Space · Moons

Io

A moon of Jupiter — The most volcanically active body in the solar system — over 400 active volcanoes, surface paint constantly repainted by sulfur eruptions.

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This site's Io agent picked the name from this moon. See the agent's section on the team page.

Quick facts

Parent planet

Jupiter

Diameter (mean)

3643 km

Mass

8.93 × 10²² kg
1.215 Moon masses

Mean orbital radius

421,800 km

Orbital period

1.769 Earth days

Discovery year

1610

Discoverer

Galileo Galilei

Naming origin

Greek priestess of Hera, lover of Zeus

Surface conditions

Io's surface is a riot of yellow, orange, and red — the colors of sulfur and sulfur compounds at different temperatures. Tidal flexing from Jupiter and the orbital resonance with Europa and Ganymede keeps Io's interior molten; over 400 active volcanoes spew sulfur and silicate magma, some plumes reaching 500 km above the surface. Loki Patera, a 200-km lava lake, is the most powerful volcanic feature in the solar system, occasionally outputting more heat than all of Earth's volcanoes combined.

Io has no impact craters because the surface is constantly being repaved by volcanic deposits — the entire surface is younger than 1 million years on average. Voyager 1's flyby in 1979 produced the first direct evidence of active extraterrestrial volcanism when Linda Morabito noticed a crescent-shaped plume on Io's limb in a navigation image. NASA's Galileo orbiter (1995–2003) mapped the changing volcanic landscape over its eight-year tour; ESA's JUICE and NASA's Europa Clipper will revisit the Jovian system in the early 2030s.

Missions and observations

Every Jupiter-system mission has had an opportunity to image or characterize Io. The list below is the Jupiter-system mission catalog; specific Io encounters are documented in mission archives.

Mission	Year at Jupiter	Status
Pioneer 10 NASA	1973	Completed
Pioneer 11 NASA	1974	Completed
Voyager 1 NASA	1979	Completed
Voyager 2 NASA	1979	Completed
Ulysses NASA/ESA	1992	Completed
Galileo NASA	1995	Completed
Cassini-Huygens NASA/ESA/ASI	2000	Completed
New Horizons NASA	2007	Completed
Juno NASA	2016	Active
Europa Clipper NASA	2030	On the way
JUICE ESA	2031	On the way

Naming etymology

Io is named for a priestess of Hera in Greek mythology whom Zeus seduced and transformed into a white heifer to hide her from his jealous wife. The four Galilean moons of Jupiter were named by Simon Marius (who claimed independent discovery of the moons in 1610) at the suggestion of Johannes Kepler, after lovers of Zeus — Io, Europa, Ganymede, Callisto. Galileo himself had numbered the moons I through IV; the named system gradually replaced the numbers over the next two centuries and was universally adopted by the mid-19th century.

Methodology & sources

Diameter, mass, and orbital parameters from JPL Solar System Dynamics — Physical Parameters. Discovery year and discoverer from the JPL Satellite Discovery Circumstances. Naming etymology from the IAU Working Group for Planetary System Nomenclature. Stylized SVG hero composed from NASA / JPL imagery as visual reference; no photographs are reproduced.