January 20, 2005

Mysterious, Methane World

Interview with Toby Owens, Part II

Astrobiology Magazine -- University of Hawaii astronomer Toby Owens spoke with Astrobiology Magazine shortly after the successful Huygens mission had completed. In this part of the interview, Owens talks about the significance of methane in Titan's atmosphere.


Astrobiology Magazine (AM): You said that one thing that was particularly intriguing about Titan was its thick nitrogen-methane atmosphere. Do you think that the data that Huygens has collected going through that atmosphere is going to enable the science community to figure out how and why Titan got its atmosphere, and why other bodies didn't?

Toby Owens (TO): I hope it will. That was one of our major goals. But it's not going to be easy. In other words, it's going to be one of those studies where you don't get a single data point that suddenly illuminates the landscape for you. You have to put together all the different results that you get. And we're just in the early stages of doing that.

For example, one of the clues to the history of any small-body atmosphere, including the Earth, is the amount of noble gases in the atmosphere - argon, krypton, xenon - and their relative abundances. Because they're very heavy, so they don't escape easily, and they're inert, so they don't combine chemically with what's in the rocks. Once you get them in the atmosphere they stay there. So they give you a kind of record of what sort of volatiles have come out of the ground or have been brought to the planet.

We're very eager to see what that record is on Titan.

AM: Will the data from the Huygens GCMS (Gas Chromatograph/Mass Spectrometer) be able to tell you that?

TO: That's one of the main goals of the GCMS, yes. Another goal is to look at the isotopes: nitrogen, carbon, and oxygen. We have some indications already from the so-called INMS, the ion neutral mass spectrometer on the Cassini spacecraft, which went through the upper atmosphere [of Titan], and we'll get more data because [Huygens went] down inside where the densities are higher. So we already know that there's been quite a lot of escape. Some gases have escaped; others have not.

The difference suggests that methane, for example, must be continually resupplied to the atmosphere - which we were pretty sure was the case already, because you can calculate that it's disappearing as a result of photochemical reactions. The lifetime of what's there, the amount of methane there, is only 10 or 20 million years. It might be that we just happened to come along just before it disappears, but nobody liked that idea very much. Now that we have the isotopes, it shows us that the carbon isotopes are completely normal, whereas the oxygen and the nitrogen show the escape. So, again, the methane couldn't have been there a long time. It must have been resupplied relatively recently.

AM: Methane seems to be a key to Titan. But it's been somewhat puzzling that methane clouds appear to be concentrated at the south pole of Titan, which is not what most researchers expected.

TO: That whole issue came up from ground-based observations just in the last five years. When people began to see these clouds over the south pole, and only over the south pole. And that was very strange, because nobody could understand why that should be. We still don't. Now we're seeing clouds in other places as well. But still there's this tendency for more clouds at the south pole. It is odd.

AM: Now that you have these images from the landing site and the initial methane data from the descent, to what extent is the picture fitting together, and to what extent is it still a little odd?

TO: Well, initially, as I guess you're aware, there was some talk that the whole surface of Titan might be covered by a global ocean. Then we came to realize that was not the case. Ground-based observations showed that there was a big bright area on one hemisphere that didn't exist on the other, so it could only be half-covered by oceans.

Then we got better ground-based data that showed that there are dark and light areas, and the best you could hope for was some seas. And now we're getting down to lakes, maybe. It's getting tough. But what we're seeing so far is that in these images we don't have a clear indication of open bodies of liquid hydrocarbons - yet. We see some intriguing possibilities, things that look like lakes or seas. But we don't know that they're liquid. They're just very dark and very smooth. So we don't yet have the evidence we want. But assuming that they are, or even without that, we do see these river channels, which indicate that liquids have been flowing over the surface. And those liquids must be the hydrocarbons.

AM: The channels couldn't be formed by ice? It couldn't be glaciation?

TO: No. It's much too cold for that. It won't make these little channels. You could get a massive movement of ice, but at these temperatures I don't think you'd even get that. You have to warm it up somehow to do that. And there seem to be places where that has happened. The equivalent of hot springs or something like that. It's not a passive world. There is some internal energy that's being dissipated and causing these geological formations. It's not rivers of water running around. But it's rivers. And it looks like it's liquid hydrocarbons, which is what we expected.

AM: I know it's a little early in the process, and I know it's difficult to interpret things right off the bat, but when you see the panoramic mosaic of the Huygens landing site, what do you see?

TO: Well, I'm from Hawaii, and what I see is surf. It really looks to me as if there is a shoreline, with waves along it. But that's just a visual impression, and my impression is no better than yours.

AM: That's what I saw, too, actually. But then it got described as ground fog, so I thought, Okay, maybe it isn't surf. Will it be possible to distinguish between those two by studying the data?

TO: Yes. Eventually it will. But I think it could well be a combination of the two. That's what we have on Earth, frequently.

AM: So this would be a hydrocarbon fog?

TO: Yeah. You just have to get used to the idea of methane and ethane being liquids - which, of course, we do on the Earth all the time. I was just reading about how people are now shipping gas as a liquid. They cool it down to condense it, and they put it in these big refrigerated tankers - my god! - and they ship it across the ocean. So Titan does it for us. All we have to do is get a hose out there.


On the Net:


ESA's Cassini-Huygens Site

Saturn-- JPL Cassini Main Page

Space Science Institute

Where is Cassini Now?