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 Earth before life took hold.

Huygens is part of the Cassini-Huygens mission to explore Saturn and its rings and moons. 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): Titan's been described as a potential pre-biology laboratory, if you will, a place to study what chemistry on Earth might have been like before life took hold. There's no way on Earth now, even by looking at ancient rocks, to see that prebiotic history. What makes us think that what we'll see on Titan is going to tell us anything about early Earth, given that the two worlds are so far apart in the solar system?

Jonathan Lunine (JL): It's kind of the best of the worst, I suppose. Suppose you want to go see a particular rock concert, but the tickets are sold out, so you just can't. You could go home and do nothing, or you could try to find the next best thing, which might not be a very good reproduction of the rock concert you wanted to see, but it's better than staying at home. And I think that's the case with Titan.

On the positive side, Titan has a nitrogen atmosphere, which the Earth does, and probably did before life. Titan has organics that are almost certainly supplied in the absence of life, and that's an important ingredient.

And it's not terribly hydrogen-rich. And to some extent, people are thinking that that's kind of what the early Earth was like. Not with the same ingredients, because the Earth probably had a lot of CO2 and Titan does not, but the same kind of chemical balance. And so in that way, it's an analog.

It's much better than Mars, because Mars has such a thin CO2 atmosphere that there are no organics. It's much better than Venus: lots of CO2 there, but temperatures are so high that you can't do organic chemistry.

And much better than the Earth, in the sense that the organic chemistry going on on the Earth is completely dominated by life today, so there's no opportunity to see a planet-wide, long-term, geologic-time experiment in organic chemistry, which we can see on Titan.

So by process of elimination, that's the place in our solar system where we're going to see those interesting reactions - if we can see them at all.

And so we'll find out from Cassini and Huygens whether those conditions really have produced a lot of organic deposits on the surface, some of which might be doing that kind of interesting prebiotic chemistry that we'd like to see.

The chance of studying that chemistry in detail with Huygens is very small: we'd have to land in the right place. But with Cassini, we can at least map the surface, see if there are lots of organics, identify those places where those organics might look different spectroscopically and plan to go back. That's a really long-term goal.

AM: You mentioned organics on the surface. Would there be any evidence of that in the atmosphere as well?

JL: Well, the atmosphere is supplying the organics that are on the surface. Hydrocarbons and nitriles are generated in the atmosphere by photochemistry. The question is: What happens to that stuff on the surface?

The atmosphere is so oxygen-poor that there's not the opportunity to make amino acids in the atmosphere to any significant extent.

But what's exciting is that if you dump the stuff on the surface and then, from time to time, an impact or some kind of ice volcanism event provides liquid water, which then comes in contact with these organics, for thousands of years, perhaps, until the water freezes.

And those are places where further organic chemistry might have taken these deposits toward amino acids, toward purines and pyrimidines maybe, towards sugars - we're just not sure.

So the first question we ask of Cassini is: Are there organic deposits on the surface, manufactured from the atmosphere?

Question two is: Are they different in some way, correlated in some way with their position on the surface; different in craters versus in areas that have apparently had geologic activity? And if the answer to that question is, yeah, there are some differences, then those become attractive targets for follow-on missions.

AM: What if there were some kind of life on Titan, though? What if, somehow, in this intensely cold environment, some series of chemical reactions that led to self-replicating molecules had occurred, and let's say that that process had left a carbon-isotopic signature indicative of life, the way biology does on Earth. Could Huygens detect that?

JL: Huygens is not really good at determining isotopes. It can do some carbon isotopes. And so, if there's a process that has led to a very exotic isotopic distribution in carbon and nitrogen (which Huygens can also do), that might raise a question mark.

In fact, Dirk Shulze-Makuch has actually argued that you can interpret the isotopic data that we see on Titan from the Earth as being the result of a biological process. There's no independent test you can do to verify whether that's true. So I think it's going to be hard to tell with Huygens.

On the other hand, if we see big differences in organic composition, which might be revealed spectroscopically, between here and there, then something interesting is going on and we'd like to go back and look at that.

If Huygens is lucky enough to land in a hydrocarbon pool and there's some interesting structure of some kind, then that might raise a question mark. Some organic chemists have argued that it might be possible to do a form of life in hydrocarbon liquids without water. Steve Benner has talked about that.

No one has quite shown how that would all work, but you can't show that it wouldn't work, either. I certainly would not rule that out, although I find it unlikely. But that would sure be a nice thing to be wrong about, because that would be a tremendous discovery.

AM: What is the most exciting result from this mission, with respect to Titan, that you can imagine?

JL: If either the imaging or VIMS system on the orbiter, or the Huygens probe while descending, takes images of liquid-filled crater basins. That, to me, would be very, very exciting, because that would sort of bring together all of the aspects of the chemistry.

It would demonstrate that there's a source and a sink on the surface, that chemistry's been going on for a long time, and so it would really tie the story together.

Finding life somewhere is the most interesting thing you can do anywhere, but in terms of the most immediate likely result that Cassini-Huygens could get, that would be the most exciting.

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