The Case Of Mars’ Missing Water
This story is a part of our spring Book Club conversation about ‘The Sirens of Mars: Searching for Life on Another World.’ Join our online community space, record a voice message on the Science Friday VoxPop app, and read along with our lineup of discussion questions, live zoom events, and more.
In the search for life outside Earth, scientists consider having liquid water one of the foremost criteria for determining if a planet could be habitable. On Mars, the evidence for a watery past has been flooding in from rovers and other instruments over the last 30 years. The contents of that water—its temperature and salinity, how fast it moved— are all now written in the planet’s minerals and rocks.
SciFri producer Christie Taylor talks to planetary scientist Bethany Ehlmann about the hunt for Mar’s water, where it all went, and whether liquid water could still, somehow, exist on the Red Planet’s surface.
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Bethany Ehlmann is a professor of Planetary Science at the California Institute of Technology, and is President of The Planetary Society in Pasadena, California.
IRA FLATOW: This is Science Friday. I’m Ira Flatow.
It’s the Ides of March this week. Or maybe I should say the Ides of Mars. Yes. As the SciFri Book Club has been digging deeply into the red planet’s potential for hosting life with the book Sirens of Mars, by Sarah Stewart Johnson. We’ve already talked about meteorites, rock samples, and the limits of life as we know it. And we’ve got another close-up on the planet Mars for you, with producer Christie Taylor.
Hey, Christie, what are we in for today?
CHRISTIE TAYLOR: Hey there, Ira. Today, we are going to talk about water on Mars.
IRA FLATOW: I remember that a couple of years ago, astronomers thought they saw lines on Mars that might be canals, right?
CHRISTIE TAYLOR: Yeah, they did. And Ray Bradbury, he even wrote about whole cities centered on those canals, in The Martian Chronicles, kind of like Venice. Those lines, unfortunately, later proved to be optical illusions. And our closer looks at Mars were first kind of disappointing.
When Mariner 4 sent images back, Mars looked like a dead, dry desert planet. But then it got really exciting. As we got closer and closer looks, they revealed a world that was probably shaped by past water. We’ve got river deltas, lake beds, valleys carved by moving water, maybe even oceans.
IRA FLATOW: Yeah. And what I really like about this story is that there are some people today who think liquid water could potentially still be there.
CHRISTIE TAYLOR: Yeah, that’s so interesting. And I talked to Dr. Bethany Ehlmann. She’s a Professor of Planetary Science at the California Institute of Technology. She’s also President of the Planetary Society. And she’s one person who is currently investigating the history of water on Mars and the potential for liquid water to still be there on Mars.
And we started by talking about why the question of water is so interesting and important in the hunt for life.
BETHANY EHLMANN: One of the reasons that I am so excited about exploring Mars, and Venus, is that we only have three planets in our solar system where we not only get a snapshot of habitats but we have the potential to understand the billion-year history of what processes sustain those habitats. And water is really key to this equation.
CHRISTIE TAYLOR: And over the last 30 years or so, we’ve been learning with increasing certainty that Mars itself once had liquid water. What were some of the biggest pieces of evidence along the way to that understanding?
BETHANY EHLMANN: The decades of Mars exploration that we’ve been doing keep getting more and more hints of this once watery world that was once far more Earth-like. The first hints came with the first missions, the first flyby missions of Mariner space probes and the Viking orbiters and landers. Because in those images we saw the evidence of past valleys and canyons, some of which were carved by liquid water.
Since then, though, the story has really taken off, in the 2000s, when we started to see the chemical and mineral fingerprints of water. So not just water maybe for a relatively short period of time in a violent flood, but over millions of years, transforming the chemistry and mineralogy, to leave fingerprints of what the environmental conditions were.
CHRISTIE TAYLOR: What are those– when you say fingerprints, what are we talking about?
