List of ocean worlds in the Solar System

This is a list of ocean worlds in the Solar System, planets or satellites known or suspected to harbor substantial volumes of liquid water, either as subsurface oceans beneath ice shells or, in the case of Io, a global magma ocean. The existence of such oceans is inferred through a range of methods, including spacecraft measurements of induced magnetic fields, tidal Love numbers, libration amplitudes, surface geology, and isotopic geochemistry.

As of 2026, three worlds have strong or confirmed evidence of present-day global oceans: Enceladus, where the Cassini spacecraft directly sampled water-ice plumes erupting from a tidally heated interior; Europa, whose Galileo-detected induced magnetic field implies a conductive saltwater layer; and Ganymede, where Hubble Space Telescope observations of auroral oscillations point to an ocean that may exceed the volume of Earth's oceans. Callisto and Pluto are considered probable ocean worlds, while the status of Titan—long thought to host a global water–ammonia ocean—was called into question by a 2025 reanalysis of Cassini tidal data that favors a slushy interior with localized liquid pockets rather than a continuous ocean.

Beyond these, a growing number of icy moons, dwarf planets, and trans-Neptunian objects are considered candidates on the basis of interior modeling, spacecraft gravity data, or recent James Webb Space Telescope observations of surface chemistry indicative of geothermal activity. Several bodies, including Charon, Rhea, and Tethys, may have possessed oceans in the past that have since frozen. NASA's Ocean Worlds Exploration Program, established in 2016, coordinates exploration of these targets, with Europa Clipper (launched 2024), JUICE (launched 2023), and Dragonfly (planned launch in 2028) representing the next generation of dedicated missions.

List + Ocean worlds of the Solar System World Confirmed? Ocean volume (vs. Earth) Activity Key observations/evidence Dedicated missions (past → planned) Earth Yes (surface + subsurface) 1× (reference: ~1.335 billion km3) Plate tectonics; hydrothermal vents; active water cycle Direct observation; only known world with stable surface liquid water and confirmed biosphere — Mars Deep crustal liquid water (aquifer, not discrete ocean) Potentially large (≥ Earth's oceans by volume, if global) Ancient surface water; no present surface activity (see also Mars ocean hypothesis) InSight seismic data: low-velocity mid-crust layer at ~5–20 km depth consistent with liquid-water-saturated rock; contested MARSIS radar detection of possible subglacial lake (see also Chronology of discoveries of water on Mars) Viking, Mars Express, InSight, Tianwen-1/Zhurong, Perseverance (all past/active) Europa Strong evidence ≈ 2–3× Possible plumes; young/chaotic ice Induced magnetic field (Galileo); reanalysis/plume hints (HST/Galileo) Galileo → Europa Clipper (launched Oct 2024; Jupiter arrival April 2030); JUICE flybys Enceladus Yes (global) ≪ Earth (tiny; sub-percent) Active south-polar plumes; hydrothermal indicators Cassini plume sampling (H₂, salts); gravity+libration ⇒ global ocean Cassini → (concept) Enceladus Orbilander Ganymede Strong evidence Likely > Earth (ocean alone may exceed Earth's) No known plumes; intrinsic magnetosphere HST auroral oval "rocking" consistent with conductive layer (ocean); interior models Galileo, Juno flyby → JUICE (Jupiter arrival July 2031; Ganymede orbit Dec 2034) Callisto Probable Unknown Geologically quiet Induced magnetic field (Galileo); later analyses strengthen case Galileo → JUICE flybys Titan Disputed (challenged 2025) Unknown (water–ammonia) Active methane cycle; cryovolcanism unconfirmed Gravity/tides (Love number k₂) & rotation anomalies initially implied ocean; 2025 reanalysis argues tidal dissipation precludes a global ocean, favoring slushy ice with liquid pockets Cassini–Huygens → Dragonfly rotorcraft (launch July 2028; arrival ~2034) Triton Possible Unknown Geysers/cryovolcanism seen by Voyager 2 Geology/topography and thermal models; possible deep ocean Voyager 2 → (no selected mission; Trident not chosen) Dione Candidate Unknown Low current activity Cassini gravity/topography; possible global ocean Cassini flybys Rhea Disfavored (possibly past) Unknown No endogenic activity observed Hussmann et al. modeled possible ocean; more recent data suggest homogeneous interior, ocean unlikely to persist today Cassini flybys Tethys Disfavored (possibly past) Unknown Extensional tectonics (Ithaca Chasma) Surface fractures attributed to past ocean freezing; no present ocean expected Cassini flybys Mimas New evidence (young global ocean) ≪ Earth (very small) No obvious surface activity Cassini astrometry/periapsis drift ⇒ ocean at depth 20–30 km; ocean <25 Myr old Cassini flybys Miranda Candidate Unknown Ancient extreme tectonics Voyager 2 imagery; Uranus-system radiation data suggests activity Voyager 2 → (proposed) Uranus Orbiter & Probe Ariel Candidate (may be active) Unknown Young resurfacing Uranus radiation-belt/ring data modeling; geology Voyager 2 → (proposed) Uranus Orbiter & Probe Titania Candidate Unknown Tectonic features Interior modeling (possible ocean) Voyager 2 → (proposed) Uranus Orbiter & Probe Umbriel Candidate Unknown Dark, old surface Modeling; possible ammonia-rich ocean Voyager 2 → (proposed) Uranus Orbiter & Probe Oberon Candidate Unknown Fractures/tectonics Modeling (possible ocean) Voyager 2 → (proposed) Uranus Orbiter & Probe Ceres Regional brines (not global) Negligible vs Earth Recent brine upwelling (Occator), cryovolcanism Dawn gravity/geomorphology ⇒ subsurface brines Dawn orbiter (2015–2018) Pluto Likely Unknown Tectonics; ongoing surface reshaping New Horizons: Sputnik Planitia reorientation ⇒ subsurface ocean New Horizons flyby (2015) Charon Past ocean (now frozen) Unknown Extensional tectonics; cryovolcanic plains Canyons and smooth plains (Vulcan Planitia) consistent with refreezing ocean; thermal-orbital models New Horizons flyby (2015) Eris Hypothesized Unknown Unknown JWST methane isotopologue D/H ratios suggest geothermal interior; tidal locking with Dysnomia implies dissipative interior — Makemake Hypothesized Unknown Unknown JWST methane D/H ratios suggest internal geochemical activity — Sedna Hypothesized Unknown Unknown Interior modeling predicts possible ocean with ammonia antifreeze; JWST observations of similar TNOs suggest internal differentiation may be common — Quaoar Hypothesized Unknown Possible cryovolcanism (methane/ethane detected) Theoretical modeling; JWST spectroscopy suggests internal differentiation and possible geothermal activity — Gonggong Hypothesized Unknown Unknown Theoretical modeling; JWST spectroscopy consistent with endogenic processes on large TNOs — Orcus Hypothesized Unknown Unknown Interior modeling predicts possible ocean with ammonia antifreeze (modeled as "2004 DW") — Io Magma ocean (silicate), not water n/a Extreme volcanism Electromagnetic/induction signatures ⇒ global magma ocean Galileo, Juno flybys

See also Water on terrestrial planets of the Solar System Extraterrestrial liquid water Origin of water on Earth Planetary habitability in the Solar System Habitable zone Habitable zone for complex life

References