Scientists Spot Signs of Salty Water on Mars

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Some cliff faces on Mars feature dark streaks that appear during the warm months, then disappear when weather turns colder. Writing in the journal Science, some researchers say that the streaks could be signs of briny water on the Martian surface. Planetary scientist Alfred McEwen discusses the findings, and what they might mean for future studies of the red planet.

IRA FLATOW, host: Moving out a bit further into our planetary neighborhood, next stop: Mars. We've known for a while that there's ice on Mars. But now researchers say that they may have seen signs of liquid water in the form of dark streaks that appear and disappear with the seasons on the - on certain Martian slopes. And the work is described in the journal Nature. You can see some photos of it up on our website, at sciencefriday.com. I've said Nature, I meant Science, in the journal Science.

And joining me now to talk about it is Alfred McEwen. He's the lead author on that paper in Science and the principal investigator on the HiRISE camera. That's part of the Mars Reconnaissance Orbiter. He's also professor in lunar and planetary laboratory at the University of Arizona in Tucson. Welcome back, Dr. McEwen.

Dr. ALFRED MCEWEN: Hello, Ira. How are you?

FLATOW: It sure does look like there's water trickling down the slopes, here.

MCEWEN: It looks like something flowing down the slopes, yes.

FLATOW: Something. You've very - being very science correct on this.

MCEWEN: I'm going to be very descriptive about this, and we're trying to - we're calling these recurring slope linea, a terribly boring name for these - intentionally to be purely descriptive so we don't bias ourselves into thinking we know exactly what they are as we investigate further.

FLATOW: But surely, you wouldn't have published the paper if you didn't think it might have been water?

MCEWEN: Maybe not in Science, but we've found these features - well, we've been searching current activity in Mars for sometime and finding all sorts of interesting things. Sand is moving around, new impact craters. And in the region that receives CO2 frost in the wintertime, there's a whole suite of dynamic processes that occur, including the gullies that are active. There's been a lot of discussion about that being due to the water.

And it was a surprise to find that they're active today in the winter, when there's CO2 frost in the ground and the temperatures are way too cold for water. And I was beginning to think, OK, that's where all the action is. It's in the winter when there's CO2 frost. But then we found these features, which are just the opposite in season. They're active in the summertime, when it's warmest.

FLATOW: Can it actually get warm enough to have liquid water there in...

MCEWEN: Oh, yeah. The surface temperatures can get up to a balmy 80 degrees Fahrenheit or so, maybe even warmer, although it is still much colder at night, and it's colder also just down an inch in depth because it's so cold at night, you know, you go down deep enough, you get the average temperature.

FLATOW: Mm-hmm. So these streaks that you see, because they're seasonal and they happen in the summer, may mean they might be water.

MCEWEN: They have a suite of characteristics. They are found only in the middle latitudes, which is - which are places where we have other evidence for the presence of ground ice. It's a little bit (unintelligible) that, but there could be ice a bit deeper than we've seen previously, but only on the equator-facing slopes. So it's high enough in latitude for there to be ice in the ground, but still places that get warm enough to get above the melting point of water, or at least salty water.

FLATOW: Mm-hmm. We're talking about the possibility of finding water on Mars this hour on SCIENCE FRIDAY, from NPR. I'm Ira Flatow, talking with Alfred McEwen. So is there any way - of course, we're not going, you know, we're not on Mars. We can't take samples back. Any way that you can strengthen this case for it being water? And what...

MCEWEN: Yeah, a great question. We - so far, we don't have direct evidence for water. There is the Compact Reconnaissance Imaging Spectrometer for Mars, CRISM, that's observed some of these sites, and continuing to observe. Now, they have an 18-meter-per-pixel footprint, and we think that that - that there's wetness only on small areas for very short periods of time. It really dries out in a hurry on Mars. They don't see water. That's not surprising, although it sure would be nice to have some sort of direct confirmation.

They're going to keep looking. But the other thing we can do, I think, is most promising, is laboratory experiments. These are small-scale features of simulating this in the lab. There are a dozen labs around the country and in Europe and elsewhere, where they can pump down the pressure to simulate a Martian atmosphere, adjust the temperature. It ought to be possible to reproduce some of the key behaviors we're seeing in the laboratory and really give us a strong answer on whether or not water is required.

FLATOW: Is there any other place on Mars where you see possible evidence of running water like this?

MCEWEN: No, not of running water. Now, at the Phoenix landing site, they saw these droplets form on the lander legs, which could have been salt. Salts can absorb water from the atmosphere. It's called deliquescence. That's one idea for where the water comes from for these slope streaks that we're seeing. But also, they discovered perchlorate. That's a particular salt that would, in solution, could have a very low freezing point, such that during the Arctic summer, there may be pods of water that form out of little salt patches. But it isn't flowing, as far as we know, and it's very cold water. This will be warmer water.

FLATOW: Mm-hmm. And this would be salty water, too, then?

