Planetary Radio: Space Exploration, Astronomy and Science - Cassini at Weird Iapetus

Episode Date: September 17, 2007

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Cassini at the yin-yang moon, 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. What a week for space exploration. On September 10th, the Cassini spacecraft flew within about 1,600 kilometers or 1,000 miles of one of the weirdest places in the solar system. What do you call a moon that is white as snow on one side and black as coal on the other? Call it Iapetus. We'll talk with Tillman Denk, a member of the imaging team that is now puzzling over some of the strangest pictures ever returned by a probe. The Phoenix lander is still

on its way to Mars as Emily Lakdawalla prepares us for the frigid end of that mission. And as for what's up, hey, did somebody get the license number of that Mars rover? Bruce Betts and I become willing victims of a hit-and-run at Wired Magazine's Next Fest show, but we'll still give away a T-shirt. Like I said, what a week. Mars rover Opportunity took her first dip into Big Victoria Crater. It was just enough to find out how slippery a slope it will have to deal with when it begins its descent in earnest. slippery a slope it will have to deal with when it begins its descent in earnest. Japan's Kaguya mission got off to a flawless start, kicking off the international lunar decade.

The spacecraft will go into polar orbit around the moon on October 3rd. Emily provided great coverage of the launch on her blog at planetary.org. Speaking of the moon, who'd like to win $30 million by putting a rover on it? Today we're challenging private teams from around the world to design and build robotic explorers and race them to the surface of the moon. The Google Lunar X Prize is a competition that will once again demonstrate that small dedicated teams of individuals can do what was thought only once possible by governments. That's XPRIZE mastermind Peter Diamandis announcing the Google Lunar XPRIZE last week at NextFest. We'll bring you special coverage in next week's show. Time for Emily's Q&A. I'll be right back with Tilman Denk.

Back with Tillman Denk. Hi, I'm Emily Lakdawalla with questions and answers. A listener asked, if Phoenix is going to land inside Mars' Arctic Circle, does that mean it'll stop working when the Arctic Very Long Night arrives? The Phoenix Mars lander is going to touch down in Mars's far north. Because Mars's spin axis is tilted like Earth's, Mars has seasons, including a winter that brings many months of continuous darkness to the poles. Phoenix will indeed experience the Martian long night and will not survive it. Phoenix is a solar-powered spacecraft depending on the sun to charge batteries from which it operates its science instruments, robotic arm, radio transmitter, computer, and internal heaters.

Without sun, the spacecraft will freeze and die. Unlike the rovers, there is absolutely no possibility that Phoenix will survive the Martian winter. The real question is, as the sun begins to sink toward the horizon with the arrival of autumn, how long can Phoenix cling to life? Stay tuned to Planetary Radio to find out. You really have to see the flyby images of Iapetus to believe them. You can do just that in Emily Lakdawalla's blog at planetary.org. It's also where we'll have links to the official Cassini mission site. What the heck is that black stuff that blankets an entire hemisphere

and has left streaks extending well into the other side of this moon of Saturn. Tilman Denk of the Free University of Berlin is one of many scientists on the Cassini imaging team who are trying to figure out this and many other mysteries of Iapetus. Tilman, thanks very much, first of all, for joining us on Planetary Radio and for doing this fairly late in the evening there. I assume, are you in Berlin or elsewhere in Germany? I'm in southern Germany, near Stuttgart area. Visiting your brother, I think you said, and hopefully sharing with him some of this amazing news,

some of these incredible images that have come back to us via the Cassini spacecraft from this mysterious moon, Iapetus. Yes, it's really great. And when I think about how much time I spent for my professional career on doing the planning of this imaging, then it's a really good result that came out. Oh, I would say it's far more than good. It's mind-boggling. My colleague, Emily Lakdawalla, who has been doing really terrific coverage on her blog at planetary.org,

she is flabbergasted, and I think anybody who sees these pictures must be. But then this moon has been a mystery ever since it was discovered. Yes, actually, maybe it might even be the oldest mystery in planetary science, because Jean-Dominique Cassini, a former Italian but later French astronomer, he was on the Paris Observatory and he discovered Iapetus, but then he lost it and he searched for it, found Rhea, by the way, and then when he found it again in 1672, it's quite long ago, then he realized quickly that one half of Iapetus must be much darker than the other because he could see this moon only on one side of Saturn. And this was a right

