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Definition: Io from Philip's Encyclopedia

Large, innermost satellite of Jupiter. It was discovered by Galileo in 1609-10, and is larger than the Moon. It is more than 3,600km (2,200mi) in diameter, and is 422,000km (262,000mi) above the surface of the planet.

Summary Article: Io from Collins Dictionary of Astronomy

The innermost of the four giant Galilean satellites of Jupiter. It keeps one face permanently toward the planet (see synchronous rotation) as it orbits at a distance of 422 000 km, within Jupiter's magnetosphere. Io is yellow-brown in color, has a high albedo (0.61), and has a diameter of 3630 km. The orbital period of Io is one half that of Europa and one-fourth that of Ganymede; the three satellites are locked into an orbital resonance, the interaction of the satellites having caused their orbital distances to adjust until their periods were common multiples. Early photographs from the Voyager 1 probe March 1979 showed the surface of Io to lack impact craters. An explanation for this was found when intense volcanic action was observed on the satellite: at least nine active volcanoes were seen to eject material over a very wide area; seven were still active when Voyager 2 arrived four months later. Following the six passes of Io made by the Galileo spacecraft during its 33 orbits of Jupiter between 1996 and 2002, scientists now know there are at least 120 active volcanic regions. Few of Io's volcanoes resemble the crater-topped mountains seen on the Earth and Mars. Instead, most of Io's volcanic craters lie in relatively flat regions. Yet half the mountains on Io are located next to volcanic craters.

Io is the most active volcanic body known in the Solar System. The activity is thought to result from heating by tidal forces exerted by Jupiter. There are several types of activity: eruptive plumes, a few of which reach altitudes of hundreds of kilometers; huge calderas (volcanic collapse craters) with associated lava flows and/or surface markings; some possible lava lakes. The first volcano observed by Voyager 1 was an eruptive plume ascending to 280 km and visible above the satellite limb; material was deposited in concentric rings up to about 1400 km across. This volcano has been named Pele. The plume of Loki was active on both Voyager passes, rising to 100 km as measured by Voyager 1 and 200 km as measured by Voyager 2. Many of the plumes, such as that of Prometheus, were 50–120 km high and also apparently long-lived. Ionized matter – sulfur, oxygen, and hydrogen – escapes from the tenuous volcanic atmosphere of Io and forms a torus centered on Io and encompassing the whole of Io's orbit. The particles in the torus glow brightly in the ultraviolet. This matter interacts with Jupiter's magnetosphere: it controls the rate at which Jupiter radiates energy and probably affects aurorae on Jupiter, radio bursts from the planet, and other phenomena. See also Galilean satellites; Jupiter; Jupiter's satellites; Table 2, backmatter.

© Market House Books Ltd, 2006

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