Distant Chemistry Sparks Thoughts Of Life
IRA FLATOW, host:
This is SCIENCE FRIDAY from NPR. I'm Ira Flatow.
The Cassini mission to Saturn has been giving scientists a peek at parts of the solar system that they really never have been able to observe before. And one thing they've been looking at closely is Saturn's moon Titan, and in looking at the amounts of chemicals like hydrogen and acetylene on Titan, what they're finding doesn't quite fit with what they expected.
And one explanation - and I emphasize, one possible explanation - could be that something might be alive there. But before we stop the presses and look forward to aliens landing, greeting us there, let me introduce my guest.
Jonathan Lunine, he's an interdisciplinary scientist on the Cassini Mission, a co-author of one of the papers that's looking at hydrogen levels on Titan and a professor of planetary science and physics at the University of Arizona in Tucson, currently at the University of Rome, in Italy. Welcome to the program, Dr. Lunine.
Dr. JONATHAN LUNINE (Cassini Interdisciplinary Scientist): Thank you, Ira, it's a pleasure to be here.
FLATOW: Tell us, the headlines are screaming all about possible life on Titan. Is that a little premature?
Dr. LUNINE: Screaming headlines are premature. The notion that we have some interesting data on the chemistry of Titan, one explanation for which might have something to do with life, is probably a better way to put it at this point in time.
FLATOW: Give us an idea of why there is that possibility.
Dr. LUNINE: Okay, so to give you a very brief overview of Titan, if you think of a planet somewhat like the Earth, but instead of water being the working fluid of the climate and the seas, we have methane, instead, on Titan, because it's very, very cold, being so far from the sun.
And methane exists in liquid form as lakes and seas in the Northern Hemisphere. It exists in the atmosphere. And it forms clouds and rains. These are all things that have been discovered by the Cassini-Huygens Mission.
What Cassini has also found is that methane is broken apart by sunlight in the very upper atmosphere, by ultraviolet light, in much the same way that water is in our own stratosphere but at a much higher rate. So that in fact, the methane - all the methane in the atmosphere, should be destroyed in a time that is small compared to the age of the solar system, about 10 percent of the age of the solar system.
So the fact that we see any methane on Titan today is very interesting. It means there's a source in the deep interior. But one of the products of that chemistry, acetylene, which instead of being carbon and four hydrogens is two carbons and two hydrogens, we expect to see in large abundance on the surface, and the paper that I was involved in found that it was actually not present in large abundance. In fact, we didn't see it at all, although one of the instruments on the Huygens Probe, which landed on Titan, did see small amounts. One of the products of acetylene chemistry itself, benzene, has been seen on the surface in the data from the Cassini Orbiter.
At the same time, hydrogen, which is another product of this methane chemistry; as the methane is broken apart by sunlight, hydrogen is formed, some of it escapes off the top of the atmosphere, but the rest we expect should form a relatively constant background gas.
But instead, another paper based on Cassini data shows that hydrogen seems to have a sink at the surface. There's something, some kind of chemistry that is removing the hydrogen.
So one possibility, because in fact hydrogen should be relatively inert, is that there's a very active chemistry. There's some kind of catalysis going on at the surface where acetylene and hydrogen are being recombined to form methane.
That would solve part of the methane cycle problem, and it would explain the relative absence of acetylene, because it's being used up, and the sink of hydrogen near the surface.
Now, can this be done by chemistry in the absence of life? Yes, it can be, but it requires a catalyst, because although this reaction releases energy in the end, the combination of acetylene and hydrogen, it requires some energy to leap over the usual activation energy barrier we're familiar with in chemistry.
So a catalyst of some kind would be required. And in fact, Chris McKay(ph) and Heather Smith(ph) in 2005, before any of these data were available, had a rather speculative paper on acetylene-eating life that would combine with hydrogen, and they made some predictions, which are just really what we see qualitatively in the Cassini data.
FLATOW: Acetylene-eating life forms, you just said.
Dr. LUNINE: Right, that's right, that there would be some form of biology that would have to exist in the absence of liquid water, because liquid water is not stable at the surface.
It would have to exist in liquid methane and its sister molecule, ethane, because those are the liquids at the surface and that the metabolism that they would undergo would be the consumption of acetylene with hydrogen to regenerate methane.
That process produces enough energy to (technical difficulties) on the Earth, but again, this is only one possible explanation. It might also be plain old chemistry.
FLATOW: Is there any other way to pin this down further? Could there...?
Dr. LUNINE: With Cassini, probably not. One of the difficulties about trying to detect life, as we say, by looking at metabolisms, is that metabolism is chemistry. We had this problem with Viking on Mars, where there was an experiment where water was added to the Martian soil to see if there were bugs that would begin to metabolize with the aid of the water.
And sure enough, there were surprising chemical reactions, which in the end were finally determined to be abiotic, although there are still a few investigators who don't believe that explanation.
So intrinsically, looking at the chemistry that life might do to generate energy doesn't really tell you whether life is there. You've got to go there and actually identify life forms, and we're not going to do that with Cassini.
