Planetary Radio: Space Exploration, Astronomy and Science - A Sea Under the South Pole of Enceladus?

Episode Date: April 2, 2007

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Is there yet another sea of liquid water in our 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. It's not quite water, water everywhere, but scientists do seem to be finding more and more evidence that the life-giving fluid may exist

in far more places than we knew. Planetary scientist Jeffrey Collins will tell us why he believes there may be a sea under the south pole of a little moon named Enceladus, a sea that is responsible for both the satellite's odd shape and its gigantic geysers. And you'll hear Jeff's thoughts about the other mysterious

and beautiful icy moons in our neck of the cosmic woods. Emily Lakdawalla is also up on Enceladus. It seems the tiny satellite is wreaking havoc on our ability to tell how long a day is on Saturn. My, my, what an influential little sphere. It's a weird week up there in the heavens. That's what you'll hear from Bruce Betts in this week's edition of What's Up. Weird or not, we'll also give away another Planetary Radio T-shirt, instantly making one of our listeners the envy of the Milky Way. At the top of our news is this apology.

All of us at Planetary Radio deeply regret that we have ignored the most important space exploration story of the last decade or more. Thousands of you have told us that you are shocked and disappointed by this glaring omission. We hear you, and we are truly, truly sorry. From here on out, we will report even the smallest detail of this monumental and still unfolding tale of scientific intrigue and the human need to explore. And the next time an astronaut demonstrates how tragically human he or she can be, diapers and all, we'll be right on top of it, bringing you around-the-clock coverage via this program and all the journalistic resources of the Planetary Society. And if you believe that, you're not aware that I'm recording this on the 1st of April.

I'll be right back with Jeffrey Collins. Here's Emily. Hi, I'm Emily Lakdawalla with a special question and answer update. Three weeks ago, I explained how we can figure out the rotation rate of gas giants like Saturn and Jupiter from the radio signals they emit. Radio signals originate in the magnetic fields of giant planets, which rotate with the planet. But there was a puzzling mystery about the radio rotation of Saturn. In the 25 years separating the Voyager and Cassini missions, Saturn's radio rotation rate slowed down by six minutes, or nearly one percent of its 10 plus hour rotation rate. This change

cannot reflect an actual change in the rotation rate of Saturn. So how did the radio rotation slow down? Last week, scientists published the answer. Enceladus is slowing it down. Enceladus is a tiny moon with geysers erupting from its south pole, spewing water ice and other molecules into space, which become ionized by Saturn's magnetic field. All that plasma acts as a drag on Saturn's magnetic field so that it can't keep up with the planet's rotation. So how will scientists figure out how long a day is on Saturn if they can't use the radio rotation rate?

Stay tuned to Planetary Radio to find out. How could a tiny moon a billion miles away generate so much attention? Because Enceladus is a very strange place. The journal Icarus has just published work by Jeffrey Collins and his colleague Jason Goodman proposing that under the south pole of the Saturnian satellite lies a sea of liquid water. This sea would neatly explain both Enceladus' strange shape and the chemically rich water geysers that the Cassini spacecraft has imaged. I recently spoke with Jeff at his Massachusetts home near Wheaton College,

where he is a planetary scientist in the geology department. Jeff, thanks very much for joining us on Planetary Radio, and let me start by saying that I love this term. It's so wonderfully scientific. Triaxial ellipsoid. Yeah, well, the triaxial ellipsoid is a fun shape to play with, actually. Basically, when we're talking about that shape, we're talking about something that looks like a football. Well, maybe a squashed football. It's got a long axis and a short axis and some other axis that's in between. The reason why we're concerned about this particular shape that you mentioned, the triaxial ellipsoid of Enceladus, is that the shape of a moon that's in orbit around a large planet can tell you about what's going on inside that moon. What we noticed was that Enceladus's shape

is kind of funny. It's not quite exactly what you would expect. We weren't the ones who were doing the shape measurements, but we noticed this in the data that the Cassini team had published. This is Carolyn Porco, the leader of the imaging team. Yeah, that's right. They had a paper in Science about a year ago where they reported the shape, and it's not exactly the shape that you would expect if Enceladus was in equilibrium.

