Spectrum from Cassini intruments indicating atmosphere over rings

During the NASA/ESA/ASI Cassini spacecraft's insertion into Saturn orbit on 1 July 2004, it passed closer to the rings than any earlier spacecraft.

The electron spectrometer from University College London (with RAL and NDRE), part of the SWRI-led Cassini Plasma Spectrometer (CAPS) instrument, saw the rings through 'electron eyes', with electron intensity inversely related to ring particle density.

The bottom panel shows CAPS-ELS data with colour scale proportional to number of electrons. The top panel shows a Cassini narrow-angle camera visble image of the ring structure for comparison.

Over most of the thick B ring, electron fluxes are relatively low. Over the relatively empty Cassini division and Encke gap, the electron fluxes are high and structured. Over the thin A ring, the fluxes are intermediate and structured. The CAPS scientists interpret the observations as photoelectrons from the sunlit side of the rings (far side from the spacecraft) attenuated by the rings themselves, with structure from ring ionospheric photoelectrons.

At the right hand side of the plot, Cassini’s magnetic footprint leaves the ‘safe haven’ over the main rings and penetrating radiation belt particles are seen at all energies.