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Io
( "EYE oh" 
) is the fifth of Jupiter's
known satellites and the third largest; it is the innermost of the
Galilean
moons. Io is slightly larger than Earth's Moon.
orbit: 422,000 km from Jupiter
diameter: 3630 km
mass: 8.93e22 kg
The pronounciation "EE oh" is also acceptable.
Io was a maiden who was loved by Zeus (Jupiter) and transformed into a heifer in a vain attempt to hide her from the jealous Hera.
Discovered by Galileo and Marius in 1610.
In contrast to most of the moons in the outer solar system, Io and Europa may be somewhat similar to the terrestrial planets, mainly made of molten silicate rock. Recent data from Galileo indicates that Io has a core of iron (perhaps mixed with iron sulfide) with a radius of at least 900 km.
Io's
surface is very different from any other body in the solar system. It
came as a very big surprise to the Voyager
scientists on the first encounter. They had expected to see impact
craters like those on the other terrestrial bodies. But there are
very few, if any, impact craters on Io. Therefore, the surface is
very young.
Instead of
craters, Voyager 1 found hundreds of volcanic calderas.
Some of the volcanoes are active! Striking photos of actual
eruptions with plumes 300 km high were sent back by both Voyagers
(right) and by Galileo (below). This may have been the most important
single discovery of the Voyager missions; it was the first real proof
that the interiors of other "terrestrial" bodies are actually hot and
active. The material erupting from Io's vents appears to be some form
of sulfur or sulfur dioxide. The volcanic eruptions change rapidly.
In just four months between the arrivals of Voyager
1 and Voyager
2 some of them stopped and others started
up. The deposits surrounding the vents also changed
visibly.
Recent images taken with NASA's Infrared Telescope Facility on Mauna Kea, Hawaii show a new and very large eruption (right). A large new feature near Ra Patera has also been seen by HST. Images from Galileo also show many changes from the time of Voyager's encounter. These observations confirm that Io's surface is very active indeed.
Io
has an amazingly varied surface: calderas up to several kilometers
deep, lakes of molten sulfur (below right), mountains which are
apparently NOT volcanoes (left), extensive flows hundreds of
kilometers long of some fluid (some form of sulfur?), and volcanic
vents. Sulfur and its compounds take on a wide range of colors which
are responsible for Io's varigated appearance.
Some of the
hottest spots on Io reach temperatures of 1500 K though the average
is much lower, about 130 K. These hot spots are the principal
mechanism by which Io loses its heat.
Though Io, like Earth's Moon , always faces the same side toward its planet, the effects of Europa and Ganymede cause it to wobble a bit. This wobbling stretches and bends Io by as much as 100 meters (a 100 meter tide!) and generates heat the same way a coat hanger heats up when bent back and forth. (Lacking another body to perturb it, the Moon is not heated by Earth in this way.)
Recent data from Galileo indicate that Io may have its own magnetic field as does Ganymede.
Io has a thin atmosphere composed of sulfur dioxide and perhaps some other gases.
Unlike the other Galilean satellites, Io has little or no water. This is probably because Jupiter was hot enough early in the evolution of the solar system to drive off the volatile elements in the vicinity of Io but not so hot to do so farther out.
... Jupiter ... Thebe
... Io ... Europa ... 