By equilibrium, I mean that the tides are pulling on Enceladus, Enceladus is spinning. That makes the tide sort of pull one axis of Enceladus out, and the spin makes it bulge around the equator. Very much like the Earth. I mean, the Earth is not quite a sphere either, right? Yeah, well, exactly. The deviation from a sphere actually tells you something. What we noticed was that the shape isn't quite right, isn't quite what you would expect. There's a couple of different ways you can explain it. One way of explaining it is that maybe the Enceladus is bulging out too much at the equator. We took the approach of maybe instead Enceladus is actually punched in along the polar axis.

We already know there's something funny going on at the South Pole. And so we thought, well, what would happen to the shape if you just took the South Pole and put a giant pit in it? And it turns out we can match the shape pretty well by doing that. In other words, you modeled it mathematically and came up with pretty much what Carolyn Porco says Cassini's found. Yeah, and the really interesting part of it is that when you try to model the shape by punching in the South Pole, you say, well, how much volume do I have to lose out of the South Pole in order to explain the shape? And one way of losing volume in an icy satellite is to melt the ice, because ice takes up more room than water does.

It's less dense. If you melt the ice, you create this big pit at the top. This happens in Iceland sometimes when volcanoes go off under glaciers and create these lakes under the glaciers and you get these pits on top. So we thought, well, what if something like that was going on in Enceladus? And so we calculated how much heat would you need to make a pit that would be big enough to explain the odd shape of Enceladus? And it turns out the amount of heat that you need is pretty close to the amount of heat

that they observed to be coming out of the South Pole, and that's when we thought we were onto something. And this is also, of course, where we see these plumes coming out of this very tiny body. I guess people thought that it was really too small to see this kind of thermal activity. Yeah, it's a surprisingly tiny body to be active. I mean, very few people, I think, ever expected this. It's a body that's only about 500 kilometers across, which may sound big, but to you living in California, you know, it's much smaller than your state.

Yeah. We're very proud of that, yeah. That's right. Maybe a little bit bigger than Massachusetts. I mean, the reason why you wouldn't expect it to necessarily be active is you expect that large planets should be able to retain their heat much more effectively than small planets. And Enceladus being a rather potato, should cool off very quickly, unless there's something that's actively keeping it warm.

So we are finding, it seems almost on a daily or weekly basis, that these icy planets, excuse me, icy moons, are far more interesting than anybody thought a couple of decades ago. Yeah, well, I personally have been interested in them, but... Yeah, no, I think that they're extremely exciting places, and one of the reasons that I think they're so exciting is that there's so many new processes that are going on there that are not necessarily Earth-like. On the Earth, we worry about what's going on in our rocky interior,

and we don't have to worry about some large layer of a completely different material that's layered on top of those rocks. So what happens when you take the rocky body and then you layer all of this ice on top of it? It's one of the things that makes icy satellites behave so differently because you have all this water on top. And water is a very strange substance. I mean, it contracts when it melts, which hardly anything else does. All kinds of amazing properties that make geology on the icy satellites somewhat different

than the geology that we expect on the Earth. And yet it's actually surprising considering how different water is. We actually do see some geological features on these satellites that we can interpret. I mean, if you think about it, if the physics was really radically different, then maybe we might see features that we just fundamentally can't interpret on the surface of these bodies. I also wonder, has anybody begun to figure out a mechanism that could have generated this kind of heat in a 500-kilometer moon. Well, people have pointed to tidal heating as being the primary thing

that's probably responsible for generating all of this heat. It turns out that Enceladus is in orbital resonance with Dione, which is a couple moons out from Enceladus. And so as the gravity of Dione tugs on Enceladus's orbit, you may get the same kind of tidal flexing on Enceladus that keeps Io warm or Europa warm in the Jovian system. The big question, though, is where is that heat deposited? You know, is it warming up the rocks in the interior? Is it warming up the ice, like at the bottom of the ice, like we think happens on Europa? Or could it be even heating the surface of the ice?

There's some new ideas about making cracks on the surface of Enceladus rub together as tidal deformation happens and sort of heating the very surface of Enceladus without heating the interior at all. Which one of those possibilities is right, or whether maybe they're all happening? It's a big question we need to solve if we're really going to understand what's going on inside Enceladus, and what these geysers mean. I mean, are these geysers pointing to a really hot interior, or are they just pointing to some kind of really strange surface process? Mention for just a moment, if you would, the work published in the same issue of Icarus by your colleague Dennis Mattson and others that says not only are these plumes coming out,

