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IRA FLATOW, host:

This is TALK OF THE NATION Science Friday. I am Ira Flatow. If you were searching for a life on another planet, what would you look for? Where would you look for it? Well, at the top of the list, of course, things you'd be looking for would be water. Life here on Earth is impossible without it, so we'd look for water, right?

And number two on the list would be some source of energy for the planet. You know, here on Earth, that would be the sun. On our planet, sunlight is necessary for photosynthesis.

Scientists looking for life on Mars have been following this plan. They're their, looking for water - scouring the planet with orbiting satellites, high above the surface and roving laboratories on the planet, photographing and sampling the Red Planet's soil. And so far they've found lots of evidence that in its early days, Mars was a water-rich planet, and hints that the water formed many of its surface features may still be there somewhere below the surface.

The latest evidence comes today in the journal Science. Detailed photos from Mars Reconnaissance Orbiter suggest that water once flowed through the bedrock in a canyon. Now that scientists have made their case for water on Mars, the next step to follow is the energy route, to figure out how the planet could provide fuel for living things.

And chances are pretty good that a living thing on Mars would use a different form of energy than most living things on Earth. While much of life here, as I say, depends on photosynthesis, there are living things on this planet that do not - they get their energy living in dark, deep rocks, well below the surface or at the bottom of the ocean where no light can penetrate. And studying these extremophiles gives scientists clues to what they might be looking for on Mars, perhaps the same kind of tough life living below the surface that doesn't depend on light for its energy.

So this hour, we're going to be talking about the search for life on Mars, just what scientists are looking for, as we broadcast from the annual meeting of the American Association for the Advancement of Science, here in San Francisco.

So if you'd like to join our discussion, give us a call. Out number is 1-800-989-8255, 1-800-989-TALK, and if you're here in our audience, please remember there's a microphone in the middle of the room there. Don't be afraid to get up and ask a question. We welcome you, and we also welcome 11th and 12th graders from Lowell High School here in San Francisco, who are eager to participate - I can see from their bright faces - in Science Friday. We'll see what kind of questions they may have.

Let me introduce my guests. David Des Marais is the long-term planning lead for the Mars Exploration Rover Mission and senior research scientist in the astrobiology program at the NASA Ames Research Center in Moffett Field, California, just down the road a piece, as we say here. Welcome to the program, Dr. Des Marais.

Michael Carr is planetary geologist at the U.S. Geological Survey in Menlo Park, California. Welcome back, Dr. Carr.

Dr. MICHAEL CARR (Planetary Geologist, U.S. Geological Survey): Thank you.

FLATOW: And Tori Hoehler is a research scientist in the astrobiology department at NASA Ames, also at Moffett Field. Welcome to the program, Dr. Hoehler.

Dr. TORI HOEHLER (Research Scientist, NASA Ames Research Center): Thanks.

FLATOW: Who wants to - if I say where are we, how is the search going, what would you say?

Dr. DAVID DES MARAIS (Senior Research Scientist, NASA Ames Research Center): Want to start with water?

FLATOW: Michael, do you want to begin?

Dr. CARR: Okay, I'll start with the broad outline of the water story. When you think about Mars, you've got to think about it in two pieces. One is a very ancient Mars, and then the rest of the story. We can recognize very ancient terrains on Mars, and they are nearly all very highly dissected by dry river valleys, and we've tried to figure out ways that these could form other than by erosion of water, and really there is no other way.

This is very compelling that at this early period, this very early period, there was - the conditions were such that water - you could have rainfall or snowfall and you could get liquid water running across the surface.

And in recent years, we've gotten supporting evidence for that model. We now see lots of areas where there were lakes. We know they're lakes because you can see deltas where these dried river valleys run into these lakes. And the problem we're having with this very compelling geologic story is we don't know how Mars could've been warmed up very early in its history, and yet it's really, really compelling.

FLATOW: So where is all that water that you talked - used to be there. Is it still there?

Dr. CARR: Well, I - probably is. Most of it probably is, but let me just touch on the rest of the story.

FLATOW: Oh, that's a good part of it.

Dr. CARR: So after this early era, the kind of water-worn feature that one sees are very different. The kinds are very different. What we see in this younger era of huge, huge floods, and they look as though water basically just burst out of the ground. And probably in order for these large floods to form, it probably had to be cold so that the ground was probably frozen very deeply. And the problem with the liquid water is either below that frozen ground and sometimes it's under pressure and it bursts out, or it's at ice in the shallow depths.

So probably, most of the water is there now but in the ground.

FLATOW: So if you want to find it, you have to go deep.

Dr. CARR: If you want to find liquid water, you've either go deep or you've got to go to a volcano. And there's strong evidence that Mars is presently volcanically active, and so one would expect the ground to be warm, and the liquid water then might be present in such areas.

FLATOW: Go ahead, David.

Dr. DES MARAIS: Yeah, and this of course is off to a great start. And I should say also, that in the area of remote sensing with satellites, we're now moving into an era where we not only can look at the ground and look at these wonderful features that Mike has mentioned, but we can begin to ask questions about their composition. What are they made of?

