During an encounter with Jupiter in 1992, the "Ulysses" spacecraft detected a stream of dust-sized particles being ejected from the Jovian system. The dust in these discrete streams travels away from Jupiter at speeds upwards of several hundred kilometres per second, has an average particle size of 10 μm, and consists primarily of sodium chloride. Dust measurements by "Galileo" showed that these dust streams originate from Io, but exactly how these form, whether from Io's volcanic activity or material removed from the surface, is unknown.