Saturn has a magnetic field whose boundaries extend 20 times farther than Earth's. The field is so large that the rings and inner moons are all affected by it. It is generated by the motion of highly compressed hydrogen and rock deep within the planet.

Cassini carries two different magnetometers: one measures the magnitude and the other the direction of Saturn's magnetic field. "Measuring subtle changes in the magnetic field allows us to see inside the planet," says Michele Dougherty, a space scientist at Imperial College London.

This Hubble Space Telescope image from 1997 pictured Saturn's ultraviolet aurora for the first time. Like Earth's aurorae, the light show is generated when charged particles from the Sun hit the planet's magnetic field.