Unlocking Saturn's Mysteries with Sound
The space probe Cassini has been exploring Saturn since 2004. One of the instruments on the two story tall spacecraft is from the University of Iowa called the Radio Plasma and Wave Science (RPWS) instrument. It picks up Saturn’s radio waves.
University of Iowa scientist Bill Kurth takes telemetry from the RPWS and converts it to audio files in the human hearing range. It’s a mix of ascending tones. Some have a squealing quality.
“What we’re listening to are radio emissions that are generated through kind of the same process that creates the Northern Lights,” says Kurth.
The sound is, for lack of a better term, time lapse audio. For some of these audio files one minute equals hours of readings. As electrons quickly travel across a planet’s magnetosphere, electromagnetic radiation emits radio frequencies. So does plasma gas. Cassini measures these emissions, and these audio files are like a voice print of the planet.
University of Iowa scientists have been analyzing this kind of data for decades. There were RPWS instruments on the Voyager probes launched in the 1970s. Usually scientists use spectrogram versions of this data to study, but creating audio makes it easier to get another perspective. Bill Kurth says hearing these readings also generates interest from the general public about science. “When you play the sound they can start to relate, wow that’s really neat,” says Kurth.
One of the mysteries scientists are trying to solve with RPWS readings is determining the length of a Saturnian day, which is approximately 10 hours, 42 minutes. “With a gas giant, you don’t really know what the rotation period is because you’re looking at clouds, you’re not looking at a crater or a mountain that you can just clock how long it takes to rotate,” says Kurth.
The planet’s emissions should reveal the exact length of a day, but Kurth can’t get a steady measurement. “At certain times we can see a different period coming from the northern hemisphere than what we see from the southern hemisphere. So it’s just a. It’s an amazing set of observations that we don’t understand I don’t think,” says Kurth.
Right now Cassini is making a series of weekly plunges through the never explored D ring layer, the layer closest to the planet. In the past when Cassini passed through the F layer, the outer rings, Cassini was bombarded with hundreds of micro size particles every second. The RPWS picks up these impacts and the audio of this data sounds like clicks and pops.
On April 27th, when Cassini made its pass through the D layer, Kurth expected a similar reading but instead the probe detected virtually nothing. This surprised Kurth. “If there is dust there, it was orders of magnitude less dust, maybe a factor of a thousand or ten thousand less dust than we had seen in the F ring. If there’s dust it must be so tiny, that our instrument’s not sensitive to it.”
Cassini has been at Saturn for 13 years, but it’s almost out of fuel. On September 15th, the mission ends when the probe deliberately dives into the planet’s atmosphere, resulting in complete destruction.
Kurth says this is part of NASA’s planetary protection plan to avoid contaminating any of Saturn’s 62 moons like Enceladus which has massive amounts of water. “If we went there and found life after Cassini had crashed onto Enceladus, we wouldn’t know whether it’s life that came from Earth on Cassini or not,” says Kurth.
While Cassini’s mission ends soon, the University of Iowa will continue to analyze radio waves from other probes like Juno which has been orbiting Jupiter since last summer. The sounds the RPWS instrument is reading from this planet are also equally mysterious.