Planetary Radio: Space Exploration, Astronomy and Science - The Mystery and Music of Saturn's Rotation with Don Gurnett

Episode Date: June 20, 2011

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The mystery and the music of Saturn's rotation, this week on Planetary Radio. Welcome to Public Radio's travel show that takes you to the final frontier. I'm Matt Kaplan of the Planetary Society. The Cassini spacecraft continues to solve old riddles about the ringed planet, even as it uncovers new and even greater mysteries. For example, we now know that the accepted rate at which Saturn turns, which has been in textbooks almost since the Voyager missions, is wrong. And that's not the half of it, as you'll hear from Don Garnett, Cassini's radio

and plasma wave science team lead. You'll also hear the eerie sounds of Saturn as it turns. Later in today's show, the eerie sounds will be coming from Bruce Betts as he introduces another random space fact, along with a new space trivia contest. Bill Nye is away this week, but I know you'll be glad to hear the return of the Planetary Society's Science and Technology Coordinator, Emily Lakdawalla. Emily, it seems like it's been ages. Welcome back. Thank you, Matt. It's nice to be back. I'm glad because we missed you and we're looking forward to hearing some of these highlights, recent highlights from the blog. I'm going to mention very quickly right up front something that is just worthwhile for the beauty of it,

and that is, amazingly, Phobos, passing through the frame with Jupiter in the background. It's really quite beautiful, and people should take a look at that. It's a June 17 entry that you got into the blog. Let's move on to talking about Messenger, which is starting to release some really interesting data about Mercury telling us that it's a unique place. That's right. You know, Messenger has now

been in orbit for one year, one Mercury year, that is, which is, of course, only 88 Earth days long, pretty short year. And they're beginning to get a sense of the shape of the planet and the elemental abundances and all the kinds of other stuff that they sent Messenger there to get. I'll talk about one particular result that they discussed at a press conference this week. And that had to do with the ratio between the abundances of two elements on Mercury's surface. They were measuring the abundances of potassium, atomic symbol K, and thorium, atomic symbol TH. And the reason that they're interested in looking at those elements and measuring their relative abundance is because potassium is what they call a volatile element.

If you heat up a rocky place really, really, really hot, potassium is one of the first elements that floats away on the solar wind and never comes back again, whereas thorium is what they call a refractory element. It only boils away at extremely high temperatures. So no matter what, the thorium is going to stick behind in your rocky body. And the reason this is relevant to mercury is because there is a lot of different ideas about how mercury could have formed.

In some of these theories, mercury had a very hot period in its history. And in some of these, mercury never got so hot. The abundance of potassium to thorium is really a critical measurement to differentiate among these theories and decide which ones are more likely to be correct. They predicted that Mercury is going to have a low potassium to thorium ratio like the moon indicating that it had a very hot period in its history. It turns out that so far as they can tell, not only is its potassium to thorium ratio not low and moon-like, it's even higher than Mars, which had the coolest formation temperature of any of the

terrestrial planets. So that's very odd. They're going to have to go back to the drawing board on their ideas of how Mercury formed, and they're not going to be able to take pieces from the formation histories of Earth and Moon. They're going to have to make up their own new ideas about how Mercury formed. No shortage of surprises left for us in the solar system. Sean Solomon must be a very excited head of the Messenger mission. We'll get him back on this show soon. Very quickly, just one more thing to mention, and that is Dawn getting closer and closer. Yeah, you know, they released another image just a few days ago, and Vesta is still pretty fuzzy, but it's getting lumpier and

bigger and craters are beginning to come into view. And I just can't wait to see it come more sharply into focus. The funny thing is that at this stage, it's still fuzzy enough that no matter where you look, people see faces on it. I've heard of monkey faces, surprised faces, fairy faces, all kinds of different faces. So I don't know what the distance we'll have to get to within Vesta in order to stop seeing faces on Vesta. But right now we're still on the faces on Vesta stage. Yes, the faces on Vesta. I'm sure there'll be a book out about that very soon. Emily, great to have you back. Thanks once again. Thank you, Matt.

