Planetary Radio • Mar 09, 2022

Water, water everywhere with Bethany Ehlmann

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Bethany Ehlmann

President, The Planetary Society; Professor of Planetary Science, California Institute of Technology; Director, Keck Institute for Space Studies at Caltech

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Rae Paoletta

Director of Content & Engagement for The Planetary Society

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Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society

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Mat Kaplan

Senior Communications Adviser and former Host of Planetary Radio for The Planetary Society

Planetary scientist Bethany Ehlmann has co-authored a paper presenting evidence that liquid surface water flowed on Mars as much as a billion years more recently than previously thought. That’s an extra billion years for possible life to have formed and thrived. We’ll also join Planetary Society editor Rae Paoletta as she explores water worlds throughout our solar system in a new article. Another great prize awaits the winner of the What’s Up space trivia contest.

Chloride deposits in Bosporus Planum, Mars
Chloride deposits in Bosporus Planum, Mars This image, from the Context Camera on NASA’s Mars Reconnaissance Orbiter, shows chloride deposits within a channel in Bosporus Planum, Mars. For scale, the largest impact crater in the scene is 1.5 kilometers (about 1 mile) across.Image: NASA/JPL-Caltech/Malin Space Science Systems
Chloride cluster in the chaos terrain in Margaritifer Terra
Chloride cluster in the chaos terrain in Margaritifer Terra Chloride cluster in the chaos terrain in Margaritifer Terra (20.74°W, 3.12°S). (a) Chloride deposits are perched above a crater and deep fractures that lie within a 10-km buffer. (b) Chloride deposit (blue tones) in THEMIS-875 decorrelation stretch image; channels are not visible at this resolution (100 m/pixel). (c) Same chloride deposit at CTX resolution (6 m/pixel), where fine channels are visible (white arrow). (d) The chloride deposit appears light purple in HiRISE color, which, when coverage is available, makes it possible to distinguish its boundaries with adjacent light toned fractured units (note greyscale (left) versus color (right) in (d)). A small chloride-filled channel is present in the lower right (white arrow). Chloride has a sharp contact with surrounding units and appears to drape SW–NE trending aeolian bedforms.Image: Ellen K. Leask / Bethany L. Ehlmann
Ocean Worlds infographic
Ocean Worlds infographic A table showing key information about the probable ocean worlds of the solar system.Image: The Planetary Society
Better Know an Asteroid t-shirt
Better Know an Asteroid t-shirt A shirt featuring asteroid Bennu designed by Chop Shop.Image: Chop Shop

To submit your answer:

Complete the contest entry form at or write to us at [email protected] no later than Wednesday, March 16 at 8am Pacific Time. Be sure to include your name and mailing address.

Last week's question:

What was the name of our solar system’s biggest mountain — Olympus Mons on Mars — before it was given that name? This was back when astronomers only knew it as an albedo or brightness feature on the surface of the Red Planet.


The winner will be revealed next week.

Question from the Feb. 23, 2022 space trivia contest:

Bruce’s Messier Math: What is the number of objects published in Charles Messier’s 1781 catalog multiplied by the Messier number of the Trifid Nebula minus the Messier number of the Starfish Cluster?


If you multiply the number of objects in Charles Messier’s 1781 catalog (103) by the Messier number of the Trifid Nebula (20) and subtract the Messier number of the Starfish Cluster you get 2022!


Mat Kaplan: Water, water everywhere, including on Mars. This week on Planetary Radio.

Mat Kaplan: Welcome. I'm Mat Kaplan of The Planetary Society with more of the human adventure across our Solar System and beyond. Think about it, if there was flowing water on the surface of the red planet for up to a billion more years than suspected, that's a billion more years for life to get a foothold and thrive. A newly published paper by Ellen Leask and Bethany Ehlmann presents good evidence for this extension. Bethany will also give us an update on her Lunar Trailblazer mission. We'll welcome her after a brief conversation with Planetary Society editor, Rae Paoletta.

Mat Kaplan: Rae has contributed a feature article to the new issue of our magazine, The Planetary Report. In it, she explores the vast amounts of H2O in the hidden oceans of water worlds throughout our Solar System. Who knew you like arithmetic? That's what we heard from many listeners who entered math maven, Bruce Bett's contest. The Planetary Society's chief scientist has another number problem for you in this week's What's Up. Call it a Martian flower. It's not, of course.

Mat Kaplan: And that weird formation Curiosity is found on Mars isn't unique either, but it sure is striking. You can see it in the March 4th edition of our newsletter, The Downlink. The Rover grabbed the image on February 25th which was the 3,397th soul or in Martian day of its mission. And that's worthy of an entire bouquet, don't you think?

Mat Kaplan: Wars have terrible consequences even for space missions. ESA, the European Space Agency has now confirmed that the ExoMars mission is, "Very unlikely to launch during this year's window." The agency is looking at its options. As Casey Dreier points out in the March Space Policy Edition of Planetary Radio, ExoMars was also intended to demonstrate ESA's ability to build and operate the rover that Perseverance will transfer its precious Martian samples to 9 billion light years away and therefore 9 billion years in the past.

