Planetary Radio: Space Exploration, Astronomy and Science - Mark Showalter: Unraveling the Mysteries of Planetary Rings

Episode Date: October 22, 2007

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Rings Around the Solar System, this week on Planetary Radio. Hi everyone, welcome to Public Radio's travel show that takes you to the final frontier. I'm Matt Kaplan. Planetary astronomer Mark Showalter keeps an eye on the rings surrounding the four biggest planets in our neighborhood. He and his colleagues believe these giant loops of dust, rock, and ice can tell us a lot about how whole planetary systems form. He'll share his thoughts and tell us about the latest observations of the rings of Uranus

as that world approaches its equinox. Mark will also talk about the recent Jupiter flyby completed by the New Horizons spacecraft as it speeds toward Pluto. New Horizons is the topic of this week's Q&A segment provided by Emily Lakdawalla, and you'll want to stick around for our What's Up Look at the Night Sky with Bruce Betts. Why? Because we're going to announce the beginning of our fifth anniversary contest

and how you might win a piece of Mars. So much space news and so little time. By the time you hear this, both a space shuttle and a Chinese moon mission may have lifted off. Discovery hopes to head for the International Space Station on October 23rd. The lunar orbiter lifts off the next day. Discovery hopes to head for the International Space Station on October 23rd. The lunar orbiter lifts off the next day. Chang'e 1 is named after a moon goddess. Out at Saturn, the Cassini spacecraft is celebrating 10 years in space.

You can join the party at planetary.org, where there's a beautiful family portrait of Saturn, its rings, and some of its moons. That's where you can also read about how the Planetary Society has honored a couple of top-flight planetary citizens, Paula S. Apsel, senior executive producer of the NOVA TV series, and Mike Malin of Malin Space Science Systems. I'll be right back with Mark Showalter. Here's Emily. Here's Emily. Hi, I'm Emily Lakdawalla with questions and answers.

A listener asked, what was the fastest spacecraft? When New Horizons launched in 2006, it was commonly stated that it was the fastest spacecraft ever. But that isn't quite accurate. It all depends upon your frame of reference. New Horizons was a tiny satellite perched at the tip of an enormous Atlas V rocket, so it did have the fastest ever escape from Earth, edging out the record set by Pioneer 10 and Ulysses. Because New Horizons achieved the Earth escape speed record, it's a common misperception that it'll eventually outrace the Pioneers and Voyagers, which are also in solar system escape trajectories. But the fastest of

these, Voyager 1, has an advantage that New Horizons doesn't. Even though Voyager 1 launched from Earth at a lower speed, it had two giant planet gravity assists and is now exiting the solar system at more than 17 kilometers per second. New Horizons did get a boost from a Jupiter flyby, but it'll miss Saturn, and its flyby past Pluto's tiny mass won't accelerate it very much, so New Horizons' solar system escape speed will only be about 80 percent of Voyager 1's. What other speed records are there in the solar system? Stay tuned to Planetary Radio to find out. percent of Voyager 1s. What other speed records are there in the solar system? Stay tuned to Planetary Radio to find out.

The rings of Saturn, Jupiter, Uranus, and Neptune are some of the most beautiful and mysterious features of our solar system. Planetary astronomer Mark Showalter has devoted decades of study to them. His observations, along with those by other astronomers we've heard from, are slowly untangling the secrets of these complex structures. He talked about recent findings at the annual meeting of the American Astronomical Union's Division of Planetary Sciences, or DPS. He spoke with me a few days later from the SETI Institute's Carl Sagan Center for the Study of Life in the Universe, where he is a principal investigator. Mark, thank you very much for joining us on Planetary Radio. Thanks.

It's good to be here, Matt. You have just come back from DPS. We had a report on that last week from my colleague, Emily Lakdawalla, and she was the one who tipped me off that we ought to get you on the show as soon as possible to talk about some of the work that you've done recently. And I hope that we can start with Uranus, which of course, as we've heard from Heidi Hamel and others, is in the midst of this wonderful period for study. Yes, we're having the equinox of Uranus, the first day of spring, if you happen to be living on the northern hemisphere of Uranus, coming up on December 8th. At that time, the sun will be passing

from the southern hemisphere across the equator to the northern hemisphere of Uranus. Meanwhile, the Earth is kind of moving back and forth across the sun, as seen from Uranus. So the Earth has been crossing the equatorial plane three times instead of just the one time for the sun. And this is a very, very rare, unusual opportunity to see the rings of the planet exactly edge on, and that gives you a window on a number of phenomena that you don't normally get to see in any planetary ring system, such as the vertical thickness and so on. We'll wish the Uranians a happy spring and impending spring. You've had two of these ring crossings to observe so far, right?