Emily Laktawal is the Science and Technology Coordinator for the Planetary Society and a contributing editor to Sky and Telescope magazine. I will be right back with our very special guest this week, Don Gurnett. How could a planet's top half be rotating at a different rate than its bottom half? It can't, even though that was a possible conclusion from recent data collected by the Cassini spacecraft as it orbits Saturn. It gets even weirder, as you'll hear from Don Gurnett, who has been making discoveries about our solar system since the dawn of the space age.

Don has been professor of physics at the University of Iowa for more than 45 years. He has contributed to planetary science missions around our solar system, almost back to Explorer 1, the first American satellite. Most of his discoveries have been related to the radio and plasma waves that emanate from our sun and many planets. His fascination with radio receivers goes back to his boyhood in Iowa, when he became an early builder of radio-controlled airplanes. They still had tubes back then. Don has been equally enthralled by the sounds made by heavenly bodies once you process and compress the signals properly.

Those sounds have even inspired musical compositions, as you'll hear if you listen to my complete conversation with Don at Planetary.org. We talked late last week. Don, it is really an honor to welcome you to Planetary Radio. Thanks for being part of the show. Yes, pleased to be with you. We've got to talk a little bit about your amazing history, which traces just about the entire history of the American effort to explore space.

Before we get to talking about your current work on Saturn, although that's certainly not the only thing you have going on, I thought that you got there even before Explorer 1, but no, apparently it was just after that that you started to work with one of the most famous actors in the American space program. Yes, I was here at the University of Iowa studying engineering as a freshman, actually, in 1957. And, of course, in 1957, Sputnik 1 was launched. And just a few months after that, Professor James Van Allen in our physics department built the Geiger tube that was launched on the first U.S. spacecraft, Explorer 1.

And that was right across the street from the engineering building, and that caught my attention, as it also caught the nation's attention. I'll say. And I went over and asked them if I could possibly start working for them in engineering work, and they hired me. So this is my 53rd year, I believe, working on space research. And Van Allen, of course, responsible for what we still know as the Van Allen belts discovered by that very first American satellite.

Yes, and also Van Allen had an instrument on Pioneer 10 and 11 spacecraft, which flew to first Jupiter and discovered the radiation belt at Jupiter, and Pioneer 11 went to Saturn and discovered the radiation belt at Saturn. Let's go to another bit of data, Voyager returned, which told us how fast Jupiter was spinning, and that has everything to do with a story that we're going to talk about in greater detail today out at Saturn. The outer planets, Jupiter, Saturn, Uranus, and Neptune, have the unusual feature they're covered with clouds and they have no surface. So there's an issue of what their rotation rate is because you can't really see any, well, you can see cloud motions, but the clouds move at different rates at

different latitudes. So historically, the way in which, if you look at an astronomy book and look up the rotation rates of these planets, it's all based on radio observations. They produce radio emissions, charged particles that are controlled by the magnetic field of the planet generates a radio emission. And the magnetic field is linked to the deep interior. So as the planet rotates, typically the magnetic axis is tilted a little bit relative to the rotation axis, kind of like at the Earth.

And as it rotates, you get a modulation in the radio intensity. And by measuring that period, you average over several years, you can get a very accurate number. Now, with Voyagers 1 and 2, when we flew by Saturn, we started to detect the radio emission from Saturn, I think about six months before closest approach. from Saturn, I think about six months before closest approach. And we came up with a rotation period of 10 hours, 39 minutes, 24 plus or minus 7 seconds. Seemingly a very accurate number. Yeah, certainly sounds like it.

Right. That's in all the textbooks, astronomy textbooks, for example. And that period was adopted as the official International Astronomical Union rotation period of Saturn. But then a few years later, quite a few years later actually, there was another spacecraft that had a radio instrument built by our French colleagues. spacecraft that had a radio instrument built by our French colleagues. And they discovered that the radio period of Saturn varied by a small amount, about 1%. Now, 1% may not seem like much, but, you know, a big planet like that,

its period just can't change by anything like that. that, its period just can't change by anything like that. It's as though here at Earth, you know, one day the rotation period were 24 hours, and the next, you know, a few years later it was 23 hours and 50 minutes. Another puzzling aspect was that from the Voyager measurements, we knew that the magnetic axis was lined up almost parallel to its rotational axis. So that was another big puzzle, and still is, actually. And so what you're saying there is that rather than Earth, as you said, where the magnetic north pole is offset from the actual north pole. That's correct.

