Unlocking Saturn's Mysteries with Sound

Jun 13, 2017

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.


Research Scientist Bill Kurth talks from his University of Iowa office about his long career of studying radio and plasma waves in our solar system. As the lead co-investigator for the Cassini’s RPWS instrument, he analyses waves detected in the Saturnian system.
Credit John Pemble / IPR

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.


A spectrograph of the radio emissions from Saturn between 30,000 and 80,000 hertz. Because the human hearing threshold ends around 16,000 hertz, a computer program lowers them to accommodate our ears.
Credit NASA / JPL / University of Iowa

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.


The D ring is closest to Saturn. It’s thin and less noticeable compared to other rings. Bright specs and streaks are image artifacts left to preserve ring details. Additional processing by IPR to bring out hard to see rings lines. Photo taken from Cassini 6/6/2013
Credit NASA / JPL / Space Science Institute

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.”


The thin F ring is one of the outer rings of Saturn. The Cassini probe has safely passed through this layer, impacted by hundreds of micro particles every second. The dot in the upper left is the moon Epimetheus. From Cassini's camera 12/16/2006
Credit NASA / JPL / Space Science Institute

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.


Enceladus is one of Saturn’s 62 moons. Geysers spray a continuous hail of ice grains coating the moon’s surface with snow. It orbits above the outer E ring of the planet. Photo by Cassini spacecraft 3/22/2006 while 810,000 miles away from the moon.
Credit NASA / JPL / Space Science Institute

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.


The sun is behind Saturn, allowing Cassini to take a backlit photograph of the planet and rings. Composite images taken using infrared, red and violet spectral filters on 10/17/2012. Also captured in the left are two of Saturn's moons Enceladus and Tethys.
Credit NASA / JPL / Space Science Institute