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Io: Facts about Jupiter’s Volcanic Moon

Jupiter’s fifth moon, Io, is the most volcanically active body in the solar system. Plumes of sulphur spew upward as high as 190 miles (300 km). The surface of Io is splotched with lava lakes and floodplains of liquid rock.

Facts about Io

Jupiter and its volcanic moon Io, taken during the spacecraft’s Jupiter flyby in early 2007.
Jupiter and its volcanic moon Io, taken during the spacecraft’s Jupiter flyby in early 2007.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Age: Io is about 4.5 billion years old, about the same age as Jupiter.

Distance from Jupiter: Io is the fifth moon from Jupiter. Its orbital distance is about 262,000 miles (422,000 km). Io takes 1.77 Earth days to orbit Jupiter. The same side of Io always faces Jupiter.

Size: Io has a mean radius of 1,131.7 miles, making it slightly larger than the Earth’s moon. It has a slight elliptical shape, with its longest axis directed toward Jupiter. Among the Galilean satellites Io ranks third, behind Ganymede and Callisto but ahead of Europa, in both mass and volume.

Temperature: Io's surface temperature averages about negative 202 degrees F, resulting in the formation of sulfur dioxide snowfields. But Io’s volcanoes can reach 3,000 degrees F. Io is often referred to as a celestial body of fire and ice.

The Discovery of Io

Artist's concept of the internal structure of Jupiter's moon Io. A global magma "ocean" (shown in orange) lies beneath a crust 30 to 50 kilometers thick. The rest of Io's mantle is shown in gold, while the moon's core is rendered in silver.
Artist's concept of the internal structure of Jupiter's moon Io. A global magma "ocean" (shown in orange) lies beneath a crust 30 to 50 kilometers thick. The rest of Io's mantle is shown in gold, while the moon's core is rendered in silver.
Credit: Xianzhe Jia (University of Michigan) and Krishan Khurana (UCLA)

Io was the first of Jupiter’s moons discovered by Galileo Galilei on Jan. 8, 1610. He actually discovered the moon the day prior, but could not differentiate between Io and Europa, another Jupiter moon, until the next night. The discovery, along with three other Jovian moons, was the first time a moon was discovered orbiting a planet other than Earth. Galileo’s discovery eventually led to the understanding that planets orbit the sun, instead of our solar system revolving around Earth.

Io’s Name

Galileo first referred to this moon as Jupiter I. In the mid-1800s, the moon was renamed Io. The name Io has a bit of a tawdry back story, as Io is named for the daughter of Inachus, whom Zeus raped. He turned Io into a cow to hide the incident from his wife.

Exploration of Io

Several spacecraft have flown by Jupiter and its moons. Pioneer 10 arrived first, in 1973, followed by Pioneer 11 in 1974. Voyager 1 and Voyager 2 returned striking photos during their flybys. The Galileo spacecraft passed as low as 162 miles (261 km) over the surfaces of the Galilean moons and produced detailed images.

Characteristics of Io

Io Creates Spots on Jupiter
Hubble Space Telescope ultraviolet image of the northern pole of Jupiter. Among many other auroral structures, the Io footprint is the most equator-ward feature close to the centre of the image. This spot is always located close to the feet of the magnetic field lines connected to the satellite Io.
Credit: LPAP/Université de Liège

The interior of Io is composed of an iron or iron sulfide core and a brown silicate outer layer, giving the planet a splotchy orange, yellow, black, red, and white appearance.

Based on data from scientific computer models, Io formed in a region around Jupiter where water ice was plentiful. Io’s heat, combined with the possibility that there was water on Io shortly after it was formed, could have made life possible, although Jupiter’s radiation would have removed the water from the surface.

The planets most distinctive features are its volcanoes. Aside from Earth, Io is the only known body in the solar system to have observed active volcanoes. While Galileo had made some cryptic notes inferring possible volcanic activity, NASA's Voyager spacecraft discovered Io's volcanoes in 1979.

The volcanic activity is a result of Io being stretched and squeezed as it orbits Jupiter. Io's rock surface bulges up and down by as much as 100 meters during the process. This impacts Io’s volcanic activity in a similar way to which the Earth’s oceans react to the moon. Io’s irregularly elliptical orbit also heightens the tidal activity.

Io’s Atmosphere

Because of the volcanic activity, Io’s atmosphere contains mostly sulphur dioxide. Io’s orbit cuts across Jupiter’s powerful magnetic lines of force, turning Io into an electric generator. As Jupiter rotates, the magnetic forces strip away about a ton (1,000 kg) of Io’s material every second. The material becomes ionized and forms a doughnut-shaped cloud of radiation called a plasma torus. Some of the ions are pulled into Jupiter’s upper atmosphere and create auroras.

- Kim Ann Zimmermann

First global geological map of Jupiter's moon Io
This map, released in March 2012 by the U.S. Geological Survey, is the first ever to chart the Jupiter moon Io's geology on a global scale. Io is the most volcanically active object in the solar system.
Credit: USGS

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