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

Up next, reevaluating the history of water on Mars. After reading news reports about the search for signs of water on Mars, I think that many of us are under the impression that some time in a distant past, Mars had huge oceans of water that gushed over the planet at times, leaving behind gullies and ravines that are still visible on the surface, but also leaving behind a big unsolved mystery. What happened to all that water?

Well, now, the Mars Reconnaissance Orbiter, or the MRO in NASA speak, has been circling the planet since March of 2006. It's sending back new high-resolution photos that are forcing us to rethink some of those ideas. A batch of papers out in today's issue of the journal Science, based on analysis of the photos, that batch calls into question some earlier conclusions about evidence for water and suggests that the Martian water mystery maybe more complex than scientists thought.

We're joined now by of those scientists trying to tease these secrets from the surface and sometimes below the surface of Mars. Alfred McEwen is the principal investigator for the High Resolution Imaging Science Experiment for NASA's Mars Reconnaissance Orbiter, and professor of planetary sciences at the University of Arizona in Tucson. Welcome to SCIENCE FRIDAY, Dr. McEwen.

Dr. ALFRED McEWEN (Principal Investigator, NASA's HiRISE; Planetary Sciences, University of Arizona): Hello, thank you.

FLATOW: Good to have you back. Maria Zuber is the E.A. Griswold Professor of Geophysics and head of department of Earth, atmospheric and planetary sciences at MIT. She joins us today from her office there. Welcome back to you, Dr. Zuber.

Dr. MARIA ZUBER (Geophysics, Massachusetts Institute of Technology): Thank you very much, Ira.

FLATOW: You're welcome. Dr. McEwen, those Science papers are based on data collected very early in the mission, but the data keeps coming in and changing things, right?

Dr. McEWEN: Yes. We are being deluged with data right now. We're at our peak data rate, the spacecraft is returning more than a terabit of data per week. So it's more than most planetary missions return in their entirety, every week, and we are really treading water to keep up with it. But that's a fun problem we have.

FLATOW: That's a very interesting use of terminology, to use that - treading water. Are these - tell us about the observations. I've been reading about them a bit and they seem to - I'll take your analogy - throw cold water on some of the observations that were made earlier or some of the conclusions about the presence of past water on Mars.

Dr. McEWEN: Well, yes and no. I see it as refining where and when water was present in order to focus each exploration, and it's a process of focusing in on where the water was. But I've been rather surprised by the reactions of the news media to this, which is, my God, it's looking like Mars is dry after all. Well, to the science community, it's always been dry. It's a question of whether the last few billion years have been 99.99 percent dry or just 99.9 percent dry.

FLATOW: Well, we saw - for example, we saw observations made last year, sent last year that showed that these very famous gullies.

Dr. McEWEN: That's right.

FLATOW: Right? And they supposedly were remnants of water flowing down the side of the gullies. And your observations are saying that's, well, we now look at these HiRISE and we see that it's not evidence of water.

Dr. McEWEN: Not necessarily. Yeah, that was a very important result that came from the Mars Global Surveyor camera and reported by Mike Malin et al. And they saw changes in two gullies, in particular, where new bright deposit appeared just in very recent times. And so this is certainly an important thing to report and for MRO to follow up on. So we have. We've gotten color stereo images, and we've gotten spectra, infrared spectra from the Compact Reconnaissance Imaging Spectrometer for Mars, another instrument on MRO.

And one suggestion was that these deposits were bright, either frost or ice or hydro mineral such as salt. The CRISM Spectrometer has not seen evidence for any of those materials present. Furthermore, with HiRISE, we see that there are bright materials up slope that have been eroded. So we think that preexisting bright materials were eroded, transported and re-deposited. Now that doesn't mean that it wasn't - there wasn't a role for water in that, but it certainly doesn't confirm water.

The other thing we've done is we - first of all, there were two examples we've seen on four other locations with very similar-looking deposits, although we haven't seen them change. So we have some better statistics. And in all six cases, these are on some of the steepest slopes at these latitudes, steep enough, we think, that dry mass wasting is sufficient to explain the motions. And so that's the alternative hypothesis that we think needs to be seriously considered.

FLATOW: Mm-hmm. 1-800-989-8255 is our number, if you want to talk about Mars and this new evidence from Mars. Dr. Zuber, we've been hearing about places on Mars where there may or may not be water. But there is a place that we know where there's water, right, at the poles?

Dr. ZUBER: Oh, there is water at the south polar regions of Mars. And, of course, we've known that there was water at both polar regions of Mars for some time, so I don't want to say that that is a new result. But the novelty of our study was that we combined information from three spacecraft - the Mars Reconnaissance Orbiter, the Mars Odyssey and the Mars Global Surveyor - to refine what the water budget and exactly how much water ice there is at the south pole. And this is the water ice that is at the south pole of Mars today.

FLATOW: Mm-hmm.

