Cassini's First D-Ring Crossing
The sounds and colorful spectrogram in this still image and video represent data collected by the Radio and Plasma Wave Science, or RPWS, instrument on NASA's Cassini spacecraft, as it crossed through Saturn's D ring on May 28, 2017.
This was the first of four passes through the inner edge of the D ring during the 22 orbits of Cassini's final mission phase, called the Grand Finale. During this ring plane crossing, the spacecraft was oriented so that its large high-gain antenna was used as a shield to protect more sensitive components from possible ring-particle impacts. The three 33-foot-long (10-meter-long) RPWS antennas were exposed to the particle environment during the pass.
As tiny, dust-sized particles strike Cassini and the RPWS antennas, the particles are vaporized into tiny clouds of plasma, or electrically excited gas. These tiny explosions make a small electrical signal (a voltage impulse) that RPWS can detect. Researchers on the RPWS team convert the data into visible and audio formats, some like those seen here, for analysis. Ring particle hits sound like pops and cracks in the audio.
Particle impacts are seen to increase in frequency in the spectrogram and in the audible pops around the time of ring crossing as indicated by the red/orange spike just before 14:23 on the x-axis. Labels on the x-axis indicate time (top line), distance from the planet's center in Saturn radii, or Rs (middle), and latitude on Saturn beneath the spacecraft (bottom).
These data can be compared to those recorded during Cassini's first dive through the gap between Saturn and the D ring, on April 26 (see PIA21446). While it appeared from those earlier data that there were essentially no particles in the gap, scientists later determined the particles there are merely too small to create a voltage detectable by RPWS, but could be detected using Cassini's dust analyzer instrument.
After ring plane crossing (about 14:23 onward) a series of high pitched whistles are heard. The RPWS instrument detects such tones during each of the Grand Finale orbits and the team is working to understand their source.
The D ring proved to contain larger ring particles, as expected and recorded here, although the environment was determined to be relatively benign – with less dust than other faint Saturnian rings Cassini has flown through.
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, Calif. manages the mission for NASA's Science Mission Directorate, Washington, D.C. The radio and plasma wave science team is based at the University of Iowa, Iowa City.