BETHANY EHLMANN: Some of the fingerprints of water that we’ve seen are from orbit, and using infrared spectra. So intensity of light coming back in reflectance, or emitted light as a function of wavelength. You see these characteristic absorptions, or peaks. And each of those indicates a particular mineral phase.
We’ve also had the enormous opportunity to send rovers to a few of the interesting spots that we’ve spied from orbit, where some of the fingerprints of water can be seen with the chemistry and mineralogy instruments on the Rover mass spectrometers, and all of the chemical tools that we can throw at the planet.
CHRISTIE TAYLOR: How does a mineral tell a story of water? What is in the mineral that really gives you that evidence?
BETHANY EHLMANN: We toss around these terms, rock and mineral. But a mineral is something very specific to a geologist. It’s an inorganic element or compound that has a fixed structure and a characteristic chemistry. And why that’s important is that minerals form in response to the conditions around them.
So in terms of minerals related to water, they’re really key indicators for what the environments were like. What was the temperature of the water? What was the pH of the water? Was it salty? Was it fresh? Was it incredibly hot?
So these are some of the things that we can tell from minerals, whether– because you get different minerals in different settings. You get salts. You get clay minerals. Iron oxides from rust. All of these are indicators of the environment.
CHRISTIE TAYLOR: What are some of the minerals that indicate like hot water or salty water or acidic water?
BETHANY EHLMANN: Yeah. Well, let’s get like nerdy with minerals here for a moment. One of the most exciting minerals that we found from orbit, when we were first looking at data from CRISM, the Compact Reconnaissance Imaging Spectrometer for Mars, we were looking at the reflected light, and we see the fingerprints of this particular mineral called prehnite. It’s this beautiful greenish mineral. It’s a calcium-aluminum-silicate mineral that has water in its structure.
And prehnite was one of these things we didn’t expect to find. It doesn’t occur in huge amounts. But it was there in small amounts. We could see it in certain rock formations and not others. And what’s important about it is it actually forms under pretty restrictive conditions that are only hot, about 200 to 400 degrees Celsius.
So when we saw this mineral on Mars, and we saw it associated with some really ancient terrains that were kind of cut into canyons, and we’d look at the wall, and then we also saw it in impact craters, in ejecta thrown out and in the central peaks, we knew that those areas had once had warm waters flowing through them. Because this beautiful greenish mineral, prehnite, that you can see in the Natural History Museum, was sending off its– had a spectral signature that was appearing in some of the rocks that we were seeing from orbit.
CHRISTIE TAYLOR: That’s so cool. Well, then what is the story of this water’s existence on Mars? We have on Earth this set of environments that include things like salt marshes and subglacial lakes and deserts. How do you describe what we know about the story of Mars’s past environments?
BETHANY EHLMANN: I would say that, minus the plants, we had all of those environments on Mars, too. And that has been the big discovery really of the last, I’d say, 20 years of exploration, driven by both the orbiters and then the rovers that have been able to follow up at some of these sites.
Mars had a diverse set of aqueous environments that varied in space, varied as a function of time, much like our own planet. So just to rattle it off, we had hydrothermal systems, we had aquifers deep underground, we had rivers on Mars, lakes, shallow ponds of salty waters. Sometimes those ponds were acidic. We know that there are a few impact craters that had volcanic hydrothermal systems coming out of the bottom, like lost city hydrothermal systems. And we had environments where soils were forming. We can see the soil horizons kind of sliced in the canyons of Mars.
So for about 2-2 and 1/2 billion years, Mars hosted an environmental diversity that was comparable to Earth. The headline, Water on Mars, has perhaps been overused over the last 20 years, but it’s because it has the evidence over and over and over again, and we get this picture of a rich, habitable world, full of potential environments that could have hosted life.
CHRISTIE TAYLOR: That’s incredible. And at some point Mars lost this liquid water. And we know some of it is in ice, but what do we think happened to it?
