--- title: Volcanic world summary: A rocky world whose surface is continually remade by eruptions, powered by internal heat far above Earth's. Jupiter's moon Io, heated by tides, is the most volcanic body known. science_status: [observed, model, sim] categories: [Planets, Planet classes, Terrestrial planets, Volcanism] aliases: [TVO, T4-V, Io-like world, Volcanic planet, Tidally heated world, Volcanic terrestrial, Io analogue] infobox: type: planet_class name: Volcanic world code: TVO legacy_code: T4-V level: type series: "Terrestrial with atmosphere (T)" science_status: [observed, model, sim] image: File:Volcanic_world_sim.avif literature_equivalent: "Tidally heated volcanic world (Io); candidate volcanic exoplanets" defining_criteria_sim: "Rocky world with intense, global volcanism and a thin atmosphere of volcanic gases" mass_earth: {observed: "0.015 (Io)", sim: "mostly about 0.3 to 3; 8% between 0.01 and 0.2"} radius_earth: {observed: "0.286 (Io, 1,821.6 km)"} density_g_cm3: {observed: "3.53 (Io)"} surface_gravity_m_s2: {observed: "1.80 (Io)"} escape_velocity_km_s: {observed: "2.56 (Io)"} equilibrium_temperature_k: {observed: "about 110 (Io)"} surface_temperature_k: {observed: "Io: about 110 on average; lava at about 1,300 to 1,600 K", sim: "about 320 to 1,500 (5th to 95th percentile)"} geometric_albedo: {observed: "0.62 (Io)"} surface_pressure_bar: {observed: "about 1e-9 (Io's patchy SO2 atmosphere, collapsing in eclipse)"} dominant_gases: "SO2 with SO, S2 and NaCl (Io)" heat_flow_w_m2: {observed: "about 2.5 (Io), against 0.087 for Earth"} internal_power_tw: {observed: "about 93 to 106 (Io), against about 47 for Earth"} interior: "Io: iron core, hot rocky mantle with a high melt fraction but no shallow global magma ocean (Juno, 2025)" tidal_state: "Synchronous; eccentricity kept up by the 1:2:4 Laplace resonance with Europa and Ganymede" typical_orbit: "Moons of giant planets in resonances; exoplanets in compact, resonant or eccentric systems" frequency_in_sim: "59 of 8,742 generated planets (0.7%), in the committed sheets as of 2026-09-27" real_examples: ["Io", "LP 791-18 d (candidate)", "L 98-59 b (candidate)"] subtypes: ["TVO-MG Magmatic", "TVO-CL Caldera", "TVO-SU Sulfuric"] sim_source: "Volcanic world physics engine and interior module; science set TVO_00 to TVO_19" last_verified: 2026-09-27 sim: entity: planet_class.TVO refs: - id: peale1979 type: article-journal author: [{family: "Peale", given: "S. J."}, {family: "Cassen", given: "P."}, {family: "Reynolds", given: "R. T."}] title: "Melting of Io by Tidal Dissipation" container-title: "Science" volume: "203" page: "892-894" issued: 1979 DOI: 10.1126/science.203.4383.892 - id: morabito1979 type: article-journal author: [{family: "MORABITO", given: "L. A."}, {family: "SYNNOTT", given: "S. P."}, {family: "KUPFERMAN", given: "P. N."}, {family: "COLLINS", given: "S. A."}] title: "Discovery of Currently Active Extraterrestrial Volcanism" container-title: "Science" volume: "204" page: "972-972" issued: 1979 DOI: 10.1126/science.204.4396.972 - id: pearl1979 type: article-journal author: [{family: "Pearl", given: "J."}, {family: "Hanel", given: "R."}, {family: "Kunde", given: "V."}, {family: "Maguire", given: "W."}, {family: "Fox", given: "K."}, {family: "Gupta", given: "S."}, {family: "Ponnamperuma", given: "C."}, {family: "Raulin", given: "F."}] title: "Identification of gaseous SO2 and new upper limits for other gases on Io" container-title: "Nature" volume: "280" page: "755-758" issued: 1979 DOI: 10.1038/280755a0 - id: lainey2009 type: article-journal author: [{family: "Lainey", given: "Valéry"}, {family: "Arlot", given: "Jean-Eudes"}, {family: "Karatekin", given: "Özgür"}, {family: "Van Hoolst", given: "Tim"}] title: "Strong tidal dissipation in Io and Jupiter from astrometric observations" container-title: "Nature" volume: "459" page: "957-959" issued: 2009 DOI: 10.1038/nature08108 - id: veeder2012 type: article-journal author: [{family: "Veeder", given: "Glenn J."