Io is the innermost of the four Galilean satellites of Jupiter, slightly larger than Earth’s Moon, and the most volcanically active world in the Solar System, with hundreds of active volcanoes revealed by spacecraft and ground-based observations. According to NASA Science, Io’s extreme activity is powered by tidal flexing from Jupiter and its neighboring moons Europa and Ganymede that pumps heat into the interior.

Physical characteristics

Io’s mean radius is about 1,821.5 km and its mass about 8.932×10^22 kg, yielding a high mean density (~3,530 kg/m³) consistent with a rocky, largely silicate composition rather than ice. These bulk parameters are summarized by the NASA NSSDC Jovian Satellite Fact Sheet. NASA NSSDC

Measurements from the [Galileo (spacecraft)] mission indicate Io possesses a large iron/iron-sulfide core roughly 900 km in radius, overlain by a silicate mantle and crust, inferred from gravity data during a close flyby in December 1995 and reported in 1996. NASA JPL

Io’s diameter is about 3,643 km (slightly larger than Earth’s Moon), and it reflects considerable sunlight with a visual geometric albedo near 0.62; its surface gravity is ~1.80 m/s² and escape velocity ~2.6 km/s. NASA NSSDC

Orbit and tidal heating

Io orbits Jupiter at a mean distance of about 422,000 km with a sidereal period of 1.769 Earth days, keeping the same face toward the planet (synchronous rotation). Britannica Its orbital eccentricity is maintained by a Laplace 1:2:4 resonance with Europa and Ganymede, which continually flexes Io and drives internal heating that fuels volcanism. NASA Science Io’s solid surface experiences tides up to roughly 100 m in amplitude due to this gravitational tug-of-war. NASA Science

Surface and geology

Io’s surface is a vivid patchwork of sulfur and sulfur dioxide frost, lava lakes, and extensive flow fields; impact craters are rare because ongoing volcanism rapidly resurfaces the terrain. NASA Science Despite pervasive volcanism, many of Io’s mountains are tectonic in origin—tilted crustal blocks uplifted by compressional stresses—with typical heights of ~6 km and maxima near 17.5 km at Boösaule Montes, documented from Voyager and Galileo analyses. NASA NTRS (Schenk et al.)

High-temperature hot spots measured by Galileo instruments show lava eruption temperatures exceeding 1,500–1,700 K (and in some events possibly approaching 2,000 K), indicating ultramafic silicate volcanism rather than low-temperature sulfur flows. USGS summary of Science article NASA Photojournal (Galileo NIMS)

Atmosphere and magnetospheric interaction

Io hosts a very thin atmosphere dominated by sulfur dioxide, largely sustained by volcanic outgassing and dayside sublimation of surface SO₂ frost; on the nightside, SO₂ condenses, producing strong diurnal variability. NASA Science As Io moves through Jupiter’s magnetic field, material escaping from the moon is ionized to form the Io plasma torus, and electrical coupling along field lines produces auroral “footprints” in Jupiter’s atmosphere. NASA Science (Hubble footprints) NASA JPL NASA notes that Io can develop ~400,000 volts across itself and drive currents of millions of amperes along this flux tube. NASA Science

Volcanism

Io’s volcanism is global in extent, with hundreds of currently active centers including persistent hot spots such as Loki Patera and prominent plume sources like Pele, Prometheus, and Tvashtar. The first confirmed extraterrestrial volcanic plumes were seen at Io by Voyager 1 in March 1979 when navigation engineer Linda Morabito identified a tall plume on the limb; subsequent imaging found multiple plumes and confirmed ongoing activity. NASA Photojournal (PIA00379) NASA JPL Voyager image page

Observations during the 2007 Jupiter flyby of New Horizons imaged a ~290–330 km-high plume from Tvashtar and additional plumes, with the nightside faintly illuminated by Jupiter. NASA Photojournal (PIA09248) NASA Photojournal (PIA09357/PIA09359)

Juno’s extended mission has provided new insights into Io’s interior and heat flow. In December 2024, analyses presented by the Juno team concluded that Io’s volcanoes are likely powered by individual magma plumbing systems rather than a global shallow magma ocean, based on tidal response and gravity constraints; the underlying research appeared in Nature on December 12, 2024. NASA (mission news) Nature (citation page)

Juno also recorded an exceptionally powerful multi–hot-spot eruption in Io’s southern hemisphere on December 27, 2024, with infrared radiance implying total power well above 80 terawatts and affected areas on the order of 100,000 km², the most intense such event reported to date by the mission team. NASA JPL Mission Juno (SwRI)

Discovery and naming

Io was discovered on January 8, 1610, by Galileo Galilei, one of the four large moons now called the Galilean satellites; the names Io, Europa, Ganymede, and Callisto were popularized later from suggestions by Simon Marius. Britannica

Exploration highlights

• –Voyager 1 and Voyager 2 (1979): revealed active plumes and a young, heavily resurfaced world. NASA Photojournal
• –[Galileo (spacecraft)] (1995–2003): long-term monitoring, discovery of an iron/iron-sulfide core, pervasive high-temperature volcanism, and extensive surface changes. NASA JPL USGS
• –New Horizons (2007): high-resolution plume imaging (e.g., Tvashtar) during Jupiter gravity assist. NASA Photojournal
• –[Juno (spacecraft)] (2016–present): multiple close/far Io flybys (notably in 2023–2024) constraining interior structure, subsurface temperatures, and current eruptive activity. NASA JPL (2025 update)

Key orbital and environmental parameters

Io’s semi-major axis is ~422,000 km; orbital period 1.769 days; synchronous rotation; mean radius ~1,821.5 km; mass ~8.932×10^22 kg; density ~3,530 kg/m³; surface gravity ~1.80 m/s²; escape velocity ~2.6 km/s. NASA NSSDC Britannica

Interaction with Jupiter’s aurora

Material lost from Io forms a doughnut-shaped plasma torus along Io’s orbit; electric currents along magnetic field lines produce bright auroral footprints where they enter Jupiter’s upper atmosphere, imaged by Hubble and modeled with joint spacecraft campaigns. NASA Science NASA JPL