Planetary Radio: Space Exploration, Astronomy and Science - Cool Science: Ice on Europa and Mars

Episode Date: March 3, 2003

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This is Planetary Radio. Hello and welcome back everyone. I'm Matt Kaplan. Last week it was fire with the volcanoes of Io. Matt Kaplan. Last week it was fire with the volcanoes of Io. This week, ice as we visit the frozen surfaces of Europa and Mars. Our guest is Dr. Elizabeth Turtle, an expert on this very cool topic. Red marks the spot on Jupiter in this week's What's Up with Bruce Betts. Bruce will have his new trivia contest question for you, too. We begin, though, with Emily, whose eye is on the mysterious moons of Saturn. That's planetary radio running rings around all the other space radio shows. Let's get started.

Hi, I'm Emily Lakdawalla with questions and answers. Did you know that Jupiter is not the only planet with an orbiting family of diverse satellites? Thanks to the successful Galileo mission to Jupiter, we now recognize that the Galilean satellites of Jupiter are each distinct worlds with unique geology and history. But these are not the only such worlds yet to be visited in the outer solar system. The Cassini spacecraft will soon arrive at Saturn to study icy moons first observed by its namesake scientist, Giovanni Domenico Cassini,

late in the 17th century. Iapetus, Rhea, Dione, and Tethys. To find out what we know about these distant icy worlds, stay tuned to Planetary Radio. Elizabeth Turtle is our guest on Planetary Radio this week. She is a Senior Research Associate in the Department of Planetary Sciences at the Lunar and Planetary Lab, University of Arizona. That's in Tucson. She has a connection to the Planetary Sciences at the Lunar and Planetary Lab, University of Arizona. That's in Tucson. She has a connection to the Planetary Science Institute, too,

which is not a coincidence in this case because it was Bill Hartman, one of our previous guests about a month ago, who said, hey, I have this terrific young scientist who knows a lot about ice elsewhere in the solar system, and you really ought to talk to her. And it turns out that that's Elizabeth. Or should I say Zibi? Zibi, welcome to Planetary Radio. Hi, thank you. How did you get that nickname?

Oh, actually, I couldn't pronounce Elizabeth when I was learning how to talk. So I started calling myself Zibette, and it kind of evolved into Zibi. Well, Zibi... Which it stuck. And you said you respond better to that anyway, so I'll try to remember to use that. Actually, Rosalie Lopez, our guest last week, said, Oh, yeah, Elizabeth, I work with her, and everyone calls her Zibby, so we will too. Interesting that last week we had Rosalie Lopez.

We told Rosalie and said on the air that we sort of wanted to think of that as our fire show because we talked about volcanology, whereas this week I'm hoping we can spend most of our time talking about much colder stuff, and that is the interesting places where you can find, we think, water, ice, and snow in our solar system, although we certainly don't mean to imply that your expertise is limited to those. I mean, in fact, you've done some work on Io as well, haven't you? Yes, actually. I've done some work on Io as well, haven't you? Yes, actually. I've investigated mountain formation on Io.

And we talked a little bit about that last week. Now, when you say mountain, do you mean volcanoes or actual mountains that are not related to volcanology? No, surprisingly, the mountains on Io are actually mostly tectonic. They don't have central calderas at the top that we see lava flows or plumes coming out of. They just seem to have formed by the tectonic motions of Io's crust. So that really is an interesting place.

Oh, it's fascinating. It's fascinating. But you're probably not going to find much water ice there. No. No. Well, let's move to its neighbor then, Europa, which has also been a focus of some of your research. And Europa, of course, is this planetary body, one of the Galilean moons of Jupiter, which is getting increasing attention and certainly generating lots of curiosity

because it is a place where there is a lot of ice and maybe even something more interesting under that ice. Yes, there may in fact be liquid water under the ice shell on Europa. There's several lines of evidence that suggest that. I guess we've certainly determined, because we've had good observations, that Europa seems to be encased in this layer of ice. But for one thing, how do we have any idea how thick that ice is, and how do we have any idea that it is water and that there's liquid water underneath it? We know from spectroscopic observations that it's water ice. And we know how much water ice, water and ice there is, liquid and solid there is, from gravity measurements, actually, that the Galileo spacecraft made as it flew by Europa.

So we know there's an outer layer of solid and liquid water that's somewhere between 80 and 170 kilometers thick. Unfortunately, the gravity can't distinguish between the phases of the water, whether that's liquid or solid. But there are other lines of evidence, such as the surface geology and the use of the surface that have suggested that there's still liquid water underlying the ice. Do we have an idea now, or is this merely a hypothesis as to how thick that layer of ice is? Well, that's certainly a subject of active debate.

