Io (moon)

Io comparison with Earth and Moon

Galileo.arp.300pix

Io VGR South polar color mosaic

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Io is one of the four Galilean moons of Jupiter, discovered in 1610 by Galileo Galilei. It is the fourth-largest moon in the Solar System, has the highest density of all the moons, and has the least amount of water of any known astronomical object in the Solar System. Its volcanic activity is due to the intense tidal heating produced by its orbital resonance with Europa and Ganymede.

Discovery and Naming

Io was discovered on January 7, 1610, by Galileo Galilei, along with the other Galilean satellites of Jupiter: Europa, Ganymede, and Callisto. The moon was named after Io, a priestess of Hera who became one of the lovers of Zeus (the Greek counterpart of Jupiter). However, the name 'Io' was not used until the mid-20th century; before then, it was often referred to simply as "Jupiter I".

Orbit and Rotation

Io orbits Jupiter at a distance of about 421,700 km (262,000 miles) from the planet's center, making it the innermost of the Galilean moons. Its orbit is slightly elliptical, which causes it to experience significant tidal forces. These forces are responsible for Io's geologic activity. Io completes an orbit around Jupiter in approximately 1.769 Earth days and is in a 2:1 mean-motion orbital resonance with Europa and a 4:1 resonance with Ganymede.

Geology and Volcanic Activity

Io's surface is dotted with hundreds of volcanoes, some of which are active and emit plumes of sulfur and sulfur dioxide that can extend up to 500 km (310 miles) into space. This intense volcanic activity is the result of tidal heating caused by its orbital resonance with Europa and Ganymede. The surface of Io is covered with sulfur in different colorful forms. As a result of the continuous volcanic eruptions, Io's surface is constantly changing, and it lacks the impact craters that are common on most other moons in the Solar System, indicating its geological youth.

Atmosphere

Io has a thin atmosphere composed mostly of sulfur dioxide (SO2), with minor components of sulfur monoxide (SO), sodium chloride (NaCl), and atomic sulfur (S) and oxygen (O). The atmosphere is extremely tenuous and is constantly being replenished by volcanic eruptions.

Exploration

The first detailed observations of Io were made by the Voyager 1 and Voyager 2 spacecraft during their flybys of Jupiter in 1979. These observations revealed Io's volcanic activity for the first time. Subsequent observations and data have been provided by the Galileo spacecraft, which orbited Jupiter from 1995 to 2003, and more recently by both ground-based telescopes and the Hubble Space Telescope.

Significance

Io plays a significant role in our understanding of volcanic processes and tidal heating. Its extreme geologic activity makes it a unique object of study in the Solar System. The interactions between Io and Jupiter's magnetosphere also create a torus of ionized particles that contribute to auroras on Jupiter.

See Also

• Galilean moons
• Volcanism on Io
• List of geological features on Io
• Exploration of Jupiter