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Io has a thin atmosphere composed of sulfur dioxide and perhaps some other gases.
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Io cuts across Jupiter's magnetic field lines, generating an electric current. Though small compared to the tidal heating, this current may carry more than 1 trillion watts. It also strips some material away from Io which forms a torus of intense radiation around Jupiter. Particles escaping from this torus are partially responsible for Jupiter's unusually large magnetosphere.
Recent data from Galileo indicate that Io may have its own magnetic field as does Ganymede.
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Some of the hottest spots on Io may reach temperatures as high as 2000 K though the average is much lower, about 130 K. These hot spots are the principal mechanism by which Io loses its heat.
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Io 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.
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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.
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Satellite
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Orbit: 421,600 km
Diameter: 3,630 km
Mass: 8.94e+22 kg
Discovered by: Simon Marius & Galileo Galilei
Date of discovery: 1610
Rotational period: 1.769138 days
Orbital period: 1.769138 days
Mean orbital velocity: 17.34 km/sec
Orbital eccentricity: 0.004
Orbital inclination: 0.040 degrees
Escape velocity: 2.56 km/sec
Visual geometric albedo: 0.61
Magnitude: 5.02 Vo
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