}, {family: "Davies", given: "Ashley Gerard"}, {family: "Matson", given: "Dennis L."}, {family: "Johnson", given: "Torrence V."}, {family: "Williams", given: "David A."}, {family: "Radebaugh", given: "Jani"}] title: "Io: Volcanic thermal sources and global heat flow" container-title: "Icarus" volume: "219" page: "701-722" issued: 2012 DOI: 10.1016/j.icarus.2012.04.004 - id: mcewen1998 type: article-journal author: [{family: "McEwen", given: "A. S."}, {family: "Keszthelyi", given: "L."}, {family: "Spencer", given: "J. R."}, {family: "Schubert", given: "G."}, {family: "Matson", given: "D. L."}, {family: "Lopes-Gautier", given: "R."}, {family: "Klaasen", given: "K. P."}, {family: "Johnson", given: "T. V."}, {literal: "et al."}] title: "High-Temperature Silicate Volcanism on Jupiter's Moon Io" container-title: "Science" volume: "281" page: "87-90" issued: 1998 DOI: 10.1126/science.281.5373.87 - id: keszthelyi2007 type: article-journal author: [{family: "Keszthelyi", given: "Laszlo"}, {family: "Jaeger", given: "Windy"}, {family: "Milazzo", given: "Moses"}, {family: "Radebaugh", given: "Jani"}, {family: "Davies", given: "Ashley Gerard"}, {family: "Mitchell", given: "Karl L."}] title: "New estimates for Io eruption temperatures: Implications for the interior" container-title: "Icarus" volume: "192" page: "491-502" issued: 2007 DOI: 10.1016/j.icarus.2007.07.008 - id: dekleer2017 type: article-journal author: [{family: "de Kleer", given: "K."}, {family: "Skrutskie", given: "M."}, {family: "Leisenring", given: "J."}, {family: "Davies", given: "A. G."}, {family: "Conrad", given: "A."}, {family: "de Pater", given: "I."}, {family: "Resnick", given: "A."}, {family: "Bailey", given: "V."}, {literal: "et al."}] title: "Multi-phase volcanic resurfacing at Loki Patera on Io" container-title: "Nature" volume: "545" page: "199-202" issued: 2017 DOI: 10.1038/nature22339 - id: khurana2011 type: article-journal author: [{family: "Khurana", given: "Krishan K."}, {family: "Jia", given: "Xianzhe"}, {family: "Kivelson", given: "Margaret G."}, {family: "Nimmo", given: "Francis"}, {family: "Schubert", given: "Gerald"}, {family: "Russell", given: "Christopher T."}] title: "Evidence of a Global Magma Ocean in Io’s Interior" container-title: "Science" volume: "332" page: "1186-1189" issued: 2011 DOI: 10.1126/science.1201425 - id: park2025 type: article-journal author: [{family: "Park", given: "R. S."}, {family: "Jacobson", given: "R. A."}, {family: "Gomez Casajus", given: "L."}, {family: "Nimmo", given: "F."}, {family: "Ermakov", given: "A. I."}, {family: "Keane", given: "J. T."}, {family: "McKinnon", given: "W. B."}, {family: "Stevenson", given: "D. J."}, {literal: "et al."}] title: "Io’s tidal response precludes a shallow magma ocean" container-title: "Nature" volume: "638" page: "69-73" issued: 2025 DOI: 10.1038/s41586-024-08442-5 - id: zambon2023 type: article-journal author: [{family: "Zambon", given: "F."}, {family: "Mura", given: "A."}, {family: "Lopes", given: "R. M. C."}, {family: "Rathbun", given: "J."}, {family: "Tosi", given: "F."}, {family: "Sordini", given: "R."}, {family: "Noschese", given: "R."}, {family: "Ciarniello", given: "M."}, {literal: "et al."}] title: "Io Hot Spot Distribution Detected by Juno/JIRAM" container-title: "Geophysical Research Letters" volume: "50" page: "e2022GL100597" issued: 2023 DOI: 10.1029/2022gl100597 - id: tsang2016 type: article-journal author: [{family: "Tsang", given: "Constantine C. C."}, {family: "Spencer", given: "John R."}, {family: "Lellouch", given: "Emmanuel"}, {family: "Lopez-Valverde", given: "Miguel A."}, {family: "Richter", given: "Matthew J."}] title: "The collapse of Io's primary atmosphere in Jupiter eclipse" container-title: "Journal of Geophysical Research: Planets" volume: "121" page: "1400-1410" issued: 2016 DOI: 10.1002/2016je005025 - id: dekleer2024 type: article-journal author: [{family: "de Kleer", given: "Katherine"}, {family: "Hughes", given: "Ery C."}, {family: "Nimmo", given: "Francis"}, {family: "Eiler", given: "John"}, {family: "Hofmann", given: "Amy E."