Galileo Flyby Discovers Immense Lava Fountain on a Jovian Moon

By an improbable stroke of good fortune, the north‐polar region of Io, the innermost of Jupiter's four Galilean moons, was putting on an extraordinary volcanic display just as NASA's Galileo spacecraft flew by to take pictures on Thanksgiving Day last year. Because its proximity to Jupiter and its orbital resonance with two neighboring moons subject it to severe tidal flexing, Io turns out to be the most volcanically active body in the Solar System. (See the cover of this issue.)