Planetary Radio • Sep 01, 2021

Liquid water under the Martian polar ice? Maybe not

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On This Episode

Jeff Plaut

Jeffrey Plaut

Mars Odyssey Project Scientist for Jet Propulsion Laboratory

Isaac smith

Isaac Smith

Assistant Professor of Earth and Space Science at York University in Toronto where he holds a Canada Research Chair in Planetary Science

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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

It was one of the most exciting planetary science announcements in 2018: Radar from an orbiting spacecraft might have found large pools of liquid water under the Martian south pole. But good science doesn’t end with first conclusions. Jeffrey Plaut and Isaac Smith are among the researchers who have found that a form of clay may better explain these reflections. We also talk with The Planetary Society’s Rae Paoletta about the Earth-like worlds found across our corner of the galaxy. Your chance to win the coveted Planetary Society rubber asteroid returns in this week’s What’s Up.

Martian south pole
Martian south pole The European Space Agency's Mars Express spacecraft captured the infrared, green, and blue-light images used to make pictures of the Martian south pole. A larger version can be found here.Image: ESA / G. Neukum (Freie Universitaet, Berlin) / Bill Dunford / Edited by The Planetary Society
Bright radar reflections at Mars' south polar cap
Bright radar reflections at Mars' south polar cap The colored dots represent sites where bright radar reflections have been spotted by ESA’s Mars Express orbiter at Mars’ south polar cap. Such reflections were previously interpreted as subsurface liquid water, but their prevalence and proximity to the frigid surface suggest they may be something else.Image: ESA/NASA/JPL-Caltech
ExoMars image of layered deposits at the south pole of Mars
ExoMars image of layered deposits at the south pole of Mars The ExoMars Trace Gas Orbiter captured this view of part of the south polar ice cap on Mars on 13 May 2018. The poles of Mars have huge ice caps that are similar to Earth’s polar caps in Greenland and Antarctica. These caps are composed primarily of water ice and were deposited in layers that contain varying amounts of dust. They are referred to as the martian Polar Layered Deposits (PLD). Thanks to massive canyons that dissect the layered deposits, orbiting spacecraft can view the layered internal structure. The ExoMars orbiter’s Colour and Stereo Surface Imaging System, CaSSIS, viewed this 7 x 38 km segment of icy layered deposits near the margin of the South PLD, which extend as far north as 73ºS. Here, CaSSIS has imaged remnant deposits within a crater at this margin. The beautiful variations in color and brightness of the layers are visible through the camera’s color filters. It highlights the bright ice and the redder sandy deposits toward the top of the image.Image: ESA / Roscosmos / CaSSIS
Clays in a cold laboratory
Clays in a cold laboratory Isaac Smith of Toronto’s York University bundled up while working in a lab, freezing smectite clays with liquid nitrogen to test how they respond to radar signals. The results have challenged the hypothesis that subsurface lakes can be found at Mars’ south pole.Image: York University/Craig Rezza

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Mat Kaplan: Liquid water lakes under the Martian south pole? Maybe not. This week on Planetary Radio. Welcome. I'm Mat Kaplan of The Planetary Society with more of the human adventure across our solar system and beyond. Mars Odyssey Project scientist, Jeff Plaut, is back. This time, joined by York UNiversity's Isaac Smith. They have contributed to a new explanation for those radar reflections from the Martian south pole that we... well, I hoped were big pools of water. Jeff and Isaac will tell us we're not yet sure they aren't, but our excitement back in 2018 may have been misplaced. We're also going to welcome back Planetary Society editor, Rae Paoletta. She has written a great article about the search for Earth-like exoplanets. There could be billions and billions of them in our own galaxy. And we'll wrap up by looking up with Planetary Society chief scientist, Bruce Betts.

Mat Kaplan: The 2021 Humans to Mars Summit from Explore Mars is now less than two weeks away. I now know that I'll be moderating three great discussions, including the climactic closing conversation that asks, why Mars? It's all online and all free. You can register at That link is on this week's episode page at It runs September 13 through 15. Really, all the best Martians will be there. What is that striking image atop the August 27 edition of The Downlink? It's a gift from the Mars Reconnaissance Orbiter. We're looking at sand dunes in the north polar region of the Red Planet. You can check out the surreal surface at Elsewhere in this edition, you'll read about a big milestone for the James Webb Space Telescope. With testing complete, it's ready for a trip through the Panama Canal for its launch from French Guiana as early as November. Mars rover, Perseverance, has made it to the citadel, a rocky outcrop where the robot will make its second attempt to collect a sample. And Blue Origin sent another New Shepard capsule above the Karman line. No passengers this time. Just a variety of experiments and thousands of postcards from students. Here's my colleague, Rae Paoletta.