That's correct. Well, let me clarify that a little bit. The first ring crossing of Earth across the ring plane was in early May. Unfortunately, the Uranus was still rather close to the sun, and it was very difficult to observe. The Uranus was still rather close to the sun, and it was very difficult to observe. So there is not, to my knowledge, any observations of the first ring plane crossing. Now, the second one was in early August, and that was just ideally suited.

We even had cooperation from the moon. The moon was out of the way, so we got nice, dark, clear skies. The third crossing is not until next February, and that happens very, very close to Uranus's solar conjunction. So that one will also not be observable. So we only get one of the three as a good moment of observing, but it's actually the entire period around the ring plane crossing that is of interest to us. So don't be too disappointed that we only got one out of three in this case. With this observation having peaked in August, has some interesting data come out of it, and any conclusions from that data? Well, sure. The thing

that's particularly unusual about the period is not just that we're seeing the rings edge on, but that for very brief periods of time, we're seeing the dark side of the rings. If you think about it, since if you're sitting on Uranus and looking at the sun, well, Earth is very, very close to the sun all the time. So normally when we look at Uranus, we're seeing the same side of the rings that the sun is illuminating because we're just so close to each other and Uranus is so far away relative to the distance between the sun and the Earth. So after that first crossing in May, the Earth was actually on the north side of the rings while the sun was still on the south side of the rings. So we were actually looking on the dark side, and that's a geometry that just doesn't happen very often. And then in August, the Earth passed back to the south side from the north side, and so we were currently on the lip side of the rings again.

We and the sun are both on the south side from the north side. And so we're currently on the lit side of the rings again. We and the sun are both on the south side. And then it's going to be very interesting in December because the sun then crosses to the north, but Earth will actually still be south. And so we'll have another period of observing the unlit side of the rings. Now, the rings are very dense, at least the narrow rings are. So you expect them to essentially disappear in this geometry. But on the other hand, there are these clouds of fine dust surrounding the rings, and those just glow very brightly in this geometry. So you get this kind of reversal of contrast where the faintest material in the system suddenly becomes brighter, and the darkest material in the system becomes very, very dark, or the densest material becomes very dark.

So that's what was so interesting to us, and that's why we were able to see some new features in the system. These conditions allowed you to see greater detail in these still fairly faint rings? Well, what we were able to see was actually rings that hadn't been known about before. So we're not just going as far as getting a better look at things that were imaged, say, by Voyager when it flew by in 1986. We're actually seeing different rings. The paper that my colleagues Imke de Potter and Heidi Hamel and I and Marcos Van Dam had in Science recently reported a quite surprising result, that we actually found a faint dusty ring kind of interior to the main narrow dense rings of Uranus. The trouble is that ring wasn't there in the days

of Voyager, and that's only 20, 21 years ago. So we're actually seeing large-scale changes of at least the dust distribution within the system over 20-year timescales. Even a short while ago, people were thinking that these rings would be stable, even the faintest, dustiest rings would be stable over centuries or millennia, and now we're finding that in our own research careers, the rings are moving around and changing. It means that studying rings is a little bit less like studying geology and a little bit more like studying weather. That's fascinating. And I suppose this has been a pretty steady progression, at least

since the Voyager flyby of Saturn, learning that these rings are far more dynamic than people thought for hundreds of years. That's right. And as we'll talk about in a moment, we're finding similar strange, very rapid changes in the Jovian ring system and the Cassini spacecraft now orbiting Saturn is finding that several of their rings have changed in a noticeable way during the past decade or two. So it's just very exciting now that there's this entire new way of looking at rings and the rings we study today may not be quite the same as the

rings we study 5 or 10 or 20 years from now. It makes the field more interesting and more exciting for us that things are actually changing on the timescales of our human lifetimes, and we can actually see new phenomena come and go. More from planetary astronomer Mark Showalter when Planetary Radio continues. Hey, Bill Nye the Science Guy here. I hope you're enjoying Planetary Radio. We put a lot of work into this show and all our other great Planetary Society projects. I've been a member since the disco era.