but they contain all kinds of interesting stuff, including some hydrocarbons. Yeah. When the initial results came out about what was in the plumes, I was extremely interested in that. I mean, there's these simple carbon compounds that are coming out of Enceladus, too. Where does that come from? Yeah, the work that Dennis Mattson published just recently points to some kind of episode where there was very hot rock in contact with the water on Enceladus. And so does that point to volcanoes that are going off right now underneath the ice on Enceladus? Or does it somehow point to some maybe previous episode

where Enceladus might have warmed in the past and you've trapped these products of interactions of the water with the hot rocks underneath Enceladus' ice only to be released later? The fact that there's some chemical evidence that the water might have reacted with hot rocks at some point is certainly something that at least jibes with what we've been talking about in terms of heating up the bottom of Enceladus' ice and melting it that way. In just a minute, we'll hear more from planetary scientist Jeffrey Collins about Enceladus. This is Planetary Radio. Hey, Bill Nye the Science Guy here. Ius. This is Planetary Radio. Hey, hey, Bill Nye the Science Guy here.

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We're talking with planetary scientist and Wheaton College professor Jeffrey Collins about Enceladus, the little moon circling Saturn that may hide a subsurface body of water like the ocean under the ice on Europa, but not exactly like Europa. People have talked about oceans on icy satellites around the solar system, and they usually mean some global layer of water trapped underneath the ice. In this case, we think that it's actually a localized sea that's just beneath the South Pole and is just trapped in sort of this bubble right beneath the South Pole and doesn't necessarily extend out over the whole globe.

If it does, it might only be a very thin layer. We're trying to make a little distinction there between what's going on in Enceladus versus other icy satellites. It seems that maybe it's just a function of Enceladus' size. On small icy satellites, maybe you just might get little localized bubbles of melt when you have heat activity going on in the interior. And on large icy satellites, perhaps those things spread out into a big global ocean like we think exists on Europa. I see what you mean, and no pun intended. You get water, liquid water, obviously a source of heat that's keeping it warm.

And you get some interesting chemistry, and you start to think, gee, we ought to be taking a closer look at this place the same way people want to look at Europa. Yeah, absolutely. Anywhere where we find liquid water in the solar system, you know, suddenly makes our ears perk up and makes us interested in what might be going on there, especially if that liquid water is in contact with something chemically very different, like active hot rock, because then you have the possibility for some interesting chemistry,

and then the astrobiologists get excited. Yes, yes, I was hoping we'd get to at least mentioning that. The Holy Grail here, of course. The Holy Grail here, of course. What can we do most immediately to develop more evidence for this sea under the Enceladus South Pole? Well, one of the things we really need to do is to get more flybys of Enceladus with the Cassini spacecraft. Luckily, the latest plans that I've seen for the extended tour that Cassini is going to take over the next few years in the Saturn system, we will have several more flybys of Enceladus.

And what I think is really important is to determine the gravity, the gravitational field. That will tell you a couple things. First of all, it will tell you, is Enceladus actually separated into a rocky layer and an icy layer? That's something we don't actually know yet. A lot of people are assuming it, but because it's hard to get that much heat coming out of the moon and have the rock not separate out from the ice. And the other piece of gravitational information that would be very useful is if there is a gravity anomaly at the South Pole.

And by gravity anomaly, I mean that there's like some extra mass that's concentrated at the South Pole that you wouldn't normally expect to be there. So, for instance, if you had a sea beneath the South Pole of Enceladus and the ice was forming this big pit over the top of the sea, you actually expect there to be slightly less gravity as you fly over the South Pole. gravity as you fly over the South Pole. And so it would be very interesting to compare the motion of the Cassini spacecraft as it flies over the South Pole versus when it's flying over other parts of Enceladus. To me, that's one of the things that's really going to be a test of what's going on beneath the South Pole.

And that would be through some of these extremely sensitive Doppler tests that can be done between Cassini and the Deep Space Network. Yes, exactly. That's exactly how they would do it. Now, I hate to do this, but with only about a minute left, I do want to get in some of these other moons that are crying out for our attention. And so I'm just going to throw things at you. And if you can give me, you know, a sentence or two on, this is so unfair.