Mat Kaplan: Two super massive black holes are in a dance of death. A new study finds that they may smash into each other in about 10,000 years. The proverbial blink of an eye in cosmic time. This and other stories await you at Here's my conversation with Rae Paoletta. I had a slight problem with my microphone, but I don't think it will bother you much.

Mat Kaplan: Welcome back, Rae. The last time we talked, it was about ice and snow around the Solar System. Let's warm things up a little bit and talk about the growing number of worlds where we are finding oceans in our solar neighborhood.

Rae Paoletta: It's so funny, Mat. I feel like every time I'm back on here, I'm like the planetary meteorologist. We just have different weather all the time.

Mat Kaplan: Nothing wrong with that. This is all started with this terrific infographic that is near the top of this piece about ocean world. And by the way, it is the feature article in the brand new issue of our magazine, The Planetary Report, which should be in the mail. Hopefully it will have reached most members. I don't have mine yet, but I imagine it's on its way. Anybody can read the digital version of this at and prominently displayed will be Rae's article.

Mat Kaplan: So it has this graphic. Eight different worlds and counting, I suppose, in our Solar System, not including our own. I'll just list them really fast, Europa, Callisto, Ganymede, Enceladus, Titan. But question marks for three others, Triton, Ceres, and Pluto. So we're not really sure about those yet.

Rae Paoletta: Yeah. Those seem to be a question mark for a reason. We just don't know what the estimated amount of liquid water those have, but there are some strong indications that suggest that many of those worlds do have some sort of liquid water. We just don't know because we really don't have enough missions to them yet. So hopefully this will inspire more.

Mat Kaplan: This infographic also introduced, at least to me, a brand new measure of volume, the zettaliter which is just cool in itself.

Rae Paoletta: Right. Sounds very high tech. Yeah.

Mat Kaplan: So 1 zettaliter I guess is a billion cubic kilometers. Wow. That's a lot of water.

Rae Paoletta: It's so much water, Mat. And then going through this and actually helping to make this infographic, it was pretty mind-blowing to be honest with you. I mean, when you look at Earth, we've got 1.3 zettaliters. Okay. When we see Earth, we know about how much water's on Earth. It's a lot. Then you go to something like Ganymede that has 35.4 zettaliters. We're like a puddle by comparison.

Mat Kaplan: But we thought we were the wet planet for all of these years. Not so much it turns out. At least, once you can get under the ice. And speaking of getting under the ice, you briefly addressed the rivalry sorts between Europa and Enceladus, and the people who believe in missions to both of these. Well, of course we have one. It's now being assembled. The Europa Clipper is coming together at, at JPL, but I don't hear many people say that we don't need a mission to Enceladus.

Rae Paoletta: I mean, I love both of these worlds. They are very special and near and dear to my ocean world loving heart. However, I want to see something for Enceladus because I'm super, super stoked that we're going to get Europa Clipper. But Enceladus does have so many incredible things. And it really does feel like an unsolved mystery. I mean, there's just some threads there that have been tied up since Cassini. I really want to know what's going on with those plumes. What's going on with all those building blocks of life that we might have detected? It's just so cool and the possibilities are really endless.

Mat Kaplan: We all love Europa Clipper. But someday, I hope there's going to be a lander visiting Europa. All this speculation that's gone on for decades of how we're going to drill through that ice. It's not going to be easy, is it?

Rae Paoletta: No, it's not going to be easy at all because the ice is extremely thick on Europa. And actually, I don't even think we know exactly how thick it is because, well, we haven't really tried to dig. It's probably uneven as well. Like I said in the article, a hypothetical journey toward the center of Europa would probably make even Jules Verne wins.

Mat Kaplan: Yeah. I love that line. And it's not soft. It's not like the stuff that comes out of the ice maker in my refrigerator. You have this great line from the student, [Muhammad Nasid 00:06:59].

Rae Paoletta: Yeah. I mean, he said that on Europa, the ice is about 110 Kelvins on the surface, which is that hardness is comparable to that of a diamond. So imagine trying to cut through those diamonds that are just packed together to get to whatever is underneath. I mean, it's terrifying. It sounds really hard, but it's also very exciting.

Mat Kaplan: I'm going to jump right out to Triton, Neptune's moon Triton, which of course has only been visited once by, we earthlings, and that was our robotic emissary, Voyager 2. What is it about this moon that makes it so intriguing?

Rae Paoletta: First of all, can we just talk about the fact that it is incredibly on the nose that Triton from Neptune, that that's going to be a water world? I mean, that just seems so perfect. Right? So I just love that for Triton personally. I think Triton is very overlooked for a number of reasons. We do have some theoretical models that suggests that Titan could hold liquid water beneath the surface. That would happen because of the tidal heating that we see in some of the other moons from other planets that have a lot of gravitational pull.

Rae Paoletta: So in theory, Neptune's gravitational pull could allow for Triton's interior to be warm enough to have something like that ocean underneath it. Actually, Europa and Enceladus are very similar and the same kind of process occurs there. I think it could be really interesting to do some more investigating of Triton. It is one of the more wild card moons that we explore in this piece, but that means it's even more worthwhile.

Mat Kaplan: Voyager 2 did see those what appeared to be geysers, those plumes coming out of the moon during its brief past through the Neptunian system.