It's ongoing. The research that I've done with my colleague, Elisabetta Pirazzo, who in fact is also at the Planetary Science Institute now, was explicitly looking into the thickness of Europa's ice layer. That's been investigated primarily upon the surface

geology, but also on thermal evolution models of Europa. One of the difficulties in using the surface geology of Europa to understand the thickness of the ice, to use that as a constraint on the ice thickness, is that a lot of the geologic features we see on Europa are very different from geologic features we see anywhere else in the solar system. So at the same time that we're trying to understand how Europa's geology works, we're also trying

to use that geology to constrain the structure in which it's occurring. And that can be quite problematic. And so what Betty Pirazzo and I did was to look at the impact craters on Europa. Because impact craters are something we've studied on many other planets in the solar system. There are several. There are, in fact, about 150 here on Earth. And so we have a pretty good understanding of the basics of how craters form and the processes involved. And so by looking at the morphologies of the craters that we see on Europa,

we can try to understand the structure of the surface in which they're forming. And so have you actually been using this impact information, the cratering, to build models that would help us to decide how thick that ice might be? Yes. What we've done is simulations of the impact cratering process. Now, impact cratering, I should say the morphologies of impact craters, the surface profiles, the shapes of the craters, they're mostly circular, but whether they're just little bowl-shaped craters, simple craters,

we call them like Meteor Crater in northern Arizona, or whether they're more complex structures with central peaks and collapsed rims, that depends on characteristics of the target. For example, the target, the gravity of Europa in this case, also the composition, the strength of the surface in which they form, as well as the structure, i.e. the thickness of the ice in which they form. The other thing that the crater morphology depends on is the size of the crater, the amount of energy involved in the impact. So smaller craters tend to be simple bowl-shaped craters, and the larger ones tend to have

this more complex morphology I spoke of, the central peaks and collapsed rims. And we do indeed see that on Europa. We look at Europa has about 25 to 30 impact craters that are larger than 4 kilometers in diameter on it, which makes it quite a young surface, actually, in the solar system. So Europa has several impact craters, and the morphologies of the impact craters do change with the size of the crater, as we see on other bodies in the solar system, for example, the Moon and other Galilean satellites like Callisto and Ganymede. There actually

aren't any impact craters on Io that we've been able to find yet, which speaks to how young its surface is. They disappear too quickly, I guess. Yes, exactly. I'm wondering, as we get fairly close to the time when we need to take a break, has this numerical analysis that you and your colleague have done, is it leading to any conclusions about just how thick that ice is? Well, what we've done is look at different stages of the impact cratering process. So Betty and I looked at the very early stage and looked at basically just how much ice is melted and vaporized in the impact.

You can think of an impact as just a huge explosion, really. It's about the same thing. And so clearly if the impact has enough energy that it melts all the way through an ice shell, then you'd expect to just have a very flat surface because the water would, if it melts all the way through, then water is exposed to the surface, and that will rise up and just freeze. And we'd expect maybe a rough but a flat floor without a central peak or these collapsed rims that we see on complex craters on, say, the moon. I see.

And the kind of craters that we observe on Europa. And so the very first thing we did was just look at how much material is melted and vaporized. The impacts that form craters that we observe on Europa to have a complex morphology, so central peaks, would have melted through if the ice were 4 kilometers or less thick. So I take it this is some indication that the ice is thicker than that. That's right. And the next step in this project was to look at the later stages of impact cratering because that's just the very early stage,

and then later as the crater actually opens up and then collapses back to form these central peak features that we see, there's a later stage of impact cratering called excavation, and then there's another one called collapse. And so what we've done is take these models further to investigate what happens in these later stages. Let me stop you there, and maybe we can pick up right after we take a quick break. Our guest is Elizabeth Turtle,

a senior research associate in the Department of Planetary Sciences, Lunar and Planetary Lab at the University of Arizona. Everybody else knows her as Zippy. We will be back with her 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. You can learn about these adventures and exciting new discoveries from space exploration in the Planetary Report. The Planetary Report is the Society's full-color magazine. It's just one of many member

benefits. You can learn more by calling 1-877-PLANETS. That's toll-free, 1-877-752-6387. And you can catch up on space exploration news and developments at our exciting and informative website, PlanetarySociety.org. The Planetary Society, exploring new worlds. Matt Kaplan back on Planetary Radio with our guest this week, Elizabeth Zibby Turtle.