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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. Planetary astronomer Mark Showalter is sharing a bit of what we are learning about the rings of Saturn, Uranus, and other planets. With these observations, are we getting a better understanding of how rings form and also why they do some of the strange

things they do, forming braids and spokes and all kinds of other strange, totally unpredicted formations. Well, we certainly are. The main place where we're learning about the very dense rings is through the Cassini spacecraft orbiting Saturn. Now, Uranus has some fairly narrow, dense rings, but those rings have actually been the one thing we haven't really seen during the ring-plane crossing period. They're fairly narrow, they're as black as tar, and they're a billion miles away or more. So those are the parts of the system that we actually are not seeing,

and what we're instead seeing is the finest material. I think Cassini is telling us something that people have suspected for some time, which is the rings of Saturn have to be fairly young. The solar system itself is maybe 4.6 billion years old, something like that, whereas the rings of Saturn really can't be more than maybe a few hundred million years old. So that's still a very old system, but it's by human standards, but it's young compared to the age of the solar system. So we still have this puzzle, and I'd say that that puzzle has not been solved yet.

In fact, it's gotten a little bit harder to solve. One of the other reports that's come out of the recent DPS meeting is evidence that there is a lot more mass, a lot more material in the rings of Saturn than we had suspected. And, of course, the more material is there, the harder it becomes to understand how it got there. If it's a small amount of material, then you can imagine, oh, a big comet came by Saturn 100 million years ago, broke up into little pieces by Saturn's tidal forces

and spread out, and that's the rings we see. But if it's 10 times as much material, say, then you need a 10 times larger comet, and those comets are less frequent in the solar system. So it becomes more and more unlikely that a ring system of the scale and size and mass of Saturn could have formed 100 million years ago. Nevertheless, that's the best explanation we've got right now. So I tend to think that we're just really lucky as humans. If the dinosaurs had telescopes and they had looked up at Saturn, they would not have seen anything that made that planet special.

And we're just lucky to be living in the era of Saturn's rings. So enjoy them now because they won't be there forever. Of course, they'll be there for a few hundred million years, which is probably long enough for us humans to enjoy them a while longer. The other part of it, though, is that these faint, dusty systems like we see at Uranus and Neptune and Jupiter, I think we're coming to realize now that they are basically old ring systems. I suspect that if you took Saturn's rings and fast-forwarded 100 million years or so, you might be left with just a few rocks and some faint dust and a few satellites floating around. And that's kind of what we see at Jupiter and Uranus and Neptune.

So those are old ring systems, and Saturn is a young ring system, and we're just lucky to have a young ring system in our sphere of the universe where we can study it so closely. Talk a little bit more about Jupiter before we run out of time. I think that you had a paper you presented or a report at TPS regarding its moons, You had a paper you presented or a report at TPS regarding its moons and, I'm sorry, its rings and the search for the moons that might be shepherding those rings. That's right. I was one of the science planners for the New Horizons mission during its Jupiter flyby.

Now, New Horizons is on its way to Pluto. It will get there in 2015, I believe, so we've still got a long wait. It will get there in 2015, I believe, so we've still got a long wait. But it was launched just a year and a half ago, and it took only 13 months to get to Jupiter on a very fast path, and then it's using the gravity assist of Jupiter to fling it onward to Pluto as fast as it can possibly get there. Yeah, we check in with Alan Stern now and then. He's a very proud PI.

I know he is. We're all very excited about that. But the Jupiter flyby was a great opportunity to test out the instruments and to also do some good science. And while we were planning this Jupiter flyby, we realized that with the sophisticated camera that it's carrying on New Horizons, we would be able to see little moons orbiting Jupiter that were only maybe 500 meters across, whereas so far the smallest moon we know of is a little guy called Adrastea at about 8 kilometers. So we can push down the limit of detectability by more than a factor of 10, and that's the kind of science opportunity you don't get very often.

than a factor of 10, and that's a kind of science opportunity you don't get very often. We also know that almost anywhere in the universe where you have particles of a given size, you have a distribution, and generally, as you go to smaller sizes, you see more particles. For example, if you see one 10-kilometer object, you might see two 5-kilometer objects, and four 2.5-kilometer objects, and so on. The smaller you go, the more bodies you see. So we really expected to see some moons in the Jovian system, particularly embedded within its very faint and dusty ring system, and we didn't. So that becomes a bit of a puzzle now that we have to understand

what process removed the moons of Jupiter that ought to be there. So that's a bit of a puzzle. But then the bigger puzzle and kind of the consolation prize from not finding any moons, was that we found little families of clumps orbiting within the rings of Jupiter. And it's very hard to make clumps in a ring and keep them around for very long. It's just the nature of these systems that if you plunk down a bunch of material in one location, it'll spread out and form a completely symmetric ring over timescales of a year or less. So we're still puzzling over whether these little clumps are young features