BETHANY EHLMANN: That’s right. So Mars, unlike Earth, was not able to sustain a richness of habitable environments over 4 and 1/2 billion years the way Earth has for its life here.
Now, there may still be Martian environments underground. The jury’s still out, whether there are underground aquifers of liquid water on Mars. And we do think there might be small amounts of liquid water that come and go on the surface even today. But Mars lost its water. And we’ve been scratching our heads, why?
But I think we’re starting to piece together that picture, that some of the water is now trapped in ice, as you said. Some of the water has been lost to space over time. And some of the water has actually been lost to the crust itself, trapped in these minerals, some of which themselves have water.
A paper earlier this year by myself and grad student Eva Scheller calculated that maybe 30% to 90% of Mars’s water was sucked into the crust. And unlike Earth, Mars doesn’t have plate tectonics to recycle that crust down into the mantle and then the water back up through volcanoes. So it’s a one-way street once that water gets trapped in minerals in the crust.
CHRISTIE TAYLOR: Wait a minute. So volcanoes are a water-recycling mechanism on Earth?
BETHANY EHLMANN: So, believe it or not, yeah. One of the ways that a lot of Earth’s atmosphere built up and acquired its water is from volcanoes. We think of them as destructive forces. But one of the other things that happens is, when a volcano erupts, it releases gases into the atmosphere. These gases include H2O.
Earth’s mantle has water in it. And so when we tap that through volcanism, water can get released. This replenishes the water in our atmosphere that’s naturally lost to space, lost by– stripped away by the solar wind, lost by thermal escape.
But also, importantly on plate tectonics, our mantle’s water is getting refreshed. And some volcanoes have more water than others because of the subduction of ocean crust, hydrated ocean crust, that has minerals in it that formed from water, gets subducted down into the mantle, carry that water in there, and then some of that comes out via volcanoes. So it’s like a giant– volcanoes and plate tectonics are a giant recycling mechanism for water.
CHRISTIE TAYLOR: Another recent piece of research that you put out actually suggests the water on Mars stuck around a lot longer than we thought, maybe as much as a billion years. How can you tell something like that?
BETHANY EHLMANN: Yeah. So one of the things that we’re trying to do is, now that we have seen all these interesting water-related environments on Mars, is we’re trying to figure out, OK, when in Mars history did Mars host water?
So one of the most enigmatic deposits were these set of chloride ponds. So chloride is NaCl, table salt. And scattered throughout the Southern Highlands of Mars– this is some of Mars’s most ancient terrain, kind of undulating hills and valleys and deep craters and some volcanic flows– scattered across there, in these kind of little irregular depressions, were chloride salts. And at one point in Mars history, they would have been shallow ponds of salty water sitting on the surface of Mars.
With grad student– former grad student– now postdoctoral scholar Dr. Ellen Leask, we took a look at how old were some of those chains of lakes, chains of salty ponds, that we were seeing. And we did that by doing a technique called crater counting, where you count the number of craters in whatever terrain is under those lakes– because those lakes have to be younger than whatever they’re sitting upon– and so you basically count the number of craters under those lake deposits, and you compare that to the density of craters elsewhere on the surface.
And using those crater chronologies, we determined that these salty lakes meant flowing water on the surface of Mars as recently as 2 billion years ago. And that is still a long time ago, but it’s about a billion years later than the terrains that we’re currently exploring with the Curiosity and Perseverance Rover. So those lakes are about 3 billion years old. These salty ponds were about 2 billion years old.
So then the question becomes, well, are there even younger deposits or younger examples of liquid water on the surface of Mars?
CHRISTIE TAYLOR: I’m Christie Taylor, and this is Science Friday, from WNYC Studios.
We’re talking about water on Mars with Dr. Bethany Ehlmann.
I mean, is it possible that there could still somehow be water on the surface of Mars even today, or is that completely out of the question?