}, {family: "Luszcz-Cook", given: "Statia"}, {family: "Mandt", given: "Kathy"}] title: "Isotopic evidence of long-lived volcanism on Io" container-title: "Science" volume: "384" page: "682-687" issued: 2024 DOI: 10.1126/science.adj0625 - id: spencer2007 type: article-journal author: [{family: "Spencer", given: "J. R."}, {family: "Stern", given: "S. A."}, {family: "Cheng", given: "A. F."}, {family: "Weaver", given: "H. A."}, {family: "Reuter", given: "D. C."}, {family: "Retherford", given: "K."}, {family: "Lunsford", given: "A."}, {family: "Moore", given: "J. M."}, {literal: "et al."}] title: "Io Volcanism Seen by New Horizons: A Major Eruption of the Tvashtar Volcano" container-title: "Science" volume: "318" page: "240-243" issued: 2007 DOI: 10.1126/science.1147621 - id: peterson2023 type: article-journal author: [{family: "Peterson", given: "Merrin S."}, {family: "Benneke", given: "Björn"}, {family: "Collins", given: "Karen"}, {family: "Piaulet", given: "Caroline"}, {family: "Crossfield", given: "Ian J. M."}, {family: "Ali-Dib", given: "Mohamad"}, {family: "Christiansen", given: "Jessie L."}, {family: "Gagné", given: "Jonathan"}, {literal: "et al."}] title: "A temperate Earth-sized planet with tidal heating transiting an M6 star" container-title: "Nature" volume: "617" page: "701-705" issued: 2023 DOI: 10.1038/s41586-023-05934-8 - id: belloarufe2025 type: article-journal author: [{family: "Bello-Arufe", given: "Aaron"}, {family: "Damiano", given: "Mario"}, {family: "Bennett", given: "Katherine A."}, {family: "Hu", given: "Renyu"}, {family: "Welbanks", given: "Luis"}, {family: "MacDonald", given: "Ryan J."}, {family: "Seligman", given: "Darryl Z."}, {family: "Sing", given: "David K."}, {literal: "et al."}] title: "Evidence for a Volcanic Atmosphere on the Sub-Earth L 98-59 b" container-title: "The Astrophysical Journal Letters" volume: "980" page: "L26" issued: 2025 DOI: 10.3847/2041-8213/adaf22 - id: davies2010 type: article-journal author: [{family: "Davies", given: "J. H."}, {family: "Davies", given: "D. R."}] title: "Earth's surface heat flux" container-title: "Solid Earth" volume: "1" page: "5-24" issued: 2010 DOI: 10.5194/se-1-5-2010 - id: nasa_galsat type: webpage author: [{family: Williams, given: David R.}] title: "Jovian Satellite Fact Sheet" container-title: "NASA Space Science Data Coordinated Archive" accessed: 2026-09-27 URL: https://nssdc.gsfc.nasa.gov/planetary/factsheet/joviansatfact.html images_wanted: - file: File:Volcanic_world_sim.avif subject: "A TVO planet from the sim: dark crust with glowing calderas and yellow-white sulfur frost, ideally TVO-CL" source: sim note: "Shot list: render a TVO plate on the lit sphere; label 'Sim render'. Confirm the plate generator's licence." - file: File:Io_Galileo_PIA02308.jpg subject: "Io in true colour from Galileo (3 July 1999): yellow and white sulfur deposits, black and red spots at active volcanic centres" source: nasa page_url: https://science.nasa.gov/photojournal/global-image-of-io-true-color credit: "NASA/JPL/University of Arizona" licence: "Public domain (NASA)" note: "PIA02308. Label 'Observation'." - file: File:Laplace_resonance_diagram.svg subject: "Diagram: Io, Europa and Ganymede in their 1:2:4 resonance, showing how repeated conjunctions keep Io's orbit eccentric and its tides flexing" source: other note: "To be drawn by us; label 'Diagram'." --- A **volcanic world** is a rocky world whose surface is continually remade by eruptions, powered by internal heat far greater than Earth's. Jupiter's moon Io is the extreme case: slightly larger than Earth's Moon, it erupts more lava than any other body in the Solar System, and its surface is so young that not one impact crater has been found on it. Io is heated by tides; most planetary volcanism, Earth's included, is driven instead by radioactive decay and heat left over from formation. Planets heated the same way around other stars are now being found, and the first hints of volcanic gases in their atmospheres have been reported. ::figure{src="File:Io_Galileo_PIA02308.jpg" size=wide alt="Io as a mottled yellow, white and orange sphere dotted with black and red spots." caption="Observation: Io in true colour from NASA's Galileo spacecraft, July 1999. Yellow and white areas are sulfur and sulfur dioxide frost; the dark spots are active volcanic centres."