that just formed in the last six months, maybe, or whether they are somehow confined by some gravitational tugs they get from Adraxer or Metis, the nearby moons, or something like that. We actually do have a spacecraft that will be launched. It's called Juno. I think it gets to Jupiter around the same time New Horizons gets to Pluto, around 2015. It's mainly focused on the atmosphere and magnetosphere of the planet, but I'm actually working with some of the scientists on that team to see what kind of Jupiter ring observations we might be able to carry out during the Juno

mission. I did not know about that mission until just a few weeks ago. It's exciting to know that we're going to be going back to that king of planets in our system. Yes, it is indeed. Just about out of time, I want to ask you something I don't think I've asked either of the colleagues that you've mentioned who've been on the show, Heidi Hamel, Imke De Potter, and that is, other than the fact that they are beautiful and mysterious,

what is it that the study of ring systems might be able to tell us about larger systems in the universe? Yeah, that's a great question, Matt. The answer that I think is most compelling is that these are just extraordinary dynamical laboratories. These are basically huge numbers of particles colliding, evolving, possibly merging, possibly breaking apart. And if you think about it, perhaps four and a half billion years ago, our solar system was a big flat disk. The sun was forming in the middle, and that was where most of the mass was concentrating.

But there was debris circling around it, and that, of course, eventually became the planets, and we're certainly grateful that one of them, Earth, happened to be one that had the appropriate environment for people like us to arise. The way that you can study solar systems in formation, well, that's very difficult because there aren't that many solar systems in formation nearby. Certainly they're very far away, and they take hundreds of millions of years to form. Well, you can see a lot of the same physics in the planetary ring systems. Saturn and Jupiter,

Uranus and Neptune have rings that are similar to the solar disks and they have satellites that are kind of the analogs of the planets that formed. So we just get to see a lot of really fundamental physics in our own celestial backyard. Fascinating. Thank you very much, Mark. I look forward to hearing more reports from you and your colleagues. One more ring crossing and that equinox to celebrate in December. Yes, we'll be at the Keck Telescope looking at the equinox as the sun crosses to the north side

and suddenly the rings turn dark again for us for the last time for about 42 years. Wonderful. Happy observing. Thank you very much, Matt. Mark Showalter is a planetary astronomer. He's the principal investigator at the Carl Sagan Center for the Study of Life in the Universe at the SETI Institute in Northern California. We'll be right back with Bruce Betts for this week's edition of What's Up. But first, here's Emily Lakdawalla returning for a little bit more Q&A.

Speed records in the solar system depend upon your frame of reference. New Horizons was the fastest to leave Earth, and Voyager 1 is on the fastest trajectory out of the solar system. The fastest spacecraft returned to Earth was a record recently set by the Stardust sample capsule, which hit the top of the atmosphere at 13 kilometers per second. In order to go faster than that, you need to get closer to a larger mass with higher gravity. The fastest spacecraft to enter into any atmosphere was Galileo, which plunged to a fiery death in Jupiter's atmosphere

at a speed of almost 50 kilometers per second. But the highest speeds of all were reached by the twin Helios probes, which travel in highly elliptical orbits around the Sun. At their closest approach to the Sun, these probes got to an incredible 70 kilometers per second. That's a seven with a zero. At that speed, they could travel around the world in three minutes. But they sit so deep in the Sun's gravity well that they'll never escape solar orbit. The pioneers, voyagers, and new horizons may be moving much more slowly, but they're going fast

enough, far enough from the sun, to escape the solar system entirely. Got a question about the universe? Send it to us at planetaryradio at planetary.org. And now here's Matt with more Planetary Radio. Bruce Betts is on the line. It's time for What's Up on Planetary Radio. He is the director of projects for the Planetary Society.

Big, big news today. We've been giving hints. Today we talk about the fifth anniversary. But first, what's up? Various planets celebrating the upcoming fifth anniversary of Planetary Radio. Notably, in the pre-dawn sky, we've got Venus celebrating in the east before dawn, looking like the brightest star-like object. And now, if you look above Venus just a little bit, the other bright star, though much dimmer than Venus, is actually Saturn. And

above Saturn is the star Regulus. And then in the south, in the pre-dawn, you'll see Mars, which is getting pretty darn bright. It's brighter than almost any star in the south, in the pre-dawn, you'll see Mars, which is getting pretty darn bright. It's brighter than almost any star in the sky, but not quite at this point. And looking reddish, it rises around 10 p.m. or so in the evening, 9 or 10 p.m. And then in the early evening, just after sunset, you'll be able to still pick up Jupiter. Over there in the west, but getting lower and lower over the next few weeks. So running out of Jupiter time in the evening sky, still looking like a very bright star-like object over there in the west. Random space fact! Did you know,

you probably did, being well-versed in both space and mythology, that the moons of Jupiter are named after lovers of the god Jupiter. Fortunately, he was quite the winner. Eventually, they have to, you know, just start counting random people who came by his house at some point. But at least with the main moons, you know, full-blown relationships, if you could call it that in the land of mythology. Quite the Randy gods. Yes, indeed, the Randy gods. Sometimes we just call that planet Randy. We go on to the trivia contest.