Ganymede, grooves on Ganymede, weird. Yeah, well, grooves on Ganymede are one of my favorite things in the world. I've been working on them ever since I was a graduate student over the past 12 years or so. One of the things that's fantastic about them is that they're very similar to extensional faults that we see on the Earth. So we can actually interpret what's going on and see how much Ganymede surface has been pulled apart. And what's cool about that is that we can then test models what might be driving the extension on the surface of Ganymede. Titan, where we are learning that water ice, frozen far, far colder than we get in our

freezer ice trays, seems to sort of be responsible for what you'd call the equivalent of geology. I know. It's the most Earth-like icy satellite we've seen so far. Just like I was inspired with the Enceladus work by seeing some of those geyser images, I was really inspired by the images from the Huygens probe that showed river channels on the surface of Titan. And I immediately became interested in how fast can you actually erode a channel into solid ice at those temperatures. So I did some quick homemade experiments, dropping things on little pie pans full of

water ice chilled with liquid nitrogen. Don't try this at home, folks. Oh, yeah, well, we did try this at home, actually. But you're qualified scientists excluded, of course. Yeah, exactly. We did some math after these little homemade experiments and found that actually you would expect Titan's surface to be eroding sort of like badlands of loose sandstone, you know, in the arid southwest of the U.S.

where you kind of have these deep channels that are incised by, you know, big thunderstorms that come through once in a while. So I sort of have this like northern Arizona Badlands model for what's going on on the surface of Titan. And what's really exciting is we're just starting some new experiments with a colleague out in San Francisco where we're actually building this Titan erosion simulator apparatus, and we're going to be nailing down much more firmly how the bedrock on Titan erodes. This has actually gotten the terrestrial people who study rivers very excited because, you know, suddenly we have a place with completely different materials that we can test some of the physical theories

of how stream channels erode on the Earth. We're going to want to check back with you on that, I'm sure. That's going to be very interesting, a Titan simulator. We're going to want to check back with you on that, I'm sure. That's going to be very interesting, a Titan simulator. Okay, finally, one sentence about Europa, where I guess it is still appropriate to talk about an ocean rather than a sea. Yeah, well, absolutely.

On Europa, you probably have a lot more tidal heating going on. And Europa is so big, the ice flows very readily at the base, and so the water sort of spreads out into a big sea. I wish I could explain Europa. There are so many features on Europa that completely confuse me. One of the ones that I've concentrated on is these areas of chaos terrain on Europa that are unearthly and fascinating. Yeah, I've worked on some models for how they might form, but I'm not sure if I believe any of them. I'm still lacking an explanation for what the heck is going on on Europa.

I'm really eager to somehow get back there someday and get some more data. You've got a fun job. I sure do. I love it. We are way out of time. Jeff, thank you so much for joining us today. And we'll be back to talk with you more about Enceladus and its icy sisters. Great talking to you.

Jeffrey Collins is Associate Professor of Geology at Wheaton College in Massachusetts, co-publisher of this recent paper about Enceladus, explaining why it doesn't look quite the way it should and the possible presence of this sea under the South Pole on that small icy moon out near Saturn. We'll be right back with this week's edition of What's Up. And Bruce Betts after a return visit from Emily. I'm Emily Lakdawalla back with Q&A. I'm Emily Lakdawalla, back with Q&A. Now that we know that plasma from Enceladus' geysers is slowing down the rotation of Saturn's magnetic field,

how will scientists figure out how fast Saturn really is rotating? This is not an experiment that Cassini will be able to perform. But one possible way to measure the rotation rate of an enormously massive planet like Saturn would involve the relativistic effects of its rotation. Recently, a mission called Gravity Probe B sent four incredibly precise gyroscopes into orbit around Earth. Analysis of the terabyte of data on the minute deflections of these gyroscopes

gathered over the one-year mission will hopefully lead to the detection of frame-dragging, the twisting of the local fabric of space-time around the rotating Earth. Saturn, being much more massive and spinning more than twice as fast as Earth, should twist its region of space-time much more than Earth does. An experiment like Gravity Probe B could determine the rotation rate of Saturn from these relativistic effects. No one is proposing to do this experiment yet,

but perhaps someday we'll use the shape of space-time to probe the interior dynamics of giant planets. Got a question about the universe? Send it to us at planetaryradio at planetary.org. And now here's Matt with more Planetary Radio. Now here's Matt with more Planetary Radio. Time for What's Up with Bruce Betts, the director of projects for the Planetary Society. And he's going to tell us about the night sky and some other stuff.

We are on the phone, as you can tell. It's been a busy weekend. Hi, Bruce. Hi, Matt. How about I tell you some exciting stuff in the night sky? It's been kind of weird, though. It's a weird week for astronomy? There's a lot of weird stuff going on.