Mat Kaplan: Rae, welcome back to the show and thank you for this great August 23rd article that people can find at It begins with 4000 or so and counting confirmed exoplanets. That is quite an accomplishment.

Rae Paoletta: It's amazing, right? It's even more amazing when you put it in the context of how recent this all is. I mean, the first exoplanets weren't' found until the '90s. That is extremely recent.

Mat Kaplan: As you know, I'm old. When I was a kid... I have said this before on the show. When I used to read astronomy books as a kid, as I did all the time, they usually said we will probably never see another star as anything more than a point of light, much less see a planet circling one of those stars. Well, I'm so glad those textbooks were dead wrong.

Rae Paoletta: Absolutely. Yeah, I couldn't agree more. Exoplanets in general. When I think about them, I always joke. I'm like, oh my gosh, they just give me the best kind of existential crisis because there's so much out there.

Mat Kaplan: I love that. And when you talk about a lot of them out there, you give a statistic that I had not seen. At least possibly one Earth-like planet for every five Sun-like stars in our galaxy. That's a lot of Earths.

Rae Paoletta: So many. Think about the ones that are waiting to be born. The ones that don't even exist yet.

Mat Kaplan: Or are just being born right now. Boy, that is an exciting thought. I was also interested to read... This came, I guess, from one of the astronomers that you talked to. Apparently, there's no category of worlds smaller than the one that our own Earth is part of. Does that mean that, when we find even something as small as Mercury, it's really classified as Earth-like? At least in some ways?

Rae Paoletta: Yeah, it's really interesting. We know that mass is one of the key deciding factors of what is or isn't considered an Earth-like world. But there's a lot more and it's kind of a murky definition, if you can even call it that. Both of the astrophysicists that I spoke to helped me enormously with this piece and both of them helped to explain how much of a spectrum this really is. That, yes, mass is important for when we're trying to understand what an Earth-like world is, but there's so many other checkpoints, benchmarks, whatever you want to call them.

Mat Kaplan: Is it in that famous Goldilocks zone? The habitable zone? It's certainly a limiting factor, right?

Rae Paoletta: Yeah. Habitability is definitely some part of that equation. Being in that Goldilocks zone. The not too hot, not too cold region where liquid water could theoretically pool. I think that that's what makes a candidate for an Earth-like world certainly more interesting.

Mat Kaplan: Well, let's talk about some of the candidates that you identify in this piece. Where do you want to start?

Rae Paoletta: There are so many that we could've talked about, but in this piece, I narrowed down there. Kepler-186f, Kepler-452b, and of course, TRAPPIST-1, which I'm personally very interested in.

Mat Kaplan: TRAPPIST-1. Now, that's a whole system of worlds, but you singled out one in particular, TRAPPIST-1e, as maybe being the most likely.

Rae Paoletta: Yeah, it's interesting. Because we don't really know enough yet about the atmospheres of the TRAPPIST planets. I think we're going to learn more, certainly, with something like JWST. Based on what we've got, it seems that TRAPPIST-1e may be the only planet in the system hospitable to life. That's because the other planets may have developed like Venus did, so they just became super hot and therefore not able to host water.

Mat Kaplan: The JWST. The James Webb Space Telescope. Just one of the tools that is almost here. Probably is going to do great things for this search, right?

Rae Paoletta: Oh my gosh. It's going to change everything. JWST is going to be one of the most powerful exoplanet hunters ever. And, yes, we're going to get more. We're going to get the Roman Space Telescope after and we're going to get awesome telescopes for sure. But JWST is going to help us search for biosignatures. So, gases associated with past or present life. We're going to really be able to look into the atmospheres of exoplanets like we've never been able to before.

Mat Kaplan: Rae, I love your closing line... a couple of sentences... in this piece. Do you have them in front of you?

Rae Paoletta: Yeah. The idea that this exactness could repeat itself doesn't subtract from how special Earth is. It amplifies the possibility that life can thrive somewhere else. It's an honor to be this common.

Mat Kaplan: I absolutely love that. Of course, we're talking here about those two big questions that our, boss, Bill Nye, likes to talk about all the time. Where do we come from and are we alone? Thanks, Rae, for bringing this to us. By the way, bon voyage. Have a wonderful delayed honeymoon celebration.

Rae Paoletta: Thank you so much, Mat. I'm really excited. I appreciate it.

Mat Kaplan: That's Rae Paoletta, my colleague at The Planetary Society. She is our editor. By the way, Rae wants to thank the two astrophysicists she mentioned. So, thank you, Moiya McTier, whose doctoral thesis was about exoplanet habitability, and Kaitlin Rasmussen, a postdoctoral research fellow at the University of Michigan.