We asked you last time around, we asked you what was the first spacecraft to do a flyby of two other worlds, other planets is what I was looking for. Key word being planets, and that's where you got in trouble with people this week. Yeah, you know, we had a few people who said Voyager 1, Pioneer 10, but no, there was an earlier mission that flew by planets. But you know what a lot of people said? No. Mariner 9.

Because in 1971, Mars, but you know what a lot of people said? No. Mariner 9. Because in 1971, Mars, but also Phobos and Deimos. But you were looking for planets. We did say worlds. We said other worlds, and I guess, you know, some people thought those counted. I don't know. I don't know. Well, I apologize for that confusion. Well, we still had at least a plurality and maybe a majority who got the answer that you were thinking of. Was it Mariner 10? Yes, indeed, Mariner 10, flying by both Venus on its way to its encounter with Mercury. 1973 launch, November of 73, reached Mars. No, excuse me, reached Mercury in March of 1974. Pretty amazing stuff.

Yeah, the Venus on the way. Yeah. The Venus on the way and then gave us our first and only spacecraft glimpses of Mercury. And exciting, January 14th, this coming year, Messenger shows us the other side of Mercury. Lindsay Dawson, though, was the winner, chosen via random.org. Lindsay is from Cobar in Australia. Congratulations, Lindsay.

We'll get a t-shirt in the mail to the Southern Hemisphere very soon. Excellent. Now, would you like me to put forth the next question, or would you like to discuss the exciting new competition? Let's talk about the new competition, what we're going to do for the fifth anniversary, because I'm very excited. You heard about the prize?, what we're going to do for the fifth anniversary, because I'm very excited.

You heard about the prize? I did indeed. Very cool. We're going to thank, first of all, Florian Knoller of SpaceFlory.com for putting up the first of the prizes that we're going to award as part of our fifth anniversary celebration. The fifth anniversary of Planetary Radio will come up during the week of November 26th this year, meaning that we've got, including the contest you're about to begin, we've got four or five, excuse me, total contests. And entering these contests is going to be key to getting in on the fifth anniversary, shall we call it a sweepstakes? Because the prize that we can announce now, thanks to SpaceFlorid.com, is a piece of a Mars meteorite.

Yes, framed on a plaque that you can put on your wall, do whatever you want with. Don't eat it. Don't be like Kim Stanley Robinson. But you can get a piece of a Mars meteorite from Planetary Radio, courtesy of SpaceFlorid.com, which is a website that has all kinds of stuff like that, but he really specializes in artifacts from historic space missions, autographs of astronauts. Anyway, he is the one who is kindly providing this first prize for us. We're going to announce more in the future. And so, everybody who enters for the next five contests, please only one entry per week,

but everybody who enters will get in on the drawing. Bruce, get us started. That's so exciting. I know! Can we enter? No. Darn. You sure? Yeah, I checked. Alright, well in that case, I'll go ahead and give everyone else their shot at a piece of Mars

in their home. This is a real, genuine piece of another planet that has gone through space, been broken into little tiny pieces, and now coming to you via Planetary Radio. So for the first question, hearkening to the recent 50th anniversary of Sputnik, what rocket, what kind of rocket launched Sputnik on its record-setting journey? Go to planetary.org slash radio, find out how to enter, and compete, and we're still giving them a weekly prize, right?

Oh, yeah, yeah, they still are up for a shirt. All right, so compete for the shirt, compete for a piece of Mars, and when do they need to get this particular contest entry in by? This year, particular one, by October 29, Monday, October 29, at 2 p.m. Pacific time. That's exciting. Well, everybody go out there, at 2 p.m. Pacific time. That's exciting. Well, everybody go out there, look up in the night sky, and think about creative things you can do with twist ties. Thank you, and good night.

I love twist ties. I feel cheated when I don't get my little piece of wire encapsulated in paper because, you know, some bread manufacturers cheap out. So here's to twist ties and a Mars meteorite, a piece of a Mars meteorite. He's Bruce Betts, the director of projects for Planetary Society. He joins us every week here for What's Up. Planetary Radio is produced by the Planetary Society in Pasadena, California. Have a great week. Thank you.