And another important dimension of water is what it does to things chemically, alteration and, of course, it supports life. And we're now entering an era with some of these exciting new missions where we can do that.

The bottom line is you've got to get your boots on the ground, though. How do we go down to the ground, because if you're really going to be in the life-detection business, you have to get close to the samples. And the Mars Exploration Rover I think has illustrated wonderfully - graphically, the challenge that we face when you're looking at a planet whose surface to a first order has been very dry and cold for a long time. And that is much of what has altered the surface at the scale that you're at when you're there with the Rover is volcanism, impacts, and just wind-blown processes, and none of this sounds very exciting for habitable environments.

So we have to sort of get past that, those things, and look at the earlier history and maybe evidence of the deep sub-surface that Mike alluded to. And the two Rovers are a classic demonstration of what we sort of encountered.

With Gusev Crater, with the Spirit Rover, we said here's a nice crater with a big channel that obviously brought a lot of water into it. Wow, let's go there. Let's literally follow that channel's water to Gusev Crater. With Opportunity, we said here is a mineral at the surface, and we think when we see this course-grained mineral on Earth, hematite, that that was formed by water. Let's go there because that may be a beacon telling us that water was there.

Opportunity, boom, right off the top, finds itself surrounded with outcrops and materials that really allowed it to make pretty substantially rapid progress on answering the question of was water there. Spirit, we had to wander for like two or three months to get away from that stuff that is sitting on what must have been a lake bed, but it was volcanic rock. And you know, if you're a Rover trying to pick, analyze things, you know, that's a step away from where you want to be.

So the landscape is wonderful because it sort of indicates where there might have been water, but the minerals, the composition, is what really tells you where the evidence lies. So we need to look at where water has really done something to rocks and created a record to make that big next step.

FLATOW: And we haven't gotten there yet.

Dr. DES MARAIS: Well we haven't detected evidence of life. We would argue that in the case of Meridiani we've detected evidence of environments that might have been habitable at some time in the past, and I would argue that even at Gusev Crater we've now found evidence that's consistent with habitable environments in the subsurface perhaps sometime in the past.

FLATOW: And we keep seeing, you know, from the reconnaissance satellites also - from Mars - the reconnaissance satellite - evidence of it was that wonderful, sort of a burp that looked like water came out of the side of a gulley. It was the white residue left, right?

Dr. DES MARAIS: That's right.

FLATOW: And how old could that have been?

Dr. DES MARAIS: Well, of course the exciting observation there was that the feature that was not there in, what, 1997, was seen in 2004. So some of these white-like, you know, streaks that came down the slopes literally, you know, less that, what, eight years old or so.

FLATOW: And that's what makes you think there's water somewhere below the surface still popping out every once in a while.

Dr. DES MARAIS: Well no good does scientific observation would be complete without a good scientific debate, okay? And Mike(ph) can perhaps give you a little more of a capsule summary about the nature of that debate.

Dr. CARR: Yeah, these gullies are really quite controversial. They're just a few meters across and they run down steep slopes. And one problem with the supposition that they formed by groundwater is that they are located or they seem to start in places where you wouldn't expect to see groundwater. Like at the top of the crater rim - right at the top of the rim or top of the central peak in the crater. And they don't all start at a stratum in the crater. And so other suggestions have been made such as destabilization of slopes by volatiles such as water, ice or carbon dioxide ice evaporating when the sun hits it or earthquakes shaking or just dry rivulets like sand. You know, there's a lot of different ways that might explain these gullies.

The problem with the idea that water is just sitting behind that slope waiting to squirt out is it's absolutely incompatible with what we know about the conditions. And so there's great reluctance to…

FLATOW: Specifically, what do you mean?

Dr. CARR: Well it's much too cold. The surface is minus - average daily temperature of the surface where these things are found - they're about minus 65 to minus 75 degrees centigrade. It's very, very cold. And the heat flow on Mars is low and so you wouldn't expect that the shallow depths that the temperatures would be anywhere near zero degrees centigrade. They're much, much lower than that. So…

FLATOW: So it's a problem. It's a problem that you don't accept that explanation.

Dr. CARR: You're getting the sense that I don't?

FLATOW: I'm sort of picking that up, you know (unintelligible).

Dr. CARR: Well let me just add one other explanation which is being proposed.

FLATOW: Well I'm going to have you hold that explanation…

Dr. CARR: Okay.

FLATOW: …because as they say in some of these TV shows, we have to go to a break. So stay with us. We'll come back and talk more and hear the other explanations, talk with Michael Carr and get your questions from the audience, from the phones. 1-800-989-8255. Talking about the searching for life, water, energy on Mars and possibly finding it someday. So stay with us. We'll be right back after the short break.

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FLATOW: I'm Ira Flatow. This is TALK OF THE NATION: SCIENCE FRIDAY from NPR News.