Rae Paoletta: That's absolutely true. There are a few different kinds of hypotheses to explain why that happened. I mean, some folks think that the geysers were ephemeral and they were just this rarity caused by sunlight. And then others just say, "Hey, I actually don't know. Maybe this is a plume that meant something else." So it would be really cool to go back and have more missions and see what's really going on there.

Mat Kaplan: Got to get back out there again. In the meantime, Rae, your article is a good place to get started. Again, it's at It's in the brand new edition of the Planetary Report, the magazine that used to only go to members on paper, members of the planetary society, but it's waiting at work for everyone. Thank you so much, Rae. I look forward to talking again soon.

Rae Paoletta: Always a pleasure. Thank you so much, Mat. Take care.

Mat Kaplan: That's Rae Paoletta. She is The Planetary Society's editor. Full disclosure, Bethany Ehlmann is president of The Planetary Society's board of directors. I'm not sure how she finds the time. Bethany is a full professor of planetary science at Caltech, the California Institute of Technology. That's where she is also associate director of the Keck Institute for Space Studies.

Mat Kaplan: She teaches, she conducts research on objects all over the Solar System including our own world. And she looks forward to the launch of the Lunar Trailblazer, a small probe that will use two powerful instruments to identify and measure water on the moon. On December 27th, she and lead author, Ellen Leask published a paper titled Evidence for Deposition of Chloride on Mars From Small Volume Surface Water Events Into the Late Hesperian-Early Amazonian. If they are right, it means water was rolling around the Martian surface for up to a billion extra years. I invited Bethany back to Planetary Radio to tell us more.

Mat Kaplan: Bethany, welcome back to Planetary Radio. Congratulations on this latest paper, at least the latest one that I've seen that you are connected with. You are a co-author with Ellen Leask, your former PhD student. I'm sorry to say that she was unable to join us for this conversation today, but I sure look forward to talking to you about the possibility. Apparently the strong possibility that there was water running around on the surface of Mars for a lot more years than was maybe thought.

Bethany Ehlmann: Thanks, Mat. It's a pleasure to be back.

Mat Kaplan: Well, some of the science, I admit was beyond me. The paper reads like a great detective story and the illustrations are absolutely fascinating how used in large part, these orbital spacecraft, the data that was returned by them, the images to do a lot of this work. I really recommend that people take a look at it and we'll put that link and other relevant links up on this week's show page. Tell us about these deposits, which I guess we've known about for a while. Are we really talking about good old table salt?

Bethany Ehlmann: We are probably talking about large deposits of good old table salt on Mars. So the subject of our paper is these former salt lakes that were discovered in 2008 by the Mars Odyssey spacecraft, THEMIS instrument and Dr. [Mickey Osterloh 00:11:57] back when she was a graduate student actually discovered these chloride deposits. So chloride is NACL, just like the table salt you might be staring at if you're listening to this over breakfast or dinner. And these have been a puzzle since they were first discovered. And that's for a few reasons.

Bethany Ehlmann: They occur in irregular deposits. They're not well organized in layers. They're kind of in irregular depressions scattered across the surface of the southern highlands of Mars. They though usually aren't in the deepest depressions. They're usually in sort of shallow depressions. So this is has always been a bit of a head scratcher.

Mat Kaplan: I love that in the very first line of the abstract for the paper, it describes these deposits as enigmatic. That seems to be backed by what you just said.

Bethany Ehlmann: Enigmatic. I mean, they're enigmatic because of how they occur in depressions, in holes in the ground, colloquially but not the deepest holes. Also, they're enigmatic because they're spectroscopic signature is not pronounced as other minerals. They were actually discovered in the thermal infrared THEMIS data by virtue of their lack of a spectral signature. They imparted some distinctive properties that were messing up the correction algorithm that was typically employed because they were unusual.

Bethany Ehlmann: But they don't have, for example, we usually... I'm a spectroscopist. I love looking at light as a function of wavelength and using absorptions, fingerprints to identify minerals, but these chlorides don't actually have distinctive fingerprints. It's more like lack of a fingerprint, but very distinctive color properties and very distinctive emission properties in the thermal data. So these are funny enigmatic deposits.

Mat Kaplan: Would we find anywhere on Earth that has something similar to these?

Bethany Ehlmann: So great question. So we get these kind of deposits on Earth where typically large, but it doesn't have to be large where bodies of water evaporate leaving behind salt deposits. So some of the places, if any of your listeners have been to Death Valley.

Mat Kaplan: My hand is up.

Bethany Ehlmann: Yeah. Hands up, hands up. If anyone has been to Death Valley or some of the other great salt lake perhaps in Utah, these areas where there's lots of water coming in and lots of evaporation are areas where we get these huge salt deposits. There are also smaller areas on Earth where we get them. And we think that this is actually the better analogy for Mars.

Bethany Ehlmann: One other place that we get them is Antarctica. There's a famous lake in Antarctica, Don Juan Pond. That's one of the saltiest bodies of water on earth. And it's salty because it's fed by seasonal runoff from glaciers that picks up salts and other materials from the surrounding rocks before it flows into this little valley at the end of the glacier leading to this very salt rich chloride pond. And there's actually another one, just uphill from it. And we think this chain of salty lakes in Antarctica is probably the best analog for what we're seeing in these chloride deposits on Mars.

