The image of Saturn's magnetosphere (top) was captured by the Ion and Neutral Camera (INCA), part of the Magnetospheric Imaging Instrument on the Cassini spacecraft, shortly after Cassini entered the planet's orbit in July 2004. On June 20-21, as Cassini approached Saturn, the energetic neutral atom emissions captured by INCA (bottom) revealed that the magnetosphere rotated around the planet once about every 11 hr.

The image of Saturn's magnetosphere (top) was captured by the Ion and Neutral Camera (INCA), part of the Magnetospheric Imaging Instrument on the Cassini spacecraft, shortly after Cassini entered the planet's orbit in July 2004. On June 20-21, as Cassini approached Saturn, the energetic neutral atom emissions captured by INCA (bottom) revealed that the magnetosphere rotated around the planet once about every 11 hr.

Photo courtesy of Johns Hopkins University Applied Physics Laboratory.


Cassini entered Saturn's orbit last July, beginning a four-year study of the sixth rock from the Sun. And among the dozen science instruments on the ship is the Magnetospheric Imaging Instrument (MIMI), developed by the Applied Physics Laboratory (APL) of Johns Hopkins University. Scientists are using the INCA to study the energetic-charged-particle environment and to obtain images of the planet's magnetosphere.

The instrument lets scientists image the plasma and radiation belts in Saturn's environment. The imaging reveals: an unexpected radiation belt inside of the "D" ring; that the belts are more intense on the night side of the planet; and a sea of ions probably resulting from radiation impacting the rings. The innovative camera measures the three-dimensional distribution, velocities, and rough composition of magnetospheric and interplanetary ions.

"Using INCA, we also discovered a radiation belt in a place where no spacecraft can go — inside the planet's rings," says APL's Dr. ED Roelof, a member of the MIMI team. "We never knew this belt existed, but we saw it and were able to determine some of its properties and characteristics."

"It's comprised mostly of oxygen and water products," says Dr. Doug Hamilton of the University of Maryland, College Park, "most likely the result of the bombardment of the planet's rings and icy moons by the radiation trapped in the magnetic field." The bombardment releases the water and it becomes charged.

The ability to visualize a planet's magnetosphere could help scientists better monitor space weather, possibly leading to advanced warning of electromagnetic storms.