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FLATOW: You're listening to TALK OF THE NATION: SCIENCE FRIDAY. I'm Ira Flatow. We're talking this hour about the search for life on Mars. We're here in San Francisco at the annual meeting of the American Association for the Advancement of Science. Last up was Michael Carr, talking with him about these pictures we've seen. And a lot of people have accepted as being true that there is water squirting out of the sides of these gullies on Mars and there's white residue leftover from when they - it ablates - when evaporated would ablate and then leaving the residue behind. And you're giving us the other side - the skeptics' debate side. Tell us. What else could it have been? You were at that part.

Dr. CARR: Well yeah, I was going to mention one other - one other possibility and Dave has yet another. And that is that Mars is - Mars has a very strange orbital - it goes through orbital perturbations(ph). And what happens periodically is that the - is that the rotation axis sort of tilts more toward the sun. And when that happens ice gets evaporated from the poles and it lands at the equator. And some people have suggested that the melting of ice could make these gullies during these periods when the tilt of the axis is more towards the sun.

FLATOW: Let me - before I get back to Dave I am going to ask to bring Tori -Tori Hoehler, you were looking for the energy side.

Dr. HOEHLER: Yeah.

FLATOW: How do you find any energy that might power life?

Dr. HOEHLER: The important consideration on Mars is that, you know, water is sort of a binary indicator for life - that tells us either this is a good place to look for life because water has been around or it's not a good place to look for life because water has never been here. But we know from our own experience that's not enough. People here in the live audience could probably tell you that life needs both water and food. And so we're asking really, what do you gain by asking and following the food on Mars. And by food, what I mean is energy - the energy that we require to keep ourselves going to grow and just to live.

FLATOW: Is there life on Earth that we found? You know, it's almost - some of it is so strong living in those hot, you know, water vents at the bottom of the oceans and other places where it seems to be pretty tough. Is there any candidate that could live on Mars and in under some conditions there?

Dr. HOEHLER: We think there are. And one of the problems in trying to look at Earth analogs to look for this sort of life is actually really hard to find a place where our surface biosphere - the biosphere that's dominated by photosynthesis - has not pervaded into the subsurface. The product of photosynthesis, if you're not looking at life itself as an energy source but rather the products of photosynthesis - the organic matter, the oxygen and so forth - it's actually hard to get away from that stuff. But we have in a few cases now and we feel like there are a couple of candidate places that could support the sort of life that we might have on Mars. So, for example, there have been studies in some deep mines that look at ultimately radioactivity as an energy source. So it turns out that radioactivity can split water in a way that transmits energy into a useful form into bacteria.

There's apparently a biosphere that's running on that. Certain sorts of rocks that can react with water that lead to support of life. So there are beginning to be a couple of places now just starting to be understood that we think could be candidates for a deep life on Mars. Would you find that - would you look for these places on Mars, too? I mean, the water guys are going to say: Well you got to - don't go anywhere where there's no water. I'll assume you're going to say: Well we have to find a place where there might be some energy source.

Dr. HOEHLER: Well I think there's an interesting issue on Mars, which is you have a couple of choices in front of you and really what you have to do is prioritize what you'd like to look for. So if you follow the water on Mars, historically you're following a trajectory - a planet dominated by surface water in its early history where life would have been available as an energy source. And we know from our own planet that's a great source of energy - leads to lots and lots of life and lots and lots of evidence that we could potentially discover. If we ask about life on Mars now or had to follow water under the surface. And so now we really are talking about needing to look at the rocks themselves as potential sources of energy. So what you're left to do is to try to prioritize. Would I rather look for four-billion-year-old evidence of potentially abundant life or much more modern evidence of potentially much less abundant and much more cryptic life.

FLATOW: Can you look for both?

Dr. HOEHLER: I think you'd have to pick and choose your spots carefully. And of course, there are question of access as well. In order to look for subsurface life, what would you need to do? You'd either need to find a way to drill into the subsurface. And potentially, we're talking about long, long way. As Mike described, prospective aquifers on Mars are really quite deep. That's logistically a very difficult thing to do, even here. On Mars it would be a real challenge. What could you rely on instead? Well maybe these catastrophic outflow channels, if that is in fact one of the explanations for them. That would be a very intriguing place to look because there you'd had a place where subsurface water had been brought to the surface.

But as Mike and Dave just described, it's very much in question whether that's what really those represent.

FLATOW: So what do you do then? You know, if you're sitting there and, Michael, Dave, you're now working for NASA trying to pick the next site - the spot. What do you look for? What's your ideal place? If I gave you a blank check, how - what would you build, the kind of craft and what would it do?

Dr. DES MARAIS: Well I'll tell you, the first thing we'd do is give ourselves a year or two to take advantage of these wonderful new images coming down from HiRISE - from the Mars Reconnaissance Orbiter with the HiRISE camera, 30-centimeter ground resolution and also an instrument called CRISM, which is a near infrared spectrometer that is going to do a pretty darn good job on mineralogy at a ground scale of about 15 meters. Give them a chance to look this place over and look for the kinds of rock types, as Tori mentioned, that, you know, might be really good places to act as energy sources for life.