Mat Kaplan: I'm also thinking of a salty area that I'm unfortunately too familiar with, and that's our own salt and sea in Southern California, which has no outlet. But I believe I've read that it's incredibly saline because it has all this stuff running into it that the water then evaporates, right?

Bethany Ehlmann: That's right. That's right. It's that concentration of water flowing in. And then the water leaves via evaporation, but the salts don't leave. They're left behind as a sort of crust on the surface. And that was actually one of the other things that Ellen sleuthed as she was doing this for her PhD. She realized that because since the discovery in 2008, the Mars reconnaissance orbiter had continued to acquire data. The later orbiter Mars, reconnaissance orbiter with its HiRISE Camera and its context imager, CTX camera had continued to acquire color HiRISE enough imagery to make stereo pairs.

Bethany Ehlmann: So she was able to sleuth out exactly how thick the chlorides were in many of these deposits. And actually they're pretty thin like maybe a meter or two at most. They seem to be kind of like crusts, just like you'd get on salt crust, but on a slightly larger scale, the upper... You can't see this on the radio. I'm making motions with my hands that show the upper foot or two is relatively salt enriched, but not beneath. You could see little impact craters punching in that pulled up clean, typical Martian basaltic soil.

Mat Kaplan: I read also in the paper or somewhere that while some of these are in these shallow depressions that you mentioned, they're even some that are on slopes, which seems odd.

Bethany Ehlmann: Yeah. It's a little hard to describe on the radio. I mean, I need my doodle whiteboard or something. Okay, so I will ask your readers to imagine a very long scale slope over hundreds of kilometers. Say you're going vague kind of downhill, but there's lots of other little depressions, and mountains, and craters, and such. Something that's interesting is that if we went down to the lowest part of that basin, the lowest part of the low, typically doesn't have the chlorides. Where they are is typically the near the highs, but in the depressions that are closest to the highs.

Bethany Ehlmann: So if we're going down slope from somewhere high, some set of hills, and we hit the first set of depressions, they would probably have chloride. If we hit the second set of depressions, they would have chloride. By the time we get to the bottom, they would no longer have chloride. So these strained asymmetric, we called them perched deposits, because they were sort of perched in little depressions, but over a bigger slope.

Bethany Ehlmann: So we think these perch deposits came to be because the source of the water was in the hills that it was surface water running off, collecting salt from the upper few meters of soil and then depositing it in this sort of chain of lakes. So imagine these little chains of small lakes connected by streams to the hills of Mars, then evaporating over time. That seems to be what it is more so than not this kind of deep lake fed system, which would lead to thick deposits in the very bottom of the depression.

Bethany Ehlmann: So this was a huge find of Ellen's and it was just through very careful detective work of really paying attention to where these were. They were downstream from the hills.

Mat Kaplan: That paints a wonderful word picture of this. I think I actually have that image in my head now. I'm wondering though, I'm curious about what took it the next step? What happened to make you and Ellen begin to think that these deposits were worthy of even more investigation that they might be able to tell us some surprising things about Mars.

Bethany Ehlmann: Yeah. So one of the questions on Mars has always been okay, so we know there's water on Mars. That has been the great finding of the last 30 years of Mars exploration. But the real question is what kinds of environments were they? How long did they last? Where did they go? Could there have been life in these environments? So to take it to the next step, so now we know that flowing downhill was surface water at some point in time making these little streams and probably more than once because salt to concentrate it, you want to do this over and over again. Fill evaporate, fill evaporate, then you get some serious salt.

Bethany Ehlmann: So we knew that there were these trickles of water. So then the next question that you ask as a scientist is well, how long ago? How long did this last? How long ago? So that was where we took the investigation next. Time is tricky on another planet. On Earth, you take the sample into the lab and you do isotope dating. On a planet, what you do is you count craters. If you imagine a surface over time, the older it is, the more craters it has.

Bethany Ehlmann: Why? Because it's been sitting there for longer being impacted into by the flux of meteorites. So more craters is older. What we can do is making some extrapolations from the moon that are tricky. But we can come up with an estimate for age by counting the number of craters on the surface. And in this case, what we want to do is we actually want to count the number of craters that are under the chlorides. So the chloride deposits were not big enough to have enough area on them that we could confidently get good statistics for the chlorides.

Bethany Ehlmann: But we could count what was underneath them because the chlorides have to be younger than the stuff underneath them. Right? So we could get this oldest bound on the age. So we looked for deposits where we could do that. We couldn't do it on all of them, but what was very surprising was that for some of the deposits that we were able to do this on the answer was about a billion years younger than most of the Martian lakes. So the answer was that some of these were on terrains that were 2 billion years old.

Bethany Ehlmann: Now, that's still a long time ago, but that's almost half a billion to a billion years later than the materials that are being explored by Curiosity and Perseverance in the ancient Martian lakes right now. So what that says is that the duration of water flowing on the surface of Mars lasted longer than we had previously kind of thought.

Mat Kaplan: That's Bethany Ehlmann of Caltech. She'll tell us more about Martian water and provide an update on her Lunar Trailblazer mission when we return.

