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JPL podcast: Lightning Crackles on Saturn

Feb. 14, 2006
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NARRATOR: It was a dark and stormy night - on Saturn. I'm Jane Platt and this is a podcast from JPL - NASA's Jet Propulsion Laboratory in Pasadena, California.

NATURAL SOUND: CRACKLING SOUNDS

NARRATOR: Those strange sounds were captured by the Cassini spacecraft when it witnessed a lightning storm on Saturn on January 23 and 24. We're going to find out more about those sounds and what they mean. With us today is Dr. Bill Kurth of the University of Iowa. He's the deputy principal investigator for the radio and plasma wave science instrument on the Cassini spacecraft, which has been orbiting Saturn and its moons since July 1, 2004. So what exactly are we hearing in those odd sounds?

KURTH: Well, we have a radio receiver on Cassini, and if you had a similar receiver in your car, which you do, an AM radio, and you were driving your car during a thunderstorm, listening to an AM radio station, you would hear cracks and pops. Most people think it's interference, what it really is, those cracks and pops are radio emissions that are generated by the lightning flashes. And so on Cassini, what we're doing is using our radio to detect the radio emissions from lightning.

NARRATOR: And it's lightning on Saturn like the lightning we have on Earth, the same phenomenon?

KURTH: Well, we think it is. It's definitely a discharge process, a big spark that releases a lot of energy in a very short period of time. And a lot of work that was done on a similar phenomenon observed by Voyager 1 has led to the conclusion it must be lightning in the atmosphere.

NARRATOR: So this wasn't the first time Cassini has witnessed lightning, but this was the biggest lightning storm, is that correct?

KURTH: That's right. Since we've been in orbit, we've detected maybe four or five such storms. This is by far the strongest that we've detected with Cassini. And it's even stronger and there's more activity than the storms that were detected by the two Voyager spacecraft back in the early 1980s.

NARRATOR: So your instrument picked up these radio emissions and you've converted it into this audio file. We won't go into the whole technology of that, because I'm assuming it's pretty complicated, but I did want to know how these audio files compare to what you might actually hear if you somehow were able to be hovering in the atmosphere of Saturn.

KURTH: Well, if you were actually in the atmosphere and you were listening, you would probably hear thunder like we do here on Earth when lightning goes off. If you're on the spacecraft that's a long ways away from the atmosphere, of course sound doesn't travel through the very tenuous gas surrounding the planets, outer space, but radio waves can. And so the radio waves from these lightning strokes would be detected by, for example, the AM radio on your car, and it would sound like cracks and pops that you hear when you drive through a thunderstorm here on Earth. The sounds that we've created are not exact representations of those same cracks and pops. They've gone through a process that's not dissimilar from someone using a Moog synthesizer, for example, and creating a lot of the music that was created back in the 60s, for example.

NARRATOR: Saturnian disco maybe?

KURTH: Oh I suppose.

NARRATOR: Okay, we've all of course seen lightning on Earth and you believe that what's happening on Saturn is similar. Just a refresher course, what is lightning, what is thunder, what causes this phenomenon?

KURTH: Well, actually this is a subject of some study even at Earth. Some of the facets of lightning we think we understand fairly well, others we don't. But at least on Earth we think the convection in thunderstorms is important in the process in basically separating charges, that is positive charges from negative charges. And if you separate enough charges, you build up a very large potential field between those separated charges. And nature doesn't like that, and it tries to correct the situation by running a current between the two separated charges. And that forms a very large spark, there's an awful lot of energy that's released, and it's meant to basically allow the negative charges and the positive charges to get back together again.

NARRATOR: So something's a little out of whack and it's nature's way of fixing it.

KURTH: That's right.

NARRATOR: Ok. Interesting because I didn’t realize that the phenomenon of lightning is not fully understood on Earth. Is it possible that what you learn from the lightning storms on Saturn could end up helping us understand our own lightning on Earth?

KURTH: Well, I think so. Presumably we'll understand what's going on on Earth even better than we do now.

NARRATOR: And you also have some visual data of the storm.

KURTH: My colleague who first detected these radio emissions, Mike Kaiser from the Goddard Space Flight Center, used to be a member of an amateur organization called the Amateur Lunar and Planetary Observers organization. And he contacted one of their members and said, "Can you find out if any of the amateurs have taken photos of Saturn recently and if there's any storm activity?' What we would expect to see is a white cloud that suggests a convective storm, kind of like the top of a thunder cloud that we would see on Earth. And lo and behold, within hours, a couple of amateur astronomers about 50 miles south of Paris, Eric Bondeau and Jean Luc Doverne, excuse my pronunciation, actually took images of Saturn that showed a cloud very similar to what we would have expected. And that cloud, we believe, was under Cassini at the time we were detecting these radio emissions. So we don't have proof, but we have a pretty good idea that this particular storm might be the storm that's associated with the lightning that we're detecting.

NARRATOR: And I'd like to remind our listeners that we have Cassini images and info online at a couple of websites: www.nasa.gov/cassini and http://saturn.jpl.nasa.gov . Before we sign off, let's take one last listen to that lightning from Saturn.

NATURAL SOUND: CRACKLING SOUNDS

NARRATOR: On that note, thank you to Dr. Bill Kurth of the University of Iowa for talking with us today, and thanks to our listeners who joined us for this podcast from NASA's Jet Propulsion Laboratory.

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