The subsurface isn't as hopeless as trying to put a drill down there. Meteorite impacts have done a good job of excavating things for us. In fact, the press -or the science paper we just talked about looked in the wall of a canyon and found a fracture that was filled with a deposit was put there in the subsurface, but here it is now on the surface. Now getting there with a rover might be a challenge, but it's an example of how the impacts and the erosion that occurred on Mars is laying bare evidence that was once far below the surface. So there's an approach there. As far as the active water today, we also are doing radar experiments where we're looking for where there might be liquid water in the subsurface using radar. You don't have to go there physically, you go there with, you know, your geophysics.

And then of course, in the ancient record, again using these same wonderful orbiters, you look for environments where the rock may be altered by water and there are types of deposits that might preserve evidence of fossils within them.

FLATOW: Is that an easier bet - to find past life, than to find something that might be living there?

Dr. DES MARAIS: Well absolutely yes. And the reason I say that is that it - geologic deposits can integrate evidence over a long period of time. So you're benefiting from things that have happened, you know, for hundreds of millions of years as opposed to something that just happened last week. Even if life is there today, you may find it by finding a fossil or something equivalent to it that got thrown to the surface some million years ago. And that's still looking for evidence of past life, but very relevant to present life on Mars.

FLATOW: Remember the old Viking Lander in the 70s. When it got there it tried to make a little experiment of sprinkling stuff on the surface and seeing if there was, you know, evidence that would - there might be something living that would chance form it into something detectable. Can you redo that? Is that a smart experiment to redo or is that just something we - hey, let's give it shot. Maybe it will work. And redo it differently today than you would have?

Dr. DES MARAIS: Well the easy - the quick answer to that is that you have to have the right sample to put into that little experiment. The second answer, which has come by research on microbiology, is that there are a lot of organisms on the Earth that wouldn't respond to that cocktail if you gave it to them. One of the sort of consequences of discovering that there's all these different lifestyles - the type that Tori referred to - is that some of them just would turn up their noses at what we served up in the Viking dinner plate. And so we have to be a little more sophisticated on how we would do that type of experiment.

FLATOW: So we wouldn't do that?

Dr. DES MARAIS: We wouldn't do Viking again.

FLATOW: No, but something like that (unintelligible) create another experiment.

Dr. DES MARAIS: This is an area of active discussion right now within NASA. You know, what would be a serious, meaningful, broad-based life detection experiment that you could do remotely. Most people would argue - got to bring that sample back to do the real-life detection experiment and you cannot argue with the power we would have in terrestrial laboratories - you know, laboratories on the earth to do those types of experiments.

FLATOW: Well there was once a mission called the sample return mission.

Dr. DES MARAIS: That's right.

FLATOW: Is that still around?

Dr. DES MARAIS: It's like a mirage. It's a financial horizon.

FLATOW: It's always in someone else's budget. Not in your budget. So was that access part of a cutback in NASA's plan to go to the moon and to Mars?

Dr. DES MARAIS: I think sample return was - we already sort of had planned it into the late 2000-teen, sort of, decade, even before the outset of the lunar. Yeah.

FLATOW: Okay. I'm going to go to the audience here. Yes, sir, step up to the mike.

GEORGE (Audience Member): Well, I'm George from Lowell High School, and I was wondering - because the atmospheric pressure on Mars is less than one percent of what it is on earth, from what I understand there wouldn't be any liquid water on the surface. And all the water that would be underground wouldn't be able to be heated like it is on Earth because Mars is so much smaller and the core is so much more cooled down. So how does that go in face of finding life there?

FLATOW: Michael?

Dr. CARR: Yes, you're right. It's very difficult to get liquid water on Mars today. There are two things that are working against it. One is temperature, and I'd mentioned the minus 65 degree centigrade average - you know, daily average temperature. And also the atmosphere is very thin. And in fact, the average pressure is below what is called a triple point of water where liquid water can exist. And so for average, Mars simply cannot have liquid water. However, there are places where liquid water could exist for very short periods of time.

For example, if you had some ice on the surface and the sun hit it and you're at a very low area so the pressure was above this triple point, you could have liquid water form temporarily. But it's so cold that it would soon freeze. And that's a big, big issue. Because how do you then explain all these features that you see that are formed by water.

And that's probably the most glaring problem that we have, climatic problem that we have.

FLATOW: Go ahead, Dave.

Dr. DES MARAIS: I'd like to answer your question specifically, though, about the subsurface. And yes Mars is probably cooler on the inside. But it's a very important point he made earlier. There's still active volcanic activity on Mars. There are places on the subsurface where it's pretty darn hot. And somewhere between that cold surface and that really hot subsurface is, you know, it's the "Goldilocks and the Three Bears" thing, you know, it's just right.

FLATOW: Well, you have founded this research, evidence that the science paper today shows that there were canyons full of water down there. I mean, you know, here you have canyons full of water, there's algae or something growing in it, you know, even in the dark. Could that be possible also?

Dr. DES MARAIS: Well, in the distant past.

Dr. CARR: Yeah, the distant past. Evidence for the canyons being full of water is really pretty good and these are huge canyons.

FLATOW: And these were how long ago where they're still filled now?

