" appears in the scientific literature, alongside ad hoc forms such as "Callistan".[^klemaszewski2001][^moore1999] ## Orbit and rotation Callisto orbits at about 1.88 million km, 26.3 times Jupiter's equatorial radius of 71,492 km, well beyond Ganymede at 1.07 million km.[^jpl-sat-elem] Its [[Orbit|orbit]] period is 16.689 days, and like most regular moons it is tidally locked, so its day lasts the same time and one hemisphere always faces the planet.[^nasa-jsat][^anderson2001] Over centuries, perturbations from the Sun and the planets move the eccentricity within 0.0072–0.0076 and the inclination to Jupiter's equator within 0.20–0.60°, and these swings in turn move the moon's axial tilt between 0.4° and 1.6°.[^musotto2002][^bills2005] The key fact about Callisto's orbit is what it lacks. Io, Europa and Ganymede are locked in the 1:2:4 Laplace [[Resonance|resonance]], which keeps their orbits slightly eccentric and so keeps them flexing and heating; Callisto's period of 16.689 days is 2.33 times Ganymede's (derived), not a simple whole-number ratio, and the moon is not in any mean-motion resonance and probably never has been.[^musotto2002] It has therefore never been heated appreciably by tides, which shaped its whole internal history.[^freeman2006] Models of the slow tidal evolution of the system suggest that Callisto may be captured into the chain in about 1.5 billion years, completing a 1:2:4:8 sequence.[^lari2020] Distance also protects Callisto from Jupiter's magnetosphere. The flux of charged particles at its surface is roughly 300 times lower than at Europa, so irradiation has altered its surface much less than those of the inner moons.[^cooper2001] The dose at the surface is estimated at about 0.1 mSv per day, just over ten times the average natural background on Earth.[^ringwald2000][^unscear2008] This low radiation level is why Callisto has been singled out as the most practical base for any future crewed exploration of the Jupiter system.[^troutman2003] In the explorer, the moons move on circles at their real periods, so Callisto can be seen falling behind the inner three without any fixed pattern of alignments; the starting positions are ILLUSTRATIVE, and the moons are drawn larger than scale so they stay visible. ## Physical characteristics ### Composition Callisto's mean [[Density|density]] of 1,830 kg/m³ implies roughly equal parts rock and water ice, perhaps with some ammonia; interior models give an ice fraction of 49–55% by mass.[^anderson2001][^kuskov2005][^spohn2003] The rock is probably close to L or LL ordinary chondrites in composition, with an iron-to-silicon mass ratio of 0.9–1.3.[^kuskov2005] Surface gravity, from G × 1.0759 × 10²³ kg / (2.4103 × 10⁶ m)², is about 1.24 m/s² (derived), the lowest of the four large moons. The surface reflects only about 20% of incident light.[^moore2004] Near-infrared spectra show [[Water|water]] ice bands at 1.04, 1.25, 1.5, 2.0 and 3.0 μm, and water ice makes up an estimated 25–50% of the surface material.[^moore2004][^showman1999] Galileo and ground-based spectra have also revealed hydrated silicates containing magnesium and iron, together with carbon dioxide and sulfur dioxide, and perhaps ammonia and organic compounds.[^moore2004][^brown2003][^noll1996] On small scales the surface is a patchwork of bright pure ice, mixtures of rock and ice, and wide dark areas of non-ice material.[^greeley2000] Unusually among the Galilean moons, the hemisphere facing forward along the orbit is the darker of the two.[^moore2004] Carbon dioxide is concentrated on the trailing side and in fresh craters such as Lofn, while sulfur dioxide is more common on the leading side.[^hibbitts2000] ### Internal structure Galileo's close flybys measured Callisto's gravity field and gave a moment of inertia factor of 0.3549 ± 0.0042. A uniform sphere would give 0.4, so density does increase inward, but far less than in Ganymede.[^anderson2001] Assuming hydrostatic equilibrium, the interior is a mixture of compressed rock and ice with rock becoming more abundant with depth, possibly around a small silicate core no more than 600 km in radius with a density of 3,100–3,600 kg/m³.[^anderson1998][^anderson2001][^kuskov2005] A 2011 reanalysis questioned the equilibrium assumption, and in that case the gravity data could fit a more thoroughly differentiated moon with a hydrated rocky core.[^castillo2011] The outer part of the moon is a cold, rigid lithosphere of ice 80–150 km thick. Beneath it there may be a salty ocean some 150–200 km deep.[^kuskov2005][^spohn2003] The evidence is magnetic: as Jupiter's field sweeps past, Callisto behaves like a perfect conductor that the changing field cannot enter, which requires an electrically conducting layer at least 10 km thick.[^khurana1998][^zimmer2000] An ocean is easier to maintain if the water carries up to about 5% ammonia or another antifreeze.