Planetary Radio: Space Exploration, Astronomy and Science - A Year of Saturn Discoveries From Cassini

Episode Date: August 1, 2005

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A big week in space and a big year at Saturn, 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. Coming up, a first anniversary review of Cassini at Saturn from one of our favorite people, Linda Spilker, the Deputy Project Scientist for the mission. And what a year it has been. Toward the end of our conversation, you'll hear Linda talk about the just announced news of relatively warm water and a possible nitrogen atmosphere at the south pole of Enceladus, one of the ringed planet's icy moons.

Speaking of news, how does this grab you? My very educated mother just served us nine pickles 2003 UB313. It's going to have to do until there's a catchier name for what may be our solar system's tenth planet, or ninth, or eleventh. Astronomers really have to get together on this planet definition thing. What may be our solar system's 10th planet? Or 9th? Or 11th? Astronomers really have to get together on this planet definition thing. Anyway, a team headed by past Planetary Radio guest Michael Brown says 2003 UB313 is bigger than Pluto and, for the moment anyway, much, much farther away. You can read about it and Enceladus at planetary.org.

The other big story is Discovery. As we finish this week's show, NASA was about to announce whether astronauts would take a walk in space to make their first actual repair of a shuttle. The return-to-flight mission has been extended by at least a day, partly so that Discovery's crew can transfer more supplies to the International Space Station, which may not see another shuttle for quite a while. Emily is up next with the tale of the biggest rock around, so long as you're not talking about planets, but let's not open that can of worms again.

And Bruce Betts will drop in with this week's What's Up and a new space trivia contest. I'll be back with Linda Spilker in a minute. Hi, I'm Emily Lakdawalla with questions and answers. A listener asked, what is the largest asteroid? The asteroid Ceres is similar in size to Saturn's moon Dione in Tethys. It's about half the size of Pluto, but not much smaller than Pluto's moon Charon. At that size, it is quite large enough for self-gravity to pull it into a roughly spherical shape. When Ceres was first discovered in 1801,

astronomer Giuseppe Piazzi thought he had discovered the eighth planet of the solar system, filling in the gap between Mars and Jupiter. His excitement faded over time, though, when further observations indicated that his new discovery was too small to be considered a planet. Ceres is less than a quarter the size of Mercury, which was the smallest planet then known. Still, Ceres is big, accounting for one-third of the total mass of the hundreds of thousands of asteroids. Despite its large size, there are as yet no pictures that resolve features on its surface. Why not?

Stay tuned to Planetary Radio to find out. Planetary scientist Linda Spilker is Deputy Project Scientist for a mission that is taking its rightful place alongside Voyager, Galileo, Viking, and other great robotic explorer missions of the past. And Cassini is expected to continue circling and observing Saturn, its rings, and its moons for years to come. Linda, welcome back to Planetary Radio, first of all. I'm very happy to be here, Matt. Well, we are so glad to have you back because we can mark

the first anniversary of Cassini at Saturn. That's right. It's very exciting. This year has just flown by. It's absolutely incredible. With an enormous amount of science pulled out of those reams of data that have been coming back, not just from Cassini, but fair coverage to Huygens as well. That marvelous little European Space Agency probe that came down on Titan. We want to review what's taken place over the last year. Can we start with Titan? Sure, that sounds great.

In fact, the Cassini orbiter and the Huygens probe have really revealed a whole new world. In a sense, we've lifted the veil on Titan. It has a dense, smoggy atmosphere, and using both the Huygens probe and special instruments on the orbiter, we've now got a chance to see what the surface of this very interesting moon looks like. Titan is bigger than the planet Mercury, and had it formed elsewhere in the solar system, it would have been a planet in its own right.

And so with the Huygens probe, we used a series of parachutes to land on the surface. And as we descended to the surface, we took images the whole way down, and we found a surface that's remarkably Earth-like, complete with drainage channels. There's evidence for erosion. We think that methane rain occurs on Titan, that methane performs the same role as water does here on the Earth. And so we saw a remarkably Earth-like landscape when we landed with the Huygens probe. We landed in a dark region, looked like a dry lake bed, complete with icy pebbles that we could see nearby the probe. And we even got a whiff of methane gas as the probe actually heated up the surface.

