IO ON THE EVE OF THE GALILEO MISSION

Io, innermost of Jupiter's large moons, is one of the most unusual obj ects in the Solar System. Tidal heating of the interior produces a glo bal heat flux 40 times the terrestrial value, producing intense volcan ic activity and a global resurfacing rate averaging perhaps 1 cm yr(-1 ). The volcanoes may erupt mostly silicate lavas, but the uppermost su rface is dominated by sulfur compounds including SO2 frost. The volcan oes and frost support a thin, patchy SO2 atmosphere with peak pressure near 10(-8) bars. Self-sustaining bombardment of the surface and atmo sphere by Io-derived plasma trapped in Jupiter's magnetosphere causes escape of material from Io (predominantly sulfur, oxygen, and sodium a toms, ions, and molecules) at a rate of about 10(3) kg s(-1). The resu lting Jupiter-encircling torus of ionized sulfur and oxygen dominates the Jovian magnetosphere and, together with an extended cloud of neutr al sodium, is readily observable from Earth.