interpretation that he made, and maybe it was one of the first interpretations of a surface feature of a moon. But then nothing more happened in terms of knowing anything in addition until the 70s, when the people started to think about possible reasons for this extreme brightness dichotomy. And so it's 335 years since the discovery of Iapetus by Cassini, and now we might be close to finding why there is this dichotomy. I know that there have been some attempts to explain this in the past, certainly since the Voyager spacecraft returned the first somewhat detailed images. But none of those Voyager images compare with what was picked up on the night of September 10th by Cassini, the Cassini spacecraft. The Voyager images had only about 9 kilometers per pixel resolution.

And this is not very good. We were able to do quite interesting discoveries there. And a lot of things that Cassini has clearly seen, there were already hints in the Voyager data for this, for instance, very large basins. They were in the dark material, the Voyager data were very underexposed. But you could see something and you could make a guess that there must be something big, but it wasn't for sure at all. All these strange mountains on the equator, they were seen in the Voyager data, but then Cassini eventually has shown that this is a large equatorial ridge or the oblate shape of an effect that's very unusual for a moon of this size,

that it has a radius in the equatorial direction that is significantly bigger, several percent bigger than in the polar direction. It's as if someone stepped on it. It's squashed. I don't know. I cannot say. Well, not only is it oblate or somewhat flattened, but it has this very strange equatorial range of mountains. Some people call it the belly band that goes all the way around the moon. Well, not really all around. This is something that's not clear yet.

It seems that there's a very big basin in the dark hemisphere on the sub-Saturn side, and it's starting somewhere east of this basin, and then it gets higher and higher, some up to the area close to the anti-Saturn point on the surface. Then there are these strange mountains that we have detected already in Voyager data, and I'm not sure yet what's going on there. Although now we have the information with about 500 meters resolution, but if you look straight from ahead on the surface, then it's very difficult to see mountains that are that high

if there are no other hints for them. And so we still have to do a very careful observation and interpretation of the data. And then when you go further back to the sub-Saturn side, then there's also not really a good hint for the ridge there. So I would say it spans maybe about one-third to a half or maybe a little bit more than one-half of the circumference of Iapetus. I see. My misunderstanding. At their highest, though, those mountains are pretty impressive.

That's right. Although what we call the informative Voyager Mountains, these might not be as high as we thought before. On the Voyager data, with nine kilometers, it was about two pixels or a little bit more in size. And this is equivalent to about 20 kilometers in size. But now in the Cassini data, it looks that the bright area that we have seen in Voyager data is more isolated, patchy, and it's not a complete mountain. And it might only be 10 kilometers high. But this is something we have to look for in more detail in the coming weeks.

Let's go back to this strange quality of this moon, not really seen anywhere else in our solar system, what causes Emily to call it the yin-yang moon, that it has this one half of it, one hemisphere essentially, that is mostly bright, very bright, bright as snow, and in fact it appears to be a lot like some of the other icy moons that we've discovered. But then there is this strange black layer that blankets one side and streaks over onto the bright side of the moon. This is really a strange thing, and there was for a long time the question,

is there a black material on the white bulge, or is it vice versa? Is the black material, the common one, and the white one just a veneer? And I think, especially with the help of the radar data, we are now pretty sure that it's the bright terrain that forms the bulk, so that the icy material is what Iapetus is made of, and also its mantle, and that the dark terrain is only thin veneer. And the question was also, how big is this blanket? We think now that it might only be in the order of a foot or something like that. No kidding. So it might be rather very thin.

So really, I mean, even if you were off by a factor of 10, we're still talking about a very thin surface layer. Yes, and this means that the process that is creating it should be ongoing today, because if it would be an old process that stopped very early in the Iapetus history, then impact cratering processes would have destroyed it completely. Or it should be very thick, and we don't believe this, because there's the information from the radar from Cassini with a wavelength of about 2.2 centimeters. You have to translate it how many inches these are.