MCEWEN: Probably salty, yeah. That's just because a couple of reasons. First, Mars is a very salty place. Every lander and rover, Martian meteorites, remote sensing show salts on Mars. So any water that flows on the surface, shallow sub-surface, should become salty, even if it doesn't start off that way. But secondly, some of this activity occurs when the surface temperatures are too cold for pure water to melt, or to remain in a liquid state. And also, pure water at these sites would actually boil. Even when it's near the freezing point of water - the atmospheric pressure is so low, it boils and is highly unstable. Salty water is a little more stable.

FLATOW: Given the fact that you've seen these streaks on the slopes, could there be others that you'd be looking for?

MCEWEN: We're looking - we've seen these - this particular activity in, oh, seven places, where we've confirmed repeat in multiple years, and another 12 or 20 or so that are likely or candidate sites. And we're going to keep looking for more of these. And that's a key characteristic, by the way, the fact that these recur year after year. It doesn't just happen and it's done. The following year, new ones form that - they may be...

FLATOW: Right.

MCEWEN: slightly different, different place, but they clearly recur. And so we're going to keep looking for more of this particular type of activity. But what surprises me is the fact that it took us so long to discover this at all. It was actually hiding in our data from several years ago. So I wonder how many more things there are that are still waiting to be discovered.

FLATOW: Mm. We'll talk - can you stay with us a few more minutes? We're going to take a break. We'll come back and talk more with Alfred McEwen, principal investigator on the HiRISE camera that's part of this satellite that's going around Mars, the Mars Reconnaissance Orbiter, taking these - snapping these pictures, sending back so much data, so many photos. It takes years to sift through them and see these photos.

So stay with us. We'll be right back after this break. Our number: 1-800-989-8255. Go to our website at sciencefriday.com. You can see the pictures of the streaks coming down the site of the slopes of these hills. Stay with us. We'll be right back after this break. I'm Ira Flatow. This is SCIENCE FRIDAY, from NPR.

(SOUNDBITE OF MUSIC)

FLATOW: You're listening to SCIENCE FRIDAY. I'm Ira Flatow.

We're talking this hour about the possibility of liquid water on Mars, some intriguing satellite images and then the satellite is the Mars Reconnaissance Orbiter, sending stuff - photos back to investigators, who are manning the HiRISE camera. My guest is Alfred McEwen, who was principal investigator on the HiRISE camera.

Dr. McEwen, I guess this shows you that you got to just keep the camera in one spot for a long enough time to see changes, right?

MCEWEN: Well, we have this interesting conflict between whether to image new sites or whether to re-image old sites and look for changes. So we just do a little bit of both.

FLATOW: Mm-hmm. And you got - would you say you got lucky here? Or you don't want to use that word?

MCEWEN: Maybe. We actually saw many of these by targeting mid-latitude craters because the gullies are there, and they're of interest. And we were mostly - we were looking for gully activity, and weren't really looking for this small-scale-type activity, and in the summer season and so forth. So we got lucky in eventually finding it, although I feel like we should have found it sooner.

FLATOW: Well, you had the prepared mind, right, as they talk about...

MCEWEN: No. If I had been smart, I would have thought: Where is the most likely place to find actual water on Mars today, and would have targeted these places for exactly that purpose. But no, we had to wait for an undergraduate student here to find it for us instead.

FLATOW: There you go. Nothing more needs to be said.

(SOUNDBITE OF LAUGHTER)

FLATOW: Let's go to the phones, Matt in St. Louis. Hi, Matt. Matt, are you there? Hi, there.

MATT: Yes. My question is where are Spirit and Opportunity? Would it be feasible or even possible to send them for samples?

FLATOW: All right. Good question, Matt. The Mars rovers. One is pretty stuck and not going anywhere, right?

MCEWEN: Yeah. Yeah, Spirit is not going anywhere. It has been - its mission has been declared over, 100 percent successful. Opportunity is headed towards Endeavour crater and - but it's right near the equator. We just don't see any of this activity near the equator, and we don't see it particularly in Endeavour crater. And it moves really slowly on a global scale compared to MRO circling the planet 13 times a day. So there's really not much hope of Opportunity getting to one of these sites.

There's another rover, MSL. It's landing in Gale crater. That, again, is near the equator. And, again, we don't see evidence of this activity. And also, if this is water, then you have to design the rover with certain planetary protection standards that weren't done for these rovers. So we really need a new mission.

FLATOW: You mean you don't want to contaminate the planet.

MCEWEN: We don't want to contaminate any water with Earth bugs...

FLATOW: Right.

MCEWEN: ...and then discover Earth bugs.

FLATOW: Yeah. So you have to have a whole new mission that's sterile when it got there.

MCEWEN: Yes. And the Phoenix Lander, for example, has an arm that dug into ice, and that arm was sterilized, not the rest of it.

FLATOW: Well, this is a new excuse to set up another mission, don't you think?

MCEWEN: Yes. They're getting harder to get, though.

FLATOW: Yeah. Well, we know about the monetary problems, and big ideas not getting funded anymore. So, thank you, and good luck to you, Dr. McEwen.

MCEWEN: OK. Thank you.

FLATOW: Alfred McEwen, principal investigator on the HiRISE camera, part of the Mars Reconnaissance Orbiter. He's also professor in the Lunar and Planetary Laboratory at the University of Arizona in Tucson.

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