New Scientist has a good <a href="http://www.newscientist.com/ns/20000108/lightfanta.html">feature story</a> on Io, one of Jupiter's most "scenic" moons. The article describes Io's dramatic and colorful aurora in fascinating detail. Unlike Earth's aurora which are powered by the solar wind, Io's is caused by Jupiter's intense magnetic field. As Io whips around the gas giant, it cuts through this field. Like a huge neon sign, the surprisingly dense atmosphere lights up as electrons from the Jovian wind race from Jupiter to Io and back.