Right, about an inch. I think it's one inch or so, right? Yes. Then we have the Arecibo radar data. There's a paper from 2004 where Arecibo, I think it was the Goldstone station that sent a signal to Iapetus and then the signal bounced back and came back in about maybe two hours or so later and was received by a 300 meter sized dish. And the signal was indicating that the dark and the bright hemisphere at 13 centimeters do look very similar for radar.

For Cassini in the 2 centimeters, they don't. They look different. And one interpretation of this data is that the thickness of the dark terrain is only in the order of a foot or so, and this means that the 13 centimeter radar, it can penetrate through the dark material and is reflected mainly by icy component below, while the Cassini radar is mainly reflected by the dark component. More from Tillman Denk of the Cassini imaging team right after a break. This is Planetary Radio. Hey, hey, Bill Nye the science guy here. I hope you're enjoying Planetary Radio.

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The Cassini spacecraft made its one and only close flyby of Saturn's moon Iapetus on September 10, returning data and images that have scientists like Tillman Denck scratching their heads in amazement. We're not likely to learn everything we'd like to from this quick encounter, but there are a few results and pictures that already show great promise. One of the most fascinating things that I saw in some of these images were little bright white spots, which appeared to at least this layman to be recent meteor hits on the moon, which apparently had blown away some of this black material. I mean, in other words, they were pretty fresh impacts, I suppose, and may have revealed the white, the ice underneath.

Is that an interpretation that is taking hold there among the researchers? Absolutely. This is what we hoped to see and that we are happy to see now. And we have to look in detail how the distribution of these bright little spots is. We look for bright spots in the data from the 2004 flyby. This was one at about 120,000 kilometers distance, which gave data at 730 meters per pixel resolution. And we haven't seen such holes in the dark blanket there. But now we see them. These are very small. And it will be interesting to count them and to put them into context to theory to see how it fits

to the idea which projectiles eventually caused the cratering, the main part of the cratering on the planet, on the satellite. Now, I do need to ask you, of course, and you can be a good scientist and defer any answer. But my guess is that there is at least some speculation, based on this new evidence, about what in the world this black material may be. That's a good question. And I think the people from the WIMS spectrometer instrument are those that might give the answer one day.

the WIMS spectrometer instrument are those that might give the answer one day. The idea is that there's some carbonaceous components therein, and maybe there's also some siliceous material, but it's not really clear, so I cannot give you an answer because I really don't know. Of course, and I wouldn't push any harder than that. Yes, the problem is if you want to find a material based on the spectral measurements, then you need an analogy spectrum that you have measured somewhere on Earth, and creating a material analogy from the outer solar system is very difficult simply because you have no idea what there is,

idea what there is and also because it's very difficult to create the production how the production was done that the situation it's four billion years old and it's bombarded by solar radiation and ultraviolet light and cosmic bombardment things like that little impacts from micrometeorites things like, and to create all these boundary conditions that's virtually impossible. And so it looks like there's some carbon-bearing material therein. And then another problem is if there are no specific absorption bands in the spectra and things like that. So it's a very tricky thing, and it still needs a lot of effort to find it out. Well, another challenge that you have, if I understand it correctly,

is that the flyby data is still coming back and I imagine will be analyzed over the next many months. I think the playback from the airspace data is now complete and this is a little bit of a down point that there are a lot of data missing for various reasons. A major reason is that there was a saving event on the spacecraft. A cosmic ray hit, apparently, just after the flyby? Yes, I don't understand all these abbreviations that are going back and forth.

It looks like a switch was tripped by a cosmic ray hit, and this was not good for the spacecraft. And we were also unlucky because there will be a new flight software on board of the spacecraft in about, it will be sent there in October. And this new space, sorry, this new flight software would not have caused this saving event. So it was a little bit unlucky. But on the other hand, we were very lucky that this did not happen half a day earlier, because then we would have lost all the data.

And maybe a little bit kidding, this is a good proof that there's no intelligent life on Iapetus, because if intelligent life on Iapetus would cause something like that, then they would have made a very bad hit. Yeah, they would have probably gone first before we got a look at them, rather than

after. Correct, correct. All right, so then we'll rule out that the black material is a result of their pollution on the moon. We only have about a minute left. You have been involved with this for a long time, and we will put up a link to some of the material that you put on the web that was done in preparation for this flyby. Really very interesting use of data and a behind-the-scenes look at the planning that goes into an event like this. Yes, I put this on the website simply that I have a reference for myself.