Jason Davis: Hi, I'm Jason Davis, editorial director for The Planetary Society. Did you know there are more than 20 planetary science missions exploring our Solar System? That means a lot of news happens in any given week. Here's how to keep up with it all. The Downlink is our new roundup of planetary exploration headlines. It connects you to the details when you want to dive deeper from Mercury to interstellar space. We'll catch you up on what you might have missed. That's The Downlink, every Friday at

Mat Kaplan: Something that I absolutely love in the paper was that you and Ellen credited the work of good old William or Bill Hartmann for this technique, the projection, I guess it's called of being able to use craters to approximately date something. Amazing to see that name coming up once again. And of course, a pioneer who is still at it, still doing science.

Bethany Ehlmann: Yeah. Bill is quite the pioneer in planetary science. I mean, I think he got his start, I mean, before I was born, really. I mean, Bill was working on the moon and on Mars, and on NASA's very earliest missions. His specialty throughout his career has been craters. Understanding how they form, but also understanding how the number, this density of craters changes over time.

Bethany Ehlmann: So building a lot of his initial work on the moon, building up these density function and then relating them to the Apollo samples that had come from these terrains. So we had that data from the lab of how old was this particular density of craters. Right? What was the correlation? And then he has worked to try to extend that to Mars with the appropriate scaling functions, for position in the Solar System to develop a chronology for the ancient cratering rate on Mars, as well as some of his more recent work has been on the more recent cratering rate on Mars to build up that curve so that we can relate age to crater density.

Mat Kaplan: There's something that just occurred to me. Are we better off using this technique on Mars because on the moon... I mean, there's no weathering of craters on the moon except by other meteor hits, I suppose.

Bethany Ehlmann: Yeah.

Mat Kaplan: But on Mars, you've got stuff blowing around all the time. There's lots of weathering.

Bethany Ehlmann: Yeah. On Mars, there's stuff going on, on Mars. There's weather on Mars. Weather creates weathering physical and chemical breaking down the rocks. I can't wait until we get some Mars samples back from the terrain to add a few more ages to calibrate this chronology. I have to say this is one aspect of science I take with a little grain of salt. We have these models. When you do them in age, pops out. It's a number. It's a very specific number. It even has air bars. But there are, I think systematic errors that we are not sure of like in how to scale the moon flux to the Mars flux. And the only way to test that is by getting samples from various terrains on Mars and age dating them.

Bethany Ehlmann: But the thing is, even if we're slightly wrong about the absolute timing, we are going to be right about the relative timing because the terrain that the chlorides are under is definitely way younger than the Curiosity and the Perseverance terrains, because there are fewer craters. Whether the correct numbers a billion years, I don't know. Maybe the correct number is 1 billion years. But we know for sure that it's significantly younger.

Bethany Ehlmann: I think that's important because that says that Mars' climate continued to be able to support surface water. My question is how young, because remember all that we were able to date was what's underneath the chlorides. We weren't able to date the chlorides itself. So all that means is they must be younger than. They could be quite young, in fact.

Mat Kaplan: Wow. I can't resist saying, okay, I'll take that salt with a grain of salt.

Bethany Ehlmann: Yes, take the salt with a grain of salt.

Mat Kaplan: If we're talking about roughly an additional billion years of flowing water on the surface of Mars, isn't that also another billion years for other interesting things that we think might happen when there's liquid water and energy?

Bethany Ehlmann: That's right. That's another billion years that there's potentially a habitable environment on Mars or at least intermittently so. I mean, as we look around on earth, Don Juan Pond in Antarctica, this incredibly salty pond in this mean annual temperature well below freezing environment is habited happily with the number of microorganisms as well as kind of larger microbial mat structures that grow from the surface.

Bethany Ehlmann: So I think as we think about writ large, did Mars have life? Does it hosts life? Really this question of how long did the water environments last? How did they come and go is so critical. I think it's an important climate piece of the puzzle as well because one of the biggest challenges in Mars science that has endured for decades is why is Mars able to host liquid water at all?

Bethany Ehlmann: It gets 40% of the amount of sunlight of Earth. The mean annual temperatures below freezing. There is some mechanism of production of greenhouse gases or the way that clouds work on Mars that we actually don't fully stand. Why could Mars sustain liquid water on its surface? It clearly did? Lakes like Jezero Lake and Gale Lake and these chloride ponds, we still haven't gotten all the pieces yet and that's why we need to keep exploring. We have never visited anything like these chloride ponds on Mars. They're a type of landing site that has so far been unexplored by rovers.

Mat Kaplan: I'm so glad that you brought up Curiosity and Perseverance now their locations, Gale and Jezero, which apparently these are quite different from, would you wish that we could send another Perseverance up there to one of these salt deposits someday and do some collecting?

Bethany Ehlmann: I think one of the amazing things from our orbital reconnaissance of Mars is that there are hundreds, if not thousands of sites to explore that record records of habitable environments, archives, where we would search for life. You realize how precious a rover is when you study the entire surface of the planet as I do and you see all of these interesting locations that we're discovering.

Bethany Ehlmann: I mean, ancient, salty lakes in an ancient chains of lakes. We've not been there. We've not been to the deposits at the bottom of Valles Marineris that look like they were formed by outflow channels and evaporating waters. We've never been to the pools of Mars. There are so many mysteries that remain at the surface. And as a geologist, I just think of all of the work over the last 100 years, unraveling the history of life on Earth.

