LYCOS RETRIEVER 
Io: Moons
built 659 days ago
If Io were Jupiter's only moon, it would not be subject to internal stresses. But there are other moons nearby which exert a gravitational pull of their own. Io's volcanic activity is caused by the powerful force of Jupiter's gravity, coupled with the gravitational pull of Io's neighboring moons--Europa, Callisto, and Ganymede. Jupiter pulls Io inward toward itself, while the gravity of the outer moons pull it in the opposite direction. These opposing forces cause the distance between Io and Jupiter to vary, making Io's orbit slightly elliptical. As a result, Io is subjected to tremendous tidal forces that alternately squeeze and stretch its interior.
Source:
The first reported observation of Io was made by Galileo Galilei on 7 January 1610. The discovery of Io and the other Galilean satellites of Jupiter was published in Galileo's Sidereus Nuncius in March 1610.[10] In his Mundus Jovialis, published in 1614, Simon Marius claimed to have discovered Io and the other moons of Jupiter in 1609, one week before Galileo's discovery. Galileo doubted this claim and dismissed the work of Marius as plagiarism. Given that Galileo published his work before Marius, Galileo is credited with the discovery.
Source:
Unlike most of the moons in the outer solar system Io may be somewhat similar in bulk composition to the terrestrial planets, primarily composed of molten silicate rock. Recent data from the Galileo probe indicates that Io has a core of iron (perhaps mixed with iron sulfide) with a radius of at least 900 km.
Source:
Io's iron core is giant, taking up half its diameter. The core was probably formed from either the intense tidal heating it undergoes from Jupiter's gravity - or the core was formed during the original formation of the moon.
Source:
Io was first discovered by Galileo in 1610, making it one of the Galilean Satellites. Of the 60 moons it is the 5th closest to Jupiter, with a standoff distance of 421,600 km. It is the 4th largest moon of Jupiter, with a diameter about 2/3 the distance across the United States, of 3630 km (2256 miles).
Source:
Io orbits Jupiter at a distance of 421,700 km (262,000 mi) from the planet's center and 350,000 km (217,000 mi) from its cloudtops. It is the innermost of the Galilean satellites of Jupiter, its orbit lying between those of Thebe and Europa. Including Jupiter's inner satellites, Io is the fifth moon out from Jupiter. It takes 42.5 hours to revolve once (fast enough for its motion to be observed over a single night of observation). Io is in a 2:1 mean-motion orbital resonance with Europa and a 4:1 mean-motion orbital resonance with Ganymede, completing two orbits of Jupiter for every one orbit completed by Europa, and four orbits for every one completed by Ganymede. This resonance helps maintain Io's orbital eccentricity (0.0041), which in turn provides the primary heating source for its geologic activity (see the "Tidal heating" section for a more detailed explanation of the process).[26] Without this forced eccentricity, Io's orbit would circularize through tidal dissipation, leading to a geologically less active world.
Source:
MORE ABOUT
Moons