She is a Senior Research Associate in the Lunar and Planetary Lab at the University of Arizona. Zibby, I had to stop you just as you were moving on to what you said is sort of the second stage of this work you're doing, investigating cratering on Europa, this moon of Jupiter, which is surrounded by an icy crust, except that we don't yet know how thick that crust is. And you said that the first phase of your work seemed to indicate that the crust might be more than four miles thick, but what about this second stage? I'm sorry, four kilometers. Sorry about that.

When we simulated the later stages of the impact cratering process, what we found is that as the impact crater opens up, the ice isn't breached. However, often as the impact crater collapses back, the water does breach the surface if the ice is thin enough. And our results indicate that if the thickness of the ice is comparable to the diameter of the initial crater cavity that's opened up, then the ice can be breached by water as the crater collapses late in the impact cratering process. Now, when I say late in the impact cratering process, I should specify that that's only a few minutes. Impact cratering is a very rapid process, so late is on the order of a couple of minutes.

And what this means is that in order for craters of the morphology we observe on Europa to form, the ice really needs to be at least 10 to 15 kilometers thick. Oh, I see. That's a fascinating finding, just to back up a little bit, this determination of the relationship of the thickness of the ice to the diameter of the initial impact crater. Was that something that emerged out of the simulations that you were doing, which had not been realized before?

Yes, that came directly out of our simulations. Those are simulations that we're continuing to refine at this point, but I should point out that that's consistent with some other impact cratering studies based on the morphologies that we see at the surface and how those change with crater size that Paul Schenck has been doing at the Lunar and Planetary Institute. And he also finds that the ice probably needs to be about 20 kilometers thick to be consistent with his morphological studies. Well, that strikes me as a finding in and of itself that is going to be useful in research elsewhere in the solar system, perhaps beyond someday,

but also that conclusion that the ice may be quite a bit thicker than some people were hoping must come as a disappointment to some folks who would love to drill down through that ice, get into that ocean, and find out what might be in there. Yes, it certainly makes it harder to get to the liquid water that we believe is beneath the ice. However, what we looked at on Europa is only a few impact craters. There only are a few impact craters, and those can only constrain the ice thickness at the locations and the times at which they formed. There may well be other places on Europa where the ice is thinner,

so it doesn't rule out the possibility that one could get through to a water layer in some locations on Europa. I see. You can't rule that out. So we should not assume that that layer of ice is the same thickness all the way around. It may not be. Yeah. Let's, as I said, we wanted to move a little bit closer to the sun in our solar system,

and that is to the planet Mars, where you've also done some investigation, not so much, I guess, of cratering or of impact, the effects of impacts, but something that has been very much in the news lately, and that is the possibility of water ice on the surface of Mars, perhaps even still existing there today. In fact, the evidence is starting to look pretty strong of that. existing there today. In fact, the evidence is starting to look pretty strong of that. But I think that your research has been more on how past ice and snow formations on Mars have actually, how should I say, flown, flowed, or shifted. That's an odd way to use the word, isn't it? And how that may have shaped the surface of Mars.

Yes. We've been looking into regions on Mars that have what is called softened terrain. If you look at images of softened terrain, what you see is craters. In fact, I'm also looking at impact craters on Mars but in a very different context, not the cratering process itself. What you see is that the craters look softened. They look like the material has flowed. And you also see around numerous mountains on Mars, you see debris aprons that, again, look like they have undergone some sort of flow away from the mountains.

So this would be very distinct, let's say, from the fairly sharp-edged craters on the moon, comparatively sharp-edged. Yes, and in fact, it's distinct from other craters on Mars that are quite pristine. So you can compare craters of the same size that have this softened topography with lower relief and gentle rims but that are still clearly impact craters and there are other craters the same size in different regions that have very sharp morphology. So talk about how your research has been looking at how ice may have caused these eroded surfaces. Well, what we've looked at is the viscous deformation, creep, of ice and, well, of ice-rich

dust, basically. So the surface of Mars is basically dusty, but there may be places where there's a lot of ice worked in, in the pore spaces of that dust. There are people that have done rheologic experiments to investigate how that, how dust-ice mixtures behave under stress, how they would flow. And so what I've done is to build computer models

that have initially the topography, the topographic profile of an impact crater, and if you put in the rheology, the behavior of dust-ice mixtures into these models, you can see what conditions are necessary to allow the dust-ice mixtures to deform under Martian gravity and Martian thermodynamic conditions. If this is taking place, and I take it that your research indicates that it is, your models indicate this, is it in any way, in even a very small way, like the movement of glaciers on Earth?