Right, right where it came down, actually might have melted some of that and vaporized some of that frozen methane? That's correct. And so the methane concentration went up around the probe. And now the challenge with the orbiter is we know one place on Titan very, very well. And so now when we look with the orbiter, we want to put that spot in a global context. And we've seen areas that are bright. We've seen darker areas,

and we now know that the darker areas are probably from these more complex hydrocarbons that slowly fall out and create the darker regions, and that the methane rain essentially wash off the icy hills and make them brighter. What about that spot, and there is a great image of this, that sure looks like a lake. Yes, we're still looking to see if there's liquid present on Titan. There's hints in the radar data. That's one of the best ways to pierce through the clouds and look at the details of the surface. But we're just not sure because those darker regions could also be very flat and very smooth,

so we can't say for sure if it's liquid or not. Another interesting discovery was a possible cryovolcano, that there might be active sort of water volcanism occurring on the surface of Titan. And so we're looking carefully at this spot and trying to see if we notice any changes. And would this maybe have something to tell us about Titan's atmosphere? That's exactly right, Matt. In fact, we think that that's how maybe the gases come out and especially methane become part of Titan's atmosphere through something like a cryovolcano. We're going to come back to Titan because at the end of our conversation, we're

going to talk about what's coming up for Cassini in its second year at Saturn. But because our time is so limited, let's move on to a topic that is fascinating, I know, to you, but to everybody else who loves this beautiful planet. And those are those wild and crazy rings. Oh, absolutely. One of my favorite topics to talk about. Of course, in Cassini's first year, in order to go into orbit around Saturn, we had to fly very, very close to the planet. And we had to perform a 96-minute burn to get captured into orbit. And after that, we turned the instruments to look down on the rings for our best ever pictures and data of the rings that actually made in situ measurements of the ring atmosphere, that sort of a water-oxygen atmosphere present above the rings. We saw

incredible detail in the rings. We saw incredible detail in the rings. We saw features called straw, groupings of ring particles that are several kilometers long, this atmosphere of oxygen around the rings. Yeah, that was fascinating. Oxygen where I guess it wasn't expected? Well, since the rings are primarily water ice, we thought we would see some oxygen or some water products, but to see an oxygen atmosphere was very, very interesting. We also saw marble-sized meteoroids impacting the rings and could see the signatures coming up in the radio and plasma wave experiment.

You mean the evidence of the actual impact as it was taking place? That's right. It would produce a signal that we could actually measure with Cassini. And when we flew over the densest ring, the B ring, we saw more of these impacts. And then when we got to a less dense place like the Cassini division, the rate fell off. And so we knew that something was hitting the rings and creating a radio signal that we could detect with Cassini. So very, very exciting. We also caught a tiny moon, Prometheus. It orbits very close to Saturn's F ring, actually stealing ring particles away from the F ring

and sort of caught in the act, if you will. And so that was a very exciting picture, and that's helping us understand the dynamics going on, how these ring particles interact with the moons and how the rings are shaped and sculpted by the activities with these tiny moons. Waves and kinks and twists in these rings and such a complex system of them. Right. And Cassini even discovered a new moon. There's a tiny moon orbiting in a gap in the A-ring called the Keeler gap. And we actually got a picture of the moon and the tiny wakes

that it creates on the edges of the Keeler Gap. So that was a very exciting discovery by Cassini. Now, just recently, there were these radio occultation experiments that told us even more about the rings? Oh, that's correct. We actually probed with three different frequencies looking at the larger sizes of particles. And we've had a series of radio occultations of the rings and of Saturn's atmosphere as well. And they've gone very, very well. Some of the results are just fascinating. It looks like, for instance, in the A ring, that the number of smaller particles might increase as you go further out in the A ring. And there are other places where the particle population is a little bit different. And the challenge will be trying

to understand how this is happening and what might be going on. It's exciting stuff. Our guest is Linda Spilker. She is the Deputy Project Scientist for the Cassini mission and very much one of the researchers who is still quite excited after a year in orbit around Saturn about the data coming back and the conclusions that we're reaching about it. We're going to come back from a break in a moment and get to more about what's happening out there at Saturn.

Specifically, we're going to talk about the planet itself, more of these icy moons and some very recent discoveries about one of them, and what to look forward to in the second year of Cassini at Saturn. That's right after this. This is Buzz Aldrin. When I walked on the moon, I knew it was just the beginning of humankind's great adventure

in the solar system. That's why I'm a member of the Planetary Society, the world's largest space interest group. The Planetary Society is helping to explore Mars. We're tracking near-Earth asteroids and comets.