Bethany Ehlmann: Dinosaurs, giant impacts, ancient ocean, fossil drilling creatures. We only know this because we've scoured the globe and looked at the rocks. That's where we are in Mars exploration. We're just at the beginning of scouring the Martian globe and looking at the rocks to really be able to read that history and answer those questions that we can't answer right now.

Mat Kaplan: Enigmatic salt deposits on a still extremely enigmatic world. It just beckons. It just [inaudible 00:29:09]. There's so much more for us to do out there.

Bethany Ehlmann: More to explore.

Mat Kaplan: Yeah. Tell me a little bit about your partner in this, in the paper, your former PhD student, Ellen Leask, who is now at the Applied Physics Lab. I understand Johns Hopkins University APL.

Bethany Ehlmann: Oh, Ellen did such an amazing job on this paper. And she was one of my most awesome grad students. Ellen came to Caltech from Canada where she had a very traditional education as a geologist, but she was intrigued by planets and wanted to study planets. So Ellen came to Caltech and she learned how to use remote sensing data, how to use orbital data to draw conclusions about planets. A while ago actually, I think it was... Let's see, when it was. At 2018, she led another important paper where she did some incredible sleuthing. Ellen is a born detective, did some incredible sleuthing to find out that a signature that had previously been in interpreted as perchloride on Mars was actually an artifact of the calibration pipeline. Meaning no large scale perchlorates on Mars, but the large scale chlorides are indeed real.

Bethany Ehlmann: And this is the study here. She also has another paper and work about a particular region on Mars, Terra Sirenum. We're going to tell the geologic history story of deep lakes with sulfate deposits kind of like Gale Crater, but then the shallow ponds later of chlorides. Another part of the planet we haven't explored. So Ellen is truly outstanding. And now she's at John's Hopkins doing a post-doc.

Mat Kaplan: We know much more now about these deposits and what they may mean for water on Mars. Where would you like to see the science go next with researching these deposits and just taking it further than you and Ellen already have?

Bethany Ehlmann: Yeah, that's a great question. I guess all science builds on the science before it. That is the nature of it. I think there are really three things. First of all, some other investigators should take a look at this to see if there are other areas that can be dated. Check out our analysis, right? Verification, check it out. See if you can take it further. So I throw down the gauntlet to other investigators to pick it up from there. I think the second thing, and I'm doing a little work on this, but I think there's much more work to be done.

Bethany Ehlmann: I'm working with my colleague, Tom McCollum at Colorado. The question is where did the chloride come from and how much chloride do you need to make the deposits that we're seeing? Where are the other salts? Why aren't we looking at sulfate and chloride deposits like we see sulfate salts from evaporation stuff in Gale Crater and in other places on Mars. What is it geochemically on Mars from the salt that lets these deposits be chloride rich and pretty exclusively chloride rich and other sulfate? What is that telling us about the water chemistry or about the processes that affected the water?

Bethany Ehlmann: That's a detailed geochemical question, but it's actually going to be important for pinning down the environment and what kind of waters would any potential Martian microbes have lived in? And then the third thing I'd love to do is I would love to see a future landed mission to these type of deposits. They're an unexplored environment, an unexplored ancient, potentially habitable environment on Mars. One of the many remaining to explore. So let's get down there on the ground and keep exploring.

Bethany Ehlmann: Now, we can't necessarily send a $2 billion Perseverance or Curiosity every time, but there's a lot of work going on now in the lunar field to bring down the cost of landed access to the surface, to standardized lands, to bring a few small rovers and thanks to having small science instruments. We could send some mobile explorers to check out these chlorides as a next step, and really get down into the details of these lake deposits. Are there organic minerals? How deep do they go? All of these questions remain.

Mat Kaplan: Maybe even a helicopter or two.

Bethany Ehlmann: Maybe a helicopter. That'd be fun. We can hop up and down that chain of lakes with the helicopter flying over the hills.

Mat Kaplan: Wouldn't that make for some great video? Speaking of lunar exploration and making it cheaper, you're not talking about landing, but give us an update on your orbiter, Lunar Trailblazer. What's the status?

Bethany Ehlmann: Well, pivoting planetary bodies to the other one that has been attracting my attention lately. In addition to water on Mars, I have been working hard with our team on water on the moon. So as Mat mentions, Lunar Trailblazer is a NASA small satellite mission. We're funded by the planetary science division and the exploration systems and science integration office that organizes NASA's lunar program. Lunar Trailblazer is a small satellite to map water on the moon that is water in ice deposits in the permanently shadowed regions.

Bethany Ehlmann: Where's the ice? How much is there? How much is at the surface and also the water, the sort of enigmatic H2O, OH that we see enriched in certain sun-lit, warm parts of the moon. That's maybe part of the rock, maybe comes from the solar wind, maybe is just H2O molecules bouncing around as a function of temperature.