Dr. CARR: Billions of years.

FLATOW: These are billions of years ago.

Dr. CARR: Right. What you see inside these canyons are layered - stacks of layered sediments and they have lots of sulfate in them. So they probably water deposited in some way. And then you see out of one end of them, huge channels as those there was a dam and the dam broke and all the water drained out of the canyons.

FLATOW: And so some mysterious event happened on Mars, that drained all this water, this warm environment that they had, and now suddenly became a cold place and froze everything.

Dr. CARR: Well, the conditions could've been cold when there was water in the canyon.

FLATOW: But not as cold as today.

Dr. CARR: Probably.

FLATOW: Yes, ma'am.

REBECCA (Audience Member): I'm Rebecca, a senior at Lowell High School here in San Francisco. And my question are there a lot of volcanic explosions on Mars as a result of the underground water pressure. And if so, does this affect the environment of Mars?

FLATOW: There were on "War of the Worlds." There were a lot of volcanic explosions.

Dr. CARR: That's a very good question. It's very relevant to the life issue. We see a connection between many of these large flood features, these large channels, and volcanoes. Some of the biggest ones are around this volcano which is called Arsia Mons. And what probably happened is you had dikes moving out from the volcano, the salt being moved through the ground and it intersects this ice-rich ground and you get these floods and probably a lot of ash and steam and so forth.

And yes I think there's been a lot of interaction between volcanoes and this water-rich surface. And it's probably going on until recent time.

FLATOW: Talking about Mars this hour on TALK OF THE NATION: SCIENCE FRIDAY from NPR News.

Here with David Des Marais, Michael Carr and Tori Hoehler. Tori, you want to jump in there?

Dr. HOEHLER: I think it's important to follow this threat about the volcanoes. That was really a great question. What makes volcanoes is not only the heat they transport but the other stuff they transport. You can sort of think of a planet like a big battery, has chemically different layers and when those layers are brought into contact, just like connecting the terminals on a battery, you bring a lot of energy and leave a lot of potential for life to survive.

And we see this demonstrated over and over again in places like hydrothermal vents and hot springs. And so what's very intriguing and interesting, I think, about volcanoes of Mars is they represent a place where maybe not only do you have liquid water a little bit closer to the surface because of the heat but you also have a potential system that's delivering energy and those represent a very intriguing and compelling place, I think, to think about life.

Let's see if I can get a phone call in here. From Larry in Philadelphia. Hi, Larry.

LARRY (Caller): Hi, thanks for taking my call.

FLATOW: Go ahead.

LARRY: Yeah. I was wondering, due to the different environment, the different energy source, have you had to look into different ways of testing and processing the information? Like sort of think outside the box, I guess, to gather your information and process what you have?

Dr. DES MARAIS: Well, first I'd like to emphasize is that we think that some of the chemical reactions on Mars that we would be looking for as sources of energy for life are very much similar to what examples we could find on the Earth. And for example, I'm thinking about iron. If you take iron that's sort of in its reduced state in rocks and you subject to water or to oxidation, you get energy out of that, chemical energy.

And we have microorganisms on the Earth that can do that kind of thing. And lo and behold, we see evidence now and places on Mars where there's been alteration by water that the iron has been oxidized. So in principle we already see of a kind of chemistry, and that is iron oxidation, that we would look for Mars and then as we look for evidence of it, look for other evidence like little tiny cells or organic matter or other features that would also characterize any life that might be there.

Another example is sulfur. If we can see evidence in like fumaroles that, as Tori mentioned in volcanoes, the sulfur that comes out that gets oxidized could be another kind of chemical reaction that organisms can use. And so if we find places on Mars where we have reason to believe that sulfur might have been oxidized, you know, the tools that we take to analyze for that reaction and then again look for other features would be something to consider.

So I think that organisms that are good at living in these places that we might also occur on Mars also tell us the types of measurements, chemical measurements, we might want to make.

FLATOW: What about these meteorites that come from Mars that land on Earth. They pick them up in Antarctica off the ice. And we once had that very controversial one that we possibly thought there were signs of…

Dr. DES MARAIS: Yeah. And what's interesting about that meteorite, if you put the life controversy to one side for a minute is - well, the thing to me that's most profound about it is its incredible age. This is a rock almost 4.5 billion years in age and was altered in some way to make carbonates four billions years ago. And all the arguments that you heard, they were arguing about features that are tiny, less than a micron. I mean a tenth of a micron.

I mean these rocks have lasted all these billions of years and preserved all this fine textural information. And if nothing else, they tell us that there is a rich ancient record of what happened on Mars and we just have to go read it.

FLATOW: All right. We're going to take a short break, come back and talk lost more with our guests, Dave Des Marais, Michael Carr and Tori Hoehler. And take questions from our audience. Talking about Mars. Maybe we'll talk about what the next Mars Rover might look like. You know, bigger than a breadbox, much bigger I'm sure. Stay with us. We'll be right back after this short break.

I'm Ira Flatow. This is TALK OF THE NATION: SCIENCE FRIDAY from NPR News.