BETHANY EHLMANN: The pendulum has gone back and forth as to whether there could be liquid water on the surface today. I think, for a long time, people thought that the answer was no. I mean, it is cold. The atmosphere is incredibly dry. Certainly, thermodynamically, at this moment, the water is unstable to both sublimation as well as evaporation if you did get liquid water on the surface.
However, when we’ve explored with the rovers, we have found these tantalizing hints of an active Mars. For example– and you’ve covered this before– methane on Mars. It comes. It goes. There’s something happening that’s driving that. Is it water? We don’t know.
We’ve also seen hydrous salts that pass through a phase called deliquescence, where they can let off their water. Now, this is small amounts of water. But water and mineral structures, in the course of a day, in the course of a season, the salt can effectively breathe in and out the water as liquid water. It’s called deliquescence.
But the evidence that I always think about, that I just think is tantalizing, is the Spirit Rover, when it was driving, and it got its bum wheel– [CHUCKLES] late in history, and it had to kind of drag this wheel around–
CHRISTIE TAYLOR: Yeah.
BETHANY EHLMANN: –it was sort of fortunate. It was like this trenching experiment on the go. And one of the days when we turned around, after the drive, it had dragged through this, like, large salt deposit a few feet wide. The upper few inches– or centimeters, if we’re being scientific– [CHUCKLES] portions of the surface that had been churned by that wheel had turned up these yellow and white salts, like just beneath the surface.
And those wouldn’t have sat there stable for 4 billion years with all the wind coming and going in the sands. So they must have been formed by water passing through the soils at least within a few million years, if not sooner.
And so I think Mars has a climate perturber that Earth doesn’t have to the same degree. Mars tilts on its axis. It tilts its obliquity, leading to climate change on the scale of hundreds of thousands to millions of years. And I think this sometimes tilts Mars into a regime where water and trapped gases are released from the polar caps, making liquid water more stable on the surface. And I think that we may be seeing hints of that.
CHRISTIE TAYLOR: Well, with that in mind, is there anywhere in particular on Mars that you want to personally walk around on with your suite of geologic instruments?
BETHANY EHLMANN: Oh, it would be incredible to be the first astronaut on Mars. In the meantime, I will settle for sending some robotic mobile explorers, like rovers or helicopters.
I always struggle to answer this question, because the reality is, is that if we want to piece together the question of Mars’s climate, explore its past habitats, and really look for life, we now have dozens, if not hundreds, of places that are ideal to explore on the surface. So I really think we’re transitioning to this point in Mars exploration where it’s important to get boots on the ground, so to speak, whether they’re astronaut boots or rover wheels or helicopter landing foot pads.
Getting to some of these sites where we have this rock record of liquid water is I think incredibly important, so that we can look for evidence of water, understand the watery environment and the conditions that sustained it, as well as look for life, in multiple times of ancient environments. And then there are places with recent salts, with ice, just at the surface, that I think are good to look for modern water, and potentially modern life.
So I’d love to go also to some of the ice deposits and some of these recent salt deposits. So many places to explore.
CHRISTIE TAYLOR: And so little time. Well, thank you so much, Bethany, for the time today.
BETHANY EHLMANN: My pleasure.
CHRISTIE TAYLOR: Dr. Bethany Ehlmann is a Professor of Planetary Science at the California Institute of Technology, and President of the Planetary Society. She’s based in Pasadena.
IRA FLATOW: Great story, Christie. Thank you. So much Mars to explore. I get it.
Well, that’s got me excited and impatient for the next mission to Mars, whenever that might be. Thanks so much, Christie.
CHRISTIE TAYLOR: Thank you, Ira.
IRA FLATOW: And if you’ve been dying to talk to Sirens of Mars author, Sarah Stuart Johnson, another planetary scientist, you’re in luck. She’s answering questions in a live Zoom taping next week, and you can learn more about the mysteries of Martian geology. Visit Sciencefriday.com/MarsEvent, to sign up. Sciencefriday.com/MarsEvent.