In January 2005, the European Space Agency's (ESA's) Huygens Probe will descend through Titan's atmosphere, sending back a detailed picture of the chemical interactions taking place there and, hopefully, giving scientists a glimpse into the chemistry that took place on early, prebiotic Earth.

The Huygens Probe is part of the Cassini-Huygens mission to explore Saturn and its rings and moons. Titan is the only moon in our solar system with an atmosphere. Organic chemistry detected in that atmosphere has sparked the imagination of planetary scientists like Lunine.

Lunine is the only U.S. scientist selected by the ESA to participate in the three-member Huygens Probe interdisciplinary science team.

Astrobiology Magazine (AM): One of the major goals of the Cassini-Huygens mission is to explore Titan. What makes this distant moon so interesting?

Jonathan Lunine (JL): If Titan were in orbit around the sun, it would have been a major target of solar system exploration, possibly before Cassini. And I say that because it's a body the size of a planet (larger than Mercury), which has a dense atmosphere of nitrogen and methane.

And so, if we were to look at it in the sky, we would say: Hey, here is a planet that has a dense atmosphere. The Earth has a dense atmosphere. Venus has a dense atmosphere, but it's hot enough to melt lead.

Mars may have had a dense atmosphere in the past, but it's cold and tenuous now. If we want to explore a planet in the solar system which is somewhat like the Earth, [Titan is] the place to go.

Because Titan's in orbit around Saturn, though, and not in orbit around the sun, it kind of gets a bad rep. People ignore it; they think of it as if it were a moon; and it is a moon, but it's a moon with an atmosphere.

And it's large enough that probably a lot of the phenomena that we see going on on Earth are going on there - except for biology.

AM: It's curious that Titan has an atmosphere, while Ganymede and Callisto (Jupiter's two largest moons), which are larger still, don't. Why is that? How did Titan end up with an atmosphere, while other large moons didn't?

JL: That's one of the three great questions about Titan: Why does Titan have that atmosphere, and Ganymede and Callisto do not?

Ganymede and Callisto and Titan are all just about the same size and the same mass, so they have the same density, which means roughly the same composition. And so they're cookie-cutter moons that are formed by the same process.

They're the biggest moons, apparently, that can be formed by giant planets, at least as far as we know. Why Titan is different in terms of having an atmosphere is one of the key questions that we don't know the answer to.

One possibility is that because Saturn is farther from the sun (than Jupiter), it was colder during formation. Gasses that could be trapped in the ice at those temperatures and that would have made their way into Titan might not have been trapped at temperatures where Jupiter formed.

And therefore Ganymede and Callisto would not have acquired large amounts of nitrogen and methane and so on.

A different possibility is that because Jupiter has higher gravity, the impact speeds [of comets] are larger at Jupiter than at Saturn. And so Ganymede and Callisto, in this other picture, would have started out with gas but it would have been ripped away by these impacts.

I personally much, much prefer the first story, which was that the ambient conditions were colder at Saturn, and that allowed these very volatile gasses to be trapped in the planet-building material and find their way to Titan.

AM: Will Cassini-Huygens be able to answer the question of how Titan acquired an atmosphere?

JL: I think it will be able to address it in a fairly significant way by being able to gauge the origin of the nitrogen on Titan. If nitrogen on Titan came from ammonia, then we know that it would have been very effectively trapped in the water ice, much better than argon, for example, which is an abundant noble gas.

If the nitrogen came in as molecular nitrogen (N2), it would have been trapped to about the same extent as argon - a little bit worse. And so one way to get at this is to measure the argon-to-nitrogen ratio in Titan.

If it's high, if it's more than 1 percent, then probably nitrogen came in as N2, and conditions then were really cold at Titan, and that would say: Yeah, this was a temperature difference issue.

If, instead, the argon-to-nitrogen ratio is very, very small, then probably the nitrogen originated as ammonia. Ammonia's not quite as volatile, and could have been present at Jupiter as well as at Titan, and so we'd still be left with something of a mystery in that case.

The best Voyager analysis, which actually took almost 20 years after Voyager to get right, puts an upper limit of about 6 percent on the argon abundance. And that's not quite good enough to do the comparison. We really need Cassini.

AM: Cassini, or Huygens?

JL: Huygens. We need the gas chromatograph on the Huygens probe. This is one example of a measurement that really only Huygens can do: the measurement of the noble gases [such as argon] in the deep atmosphere.

AM: There's a lot of methane in Titan's atmosphere. I've heard people say that if we saw methane in the atmosphere of an extrasolar world, it would be a possible indicator of life, because methane dissipates unless there's a source constantly replenishing it.

On Earth, microbes play an important role in generating atmospheric methane. If you don't believe there's biology going on on Titan, where's the methane coming from?

JL: Well that's the second great question about Titan. The first great one was where does the atmosphere come from, why does it have one and Ganymede and Callisto don't?

The second great question is how much methane is in the Titan surface-atmosphere system, how much was there at the beginning, and therefore has this chemistry of destroying methane been going on for billions of years, or has it stopped and started in some way?

AM: How will the Cassini-Huygens mission address this question?

JL: Cassini will do the mapping from orbit, looking at large numbers of crater basins, to see if they're filled with liquid; Huygens will get a local view of one area to see if there are on small scales lots of liquid patches.

If there are a lot [of these patches], if there is a lot of methane, then the question is: Where did that come from? Almost certainly it was produced abiotically.

On the other hand, if we find evidence that the chemistry has gone on for a long time in Titan's atmosphere, but we don't see methane reservoirs on the surface, that's going to be kind of an oddball situation.

Where was the methane that was photolyzed and became ethane or very deep solid-organic deposits? Are we just looking at the last gasp, for example? Or is it buried underneath the surface somewhere and we just can't see it? That's going to be an interesting puzzle.

Is methane, by itself, a good indicator of life? I think the answer is no. What you really want to look for are major chemical constituents that are existing together in an atmosphere that would actually react quickly and destroy each other. So, methane doesn't exist in an oxygen-rich atmosphere.

It gets destroyed too quickly. [Methane is present in Earth's oxygen-rich atmosphere because] life provides the methane, and keeps making it fast.

If we see oxygen and methane [in the atmosphere of an extrasolar world], then that might well be a slam dunk for life.

Less dramatic, if we saw a lot of methane in an atmosphere of a planet that was at 1 A.U. [astronomical unit, the distance of the Earth from the sun], from a solar type star, and not, like Titan, very far away, then we might say: Well, gee, how do you get methane to a planet like that, which is too warm?

Maybe that is biologically produced. That might be one situation where methane by itself would give you a potential indicator that there was life, if it was around a warm planet.

AM: What's the third great question about Titan?

JL: The third great question for Titan is the question of how far the prebiotic chemistry has gone. And how far can you go, in an environment like that, toward organized chemistry? It will be hard for Cassini-Huygens by itself to answer that question. It may be able to set the stage for answering it in a future mission.

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