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FLATOW: You're listening to TALK OF THE NATION: SCIENCE FRIDAY. I'm Ira Flatow.

We're talking this hour about the search for life on Mars with my guests David Des Marais of NASA's Ames Research Center. Also with NASA, Tori Hoehler research scientist in master biology. And Michael Carr, planetary geologist at the U.S. Geological Survey in Menlo Park, California.

1-800-989-8255. Let's hear - right to the podium. Yes, ma'am.

Ms. LUTZ (Audience Member): I have a question on behalf of my son Patrick Lutz. He's a middle school student in Lexington, Kentucky. He's very interested in the question of is there some other life out there in the universe. But he's bothered by the way we assume that life would be life as we recognize it and know it on this planet. For example, if you define it by its requirements, we require water and energy and some other things. But could there be life that doesn't require water?

We know Stanley Miller, Harry Urey, 1953 - their classic experiment, Abiotic Synthesis of Organic Molecules - from non-organic molecules in water or using water. But could life arise without water. Are we looking for something and missing even though it's there because we're defining it the way we define it?

Now the real question is how do we define life so that we can exclude fire, which reproduces, uses oxygen, transforms chemical energy to heat energy and so on, but doesn't require water, obviously.

FLATOW: Tori, do you want to answer that?

Dr. HOEHLER: Yeah, I can give it a shot. I think, first of all, Patrick should come to work with us at NASA Ames Research Center. We could really him use him…

FLATOW: I think his mom already does it sounds like. Go ahead.

Dr. HOEHLER: You know, it's a great question and it actually bothers me as well. That we look for life based on definitions that very much match up with what we have here. I think an answer to the does life need water question is there are some very bright chemists are there who have come up with what I think are reasonably plausible schemes for other types of life.

It's important to ask what it is that we need water for. We need water as a medium to allow the complex bits and pieces that we're made up of to interact. We need it as a thing that dissolves some of our biology and not other parts of our biology. There may be other things that can play that role.

Interestingly, I think that none of the plausible alternatives are reasonable for Mars. So for Mars, it's actually not bad, I think to really pin it all on water. When talking about life elsewhere in the universe, I think it really is necessary to think as broadly as possible and to avoid very narrow definitions of life. In fact, what I favor is to define life only to the extent that you gain some useful information with which to mount a search or to understand a little bit more about where that might be.

FLATOW: Could it exist without oxygen on Mars?

Dr. HOEHLER: Certainly. Yeah, we have organisms that exist without oxygen here. In fact, the first half of our planet's history was essentially an oxygen-free history. And so those are our roots. The appearance of oxygen on Earth was very nearly a catastrophe for life in fact because it's just so reactive and so hard to deal with.

FLATOW: If you were looking for life on Mars and you were looking for on some of the planets - the moons of Jupiter, Saturn, you know, places like Europa - would you be looking for something different?

Dr. HOEHLER: I think in the case of Europa, so much of the excitement of Europa has been for similar reason as to Mars. And that is that it's a place where liquid water seems to exist as best we can tell. I think that in order to find something very different, you'd be interested maybe in a place like Titan, a place where the chemistry is not dominated by water by dominated by something fundamentally different.

Lakes of ethane and atmosphere…

FLATOW: We've seen these new pictures that have come back. Yeah, very interesting. All right. Let's go to right here at the podium.

Unidentified Man: Hi. I'm a student from Foothill High School and I was just wondering do the erosions on Mars have to be from water? It could be from a different liquid.

FLATOW: Somebody once said carbon dioxide in a paper I read a few months -years ago. You guys, should be look at the CO2 could be liquid form on Mars also.

Dr. CARR: Well, in the early 1970s when we really got to see what these, in quotation marks, "water-worn features" were like, a lot of other kinds of liquids were - for example, lava. Lava cuts channels. Could these be lava channels? Some suggested liquid carbon dioxide.

But all these other fluids created many more problems than water. And in fact, we have really apart from the morphology of the surface, we know there's water on Mars. We've measured it. And we know that some of these mineral molds that the MIR Rovers, for example, are finding in the sediments, had to be dissolved by water.

We know that the sulfates there, all over Mars, that's a water-soluble salt. So there's lot of other supportive evidence that water is…

FLATOW: What about the poles? We see them growing, the ice and the…

Dr. CARR: Yeah. There's water ice at the poles and we know at high latitudes there's water and ice just below the ground. So…

FLATOW: Why not go to those places?

(Soundbite of laughter)

FLATOW: Dave? You're the long-term planner. Why not go to those places.

Dr. DAVID DES MARAIS (Senior Research Scientist, NASA Ames Research Center): And these Rovers are solar-powered. One of the big challenges with planetary exploration is you just follow the - you've got to bring the energy - you know, have the energy. And you pretty much have to stay within maybe 20 to 30 degrees of the equator on Mars, and you sure don't want to go beyond the orbit of Mars unless you have some other source of energy for you.

So going to a polar environment for us is a challenge. Now of course the Phoenix Mission is going to launch this August, and it's…

FLATOW: Tell us about that.