And therefore, there's no reason, given that it's exactly aligned, there's no reason there should be a modulation in the radio period. Do you sort of follow that line of thinking? Yeah, sure. Because it takes a tilt to cause the radio beaming to oscillate as the planet rotates. Well, the next kind of step in the story is the Cassini spacecraft, which was launched in, let's see now if I can remember the date, 1997. It arrived at Saturn on July 1, 2004.

And we have a radio instrument on Cassini, a very sensitive instrument that covers a wide range of frequencies. We started picking up the radio signal from Saturn about two years before we arrived at Saturn. And we carefully measured the period. In fact, I think you have a recording that I sent you where you can actually hear the modulation as the planet rotates. I don't know if you're going to play that or not. You bet. In fact, tell us a little bit more about that recording, and then we're going to play it for the audience.

Well, we did there in the recording you're going to listen to. The radio emission is actually up at about 100 to 300 kilohertz, which you can't hear, but we've shifted signal down to the audio range, kind of like you do in your car radio in some respects. And then we took five days and we compressed it into about 15 seconds. And then if you listen to that recording, you can hear this kind of... every rotation. Simple matter to get the period, then. You just count the rotations over maybe 100 rotations,

and you can get a very accurate number. maybe 100 rotations, then you can get a very accurate number. We came up with a number of 10 hours and 45 minutes, 45 minutes, 45 seconds. So we confirmed that the radio emission indeed is varying by a small amount. Stay with us for more of the sounds of Saturn when we return with Cassini scientist Don Gurnett. This is Planetary Radio. I'm Sally Ride. After becoming the first American woman in space,

I dedicated myself to supporting space exploration and the education and inspiration of our youth. That's why I formed Sally Ride Science, and that's why I support the Planetary Society. The Society works with space agencies around the world and gets people directly involved with real space missions. It takes a lot to create exciting projects like the first solar sail, informative publications like an award-winning magazine, and many other outreach efforts like this radio show. Help make space exploration and inspiration happen. Here's how you can join us. You can learn more about the Planetary Society at our website, planetary.org slash radio, or by calling 1-800-9-WORLDS.

Planetary Radio listeners who aren't yet members can join and receive a Planetary Radio t-shirt. Members receive the internationally acclaimed Planetary Report magazine. That's planetary.org slash radio. The Planetary Society, exploring new worlds. Welcome back to Planetary Radio. I'm Matt Kaplan. Space science pioneer Don Garnett has been telling us about the mysteries of Saturn's rotation. Again, you can hear more of my fascinating conversation with Don at planetary.org. Just click the show link.

Even before it went into orbit around the ringed planet, the Cassini spacecraft was able to learn that the long-accepted rotation rate was not just wrong, it seemed to vary. And then, of course, we put Cassini in orbit around Saturn, which allows us to make long-term observations, you know, over several years. It took a while, but after about four years, we had enough data that we could really analyze these variations in the rotation period. And we discovered another amazing thing, which is that there were actually two periods, approximately 10.6 hours and 10.8 hours. Which sounds impossible. I mean, was it tempting to think, okay, we got a planet with one hemisphere rotating faster than the other? Oh, that's the big puzzle here. The basic picture is that the radio emissions are produced in what we call the auroral zone,

up at high latitudes, approximately 15 degrees or so from the pole. In fact, these radio emissions are associated with the aurora. There's electrons that come down the magnetic field line. They produce radio emission. They also produce the aurora. And you can see these emissions. You can see the auroras wax and wane along with the emissions, right? Yes.

The imaging of the camera on Cassini, and also Hubble can take pictures of the aurora. It's very clear. So that was a big puzzle. Then about a year later, actually last year, 2010, I discovered that these two rotation periods reversed shortly after the equinox, which occurred in August of 2010. We went through equinox, where the rings are edge-on as viewed from Earth. What happened is that the radio periods reversed. By reversed, I mean they sort of interchanged between the north and the south, rather slowly, but that was the crossing point. And the crossing point didn't occur exactly at equinox, but it was about seven months after equinox. We went back and analyzed the Ulysses data, and we discovered a similar reversal at the previous equinox.

