Astrobiology Magazine (AM): The Cassini-Huygens mission was planned before astrobiology officially existed.

Toby Owens (TO): Yes, that's right.

AM: Did people at that time think that Titan might be a world where life could have gotten started?

TO: Oh, sure. From the beginning. Before there was astrobiology, there was exobiology. That was very much in people's minds. We had just completed the Viking mission to Mars, where there had been an intensive search for evidence of life, or organic compounds, and we hadn't found any organic compounds on Mars.

It looked as if the entire planet on Mars was sterile and completely oxidized, that there was nothing on Mars that would burn. It had all been oxidized. And here we have a world where everything would burn. I mean it's totally flammable. The aerosols are flammable, they're dropping out on the surface, there could be pools of liquid ethane there. I mean it's the Exxon-lover's dream: free hydrocarbons.

AM: So the whole place is a vast oil spill?

TO: Well, not really, but it certainly a wonderful source of energy. All that energy locked up in these compounds, and so, yes, people certainly thought about the pre-biological chemistry, and, of course, there was a lot of humorous discussion of the possibility of life.

AM: Liquid plays a central role in life on Earth. Could liquid hydrocarbons play a role as the basis of life on Titan, without water?

TO: Your guess is as good as mine.

AM: But looked at theoretically, when you think about the chemistry involved, can you imagine an alternative biology based solely on hydrocarbons?

TO: People have speculated about that, and the favorite is to use ammonia as a substitute for water. Hydrocarbons, I haven't really seen anyone work on that, hydrocarbons by themselves. But we always have to keep a certain amount of humility in this question of life.

Because, for all its diversity, life on Earth is really different manifestations of the same thing. It all uses the same 20 amino acids, considering how many are available. It all uses the same DNA and RNA. That's a very limited set of molecules.

When we think about life elsewhere, we certainly have to keep an open mind, that it could be very, very different from life on Earth. So, if one of those icy blocks that we see in the picture [from the Huygens landing site] were to get up and walk away, we would concede that it was life, even though we had no idea how it did it. But, as I say, we need to keep an open mind.

AM: You mentioned ammonia. Could it exist as a liquid on Titan?

TO: At these temperatures it would be a solid. So on Titan, it doesn't really make sense - although people talk about maybe ammonia-water mixtures in an ocean below the surface. And this is something else that has gained a lot of favor lately, as we've become aware of colonies of bacteria living below the surface on the Earth, out of touch with sunlight, and living in a very hostile environment (we would think).

So people have been asking, Well, what about bacteria under the surface on Mars? And then you can make another leap, and if you've got oceans on Titan below the surface, what about life in those oceans?

Well, we don't even know if there are oceans there, so that's already a leap. So there's a lot of leaping to do before you get to life.

AM: If you were going to propose a new mission to Titan, what would you want it to do?

TO: I don't know yet. Ask me that question after we've done a little more exploring. Ask me in 3 years, after [Cassini] has really surveyed [Titan].

I keep telling people that, so far, we've looked at one little piece on Titan, twice. And radar's made a tiny strip somewhere else.

In the case of Mars, exploration, we went there 3 times with 3 different spacecraft, looking at 3 different parts of the planet. We never saw the volcanoes, the riverbeds, the big canyon. All we saw were craters. Some scientists were so disappointed with that, they said, Let's cancel the program. Mars is just like the moon. There's no point in spending this money.

But fortunately, we had an orbiter in the works, and it discovered that we had visited the 3 most boring places on Mars - just by chance. And maybe all the really exciting stuff is on the other side of Titan.

We haven't been there yet. So let's wait and see. Then I'll be able to answer your question.

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