} ## Characteristics ### Heat from tides Io orbits Jupiter every 1.77 days on an orbit that is almost, but not quite, circular, with an eccentricity of 0.004 :cite[nasa_galsat]. Jupiter's gravity raises a tidal bulge in Io's rock about 100 metres high, and as Io's distance from Jupiter changes over each orbit, the bulge grows, shrinks and shifts. The constant flexing is dissipated as heat inside the moon. The heating rate for a synchronously rotating moon is $$ \dot{E}_\mathrm{tide} = \frac{21}{2}\,\frac{k_2}{Q}\,\frac{G M_p^2 R^5\, n\, e^2}{a^6}, $$ where $M_p$ is the planet's mass, $R$ the moon's radius, $a$ and $e$ its orbital distance and eccentricity, $n$ its orbital angular speed, and $k_2/Q$ a measure of how easily the moon deforms and how much of that work becomes heat. The steep dependence on distance, $a^{-6}$ with $n$ adding a little more, is why tidal heating matters only for close orbits. A lone moon would lose this heating quickly, because the same tides would make its orbit circular. Io's eccentricity is kept up by an orbital resonance: Io completes four orbits for every two of Europa's and one of Ganymede's, so the three moons line up at the same points again and again, and their regular tugs keep Io's orbit slightly stretched. Peale, Cassen and Reynolds worked this out and predicted widespread volcanism on Io in a paper published days before Voyager 1 flew past in March 1979 :cite[peale1979]. Days later, a Voyager navigation image showed an erupting plume :cite[morabito1979]. ### How much heat Io's orbit is slowly changing as it loses energy to tides, and measuring that drift from centuries of observations gives a dissipation of about 93 ± 19 terawatts :cite[lainey2009]. Adding up the infrared glow of its volcanoes gives about $1.06 \times 10^{14}$ W, or about 2.5 watts per square metre of surface :cite[veeder2012]. For comparison, Earth loses about 47 terawatts through its whole surface, about 0.087 W/m² on average :cite[davies2010]. Io, with less than a tenth of Earth's surface area, radiates about twice Earth's internal heat, nearly thirty times as much per square metre. ### Volcanoes and lava Juno's infrared camera has catalogued 242 hot spots on Io, 23 of them seen for the first time :cite[zambon2023]. The largest, Loki Patera, is a lava lake about 200 km across that alone produces nearly a tenth of Io's heat :cite[veeder2012]; its crust founders and sinks in waves that sweep across the lake over a period of months :cite[dekleer2017]. Galileo measured eruption temperatures of at least 1,700 K, hotter than any lava erupting on Earth today and suggesting magnesium-rich lava like the komatiites of Earth's early history :cite[mcewen1998]. Later analysis revised the typical temperature down to about 1,300 to 1,600 K, closer to hot basalt :cite[keszthelyi2007]. Plumes of sulfur and sulfur dioxide rise hundreds of kilometres; New Horizons photographed one from Tvashtar reaching about 330 km as it passed Jupiter in 2007 :cite[spencer2007]. ### Atmosphere Io's atmosphere is sulfur dioxide, first detected by Voyager's infrared spectrometer :cite[pearl1979]. It is thin, about a billionth of Earth's surface pressure, and patchy, fed partly by volcanoes and partly by frost evaporating in sunlight. When Io passes into Jupiter's shadow and the surface cools, much of the gas freezes out within minutes, and it returns when sunlight does :cite[tsang2016]. ### Interior A magnetometer on Galileo detected a signal at Io that was interpreted as a global layer of partly molten rock at least 50 km thick beneath the crust, a so-called magma ocean :cite[khurana2011]. Juno's close flybys in December 2023 and February 2024 