Dr. DES MARAIS: And it's just a fixed lander that will land at - what is it, some 60 degree or so latitude? Anyway, up to a point to where, you know, vapors and volatiles moving in and out of the surface will be something that it can measure, and it'll actually paw down a bit and try to analyze the material and even see maybe if there's organics in there or something.

But that mission is guaranteed not to last beyond the fall that - you know, the season. It's going to be a short-term mission, and that'll be pretty much it.

FLATOW: Why is that guaranteed not to last?

Dr. DES MARAIS: Well because after that time, the temperature - as the seasons change, it will get so cold that not only will it run out of power, but the low temperatures will damage the spacecraft

FLATOW: Well, your rovers were supposed to be a three-hour tour, also.

Dr. DES MARAIS: A three-month tour?

FLATOW: Right, and they went for three years.

Dr. DES MARAIS: Well, there were times when we thought it might only be three hours.

(Soundbite of music)

FLATOW: You know, it's three years now, right?

Dr. DES MARAIS: That's right. And one of the serendipitous things that happened for us was that wind blew the dust off our solar panels. You know, a little dust devil or whatever. But you know, I think for Phoenix, it's just a much more profound seasonal challenge.

FLATOW: Yes, sir.

ELAN(ph) (Audience Member): Hi, my name's Elan. I'm a senior at Lowell High School here in San Francisco, and I was wondering what implications there are to possible future human civilizations on Mars due to the existence of water. Like, would it be possible to drill into the aquifers in the subsurface and harvest the water for our own use?

FLATOW: Yeah, say you get there. Let's say we do get there. Can we live there?

Dr. DES MARAIS: Well, that's an interesting question. Actually, the notion of a human station on the moon is to go to the pole in the moon, where they think there might be the greatest chance of having water as an accessible resource.

There's an interesting little twist with Mars that I have to mention at that point, and that is there's an international agreement that we will not go and contaminate Mars with U.S. terrestrial organism because if there is Martian life there, then you know, this would be a scientific tragedy, and there's a lot of ethical issues involved, as well.

So the idea for the first human station on Mars would be to pick a place where the chances of intersecting the Mars biology would be minimized, and that's probably somewhere near the equator, probably in an area which, from an astro biological point of view, wouldn't be terribly interesting, but you could operate there a series of robots, perhaps, to other parts of the planet.

So the question is - what's the water resource that we can get at that place? You know, drilling in the sub-surface is getting to be a little no-no here, too, right, because if you get down to where that aquifer is, you're running up against the same problem.

That's actually - the answer to your question is the topic of a very important line of investigation within NASA right now, and that is in-situ research utilization. How can we utilize resources there that are both enough to give the astronauts what they would need but also satisfy these concerns about, you know, keeping away from Mars biology, if its there.

FLATOW: There was also talk, Tori, of actually sending reactors to the surface that could create - take hydrogen out of the soil in advance, even before people got there, so you create fuel to go home. Are you familiar - am I making this up?

Dr. HOEHLER: Yeah. No, I don't think you're making it up. I think, you know, sort of a simple answer to that question is to do anything substantial with people on Mars, we almost have to look to the natural resources, and I think that all of these things are going to have to be thought through.

The energy is a very big one because it's - every little bit of mass that you take with you costs you an awful lost in terms of how much it takes to get there, and so looking not only for water resources but also for the sorts of energy resources that would get you home is an important thing, and really what we're talking about there is the sun is still a pretty decent energy source at Mars. It's not as bright as here, but it's still not bad, and what we're looking for are ways to take that solar energy and transform it into a form that actually becomes fuel for us to do what we want to do.

FLATOW: Dave, you knew what I was talking about, right?

Dr. DES MARAIS: In-Situ Resource Utilization, yeah.

FLATOW: Go ahead, Mike.

Dr. CARR: Yeah, it's called ISRU. Yeah, NASA has all these…

FLATOW: Acronyms. It's because they're connected to Washington. That's why. It's easy.

Dr. CARR: In-Site Resource Units, a pilot program - a pilot to test out the possibility of getting hydrogen.

FLATOW: All right, sir at the microphone.

JAKE (Audience Member): Hi, my name is Jake. I'm a senior at Lowell High School here in San Francisco, and my question is how long ago was the climate shift that cooled Mars and moved the water from the surface to below the surface, and what caused that shift?

FLATOW: Michael, you want it?

Dr. CARR: Yeah. Well, the timing of this change from where we see lots and lots of evidence of the dry river valleys and the implication being that there was a warm climate, precipitation and so on - the timing of that to the later, colder period is really quite well established because it coincided almost with what we call a heavy bombardment, the period when early Mars was being hit by large meteorites at a very high rate, and that dropped off rapidly.

And about the same time, these - the rate of formation of these dry river valleys dropped off, and that was about 3.7 billion years ago.

FLATOW: All right. Thanks for the question. Yes, ma'am. You're next.

CHELSEA (Audience Member): Hi, my name is Chelsea, and I attend Lowell High School. I'm a senior, and my question is - has there been any evidence suggesting the occurrence of other natural disasters and volcanoes and if so, how could that affect the possibility of life on Mars?