So what we've discovered is that there's a, I'll call it a seasonal effect. I mean, that's the significance of the equinox. You essentially go from in the southern hemisphere to approaching summer in the northern hemisphere. This, I think, is a very important clue. And it's led us to think that what's happening is this magnetic field, there's been a big debate actually going on about whether this rotation period has something to do with the internal magnetic field, i.e. something to do with some kind of rotating feature internal to the planet,

or whether the magnetosphere, that's the region around the planet where the magnetic field controls the electrons that produce the radio emission. Which is huge. It's huge, that's correct. So we came up with this idea that the magnetosphere in the north and the south are somehow slipping in their rotation relative to the interior. That's our current concept. And that the slippage rate is different in the two hemispheres, the north and the south.

And that's a big puzzle these days that people are trying to understand how that happens. I've advanced a specific idea, which we're now in the process of testing, and that is the rotation of the magnetosphere is driven by high altitude winds, azimuthal winds, you know, the east-west winds. Because of the delay time after equinox, I made the point that that seems to point to an atmospheric effect. You know, it actually gets warmer later in the summer, a delay about a month or so. So when you see a delay effect like that, it points to an atmospheric phenomena. So the thing we're pursuing is whether it's got to do with high-altitude winds, which are different in the northern-southern hemisphere,

which are in fact driving the rotation of the aurora and the magnetosphere. That's our current concept. You know, one of the points that has come up in our conversations with Linda Spilker over and over is the enormous value of the fact that this spacecraft, Cassini, is out there at Cassini and is returning data over so many years, and I am sure you would agree. data over so many years, and I'm sure you would agree. Yes, that is a crucial aspect because, like I mentioned earlier, it takes 29 and a half years for Saturn to go around the sun, so to investigate seasonal effects takes many years of observations. At Earth, of course,

one year would do it, but some of these subtle effects, maybe not so subtle, this is actually a major effect having to do with the rotation period of Saturn. And we still don't accurately know the internal rotation rate. So the number that's quoted in astronomy textbooks is certainly not correct. In fact, we don't know exactly how to come up with the precise number. That's unique, as far as we know, to Saturn. Jupiter, which has a substantial tilt of about 10 degrees, has been just rock solid in the radio rotation rate for well over 50 years. I mean, down to a second or so. Uranus and Neptune also have a substantial tilt to their magnetic axis,

and they also have radio emissions. We don't know quite so much about them. The only thing we really know is what we got from Voyager 2, and we went by Uranus and Neptune. So we're talking about really two different signals, two magnetic beats, one from the northern hemisphere, one from the southern. And there is a recording that you've made where you've compressed the time. And there is a graphic representation as well.

And we will suggest to our listeners that they go to planetary.org where we will put up a link to that graphic representation. But we can play this for them now in this compressed form. And I think that people will find that it's rather musical, which is something that has, I think, fascinated you for many, many years. That's true. Actually, when you listen to that, we depicted the rotation rate, the two rotation rates as a tone,

which is actually proportional to the rotation rate, but we've had to shift it down into the audio range so you can hear it. So there is definitely some modifications we've made to the raw data to be able to produce an audio thing which you can listen to. But it is kind of musical. You can hear these two tones that gradually change in frequency. Then you can hear them come together and reverse shortly after the August 2010 equinox. Let's play that for people right now. So there is the recording, enhanced somewhat, as you heard from Don. And if you may have noticed, if not, definitely go to the website where you can hear it again, because I didn't really notice the first time. You can hear at just before the crossover point,

those few months after the Saturnian equinox, you can hear the higher frequency drop down, the lower one come up, they cross, and then you hear one going up and one going down. It is really fascinating. Yeah, I think it was an amazing result when we first uncovered this. Don, it has been a great pleasure. Thank you. Don Gurnett is Cassini's Radio and Plasma Wave Science