Bethany Ehlmann: So Lunar Trailblazer is in the process of being built. The PowerPoint that we've had for two years are now turning into hardware and the hardware looks like the PowerPoint charts, which is really amazing to see what we've worked on so long, come together. Both of our instrument teams are in the middle of instrument integration and tests. So putting hardware together, we have a thermal vacuum chamber test in about two and a half weeks for the fully assembled spectrometer of our imaging spectrometer instrument, the high resolution volatiles and minerals moon mapper, HVM cubed.

Bethany Ehlmann: So it is really exciting to see this all come together and will be our spacecraft integration and test starts in the early part of the summer. And we will be done by November. So we will have a spacecraft ready to launch by the end of the calendar year. So getting to the moon and getting the maps of water there is the next thing.

Mat Kaplan: Very exciting. Since you mentioned one, what is the other instrument?

Bethany Ehlmann: Yeah, we have two instruments and we're a small, but mighty small satellite. We have two instruments that are nested with each other. The imaging spectrometer that I mentioned, HVM cubed and nested just atop of it is the lunar thermal mapper instrument from my colleagues at the university of Oxford. So one instrument from JPL, one instrument from the University of Oxford. The lunar thermal mapper is a multi-spectral thermal camera.

Bethany Ehlmann: So it will have four temperature channels to be able to sense what exactly the temperature is as we're measuring water with the other instrument. It also will have 11 compositional channels. So we'll be producing some of the maps of the rock composition on the moon in terms of its variability kind of building off what the diviner instrument has done before, but with a much higher spectral and spatial resolution.

Mat Kaplan: Great stuff. Still following the water.

Bethany Ehlmann: Still following the water. Following the water across the Solar System. That's kind of my theme.

Mat Kaplan: Before I let you go, I have to ask because I'm reading three different books right now, while I finished one of them. All of which talk about planetary science as this multidisciplinary discipline, which was largely invented at Caltech, your institution and other places. It's exciting to talk to planetary scientists like you because you can literally talk just about anything. You have to be able to do this work, right?

Bethany Ehlmann: This has been the singular most fun thing about being a professor at Caltech is just to continue to expand into new areas of knowledge and to continue to learn. So for me, coming with a geology background with a strong dose of math and computer science to do remote sensing, the thing that I've been driving into these past few years is sort of the systems engineering side of things in order to be able to execute these space missions, to realize the dreams of scientists. In order to answer the questions, we have to have the instruments, we have to have the spacecraft. So it's at that science engineering interface of really making it happen. How do you make the measurement then? How do you build the actual instruments to make the measurement that you are sure is going to work in space? That's what's been really exciting to do and to learn about over these last few years, working the rovers and now leading Lunar Trailblazer in its mission to the moon.

Mat Kaplan: Thank you, Bethany, for this conversation, for all this great work that is underway and best of continued success with it. Thanks also for finding time to serve as president of The Planetary Society.

Bethany Ehlmann: Always a pleasure. Keep exploring, and I'm happy to help enable everyone who's listening to the show to keep exploring.

Mat Kaplan: That's Bethany Ehlmann, professor of planetary science, also the associate director of Caltech's Keck Institute for Space Studies, and the principal investigator for that upcoming exploration of our own big satellite, the Lunar Trailblazer.

Bethany Ehlmann: Thank you, Mat.

Mat Kaplan: It is time again for What's Up on Planetary Radio. So we've got the chief scientist of The Planetary Society here. That's Bruce Betts. Welcome. I have a fun little opening message for you from a listener.

Bruce Betts: Oh, fun.

Mat Kaplan: It's sweet actually, and very rewarding. Melanie Podbielski from Edinburgh, Scotland-

Bruce Betts: Edinburgh.

Mat Kaplan: ... wrote to us. She says, "Hi, Mat and Bruce. Thanks as ever for your show. Planetary Radio has introduced me to so many fascinating scientists and topics. I just returned to university to study planetary science and you and your show were not insignificant in that choice.

Bruce Betts: We're not insignificant. Yay.

Mat Kaplan: It's always good to know. I know exactly what you mean.

Bruce Betts: No, that's very nice. Yes, congratulations.

Mat Kaplan: Best of luck to you in studies and we look forward to having you as a guest on the show someday. What's up?

Bruce Betts: Well, it's hard to compete with that, but I'll try in the predawn sky. Predawn is where it's happening over there in the east. Check out super bright Venus, and it is near much dimer, reddish Mars. And to their lower left, we've got Saturn. You look in yellowish and coming up higher, they're getting closer and closer as we move towards the end of March. So that's definitely the sky thing to check out. Evening sky, keeps saying it, but Orion beautiful over in the evening sky in the south. And you can find all sorts of other stuff. So enjoy the night sky. We move on to this week in space history. 1781, Mat, was a big year. William Herschel discovered Uranus. It seems significant.

Mat Kaplan: I'd say so. There aren't too many people who can say that.

Bruce Betts: We'll move forward a lot. In 2006, Mars Reconnaissance Orbiter arrived at Mars into orbit to do reconnaissance. And boy, has it done that. Still there, still doing great stuff.

Mat Kaplan: An amazing catalog of images that it has returned from. Both of it is still wonderful cameras. Hey, before you go on to Random Space Fact, I got another listener message for you. This one comes from Michael Lloyd in Texas. He says, "Mat, Bruce, I love everything you do. I love passing along your Random Space Facts to my kids. It makes me smile and feel smart. When out of the blue they ask, 'Daddy, Random Space Fact please.'"