Dr. ZUBER: And we've seen from telescopes and orbiting spacecraft that Mars has a south polar cap. It's covered with a very thin veneer of carbon dioxide, which is the same composition as the Martian atmosphere. But there are depressions in the south polar cap that indicate that there's water ice below that. And then around the south polar cap, there is a region that is dark, like the surrounding terrain but it's elevated by a couple of kilometers over the surrounding terrain that's much bigger than the south polar cap itself.

And the question is, was that sand and dust and dunes, or was that predominantly water ice, or was it CO2 ice? And what we've been able to measure by looking at the gravity field, which give us the mass and then using altimetry to give us the elevation of the top of these deposits and then a sounding radar to give us the bottom of these deposits. We're able to measure the volume very precisely, and mass divided by volume gives you density.

What we find out is that the density of these deposits we've now been able to measure very precisely and it gives you constraint that it's almost pure water ice maximum of 15 percent dust. So when you put all that together, what you have is that that's the largest reservoir of surface water, water ice on Mars today.

FLATOW: Would that then be a place you'd like to send a spacecraft?

Dr. ZUBER: It would be a wonderful place to send a spacecraft. In fact, NASA tried to send a spacecraft there, the Mars Polar Lander.

FLATOW: Oh, yeah. That was a - sad story on that one.

Dr. ZUBER: It would have landed in the south polar layer deposits. And one of the goals of that mission was it had a little arm on, a little robot arm that would have scraped away the dust at the top to see what was beneath that. And there were questions about how much ice versus how much dust. And we now know from remote methods that it's primarily water ice.

FLATOW: What about the, you know, I'll ask this both of you, what about the concept that I mentioned before that we've had this picture that, at one point in time, Mars was covered with these oceans. And there were these gushes and flowing that created the crevasses or the gullies. Your new evidence seems to throw a little bit doubt on that, that there may have been episodic times, but not really sustained times. Would that be describing it the right way?

Dr. McEWEN: Possibly, yes. But let's distinguish that with the gullies. The gullies are a different phenomena. It's much more recent and not associated with the potential oceans. But there has been a lot of discussion of potential oceans in Mars' past. And it's been a rather - my first impression when this debate came up 10 years ago was that with new orbital remote sensing, surely, we can figure out whether or not there was an ocean. That's a big thing. It has major effects in the landscape.

But we're still debating it. It's been a very frustrating ocean to pin down and find definitive evidence for. And there has been a lot of debates about shorelines and different interpretations of those features, whether they were really shorelines or not. And according to a review paper by Mike Carr and Jim Head, a couple of the real experts and leaders in our field. They thought that this unit in the north pole are called the Medusae Fossae Formation.

I mean, Vastitas Borealis Formation, which is a unit that's in the lowest area of the lowland, such where an ocean would have gone if there was one. They thought that the nature of that unit could be key to determining whether or not there was an ocean. So it's HiRISE. We got some high-resolution images. We thought we might see a fine grain deposit, perhaps with layering, perhaps with bright layers similar to a sedimentary, aqueously altered deposits we see elsewhere. Instead, what we see are no layering and lots and lots of boulders.

So we've been puzzling over what that means. And I think it makes it difficult, certain concepts for an ocean, for example, one that would deposit a 100-meter thick layer of fine grain sediments similar to deep oceans in Earth. That seems difficult to reconcile with all of these boulders. There are other concepts of oceans as well, though that can be reconciled with it. So I don't think we've settled that one, yet. It's a very interesting question and we don't know the answer yet, but we're certainly aren't finding the smoking gun at this point.

FLATOW: Do we doubt that there is water there some place, though?

Dr. ZUBER: Yes. If I could, you know…

FLATOW: Yes. Please, go ahead.

Dr. ZUBER: What - Alfred talked about the surface; I'll talk about the subsurface. There certainly is water beneath the surface of Mars or water ice beneath the surface of Mars. One of the primary results from Mars Odyssey - there were instruments on that spacecraft that measured gamma ray fluxes and neutron fluxes, which showed that within about the top meter of the surface of Mars - that there were concentrations of water in the polar regions but also in midlatitudes as well, that were too large to be explained by water that just simply bound up in mineral structures. And that's - so that it has to be free water and it's in this form of water ice today because Mars is cold.

But the idea was that if there was water on Mars early in its history, that it - as the planet cooled off, the crust of the planet being very rubbly from being broken apart by impacts, that water would have seeped beneath the surface. And so, you would expect to find water ice beneath the surface of Mars today. And, in fact, when you look at it remotely, you do see it there. So…

FLATOW: Mm-hmm.

Dr. ZUBER: But a lot has happened in, you know, the last four billion years. So as the HiRise and other spectral observations are showing, it's gotten covered up and the story at the surface is very complicated.

Dr. McEWEN: I think this is one of the most important - actually, definitive new results from recent years is that we definitely have found the water ice, that Mars is a water ice-rich planet. That was under debate 10 years ago. So maybe the polar - residual polar caps that's all there was. But no, there a lot more water than that, water ice that is.

FLATOW: Mm-hmm. If it gets impacted with, you know, Mars is pot-marked with meteors, meteorites, whatever, hitting them, could that liquefy the water for a brief period of time from the impact?