We sponsor the search for life on other worlds, and we're building the first-ever solar sail. We didn't just build it. We sponsor the search for life on other worlds and we're building the first ever solar sail. We didn't just build it. We attempted to put that first solar sail in orbit and we're going to try again. You can read about all our exciting projects and get the latest space

exploration news in depth at the Society's exciting and informative website planetary.org. You can also preview our full color magazine The Planetary.org We're back with Linda Spilker, Deputy Project Scientist for the Cassini mission at Saturn. And we are sort of doing a best-of show. The first year in orbit at Saturn, we've already talked about Titan and the rings. And we're going to move on to the planet itself, which, for some reason, impressive as it is,

doesn't seem like where most of the big news is coming from. Right. I think part of that, Matt, might be that we were there with Voyager. We have a lot of data from previous missions, although there are some exciting discoveries that Cassini has made about Saturn. And one of those is a new radiation belt that's new, completely unexpected. It's a radiation belt that's in between the inner edge of the D-ring and Saturn's atmosphere. And that was discovered with our magnetospheric imaging experiment. And another very peculiar thing about Saturn is that when we got there and we used the same

sort of radio beacon coming from Saturn to measure what we think is its internal rotation rate, it had slowed down by six minutes compared to what the Voyager spacecraft measured. And even if you crash the rings and the moons and everything into Saturn, there's just no way you could slow the planet down by that much. So we're starting to think that maybe that rate that we're using from measuring the radio signal coming from Saturn isn't quite the right metric, if you will, or right way to measure the internal rotation rate. So now we have to say, we don't know what a day on Saturn really is at this point, and we'll just keep looking for it and trying to understand it.

The season on Saturn is very different. You see the ring shadow going up over the northern hemisphere of the planet, creating a very different kind of a weather system. And we detected lightning and a big storm. We nicknamed the Dragon Storm coming from Saturn. Is that the Dragon Storm? Is that one of these long-term storms like Jupiter gets, the red spot and so on? Yeah, probably not as long-term as the red spot.

That's a really long-lived storm. But certainly we'll be watching as the storms develop in what we call Storm Alley on Saturn. Last bit about the planet. I read something about the North Pole having the blues. Right. It looks much bluer now. We think that with the ring shadow and the way the seasons are, it's cooler. And maybe some of the clouds that we'd seen with Voyager sort of condensed down, and it looks very blue and very pretty.

Now almost a little bit like Neptune. Well, let's go back out away from the planet, past the rings, to those moons, the icy moons in particular. We talked about Titan, not an icy moon, but you're learning lots in many, many flybys of these moons. Yes, we've had close flybys in this first year of several of them, and the first flyby we had in June of 2004 was with tiny Phoebe. And we found a scarred and battered world with lots of water and hydrocarbons on the surface.

And Phoebe is actually a captured object. It orbits retrograde or backward compared to the other satellites. And Saturn captured it at some point in its history. So that was the first icy moon we got a look at. A really strange one is Iapetus. Iapetus has one very dark side and one bright side, and we always wondered how did the dark material get laid down on the surface of Iapetus, and it appears to be like a fine coating of dust, and we're still not sure exactly how it got there. But even more impressive is Enceladus has a mountain range over 20 kilometers high that goes around the equator.

It's like a belt. Like a belly band, as we call it. And we can see it going at least halfway, probably more than halfway around Iapetus. And the question now is, how did it get there? And if you look at the shape of Iapetus, it looks like it froze with the shape that it would have had for a 17-hour rotation rate. So it must have solidified early, very early, before it had totally stopped or despun, because now we know it keeps one side always facing toward the planet. So very, very fascinating world. And we've also looked at another moon, Dione. From Voyager, we had these fingers of wispy terrain, very bright and looked frosty, and we weren't sure what their source might be.

And it turned out to be tectonic features, sort of an uplift with the side of the cliff being bright white and icy when we got in very close with Cassini's cameras. Probably the biggest surprise is tiny Enceladus, that we've had two close flybys in the first year, and we detected a tenuous atmosphere with a magnetometer, actually keeping the field lines from going down to the surface of Enceladus, and it looked like it was a very, very intriguing place. And more intriguing as more data has been gathered. And we're now talking second year, I suppose, because this just happened in the last couple of weeks. Very dramatic flyby, getting within, what, about 100, just over 100 miles, 175 kilometers of the surface,

and got some very close-up pictures as you literally zipped by this little moon. some very close-up pictures as you literally zipped by this little moon. Right. This was Cassini's closest flyby to date, and we were intrigued with what we'd learned at Enceladus, so we actually moved this flyby even closer. We were going to go by it 1,000 kilometers, decided to push it all the way into 175. And lo and behold, we discovered that Enceladus had a warm South Pole. What a surprise. Here's this tiny world.

It's only 250 kilometers in radius, and yet processes are internal heating, are allowing material that looks like water to come out and a water-like atmosphere to form just over the South Pole. Now, we should say, as we're speaking, this news has just been released, and it is, of course, on the Cassini JPL website, and came as quite a surprise. Right, right. The composite infrared spectrometer measured a warmer temperature at the South Pole.