[^spohn2003] ### Surface features Callisto's crater density is close to saturation, meaning new craters largely erase older ones, and the moon has no large mountains, volcanoes or tectonic belts.[^zahnle1998][^bender1997] Its surface divides into cratered plains, which cover most of it, lighter plains of icy impact deposits, smooth plains within large impact structures, and small dark smooth patches that may be cryovolcanic.[^greeley2000] Crater shapes change with size: simple bowls below about 5 km, central peaks between 5 and 40 km, central pits between 25 and 100 km, as at Tindr, and central domes in some of the largest craters.[^greeley2000] The largest features are multi-ring basins. Valhalla has a bright central region 600 km across surrounded by rings reaching as far as 1,800 km from its centre, and Asgard is about 1,600 km across.[^usgs-callisto] These concentric fractures probably formed as the brittle lithosphere responded to an impact above a softer or liquid layer, possibly the ocean.[^klemaszewski2001] Straight chains of craters called catenae, such as Gomul Catena, were probably made by bodies torn apart by Jupiter's tides just before impact, as comet Shoemaker–Levy 9 was before it struck Jupiter.[^greeley2000] At the smallest scales, Galileo images show bright frost on crests, crater rims and knobs, surrounded by smooth dark material in the lows.[^moore2004] Craters smaller than about 1 km are scarce and small knobs abound. The favoured explanation is slow sublimation of ice warmed to as much as 165 K where the Sun is overhead: the ice escapes, leaving dark debris that slides downslope and buries small relief.[^moore1999][^moore2004] Without absolute dates, the cratered plains are estimated from theory to be about 4.5 billion years old, while the ages of the large basins range between about 1 and 4 billion years depending on the cratering rates assumed.[^greeley2000][^zahnle1998] ### Atmosphere and ionosphere Callisto has an extremely thin atmosphere so tenuous that its molecules rarely collide. Carbon dioxide was detected first, through a Galileo absorption feature at 4.22 μm; its surface pressure is estimated at about 0.75 μPa.[^carlson1999] Such an exosphere would be lost in about four years unless it were replenished, probably by sublimation of carbon dioxide ice from the regolith.[^carlson1999][^cartwright2024] Galileo radio occultations then found an ionosphere with peak electron densities of about 1.5–1.7 × 10⁴ per cm³, too dense to be produced from carbon dioxide alone, which implied that molecular oxygen is 10 to 100 times more abundant.[^kliore2002][^liang2005] A 2015 analysis of Hubble ultraviolet spectra found emission from an oxygen atmosphere, and in 2023 forbidden oxygen emission lines seen while Callisto was in Jupiter's shadow confirmed gaseous oxygen directly.[^cunningham2015][^dekleer2023] The source of so much oxygen is unresolved: radiolysis of surface ice falls short by two to three orders of magnitude, a problem called the "oxygen enigma".[^carberrymogan2023] Atomic [[Hydrogen|hydrogen]] forms a faint corona, found in 2017 in Hubble data from 2001.[^roth2017] ## Origin and evolution Partial differentiation means Callisto was never hot enough for its ice to melt throughout. The preferred explanation is slow accretion in a low-density disc of gas and dust around the young Jupiter, which allowed the heat of impacts, [[Radioactive_decay|radioactive decay]] and contraction to escape almost as fast as it built up; the allowed formation time is 0.1–10 million years.[^canup2002] Ganymede, formed nearer Jupiter where the disc was denser, may have grown faster and melted.[^mosqueira2003] After accretion, Callisto's thermal history was set by radioactive heating, conduction through its cold outer shell and slow solid-state [[Convection|convection]] in the ice beneath, at speeds of the order of a centimetre per year.[^freeman2006][^mckinnon2006] In this "stagnant lid" regime the rigid outer layer, about 100 km thick, conducts heat without moving, which explains the absence of internal activity at the surface.[^mckinnon2006][^nagel2004] Early convection cooled the interior efficiently enough to prevent large-scale melting, while very slow separation of rock and ice has continued over billions of years.[^nagel2004] An ocean remains possible because the melting point of ordinary ice falls with pressure, to about 251 K at depth. In realistic models the layer between 100 and 200 km deep sits at or slightly above that temperature, and 1–2% of ammonia by weight would almost guarantee liquid.[^spohn2003][^freeman2006] Callisto and Ganymede are similar in size and bulk, yet one is heavily cratered and inert and the other grooved and differentiated. Proposed explanations are different formation conditions, stronger tidal heating of Ganymede, or more energetic impacts on Ganymede during the [[Late_Heavy_Bombardment|Late Heavy Bombardment]].