Another of Cassini's unexpected discoveries was a radiation belt existing much closer to the planet than scientists had thought possible. Saturn has one other radiation belt, outside the rings. Cassini's magnetospheric imaging instrument (MIMI) discovered the new belt encircling the planet between the cloud tops and the inner edge of the D ring, the innermost ring. Before the Cassini mission, scientists had thought that a radiation belt could not exist between the D ring and Saturn's atmosphere because Saturn's magnetic field would prevent the ions from entering the rings. Apparently, however, as particles trapped in the main radiation belt collide with Saturn's upper atmosphere, some of those particles “leak” into the region inside the innermost ring, creating a second radiation belt.
In early 2004, while Cassini was still en route to Saturn, the spacecraft's ultraviolet imaging spectrometer (UVIS) detected the sudden and surprising appearance of an immense cloud of oxygen surrounding Saturn. Scientists theorized that the oxygen atoms emerged from ice particles created as small moonlets in Saturn's E ring collided and broke apart. Radiation given off by Saturn freed the oxygen atoms from the ice particles. The oxygen cloud, equivalent to the total material in Saturn's E ring, dissipated within about two months. Cassini scientists will be watching to see if this startling process occurs again.