It's similar. Many glaciers on Earth are actually able to slide along the bottom, along the bottom surfaces if they're warm-based. And that may not be possible under the temperature conditions on Mars. I see. But nonetheless, the ice can creep. The ice itself can flow, and that also occurs in glaciers. So I kind of jumped over it there,

but does your model seem to indicate that this is very possibly an accurate model of what has happened on the Martian surface? It is rheologically accurate. The constraints we have on the temperatures at the base, we don't know, for example, if these features on Mars have actually slid across the surface the way glaciers do on Earth or if they're just creeping internally. That's one of the things that we're actually trying to understand right now. But we've used the most recent rheologic experimental data to put into the models,

so hopefully they're as realistic as we can make them right now. So hopefully they're as realistic as we can make them right now. What additional real-world data that might be gathered, let's say, by a rover, would you most look forward to in helping you to make this model more accurate? One of the things that would be wonderful to know for Mars is the heat flow, how much heat is coming out of the surface, and what the temperature profile is with depth on Mars. And landers would be able to get at that to some extent.

What you really want to do with Mars is send some geologists there, to be honest. Do you want to go? Oh, I'd go, yeah. I'm not a geologist, but I'd go. Yeah, me too. And there's so much more that we could talk about. We haven't even touched on a lot of your other research. For example, in the little synopsis that you sent me, you talk about finite element modeling of mountains on Io,

and it sounds very similar to research you've done looking at the formation of your open double ridges. And I wish we had time to find out. I guess the only solution is that we'll have to have you back on Planetary Radio again sometime soon. Thanks. That sounds great. I'd love to come back. Her friends know her as Zibi. Elizabeth Turtle is Senior Research Associate at the Department of Planetary Sciences in the Lunar and Planetary Lab, University of Arizona, Tucson, Arizona.

Zibi, let me ask you one more thing. You said you split your time between the Lunar and Planetary Lab and the Planetary Science Institute. Yes, that's true. I work part-time at the Lunar and Planetary Lab and the rest of my time at the Planetary Science Institute. Yes, that's true. I work part-time at the Lunar Planetary Lab and the rest of my time at the Planetary Science Institute. A couple of fun places. Yes. Thanks again very much for joining us. Thank you.

And Planetary Radio will continue in just a moment. Hi, I'm Emily Lakdawalla, back with questions and answers about the icy moons of Saturn. Like the Earth's moon, Iapetus, Rhea, Dione, and Tethys all orbit their planets synchronously, meaning that one side of the moon always faces the planet. Synchronous rotation also means that one hemisphere of the Moon, called the leading hemisphere, is always facing forward along the orbit, and the opposite hemisphere, called the trailing hemisphere, is always facing backward. Because the Moons always face the same way in their orbits around Saturn, nearly all of the impact events occur on the leading hemisphere, just as more rain falls onto the front than the

rear windshields of a moving car. Rhea and Dione are similar worlds that are both pockmarked with craters on their leading hemispheres, while their trailing hemispheres have bright swathes of material on a dark background. Iapetus, though, is very different from Rhea and Dione. It's a strange black and white world. The leading hemisphere is darker than soot, while the trailing hemisphere is one of the brightest and most reflective surfaces in the whole solar system. Because of this radical contrast, Cassini could see Iapetus only on one side of Saturn, when Iapetus was moving away from Earth in its orbit, and not the other. Scientists currently have no good explanation for the sharpness of the contrast between Iapetus's two hemispheres.

Finally, there is Tethys. Tethys is a bright-colored moon whose surface is dominated by a gigantic impact feature nearly 40% the width of the moon itself. The only way that such a large impact would not have destroyed the moon is if the event took place while the moon was liquid. Hopefully, the Cassini mission will reveal as much about Saturn's moons as Galileo has about Jupiter's moons. Got a question about the universe? Send it to us at planetaryradio at planetary.org. Be sure to provide your name and how to pronounce it and tell us where you're from. And now, here's Matt with more Planetary Radio. Bruce Betts is back for this week's installment of What's Up?

Welcome, Bruce. Thank you. I'm excruciatingly happy to be here. It must be the wonderful weather here in Southern California. Oh, and the charming personalities. Shall we start with well beyond our weather, and that's what's up in the sky this week? Let's do that. Well, it's related. If you have cloudy weather, you won't see them.