Dr. ZUBER: Oh, it certainly would have. And the question - and I think you got this earlier in your comments, Ira, was a question of persistence. Did water in liquid form stay at the surface of Mars for a long period of time or was there considerable water in the subsurface, which I think we don't argue about because those are physical measurements that are rather definitive.

FLATOW: Mm-hmm.

Dr. ZUBER: Were they - were there episodic impacts or volcanic intrusions that would liberate water to the surface where it stayed for a short period of time and then froze over? And those are the questions that we're grappling with now.

FLATOW: Let's get another…

Dr. McEWEN: This - well, I think this is one of the most interesting new results from HiRise is what see from some young, but fairly large impact craters, greater than a kilometer in diameter. And that's evidence for water in these things that are young, not ancient. There's - there are flows, like debris flows. There's evidence of impact melt, which is made more easily in the presence of volatiles. There's even erosion of sand, erosion of gullies and sands or channels and sands over low slopes and so forth. It looks very much like - there's a critical Mojave crater, it looks like the Mojave Desert - a spitting image of it.

So that, I think, supports the - this clearly supports the idea that impacts can liberate water from the subsurface and lead to some of the effects we've seen in modifying the landscape, may be explaining much of what we see back in the ancient Mars when, you know, the channels are much bigger, the erosion is much greater, but also the impacts were much greater back then.

FLATOW: Talking about Mars this hour on TALK OF THE NATION: SCIENCE FRIDAY from NPR News. Talking with Alfred McEwen and Maria Zuber. 1-800-989-8255 is our number. Let's see if we can take a phone call or two. Hi. Let's go to Jeff(ph) in Grand Rapids. Hi, Jeff.

JEFF (Caller): Hi, there.

FLATOW: Quickly.

JEFF: Yeah. My question is - I'm just wondering if your guests think that we could get more science or less science compared to bringing actually people to Mars as opposed robotic missions?

FLATOW: Yeah. Good question.

Dr. ZUBER: Okay. I'll start that, Alfred.

Dr. McEWEN: Okay.

Dr. ZUBER: And you can pitch in. There's - certainly, it is more cost-effective to do science with robots than with humans on the surface. It's very difficult to send humans. It would be a big investment and we have, I believe, not reached the point where robotic science has limited, in any way, in discovery. At some point, it may make sense to send humans, but - that they would - and they would certainly make scientific discoveries, I think, that robots would not be able to make. But you can't argue with this point that you need humans to make discoveries on Mars.

FLATOW: Dr. McEwen?

Dr. McEWEN: Yeah. I echo that. The Mars exploration program of NASA and Europe has been a huge success. We've learned so much about Mars. And clearly, this is possible and the cost is quite modest compared to human exploration. And when asked if I favor human exploration on Mars, my answer is yes, because I'm assuming that this will only happen we've solved problems like global warming on this planet. So I'd like to see that happen. But I, realistically, don't you think it's on the horizon.

FLATOW: I've only got about a minute left. And I want to ask this question about future Mars missions. Does new knowledge that keeps coming in and you get a terabyte of data like this. Does it change the way do you think should look and how - what you should be looking for and how you should look for it?

Dr. McEWEN: Yes. And we have to hurry up and figure this out because the selection of the Mars Science Laboratory landing site is coming up really fast here. And we've got so much new data and we've barely analyzed it. That's quite a challenge that's ahead of us. And then there's also the European ExoMars Rover that we have little more time to prepare for that. But this is a very important issue, yes. And that's one of the major goals of the Mars Reconnaissance Orbiter. That's why it called reconnaissance, reconnaissance of the most important places to land and explore in detail.

FLATOW: Dr. Zuber?

Dr. ZUBER: And we're engaging as much of the scientific community as possible. NASA decides where the landing site should be, but they open up to the entire worldwide community to provide input.

FLATOW: Mm-hmm.

Dr. ZUBER: So it isn't just the Mars Reconnaissance Orbiter and the other mission teams that are analyzing the state of the - the state is released to the full scientific community. So anybody that has wisdom to add to this fascinating question has the opportunity to do so.

FLATOW: So the landing site is not cemented yet, so to speak?

Dr. ZUBER: That's correct.

Dr. McEWEN: Yes.

FLATOW: And you'd - you'll be very excited to do decide when that is happening. Thank you very much both of you for taking time to be with us today. Good luck to you. We'll be following what's happening.

Dr. ZUBER: Okay.

Dr. McEWEN: Okay. Thank you.

FLATOW: Welcome. Maria Zuber who is E.A. Griswold Professor of Geophysics and head of the Department of Earth, Atmospheric, Planetary Sciences at MIT. And Alfred McEwen who is a principal investigator for the high resolution imaging science experiment for NASA's Mars Reconnaissance Orbiter. He's also a professor of planetary sciences at the University of Tucson.

We're going to take a short break. Switching into third gear. Up next, talk about why you feel awful sometimes when you get off a long flight? And then you say, oh, the air is so stale. Is it just my imagination? Is there's something in it? New research talks about what's in the air and how you make may be making worse? Stay with us.

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

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