Mat Kaplan: Is Mars Earth-like? Well, kind of. But once upon a time, it was much more like our own warm, wet world. Scientists have found lots of evidence for this past state of grace, including the water ice that lurks below the surface. But is there still liquid water somewhere on Mars? Remember the recurring slope lineae or RSLs? Those downhill dark cascades that appear quite suddenly are now thought to be just be newly exposed sand with no more than a tiny bit of moisture at best. Okay, but those radar pings bouncing off salty lakes under the south pole? Jeff Plaut of NASA's Jet Propulsion Lab and Isaac Smith of York University in Toronto are among the researchers who may have thrown a dry towel on this interpretation. Jeff joined us in November of last year when he and Rich Zurek helped us celebrate Mars Odyssey and the Mars Reconnaissance Orbiter, two of the oldest spacecrafts circling the Red Planet. Jeff is also co-principal investigator for the radar on the European Space Agency's Mars Express orbiter. The same radar that delivered those tantalizing data in 2018. And, as you'll hear, Isaac has been simulating the conditions under the ice in his lab.

Mat Kaplan: Jeff Plaut, Isaac Smith, thank you very much for joining us on Planetary Radio. Jeff, of course, welcoming you back. Isaac, very happy to have you make your first appearance on our show.

Isaac Smith: Thanks very much for having me. It's so exciting to talk to you.

Jeffrey Plaut: Yep, and I'm glad to be here as well.

Mat Kaplan: I'm going to warn everybody that, in case we hear a small child, a baby crying, there's good reason for that. Isaac is still fairly new dad and so there may be some fatherly advice offered during the program as well talking about what is going on at the poles of the Red Planet. Guys, I told you what I put in my newsletter when I was letting people know about this conversation coming up. The line I used was, "Disappointing, yes, but each new data point takes us closer to understanding the true nature and potential of our solar system." But it is kind of disappointing. I mean, you had to come up with this data, right?

Isaac Smith: It is disappointing. It wasn't my intention or my team's intention to play spoiler to the fun. It just happens that we think there's a better explanation for this than liquid water. It doesn't mean that there's no liquid water on Mars. We just... I think we haven't found it yet. So, we're going to keep looking. That's part of the goals of these missions. Again, we always want to get closer to the truth. Understanding our solar system, understanding each planet, is really the goal here. Flashy headlines is really nice, but it doesn't mean that we haven't gotten closer. So, the goal is that.

Mat Kaplan: Jeff, I saw a quote from you that said pretty much the same thing. That we are often inching our way toward the truth in planetary science. This seems like a great example.

Jeffrey Plaut: Yeah. I think so. And I think Isaac and I are of a like mind on this. That the presentation of hypotheses, whether they're mundane or extraordinary... That's how science works is that you put these things on the table and you study them carefully. You present your evidence and sometimes somebody else might come along and have a different idea. It's not proven that either interpretation is the correct one. At least not yet. We use this term frequently in planetary sciences. We're putting constraints around the problem. We're tying to narrow the possibilities of the solution space. I think maybe that's what's happening here.

Mat Kaplan: Before we get into the details of what actually may have been found by the two of you and many other researchers, or at least where the evidence seems to be pointing now, what will it take to really answer this question? Are we going to have to go someday and drill down through the ice at the pole?

Isaac Smith: I think that would be a fantastic mission. I'd love to get core samples of the ice. And as long as you're getting cores, going to get the bottom would be really very interesting. So, that's something we can think of very long term. I don't think it's going to happen in the next decade or two decades, unfortunately.

Mat Kaplan: Yeah. Yeah. Doubtful. Doubtful. But would that be the way? Will we not really have the answer until we can dig down like that, Jeff?

Jeffrey Plaut: Well, you're asking for the ultimate truth. The answer. And I'm not sure if we ever get there. Certainly, yeah. Without sampling and actually touching the water, you can never be sure that there's water there. There are some mission proposals out there to visit these terrains and to look at the ice that's exposed near the surface and maybe drill down some modest depth, but not kilometers to where these reflective patches are. But I think that would take us a long way towards understanding what materials might be responsible.

Mat Kaplan: Let's back up now. Back to 2018. Jeff, we'll stick with you because you are the co-principal investigator for this instrument on Mars Express called the MARSIS. The Mars Advanced Radar for Subsurface and Ionospheric Sounding. The other co-principal investigator, your colleague, Roberto Orosei... I hope I'm pronouncing that correctly. He's the one who first used this radar and saw these interesting reflections, right?

Jeffrey Plaut: Well, yes. The thing that was new was not so much that these strong reflections were detected, but it was their analysis and their interpretation. Also, they did utilize the radar, the MARSIS radar, in a slightly different way than we had up until that time by using a higher resolution mode. They were able to identify the boundaries of some of these reflective patches to a much greater detail than we had seen before. But I should say that we have seen these high reflectivity areas going all the way back to the beginning of the MARSIS experiment back in 2005.