But if it's not, you will see Saturn and Jupiter in the evening sky once again. Early in the evening, if you look up, you'll see Saturn almost directly overhead, kind of above Orion, and you will see Jupiter extremely bright in the east, southeast, and by 10 or 11 p.m., up straight overhead in the morning for the early risers or late going to betters. Look for Venus looking extremely bright right before dawn in the east and Mars considerably to its upper right looking faint, dim, and red. And they're growing every week. Everybody should keep Jupiter in mind because it's going to come up again in just a few seconds. What's next?

Well, this week in space history, March 5th, 1979, Voyager 1 executed its flyby of Jupiter. And I didn't realize it was coming up quite that soon, but there it is. We don't lie. This is Planetary Radio. Still one of the most amazing

dates, flybys, in the history of space exploration. Yes, it is. Learned a lot. It was the beginning of a lot of great stuff from the Voyager spacecraft. Now what do we move on to? Random space fact! Speaking of Jupiter, the Great Red Spot on Jupiter is a hurricane-like storm system.

It is large enough that two Earths could fit across it. But wait, don't order yet. The other amazing thing about the Great Red Spot is it's been around since at least the early 1600s when it was first detected shortly after the invention of the telescope. Hasn't it only recently been determined that, yes, the Big Red Spot is sort of a big storm, and yes, it actually can be stable for that long. We kind of understand the dynamics of that now? Yes, the much more extensive modeling and more detailed imagery, such as from Voyager 1's flyby in 1979,

have given us a much greater understanding of both what it is and how long it can be around. Well, fortunately, our storm over Southern California is not going to last quite that long, although we all love the rain. Let's go on to the trivia contest. All right. Starting with last week's. Last week's trivia question was Apollo 11 and Viking 1, two breakthrough landing spacecraft. Both landed on July 20th. Of course, Apollo 11 on the moon with the first astronaut to set foot on the moon. Viking 1, the first successful Mars lander. How many years separated their landings? Let's see.

Apollo 11, 1969. Viking 1 landing, 1976. Sounds like seven years. That is correct. Oh, what did I win? Nothing. Nothing. But we do have some other winner, hopefully, who wasn't quite as closely associated with the radio show.

Well, Beth Morris in Sewell, New Jersey, you are our winner this week, randomly chosen from among all the correct answers that were submitted, and we had lots of them. So, Beth, you will be receiving that Carl Sagan Memorial Station T-shirt in the mail real soon. And she even told us what size she needs, so we're all set. Excellent. Let's go on to this week's trivia contest. What gas is primarily responsible for the bluish color of both Uranus and Neptune? It's not cotton candy.

We tried that once before, I think. It's blue. Yeah, blue is a weird color. It's a beautiful blue. Yes, blue. What, blue is a weird color. It's a beautiful blue. Yes, blue. What's up with that? That's basically our question this week, ladies and gentlemen. Go to our website, planetary.org, to submit your entry.

Follow the Planetary Radio links to find out how you can give us the answer. I want to mention one other thing this week, as we are rapidly closing to the end of our student astronaut contest, an international contest to select students who will actually be in operations doing real things at JPL during the Mars Exploration Rover mission as part of the

Planetary Society's Red Rover Goes to Mars project. So, if you're interested and if you or someone you know has a birthday between September 1st, 1986 and September 1 1, 1990, please go to our website, planetary.org, and follow the links to Red Rover Goes to Mars. Amazing how much you can do with that website. What was that URL again, Bruce?

Why, that was planetary.org. Now, we actually finished a few seconds early, so maybe we can mention one more thing, which is the passing of a space pioneer. In fact, Pioneer 10. Pioneer 10 launched in the early 70s, functional for 30 years roughly, and just recently has been declared out to pasture, and if not dead, at least barely able to communicate with. And so the decision made not to try to continue communication because the communication coming back was so limited and also had been a month or two now since they last got it.

But one of the four great explorers whizzing out of the solar system along with Pioneer 11, its sister spacecraft in Voyager 1 and Voyager 2. A little robotic representative of humanity headed out toward the stars. We wish it well. We wish it well and hope it's okay. Bruce, thanks very much. We'll see you again next week for What's Up. Thank you.

And you pedestrians, be careful out there. And once again, thank you. Good-bye. Bruce Betts, the Director of Projects for the Planetary Society. Back to Mars next week with planetary scientist Chris McKay. We hope you'll join us again and tell your friends, even if they'll try to beat you in the trivia contest. Send us a note at planetaryradio at planetary.org. We'd love to hear from you.

And remember that you can hear all of our past programs at planetary.org. Have a great week.