The ultraviolet experiment, as they watched a star move behind Enceladus, detected an atmosphere. The magnetometer saw strong signals of an atmosphere and of currents flowing in this atmosphere. And there was even an instrument that could, ion and neutral mass spectrometer, that could sort of sniff the atmosphere of Enceladus and, of course, found lots of water and possible evidence for maybe nitrogen N2 or CO, lots of interesting constituents. So all of the instruments on Cassini worked together, both the optical remote sensing from a distance as well as the in situ to make this discovery.

And then finally, with the images that we had, the really beautiful images of these tiger stripe blue regions on the South Pole, we found that right at the tiger stripe, that's one of the warmest places that we could see. So we knew now that that material that's coming out, being outgassed, is coming from these long fractures we call tiger stripes that look blue, that are almost reminiscent of the blue of glacial ice. So what a wonderful discovery.

People have got to go to the website, of course, and take a look at some of the images that we've been describing. And we will provide that URL. We'll put it up at planetary.org slash radio, where you can also hear this program and maybe listening to it right now. Linda, it's just the first of, we hope, many years at Saturn for the Cassini spacecraft. We only have about a minute left. What else can we expect just in this upcoming year? Well, we have six worlds in 80 days.

That includes two flybys of Titan and very close flybys of Tethys, Hyperion, Dione, and Rhea. So stay tuned. Who knows what we'll discover on these icy worlds. Six worlds in 80 days. Phineas Fogg would be so proud. And then finally, our Titan flyby in October will have a radar swath across the probe landing site. And this will help us start to put together that one small piece of Titan with the rest of this very fascinating world.

And then from there on to many, many orbits of Saturn and many flybys of Titan. And the spacecraft is in good shape? The spacecraft is in excellent health, and we're looking for, like you say, to many years of very exciting science discoveries in operation. Excellent. Thank you so much, and we'll look forward, I hope, to getting another report from you. I think we ought to get them semi-annually, if you don't mind. Oh, sounds great.

Linda Spilker is Deputy Project Scientist on the Cassini mission, and she's also part of that composite infrared spectrometer team that made that great discovery about the southern region of that little icy moon just a couple of weeks ago as we speak, and we'll look forward to talking with her again. We're going to go to a little bit more Q&A with Emily now, followed, of course, by What's Up with Dr. Bruce Betts. Stay with us.

I'm Emily Laktawalla, back with Q&A. If the asteroid series is big enough to be a spherical world, why aren't there any good pictures of it? The answer is simple. We've never been there to take them. Only a few asteroids have been visited directly by spacecraft. Gaspra, Ida, Dactyl, Matilda, Eros, Braille, and Anne Frank. The largest of these, Matilda, is only a few percent the size of Ceres.

Our visual inspection of asteroids has so far focused on smaller bodies that better represent the objects that have caused impact extinction events on Earth in the past. Next year, though, NASA will launch the Dawn mission to rendezvous with Ceres and also Vesta, the next largest of the asteroids. The two objects are very different. Ceres is round, while Vesta is pumpkin-shaped. Ceres seems to have water ice, while Vesta is dry. Ceres is dark, while Vesta is the brightest asteroid in the solar system. Dawn will seek to understand why these two objects,

in similar orbital positions in the solar system, are so different. Got a question about the universe? Send it to us at planetaryradio at planetary.org. And now here's Matt with more Planetary Radio. Well, we said it was going to be time for What's Up? And sure enough, here he is, Dr. Bruce Fetz, the Director of Projects for the Planetary Society.

What's up? Hey there, Matt. We've got cool planets in the evening sky. Let's start with those. And we've also got a meteor shower. But starting with planets, we've got Jupiter and Venus low in the west shortly after sunset. You can see Venus getting higher in the sky over the coming weeks and Jupiter getting lower. And you know what that means, Matt? Uh, uh.

Okay, you don't. It means that I'm going to get excited again about planets coming close together in the sky. Oh, yes, I should have known. On September 1st, Venus and Jupiter, the two brightest planets, are going to be snuggling up near each other. And you can watch them grow together if you watch in the evening sky. Easy to see. They'll be blinding.

Blindingly bright. Blindingly. Well, maybe not blindingly, but they are the two brightest star-like objects up there. And they're getting closer and closer. Look in the west after sunset. Mars rising around midnight in the east. We high overhead in the pre-dawn sky looking orangish and getting brighter and brighter through late October, early November. We'll keep you posted on that.