tested this by measuring how much Io's shape and gravity respond to Jupiter's tides. The response was too small for a shallow liquid layer: Io's mantle is hot and partly molten, but it has no global magma ocean near the surface :cite[park2025]. ## Formation and evolution A volcanic world needs a source of internal heat stronger than a rocky planet's slowly fading radioactivity. The long-lived examples are tidally heated: moons of giant planets locked in orbital resonances, or planets in tight systems where neighbours keep each other's orbits slightly eccentric. Young planets are also volcanic, because they still hold much of the heat of their formation, and any rocky planet close enough to its star can have a molten surface regardless of its interior (see [[Lava world]]). Io's volcanism is old. The isotopes of sulfur and chlorine in its atmosphere are heavily skewed toward the heavier forms, the result of billions of years of lighter atoms escaping from the top of the atmosphere while volcanoes kept cycling material through. The measured skew implies that Io has been volcanically active for most of the Solar System's history, and so that its resonance with Europa and Ganymede is also ancient :cite[dekleer2024]. ## How we know Io was discovered by Galileo Galilei in 1610, but its volcanism was a prediction until Voyager 1 photographed an erupting plume in March 1979 :cite[morabito1979]. Galileo orbited Jupiter from 1995 to 2003 and made several close passes of Io. Ground-based telescopes with adaptive optics now track its volcanoes from Earth, which is how Loki Patera's overturn cycles were followed :cite[dekleer2017]. NASA's Juno flew within 1,500 km of Io in December 2023 and February 2024. Around other stars, tidal volcanism can only be inferred so far. **LP 791-18 d** is an Earth-sized planet around a red dwarf, about 1.03 times Earth's radius. A more massive neighbour keeps its orbit slightly eccentric as it circles its star, so its tidal heating may make it as volcanically active as Io :cite[peterson2023]. **L 98-59 b**, with 0.84 times Earth's radius and 0.40 times its mass, is the first small exoplanet with evidence of a sulfur dioxide atmosphere: JWST spectra favour one at 3.6 standard deviations, which the authors interpret as a sign of volcanic outgassing on a tidally heated world :cite[belloarufe2025]. Neither result is yet a confirmed volcano. ## Notable examples | Body | Radius | Mass | Internal power | Notes | |---|---|---|---|---| | Io (Jupiter) | 1,821.6 km | 8.93 × 10²² kg | about 100 TW | 1:2:4 Laplace resonance; about 2.5 W/m² | | Earth, for comparison | 6,371 km | 5.97 × 10²⁴ kg | about 47 TW | 0.087 W/m², mostly radioactive decay and primordial heat | | LP 791-18 d | 1.03 R⊕ | about 0.9 M⊕ | unknown | tidally heated by an outer neighbour (candidate volcanic world) | | L 98-59 b | 0.84 R⊕ | 0.40 M⊕ | unknown | SO₂ atmosphere favoured by JWST (candidate volcanic world) | :::callout{type=sim title="In Pax Abyssi"} The sim's volcanic worlds are rocky planets with intense volcanism and thin atmospheres of volcanic gases. Most are of roughly Earth's mass, with a small share of Io-sized bodies between 0.01 and 0.2 Earth masses. Three subtypes are set by surface temperature: **magmatic** worlds (TVO-MG) at 1,400 K and above, laced with lava; **caldera** worlds (TVO-CL) between 400 and 1,400 K, with scattered active calderas on a dark crust; and **sulfuric** worlds (TVO-SU) below 400 K, frosted yellow and white with sulfur like Io. A volcanic world that ends up too hot for a solid crust is moved to [[Lava world|lava world]] when its system is built. In the committed system sheets, 59 of 8,742 generated planets are volcanic worlds, most of them caldera worlds. In the game each is drawn as a prebaked texture plate chosen by its subtype code. ::: ## See also - [[Lava world]] - [[Subsurface ocean world]] - [[Tidal heating]] - [[Orbital resonance]] - [[Natural satellite]] - [[Super-Earth]] (volcanic super-Earths, SEV) - [[Planet classification]]