All right. Go for it. I'm ready. We've got Venus looking really bright in the evening sky, but for reasons that are unclear, it's looking a lot more reddish than usual. We've got Mars in the pre-dawn, and it's looking a lot yellower than usual. You sure you don't have those backwards? Well, I'll check it later. Okay. We've got Jupiter that is up around midnight, and then it tends to just disappear somewhere around three in the morning.

Saturn is looking great, rising up high in the early evening. Yeah. And if you take a look at Saturn through a telescope, which I know you have, on April 15th, the cloud patterns and the rings will actually form a giant smiley face. Is that right? Isn't that, didn't Galileo note that? Well, it does recur approximately every 200 years. Yeah, it was probably a couple times ago.

Oh, so yeah, there's only been, what, like you said, two opportunities in the past, and one of them only Galileo saw, so this is a pretty big opportunity. April 15th, eyes on Saturn. Okay. We have this week in space history, in 1932, Robert Goddard, of course the father of American rocketry, did the first launch of chipmunks on a rocket, two chipmunks, and they intentionally launched them with fleas to see what would happen. Only two, not three?

No, there were two. And when it came back, however, the fleas were gone. Oh, really? Yeah, yeah. It was an original way, I guess, of trying to de-flea small mammals. Fascinating. We could probably make use of that. It's a little expensive, though.

Yeah. Random Space Facts! Did you know that the outer layer of the sun is actually referred to as rayon? The rayon layer? It can only be seen during eclipses, right? Yes. During eclipses, it comes out,

and occasionally you can see to the next layer, which, of course, is polyester. Of course, of course. Okay, so far so good. Let's move on to the trivia contest, shall we? First of all, note that it is, at least in crazy America, April Fool's Day, basically. Yes, yes, this is the show closest to that most hallowed of American holidays. Exactly.

So I'd listen to less of what I said or at least follow it less than usual. Oh. I'm looking for the smiley face on Saturn. I'm sorry. I mean, if you use your imagination, there's an element of truth in everything. Well, except maybe the chipmunks. But the rayon layer, that's for real. Totally true.

Yeah. Polyester I made up, but rayon, there's a good layer. And it's good to layer, especially if it's cold, which is not on the sun. I hope this was useful to everyone. Most of the planet information was right, though, except for the whole color. Shall we go on to the trivia contest? Why not? Shall we actually do a legitimate trivia contest?

Does it have to be legitimate? Well, at least last time. All right. Which two Gemini missions were in orbit at the same time? This is what I ask people. How'd they do? Were they not fooling around? Shall I give them the answer?

Oh, please do. Gemini 6A, to be precise, with Schirra and Stafford, met up with Gemini 7 with Borman and Lovell on December 15 and 16, 1965. They didn't actually touch, but they did get really, really intimate. Okay, yeah. So did we have a winner? Oh, I'm sorry. We sure did. John Lease.

And John is from Moorpark, California. We could just about throw a rock over his way. He got them both right, Gemini 6A and Gemini 7. And for that, John, we're going to, we might as well just drive it over there. We'll drop off a Planetary Radio t-shirt. Yeah, well, if that happens, that'll be Matt. Let's go on to another trivia contest, and I felt like I couldn't just ask a bogus contest question,

because, you know, we give away real T-shirts, real cool Planetary Radio T-shirts. So I have to ask something legitimate. But with that April Fool's Day twist, I thought it should be something, you know, a little creative. And people may actually have to, well, not think, but do a little more research here. Who was the first person in orbit around the Earth, orbit, with the letter Z in their name? Okay. And in the case of non-English alphabets, we'll go with the most standard transliteration to English. First person in orbit with the letter Z in their name.

Okay. Go to planetary.org slash radio. Find out how to enter. And you've got until April 9, Monday, April 9 at 2 p.m. Pacific time. Happy April Fool's Day. Happy April Fool's Day, everyone. Sorry to mess with you once a year. And you really can't find those planets, I told you.

Hey, guys, what should the listeners think about? Aliens in underpants. All right, everybody, you heard it. Go out there, look in the night sky, and think about aliens in underpants. Can we say that on the radio? You just did. Oh. Thank you, and good night.

That's Bruce Betts, director of projects for the Planetary Society, and his boys with underpants on their mind. That's really not where they should have them, but really it's a good thing. That's probably how the Air Force keeps the aliens under control in Waswell. They just don't give them back the rest of their clothes. I think we're done. Gosh, I hope so. Their grandparents don't listen to this, do they?

Planetary Radio is produced by the Planetary Society in Pasadena, California. Have a great week, everyone. Thank you.