On August 12th, the Perseid meteor shower peaks at 1800 UT. But basically the point here is any time around August 12th, within a few days either way, you can go out, stare up at the night sky, and expect to see at a dark site perhaps one meteor a minute. Okay. So go see them. This week in space history, on August 2nd,, Apollo 15 Lunar Module Falcon leaves the moon in the first televised lunar liftoff, watching them blasting off the moon. Just spectacular. Bits of stuff, debris flying off of the lower part of the lunar module.

I remember it. And, of course, no flame because it's just a rocket engine going off in vacuum. It was terrific. Yeah. It's very cool. So speaking of bits of stuff flying off things, random space fact! We will talk about the external fuel tank of the shuttle, some spiffy stuff, that big giant tank in the middle. It turns out the top is where they store the liquid oxygen, the bottom, the liquid hydrogen,

and in between they have an empty ring where the things come up and then flow into the space shuttle through two big 17-inch diameter tubes to put the liquid oxygen, liquid hydrogen together in the main engines of the space shuttle to power that puppy. It's cool stuff. Certainly. In the news. In the news. They're in the news a lot. There's a fun aside on them.

But yes, they are in the news with that Discovery mission. And the also interesting thing I enjoy is that they're manufactured in Louisiana and then sent on giant barges around to Florida. I've seen pictures of those, too. I get around. You do. You've seen pictures of a lot of things, Matt. We won't even talk about some of the things I've seen pictures of those, too. I get around. You do. You've seen pictures of a lot of things, Matt. We won't even talk about some of the things I've seen.

Oh, God, please don't. Please don't. Moving on to the trivia contest as quickly as possible. Mm-hmm. Woo-hoo! We asked you to provide us with at least one piece of evidence of why we believe that Europa probably has a subsurface ocean. How'd we do, Matt? Well, I particularly like the excellent answer from Thomas Cristello, who said, how about

the subsurface mermaids? True, that is one. That's one of those, you know, that people have been, NASA's been trying to keep under wraps because, you know, not that it would panic people, but it might cause people to want to get to Europa really fast. Have you seen the pictures from Galileo with the mermaid tail popping up through the ice? No, I did not see that one. I guess that's one I missed. You'll have to look for that.

Anyway, did we have anyone else who might have been randomly selected as the winner? We actually did. Yes, did we have anyone else who might have been randomly selected as the winner? We actually did. Yes, we actually did. Rich Ellersich, I guess is how it's pronounced, of Lancaster, California. He wrote in with this. One piece of evidence for a subsurface ocean on Europa is the scalloped crack features found on its surface. According to scientists, scalloped cracks can best be explained by a diurnal tide created by nearby Jupiter raising a subsurface ocean and the icy surface by as much as 30 meters

each day. Guess where he found this information? I don't know, Matt. Where did he find this information? January, February 2000 issue of the Planetary Report from the Planetary Society. Well, that must be true. You've got to go with it, right? Yeah. Then you win. You win a Planetary Radio from the Planetary Society. Well, that must be true. You've got to go with it, right? Then you win.

You win a Planetary Radio t-shirt. There are basically a couple of categories of main evidence for our belief that Europa has a subsurface ocean that starts many kilometers below the waterized surface. One of those is in a variety of surface features, like he's referring to, that appear to show everything from those cracks to things that look like ice flows on earth, where different pieces have been rotated around and almost like they were floating on a subsurface

ocean. Now, you can actually do this with very soft ice underneath, but you've got that, you've got evidence of things coming up through the ice that are most easily explained, at least, by a liquid water erupting. And then you also have the fact that it has a magnetic field, which could be caused, in this case, by movement of the salty water down there. That's the one we heard from a lot of other listeners about the magnetic field. What have you got for us next week? For next week, I've got something near and dear to my heart, sleep.

Who was the first person to sleep in space? Oh, interesting question. Okay. You know, one of those where I immediately want to start speculating on my own, but that would be wrong. That would just be wrong. Right. How do they enter?

So everyone else, go to planetary.org slash radio, find out how to email us your clever and correct answers of who was the first person to sleep in space. Get those answers to us by August 8th, Monday, August 8th at 2 p.m. Pacific time,

and you will be part of this next big trivia contest in What's Up. We're done. Cool. Everyone go out there, look up in the night sky, check out things like meteors and planets, but also think about sleep. You're really tired, aren't you?

Thank you, and good night. Bruce Betts is the director of projects for the Planetary Society. He joins us each week here on What's Up. Shh! Planetary Radio comes to you from the Planetary Society in Pasadena, California. And we'll do it again next week. We'll see you then.