These were more or less the files that I used in the planning month and that always changed when I had to do some change in the planning. We had the star occultation, for instance. This was an experiment where the ultraviolet instrument was looking for a star and the star hiding behind Iapetus and hoping they see some kind of a short blinking before it disappears. And this would indicate some very tiny atmosphere. When the first rough plan of the flyby, the rough schedule of the flyby, was created in

the year, I think it was 2000 or so, star occultation was taking place about 10 or 15 minutes before closest approach, and this would have ruled out all the inbound observations. I didn't like this for all these years, and I was asking the imaging team and later the Cassini navigation people, and then they eventually looked last summer if there might be another trajectory with a different star occultation, and I was very pleased to hear that they found one that was more than an hour before closest approach. Congratulations, and thank you once again for joining us on Planetary Radio. Yes, thank you for the invitation.

Tilman Denk is at the Free University of Berlin. He is an imaging team associate on the Cassini mission, and much more to come from that mission, which will continue to circle Saturn and return unprecedented data. All of us hope for years to come. We'll be hearing from Emily Lakdawalla next. It's time for her second half of today's Q&A, and that'll be followed by Bruce Batts with this week's edition of What's Up. I'm Emily Lakdawalla, back with Q&A. The Phoenix Mars lander cannot survive the Martian winter,

back with Q&A. The Phoenix Mars lander cannot survive the Martian winter, so the question is how long it'll last into the autumn. Phoenix will land on May 25, 2008, about 30 days before the northern summer solstice, so there will be plenty of power available throughout its 90-day primary mission. The long night begins at Mars' North Pole another 90 or so days later on December 26. So the spacecraft will work through June, July and August. It'll probably make it through September and possibly October. November is questionable and December probably not. It'll depend on the performance of the solar panels and also on how aggressively the Phoenix team decides to use the spacecraft.

They could survive a relatively short time and continue to use the robotic arm and chemical analysis tools, or they could survive a long time as a more passive weather station, perhaps only operating for one day in three and going into a sleep state for the other two. Hopefully, Phoenix will survive long enough for its cameras to catch the first crystals of carbon dioxide frost condensing on the surface, a frost that will eventually grow to encase the lander in an icy tomb.

When spring arrives and the frost sublimes, Mars Reconnaissance Orbiter will grab a photo to show us what remains of the silent Phoenix lander. Got a question about the universe? Send it to us at planetaryradio at planetary.org. And now here's Matt with more Planetary Radio. Time for What's Up on Planetary Radio. Not at the Planetary Society this time, someplace ever so much cooler, or maybe it isn't. It's the Wired Next Fest in downtown Los Angeles, and we are surrounded by a technology and science of the future. We are indeed. Clearly, we are here because we represent the cutting edge.

You are a sharp guy. Thanks, thanks. Let's try to avoid anything dull, and maybe I should go on to what's in the night sky. Yeah, and we should say, by the way, that we are, of course, in the NASA pavilion here at NextFast. We are. We're surrounded by spacecraft and astronaut gloves and big video screens and all sorts of spiffy things. And right next door is the XPRIZE Foundation's little pavilion, And right next door is the X-Prize Foundation's little pavilion, where we will be doing much more coverage next week of the just-announced Google Lunar X-Prize. They also are running rovers over children, apparently.

I saw that. It seems very cruel, but, well, that's the future for you. Yeah, I guess so. On to the night sky, which is the past, present, and future. We've got Jupiter still looking lovely in the evening sky. Off in the south, they're moving towards the west just after sunset. We also have Mars rising right around the middle of the night in the east and high overhead before sunrise, looking reddish. And then we have Venus now dominating the early morning sky over there in the east, the brightest star-like object up at that time.

And that's our roundup for simple sky tonight. I decided to keep it simple. You know, it's clean design of the future. Our compliments to the designer. Okay, go ahead. Let's go on to random space fact. Hi, everyone. Darn it, people weren't giving us enough attention.

