Io is the strangest of JupiterΓÇÖs moons. At three quarters the size of the planet Mercury, it is gigantic for a satellite. It is also one of the few moons in the solar system that has active volcanos. The constant erup- tions indicate that IoΓÇÖs surface is one of the youngest in our planetary system. Its interior must be so hot that its most abundant chemical, sulfur, melts within it.
IoΓÇÖs atmosphere, like those of Sat- urnΓÇÖs moon Titan and NeptuneΓÇÖs Triton, is very thin. Io, Titan, and Triton are the only moons in our solar system with this property. IoΓÇÖs atmosphere, which is probably pro- duced by volcanic activity, seems to consist mostly of sulfur dioxide and glows with a yellow halo of sodium atoms. The halo extends so far that it leaves a ring of gas around Jupiter.