Instrument Team Lead and a long-time professor of physics at the University of Iowa in Iowa City, out there near the middle of the United States of America. We'll be right back for another look at the night sky with our friend Bruce Betts. Bruce Betts is the director of projects for the Planetary Society, and he also is here every week to tell us what's up in the night sky and talk about trivia, space trivia, and do other fun things. And we're going to do some of that right now. Welcome back. Good to be back. It's a special

week. No trivia answer this week, but I imagine that you have a question that you will lay on us in a few moments. I do. Fear not. There is a question. Observe a moment of silence for no trivia answer. What's up? All right, maybe we won't. What's up is Saturn in the evening sky over in the southwest, looking yellowish and kind of interesting right now it's got a little friend, Poryma, which is a star, which is a little bit dimmer, makes it look like kind of

a fake double star thing. And just to make it more exciting, Poryma with a telescope is actually a double star itself. There's just such doubling, it's very exciting. We've got in the pre-dawn. Check out Venus. Hard to miss over in the... No, no, don't check out Venus.

I keep wanting you to. You might be able to see Venus. It's very low in the east. Check out Jupiter. Super easy to see. Very bright. High in the east. And to the lower left of Jupiter,

you will have more trouble seeing Mars. Looking kind of dimmish and red, but it'll keep getting brighter over the coming months. And I got something else exciting, Matt. What's that? Supernova. Ooh, not a new one. Actually, it is a new one. Relatively new.

It's been brightening recently. It actually occurred on May 31st. Well, it occurred a really long time ago, but it was observed on May 31st. It's been brightening ever since. Not a naked eye object, but always cool when you get one of these. It's all the way over in the Whirlpool Galaxy. brightened up so you can actually see it where you could never see it before. It's magnitude 12.6 right now for those playing the magnitude game, which means you can see it in an amateur telescope. You probably need like an 8-inch telescope to get to it. Still an interesting kind of thing going on.

And 51, that's the Whirlpool Galaxy. Well, you mentioned telescopes, and I thought of it when you said Saturn, because I was lucky enough to be back up at the Griffith Observatory here in L.A. just yesterday, just last night. What a great place. All sorts of people coming out to look out over the city of Los Angeles and look up at the stars. Long line at the refractor telescope there that was pointed at Saturn. Oddly enough. Yeah, we didn't wait in line because it was going to be like an hour, but just warms my heart to see people waiting to look at what's up.

Yeah. Do check out Saturn in a telescope if you get a chance. That, pretty much any small telescope, even a good set of binoculars held still, you can see the rings. What else you got for us? We got this week in space history. It was this week in 2004 when spaceship one launched and became the first privately funded human space flight on its suborbital jaunt i was there yeah getting close to spaceship two now yeah getting there we move on

to random space fact supernova let's talk about them. They're cool. They're extremely luminous and bright. They often cause these bursts of radiation that often briefly outshine an entire galaxy before they fade after several weeks or months. weeks or months. Over those several weeks or months, a supernova can radiate as much energy as the sun is expected to emit over its entire lifespan. Let me just say that this is why I don't mind hearing that there's one in a neighboring galaxy. I never want you to tell us, hey Matt, I've got big news. Alpha Centauri's gone supernova, because that might be the last of our What's Up segments. Hey, Matt, I got a great tan today.

Really? Enjoy that. Oh, really. All right, we move on to the trivia contest, and I can't get enough of this. So, Whirlpool Galaxy, how far away is it? That's your question for this time around. Approximately, since we only know it approximately. How far away is the Whirlpool Galaxy or M51? Go to planetary.org slash radio.

Find out how to enter. And you already know my answer. Not far enough. I would not lose sleep over this one. Okay. It's pretty darn. I'll give the qualitative answer. It is pretty darn far. Always with the technical terms.

You have until the 27th of June, June 27th, Monday, 2 p.m. Pacific time to get a free answer. Just trying to make it so you'll understand, Matt. Yeah, thank you. Thank you so much. Bring it down to my level. All right, I think we're done. All right, everybody, go out there, look out at the night sky, and think about A, B, C. Thank you, and good night.

He's Bruce Batts, the Director of Projects for the Planetary Society. Every week, he brings us the A to Z of what's up in the night sky. The Juno mission lifts off for Jupiter in August. We'll talk with Principal Investigator Scott Bolton next week. Planetary Radio is produced by the Planetary Society in Pasadena, California and made possible in part by a grant from the William T. and Eileen L. Norris Foundation. Clear skies. Thank you.