[^barr2008][^showman1997][^barr2010] Callisto's simple history makes it a reference for comparing more active icy worlds.[^showman1999] ## Habitability Like those of [[Europa_(moon)|Europa]], [[Ganymede_(moon)|Ganymede]] and Saturn's moon [[Enceladus]], Callisto's possible ocean is thought to be salty; the magnetic evidence for it in fact requires dissolved salts, since pure water would conduct too poorly to produce the observed response.[^nimmo2015][^khurana1998] The idea that such an ocean might host microbial life, for instance salt-tolerant organisms, has therefore been raised.[^lipps2004][^nasa-splash1998] Most assessments rank Callisto below Europa. Its ocean, if it exists, is thought to lie between layers of ice, so it may never touch rock and would lack the chemical energy that water–rock reactions supply. It is also warmed only by the slow decay of radioactive elements in the rock, with no contribution from tides, so the heat flowing through it is small.[^lipps2004][^nasa-splash1998] Among Jupiter's moons, Europa is considered the likeliest to support life.[^lipps2004][^raulin2005] Callisto's value to astrobiology lies more in comparison: it shows what an icy ocean world looks like when tidal heating is absent. ## Exploration ### Pre-spaceflight Before any spacecraft reached Jupiter, Harold Jeffreys in 1923 and Gustav Tammann in 1931 suggested that Callisto was made of lighter material than rock, and in 1951 Jeffreys proposed that it contained water.[^cruikshank2007][^greenberg-history] ### Space age The Pioneer 10 and 11 flybys of 1973 and 1974 added little to what was known from Earth. [[Voyager_1]] and Voyager 2 in 1979 imaged more than half the surface at 1–2 km resolution and measured the moon's temperature, mass and shape.[^moore2004] The Galileo orbiter, in orbit around Jupiter from 1995 to 2003, made eight close encounters, the closest, during orbit C30 in 2001, at 138 km; it completed global imaging and took some pictures at resolutions as fine as 15 m.[^greeley2000] Cassini recorded infrared spectra of Callisto in 2000 while passing Jupiter, and New Horizons imaged it in 2007 on its way to [[Pluto]].[^brown2003][^morring2007] ### Future exploration ESA's Juice, launched on 14 April 2023, is to make many close flybys of Callisto during its tour of the Jupiter system before settling into orbit around Ganymede.[^esa-juice-fs] NASA's Europa Clipper, launched on 14 October 2024, will also pass Callisto repeatedly while it uses the moons to shape its orbit.[^nasa-clipper] China's Tianwen-4 mission is planned to launch toward Jupiter around 2030 and to enter orbit around Callisto.[^jones2022] ### Old proposals The joint NASA–ESA Europa Jupiter System Mission, planned for launch in 2020, was given priority over a Titan mission in February 2009; it was to consist of a NASA Jupiter Europa Orbiter, an ESA Jupiter Ganymede Orbiter and possibly a Japanese magnetospheric orbiter, and the ESA part later evolved into Juice.[^rincon2009][^esa-l1-2012] ### Potential crewed exploration and habitation In 2003 NASA's Human Outer Planets Exploration (HOPE) study chose Callisto as the destination for a notional crewed mission, because of its low radiation and geological stability. A surface base could make rocket propellant, support remote exploration of Europa, and serve as a waystation for flights farther out.[^troutman2003] The study considered such a mission possible in the 2040s.[^nasa-grc2003] ## See also - [[Galilean_moons]] - [[Jupiter]] · [[Io_(moon)|Io]] · [[Europa_(moon)|Europa]] · [[Ganymede_(moon)|Ganymede]] - [[Late_Heavy_Bombardment]] - Moons of Jupiter · List of geological features on Callisto - [[PORTAL_Solar_System|Solar System portal]] ## Notes Derived numbers are computed from the cited values: Callisto's mass of 1.0759 × 10²³ kg is 0.33 of Mercury's 3.30 × 10²³ kg; its orbital distance of 1,882,700 km is 4.9 times the Moon's mean distance of 384,400 km; its period ratio with Ganymede is 16.689017 / 7.154553 ≈ 2.33; and its surface gravity is GM/r² with G = 6.674 × 10⁻¹¹ m³ kg⁻¹ s⁻² and r = 2.4103 × 10⁶ m, ≈ 1.24 m/s². ## References [^nasa-jsat]: Williams, D. R. "Jovian Satellite Fact Sheet". 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STAIF-2003-177. http://trajectory.grc.nasa.gov/aboutus/papers/STAIF-2003-177.pdf ## External links - NASA Science: Callisto — https://science.nasa.gov/jupiter/moons/callisto/ - USGS Gazetteer of Planetary Nomenclature — https://planetarynames.wr.usgs.gov/ - ESA Juice mission — https://sci.esa.int/web/juice - NASA/JPL Photojournal: Callisto images — https://photojournal.jpl.nasa.gov/ ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Callisto_(moon)) : [Wikitube](https://en.wikitube.io/wiki/Callisto_(moon)) · pinned revision [1372366198](https://en.wikipedia.org/w/index.php?oldid=1372366198) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-060 · explorer state `?obj=Callisto`.* <!-- hub_tags: Life_Physics · PORTAL_Solar_System -->