Bruce Betts: That's so cool.

Mat Kaplan: Isn't that? I knew you'd like that.

Bruce Betts: I do. That makes me very happy. Well, here's another one, because we're going on to Random Space Fact. So we're talking escape velocity now, the velocity required in ignoring atmosphere and such to escape the gravity of an object. Crudely stated. And here's my old factoid or double factoid. The escape velocity for Jupiter from the top of the atmosphere, the one bar level is about five times that from the surface of the Earth. And Earth's escape velocity is about five times that of the surface of the moon.

Mat Kaplan: That's unique. We haven't done one about escape velocity before I don't think.

Bruce Betts: Well, I'm glad you liked escape velocity because I liked it a lot this week. We'll come back to that. But let's move on to the trivia question that I asked you to do some Messier math, referring to the Messier objects, the catalog of objects. I asked you to do the following problem. The number of objects published in Charles Messier's 1781 catalog times the Messier number of the Trifid Nebula minus the Messier number of the Starfish Cluster. And how do we do and what do we get?

Mat Kaplan: We got a huge response and a lot of first timers. Welcome to the contest those of you who had not visited or dropped in before. A pleasure to read all of your entries, including the one from our poet laureate, Dave Fairchild in Kansas. Here is the response he gave us. Okay, class. Please listen. And your math will Messier. Start with 103, and that's our catalog today. Multiply by Trifid, that is 20 in our skies. Minusing the Starfish gives the current year. Surprise.

Bruce Betts: Surprise. Wow. Nice way to turn that into a poem. Impressive.

Mat Kaplan: 2022, right?

Bruce Betts: 2022 is the answer. Oh, that is the current year, isn't it? What a coincidence.

Mat Kaplan: Glad to bring you up to date there, chief scientist. Lot of people came up with 2162 because they went with the current Messier catalog total of 110 objects rather than what you had specified, the 1781 original. You expected that didn't you?

Bruce Betts: I did. Well, yeah. I mean there are a number of objects in Messier catalog. You have to be a little more precise, and the other ones get kind of fuzzy. So, yes, that's why I specified in a seemingly, overly detailed way the 1781 catalog, which is a classic. I've got a first edition signed by Messier. No, I don't.

Mat Kaplan: You wish.

Bruce Betts: Yeah, I do.

Mat Kaplan: If you were one of those who came up with 2162, well, you can get some solace from the fact that you got the math right. Here is our winner, and he is a first time winner. Steve Sheridan, who comes from, well, it's a beach town in California where I used to actually hang out when I was a kid. That's where our mom would drop us off to go to the beach. I won't say which town. He added, "I very much appreciate what Mat, Bruce and the entire Planetary Society do for the space community. Per aspera ad astra." You said it, Steve. And we are going to send you a Planetary Society kick asteroid, rubber asteroid for your trouble.

Mat Kaplan: Thanks for entering. Galen Drennan in Ontario, Canada, he said, "This is evidence that even a simple math question isn't exempt from a bit of Bruce's classic cheek."

Bruce Betts: That is I. Some people call me the classic cheek. I asked him not to.

Mat Kaplan: And Kent [Merly 00:45:25] in Washington. He says, "Pluto was not completed even one orbit since Messier's catalog was first published with those 103 objects. Messier might have added an apparently non-moving Pluto to his list if his tech of the day had included mirrors instead of speculum metal, unobstructed tubes, swappable eye pieces, and photographic plates for a blink comparator. Messier forged on also without a thermos for hot cocoa. What will people one Pluto year from today wonder how we coped without?" I don't know. I think they'll still be enjoying hot cocoa.

Bruce Betts: Yeah. I mean, once you discover that it's not going away. That's what I figured.

Mat Kaplan: We're ready for another one of these.

Bruce Betts: Little more math, because people want it. Not as much. Approximately what is the ratio of the surface escape velocity from Mars compared to the surface escape velocity from Earth? Their ratio. Go to

Mat Kaplan: Okay, you mathematicians or arithmeticians, you have this time until the 16th, March 16th at 8:00 AM Pacific Time. And we have another Chop Shop Store prize for you. This week it is a terrific T-shirt. I'm looking at it right now. Very cool at It's a Better Know an Asteroid T-shirt that he has them for women and men. It says that right on the shirt and round a little asteroid that looks an awful lot like Bennu is OSIRIS-REx coming in for the kill. You like that too?

Bruce Betts: Well, I was a little disturbed by it, frankly, but okay.

Mat Kaplan: Anyway, like I said, you got until the 16th. We're done.

Bruce Betts: All right, everybody. Go out there, look up in the night sky and think about your favorite pattern of bark. Tree bark, dog bark, take your pick. Thank you and goodnight.

Mat Kaplan: He's Bruce Betts. He's never barking up the wrong tree. He's the chief scientist of The Planetary Society who joins us every week here on What's Up.

Mat Kaplan: Planetary Radio is produced by The Planetary Society in Pasadena, California. That is made possible by its members who finds solace and wonders of the universe. Mark Hilverda and Rae Paoletta are our associate producers this week. Josh Doyle composed our theme, which is arranged and performed by Pieter Schloser. Ad astra.