FLATOW: And so this is not - if this is a life form, then this is not something that breathes oxygen because it's not there, correct?
Dr. LUNINE: Right, although, you know, the breathing, I mean, most life on Earth doesn't breathe oxygen, either. The majority of the cells on the Earth are bacterial and archaeal cells that like to do fermentation.
The essential ingredient for life on Earth is the presence of liquid water, not oxygen but liquid water, because that's the medium of our cells, and that's the medium in which all of the biopolymers function.
On Titan, there is no stable liquid water. It's just too cold. And so again, one would be postulating - and I say postulating - a form of life that could exist in liquid methane and ethane. One at the moment doesn't know how to construct such a form of life, even on paper, because we have no experience with that kind of biology. But there's nothing a priori that would rule it out.
FLATOW: You just haven't been creative enough. (Laughs)
Dr. LUNINE: I guess that's it. Human beings aren't smart enough to, you know, inductively create a model of another form of life. Although I have to say that Steve Benner(ph), an organic chemist in Florida, has gone farther than anyone else in identifying possible bonding mechanisms and compounds and so on that that type of life might, in fact, be composed of. But again, these are really speculations, and one would have to eventually go back to Titan and sample in the lakes and seas to actually test for life.
FLATOW: And we have no mission to do that.
Dr. LUNINE: We have no mission to do that. There are various proposals. Of course, Cassini-Huygens itself, which has been probably far and away among the most successful planetary missions in terms of number of discoveries in the Saturn system and on Titan, will continue until 2017. And we'll get a much better insight into the climate and chemistry, and that will help us with all this.
One can imagine missions, and these have been thought of and may be proposed to NASA, where you simply land something like the Huygens Probe, instead of at the equator, in one of the northern lakes, and you carry chemical instruments to sample and look for compounds that might be indicative of life forms. That would be the test, but many years in the future, of course.
FLATOW: And so this is just one of the possibilities, but a possibility because it could be possible, of what's going on on the surface there.
Dr. LUNINE: It's - the one attractive thing about the biological hypothesis is that it explains three things at once: the sink of hydrogen, the dearth of acetylene and also the relative lack of the primary product of methane chemistry, which is ethane.
However, you can explain all three of these with separate mechanisms, abiotically. So let me just offer one quickly: There is a catalyst that we're not aware of at the surface that is fixing the hydrogen with methane. The acetylene is just converting to plain old benzene, abiotically. We have evidence for that. I mean, that's something that in fact we see in some of the Cassini data.
And finally, the ethane, after being produced by methane, is simply being buried in the crust. It's percolating through the icy crust of Titan.
That set of hypotheses requires no life whatsoever, and in fact given the fact that an extraordinary explanation, which would be a biological one, requires extraordinary evidence, one really ought to focus, I think, on developing the abiotic explanations and trying to test those.
FLATOW: That was Carl Sagan's famous quote.
Dr. LUNINE: Yes, indeed. I paraphrased Carl, exactly, and I think he would agree with that, and I'm sorry he's not here now. We all miss him, in fact, because he contributed so much to the science of Titan while he was alive.
FLATOW: So we can't advance this any more except that it's a theory. I mean, there's no way to move it forward even as a scientist, I guess.
Dr. LUNINE: No, I think we can, and I hate to use this phrase, which puts everyone asleep, but we can collect more data.
(Soundbite of laughter)
Dr. LUNINE: And, you know, Cassini will be making many more fly-bys of Titan. So we'll have an opportunity to take more spectra with the VIMS instrument. This the spectrometer that found the benzene and the dearth of acetylene, and there's much more (technical difficulties) for that instrument yet to come.
The Sears(ph) instruments, which is the mid-infrared, should get us more measurements of hydrogen, and I think that essentially by getting more of this data and developing a clearer picture of Titan as a working world with its methane cycle, we probably will be able to narrow down, I think, whether the abiotic explanations will work or not. That's what we have to do. It's just slogging through the science.
FLATOW: Now we've got the moons of Saturn to add to the moons of Jupiter for possible...
Dr. LUNINE: I, you know, my own bias is that these, that the moons of Saturn are very promising. We've talked about Titan. Let's briefly mention Enceladus, which has a plume fed by geysers at the surface. That plume is water ice, but methane and ammonia and other organic compounds are coming out, as well. They've been measured directly by Cassini.
FLATOW: All right, John, I'm going to have to leave it at that one because we've run out of time.
Dr. LUNINE: It was good to talk to you, Ira.
FLATOW: We'll have you back. We'll talk about the other moons and the possibilities. This is all fascinating stuff. Thanks for taking time to be with us.
Dr. LUNINE: My pleasure, Ira, thank you.
FLATOW: You're welcome. Jonathan Lunine, talking about Titan. We're going to change gears and talk about food allergies. So stay with us. Tell us about food allergies when we come back after this break.
(Soundbite of music)
FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY from NPR News.