Little applause over there. Oh, thank you, thank you very much. Do you know, well, I'm sure you know, and a lot of our listeners know, but for those who don't, there are hardly any mare on the far side of the moon. Those dark, basaltic areas you see on the near side where Apollo liked to land? Almost nothing on the far side. You know, I had heard that. I'd forgotten, of course. But does anybody have any idea why?

They do. It's a mass distribution thing and thickness of crust and things like that. So the basalt oozed out on the one side. When you tidally lock a body like the moon, you tend to line it up like if there were a dumbbell, you'd line it up. Okay, too many jokes to be made. You'd line it up on the long axis pointing away from you, so the moon has got some of that going on. But I was surprised when those first spacecraft went around the backside and saw that. Very interesting. Thank you.

Okay, you're welcome. We go on to the trivia contest, and we asked people, when, what year, were Phobos, in this case Phobos and Deimos, discovered? How did we do? Man, we got a huge response. I don't know. We must have had 50%, 60%, 70% more entries than usual. Maybe this one was just really easy to find in Wikipedia or something because I think a lot of people went to the same source because we were getting a lot of the same text.

But not quite everybody. Craig Journet pointed out that he got the answer right, first of all, but Phobos and Deimos, Greek mythology, they accompanied their father, Ares, into war. Ares, known as Ars to the Romans. That was pretty interesting. Indeed, indeed. You got more? Yeah, several people. Lindsay Dawson was the first one to point it out, mentioned that, yes, the moons were actually first seen in 1877. But strangely enough, quite weirdly, 150 years earlier, Jonathan Swift talked about two moons of Mars in Gulliver's Travels

and got the parameters of Phobos just about right. Yeah, that's amazing. Did you know he also predicted the Lilliputians would be found by Viking landers? And they were. They're tied to the surface now. Did I get that reference right? Yeah. Okay, but you want to know who really won, right?

Oh, I do, I do. It was John Randall. John Randall of Portsmouth in the United Kingdom, who indeed said Phobos and Deimos, discovered by Asaph Hall in 1877. John says, love the show, all the best to all of you. Well, John, a Planetary Radio t-shirt to you, and here's how to win another one. Here's the question for this coming week. It's time already, once again, to play Where in the Solar System?

Tell us, people, where in the solar system is Hesperia Planum? Hesperia Planum, tell us where in the solar system you'll find that, and you'll have a chance to win a Planetary Radio t-shirt like the one Matt is sporting so proudly here at NextFast. Go to planetary.org slash radio, find out how to enter. When do they need to get that in by, Matt? September 24 at 2 p.m. Pacific time, September 24. That's a Monday, and we would love it if you would apply and win one of these t-shirts like the one that, indeed,

I am wearing proudly at this very moment. Do you know that Voltaire apparently also predicted two moons on Mars, and that in their honor, there are craters on, I forget if it's Phobos or Deimos, there are craters, Voltaire and Swift. When do we get ours? Maybe when we predict something that comes true 150 years later. I want mine sooner, though. Yeah, it'd be nice to have it around while we are. All right, well, there's some, I think, that aren't named. We could just go through the pictures and declare them. Craters, Kaplan, and Betts. They won't be big, but still, it's going to be cool.

Huh? I think IAU would go along with that. Let's go for it. All right, we will. We're being driven over by rovers yet again. Yeah, I think it's going to come back the other way. Oh, I hope so, because my lower lumbar could really use a little bit more recording. Doesn't this actually feel great, doesn't it?

It does, it does. And people, you know, hopefully will be able to see a picture of this on the website of Matt and Bruce doing it all for their show. I see a future in this for, you know, Steve Squires and the Sharper Image or, you know, Brookstone, somebody like that. Yeah. Oh, that feels good. Oh, yeah. Right there. Oh, yeah. Oh, yeah. That's good. All right, everybody, go out there and look up the night sky and think about being run over by a rover.

Thank you, and good night. It'll never happen. He's Bruce Betts, the Director of Projects for the Planetary Society. He joins us here, prone, for each week for What's Up. Thank you. A little lower. Planetary Radio is produced by

the Planetary Society in Pasadena, California. Next time, the Google Lunar X Prize. Have a great week. Thank you.