FLATOW: Volcanoes. Dave?

Dr. DES MARAIS: …large events on Mars that might have affected its - yeah, well, again we have, as Mike mentioned, evidence of explosive volcanism, where it seems that volcanic activity has intersected water bodies in the sub-surface. We have, of course, Olympus Mons, which in the Tharsis Region, which have summit calderas that, you know, indicate - you know, the tops of the volcanoes, that may have been active within the last few hundred millions of years.

I don't know if anyone has addressed what the climatologic implications of a large eruption from those would be, but they have happened, and certainly on time scales of hundred million years or so, and I guess you could look to Earth to see what the consequences of a large eruption would have been and say that it would be a big challenge, I think, for the kinds of life that we think about, plants and animals and so forth.

I think the microbes could probably ride these through. In fact, in many cases, the volcanic activity, as Tori mentioned, is a source of energy for them. So they happened, they would be a big problem for any astronauts or plants on Mars. The microbes could probably ride them through and in some cases benefit from them.

FLATOW: We're talking about Mars this hour on TALK OF THE NATION Science Friday from NPR News.

That's take a last question from the audience here.

CHRIS(ph) (Audience Member): My name's Chris. I am not a senior at Lowell High School.

(Soundbite of laughter)

CHRIS: I have an overview question, not from a Luddite point of view, but I'd just be curious from the three of you - why do this work? When you look into the future, what - are there practical aspects that are going to come back to us? If so, when? What might that look like? What's in it for you to do what you do?

FLATOW: Good one. Do you agree with Stephen Hawking when he said if we're going to survive, we have to leave this planet?

Dr. DES MARAIS: I hope not. I don't happen to agree with him.

(Soundbite of laughter)

FLATOW: Are we using it up, and we have to go someplace else? So what, as the lady said, what motivates you?

Dr. DES MARAIS: Well, I think what motivates me is a chance to understand how life might have become and how different it could be. You know, in the end, at the moment, we just have one example of life, and some of the questions indicated that if there's any possibility of life elsewhere, it could be different in important ways.

I think to understand really what life is, you need more than one example because in the end, the example that we have here on the Earth really reflects the outcome of what happened on the Earth, a particular history of this planet.

In that sense, planets are like organisms. They're like life itself. Each one not only is the physics and the chemistry, but it's also a history. Life has remembered its history, and that's much of what it is when you see it.

Same thing with planets, and if you believe that with planets, then the kind of life we have on the Earth is inseparably related to the Earth's history. Another planet, another kind of life.

FLATOW: Michael, you agree? Different for you? Because it's there.

Dr. CARR: Well, because it's there. There's a natural curiosity. How - it's like - does knowledge of the formation of the universe have a practical consequence? I don't think - probably not, but it's inherent in us to want to know how everything came about. And life, of course, is particularly personal need.

FLATOW: Tori?

Dr. HOEHLER: Yeah, I think maybe another way to view that question is that you often in this line of work get the question, well, what do you honestly think? Is life out there? Do you think that life is on Mars? And the answer that I usually give is I honestly don't know, but I wouldn't be doing this if I didn't think that it was at least possible.

To address that question scientifically, there are so many unknowns in this question. I sort of divide Mars' history up into two parts. There's the part when things were good at the surface, and water was around, and sunlight was present, and there's a question as to whether or not life could take hold at that point.

There's a question about the subsequent history of Mars. Could life survive that subsequent 3.5 billion years in the sub-surface. I almost think it's easier to address that second part of the question and more plausible to get to the answer. The first part really gets to issues of: Is life a rarity, or is it a commonality in the universe? If the right set of conditions come together for a long enough time, is life something that will always originate, and there are very smart people who debate opposite sides of that question.

I think in trying to find some empirical evidence for life out there, we're actually looking for a way to address that question.

FLATOW: I have about 30 seconds left. Do we have to go personally there, or do we have robots that are smart enough, David, that we could send?

Dr. DES MARAIS: I think, ultimately - I mean, the robots would be incredibly capable. Already, it's 50 times cheaper to get a sample back from the moon than humans could do it, compared to the way it was in 1960. So I think the future is - for robotic exploration is really very bright. But I think there's an engagement, there's an involvement that humans going to place brings to the thing, and given that it's such an important issues psychologically and in many ways, I think that's an important aspect.

FLATOW: All right, and that's where we're going to have to leave it. I want to thank all of you gentlemen for taking time to be with us this hour. David Des Marais, long-term planning lead for the Mars Exploration Rover Mission. As we get closer to that, we'll talk more about that, what's coming up with the next Rover. He's a senior research scientist in the astrobiology program at the NASA Ames Research Center.

Also at NASA Ames is Tori Hoehler, a research scientist in the astrobiology department, and Michael Carr, planetary geologist at the U.S. Geological Survey in Menlo Park, California, because when you talk about planets, you're talking about the geology. The astronomers are not looking at planets anymore. Thank you all for taking time to be with us today.

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