Lakes on Saturn's Biggest Moon

Narrator: Lakes on Saturn's biggest moon.
I'm Jane Platt with a podcast from JPL -- NASA's Jet Propulsion Laboratory in Pasadena, California. Our guest is Dr. Ellen Stofan. She's with Proxemy Research in Washington, D.C., and she's on the Cassini radar team. Right now, she's teaching a class over in London, and we've got her with us today. Welcome.

Stofan: Thank you.

Narrator: You have a cover story, you and your colleagues, in this week's journal "Nature" about lakes on Titan, which is Saturn's largest moon. What about those lakes, tell us briefly what you now know that they're like.

Stofan: In late July, we had a pass across the surface, we did a narrow strip about 250 kilometers wide and over a thousand kilometers long. This particular strip went the furthest north that we had ever seen on Titan. Now it has been long hypothesized that Titan might have oceans on the surface or some sort of lakes. So far, none of the instruments on Cassini had been able to detect any evidence of current liquids on the surface. We'd certainly seen evidence of past liquids, both in the radar data, where we saw all kinds of what looked like dry river beds, and also the Huygens lander as it was descending had identified some of these same channel-like features. But when we took this radar pass back in July, very high up into the northern hemisphere, once we got into the region above about seven degrees north on Titan, there were just lakes everywhere. We detected a whole lot of lakes, in fact over 75 lakes, ranging in size from about a mile-and-a-half across to over 40 miles across, so these are quite a lot of lakes, and some of them quite substantial in size.

Narrator: And tell me about them, first of all, how are they unlike those on Earth?

Stofan: Well, here on Earth, our lakes are filled with water. On Titan, it's 90 degrees Kelvin, which is just extremely, extremely cold. Remember Saturn is very far out from the sun, so it's very cold out there. It turns out at those temperatures, what's liquid is methane, which we think of as being very different, a gas on the Earth. But on Titan it's actually cold enough that methane is a liquid. We also think that the lakes have some sort of another hydrocarbon compound called ethane in then. So again, things that on Earth you just don't find in a liquid state, out at the cold temperatures of Titan, they're a liquid. But despite this sort of very exotic chemistry, in other senses the lakes are very like those on the Earth. The methane-ethane would become transparent, the way water is on Earth, it would be behaving like water, the lakes could have small waves on the surfaces. So if it was possible for you to stand on Titan and look at the lakes, you wouldn't really know it's this weird chemistry, but that's the major difference. The other similarity is part of the reason we can identify these as lakes is the way they look. They look very similar to lakes on Earth, they have channels feeding into them, just like you have rivers feeding into lakes on the Earth, the shapes of them, their shorelines, all of those geologic aspects of the lakes are actually very familiar.

Narrator: Interesting, so if you actually could be on Titan, you might sit there and see a nice, beautiful-looking lake, and maybe you could watch the sunset over it?

Stofan: Well, the one thing, we were talking about this on the radar team because we were thinking, what would you see if you were standing by the edge of one of these lakes, that is assuming you could take the 90 degree Kelvin and all the other stuff. Ya' know, of course it's winter in the northern hemisphere of Titan, and you're quite far away from the sun, so it would be rather dim light and instead of being blue, the way our sky is and the way the water looks on Earth, everything would have sort of an orangey-reddish hue. So in that sense you would certainly know you weren't on the Earth, but again the landscape might not be as exotic as we would think.

Narrator: So what is the significance in terms of the science discovery, what is the excitement about, what does this tell you, or what path does it put you on?

Stofan: Titan is right now really the first body in the solar system that we've been to that has an active, fluid, liquid cycle. On the Earth here, it's the hydrologic cycle. We almost have to make up a new word for it on Titan, it's the "methane-ologic cycle." But it's the first place where you have rain, you have erosion, you have lakes, they probably vary seasonally. You know, obviously at some point in the past, Mars had that, but on Titan, it's happening right now, and that's extremely exciting from a scientific point of view, because our ability to study climate and our ability to understand how these cycles work, if we only had the Earth to study, we can only have a limited understanding. And so here's this body so far out in the solar system made of such exotic stuff and yet the processes are ones that are so important here on the Earth. And so I think in the long run understanding how climates evolve, how bodies evolve, Titan has an awful lot to tell us.

Narrator: Interesting, so it's relevant to us on Earth as well.

Stofan: Yes. But one of the other points on Titan that's so interesting is, I mean Titan's long been a subject of interest because of the fact that it has all these hydrocarbon chemicals, it has water ice, which we know from having found volcanoes earlier with the radar on Titan, we know that that ice, that water ice at times melts and must at times have water in liquid form, though it would freeze extremely rapidly. Titan has all these chemicals on it that are the building blocks of life, and so because of this kind of similarity to a lot of the chemicals that were around on the very early Earth, Titan's always been of interest in this whole question of how did life evolve, what were the processes going on, what were the chemicals involved? And we've long thought that Titan had important things to tell us about that. And our discovery of these lakes, the fact that they exist in liquid form, we think that they persist, they vary over seasons, but they probably persist on the surface for some amount of time, I think confirms that we're on the right track. And, boy, if we could get access to those lakes, analyze the liquids that are present in those lakes, the organic sludge that probably exists at the bottom of those lakes. I think again, Titan just has an awful lot to tell us, not just geologically, but something about how life originates.

Narrator: You would obviously like to know a lot more about these lakes. Will you have opportunities in future flybys by Cassini?

Stofan: Well, I will say we already went back over some of those lakes that were presented in this paper, we already had one look at them, and that data's currently, and we've already had two looks at them. But again, over the next coming years, we'll go back and revisit them several times.

Narrator: Alright, well thank you very much for joining us today.

Stofan: Sure, thank you.

Narrator: More information on those lakes on Titan and other Cassini discoveries at www.nasa.gov/cassini and http://saturn.jpl.nasa.gov . Thanks for joining us for this podcast from NASA's Jet Propulsion Laboratory.

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