Mat Kaplan: Huh. This really does... From this point on, after that announcement was made and people like me became so excited, it really is a great story of how science works. How it is supposed to work. Because wasn't it not long after that that others... like you, I assume, Isaac... started to work wonder, hmm, could there be other explanations for what we seem to be seeing? For this interpretation, anyway.

Isaac Smith: That's absolutely true. Almost immediately, the scientific community became skeptical. The first paper that came out was in the same year, questioning can we even get to a temperature that would allow the ice to melt? How much salt would be required? Earth and Mars have heat coming out, but from what we know, Mars doesn't produce enough heat, so it would have to be a local heat source. It really made the whole story more complicated than just, "We found liquid water." From that, the community kept thinking on it. Chewing on it. In 2020, in January, we had a meeting in Patagonia in South America. I was one of the main organizers for that. The question came up. What are we going to do about this? Are we going to test it? How are we going to test it? Who's going to test it? And so we've been thinking about it as a community for all those years.

Mat Kaplan: I read about that conference. Seventh International Conference on Mars Polar Science and Exploration in Tierra del Fuego. Interesting place to hold a conference. You attended that, didn't you, Jeff?

Jeffrey Plaut: Oh yeah. I was there. It was my last overseas trip prior to the pandemic and quite memorable for that and for many other reasons. We had a great exchange there of ideas on this topic and many other topics in Mars polar science. This conference, which Isaac has been instrumental in organizing recently, occurs every few years going back maybe 20 years or more. Some of us have been there through all of it. We keep on getting fresh bodies and new blood into the community and it's great because it's a really lively kind of community where we can bounce ideas off of each other and really stimulate people to go and find out some exciting new stuff, like this.

Mat Kaplan: Jeff, was it after that meeting or were you already beginning to use MARSIS and do even more radar sounding of these polar regions? I read about work that you did with a doctoral student.

Jeffrey Plaut: Yeah, so the recent publication was by a graduate student, Adi Khuller. He's at Arizona State University. He was doing some internships with us at JPL and that allowed us to collaborate on this. Yeah, I've been looking at this problem in several different ways for many years. One of the new tools that we have at our disposal just recently was to take all of the data that MARSIS had collected over the south pole and also the north pole and compile it all into, basically, a three dimensional image. A three dimensional image volume of the polar mountain, basically. The polar mound. This allowed us to map everywhere. Basically, we have such good coverage in the polar regions because it's a polar orbiting spacecraft that this compilation of literally thousands of different observations allowed us to make very detailed, complete maps of this boundary at the base of the ice where it's in contact with the rocky Mars surface. That's the target that we're all interested in for this study.

Mat Kaplan: What did you find in the data that made you think, gee, this may not be liquid water?

Jeffrey Plaut: Well, we didn't really take a position on the water question. We were really in the data collection mode and reporting on the sum total of the MARSIS observations rather than just picking and choosing small, little bits. What we found was that there were literally dozens of areas at the base of the ice where the reflective signatures in the radar are comparable to what was reported as potentially liquid water. Not only were there many locations, but many of these locations were at very shallow depths near the edge of the icecap, where we know the surface temperatures are extremely low. If you are only going a few hundred meters below that extremely cold surface temperature, you really can't find a temperature that's warm enough to even keep very salty water liquid.

Mat Kaplan: So, even with the perchlorates and other stuff up there, which would act like antifreeze, it just wouldn't make sense to see liquid?

Jeffrey Plaut: That's right. In these particular locations, you have to invoke a few more extraordinary processes like some kind of thermally insulating blanket of material or something like that. Things that we don't really have any evidence of.

Mat Kaplan: While you were attacking this from that direction, other researchers were coming at it in other ways. Isaac, I think of what you were doing in your lab. There is a great photo of you all bundled up because you're working with liquid nitrogen and I guess it made the room pretty cold up there in Toronto. Tell us about what you were after as a possible explanation of this if it wasn't liquid water. If it isn't liquid water.

Isaac Smith: I didn't go into it looking for another explanation, actually. Like so many things in science, I was looking at something else and that inspired me on this topic. I was working with a graduate student named Craig Rezza. Craig was looking at clays at Valles Marineris. That's the largest canyon in the solar system. It's on Mars. We were looking at reflections there. In order to study those, we were looking at the reflections in the lab, using some equipment we have. The reflections we found in that equipment with these clays was very bright. He was able to extract the dielectric properties. That's the numbers that we use to characterize these materials. He showed them to me and I said, "That's exactly in the range that we'd expect to find to answer this question on lakes on Mars."

Mat Kaplan: Fascinating.

Isaac Smith: Yeah. So often, science works that way. You think about something else and it gives you an idea. That happened in January and, by March, we had submitted the paper. The story is that there's several types of clays on Mars. One of the clays is found over a large part of the surface. Curiosity has found it and has drilled into it. We can see it from orbit with spectrometers. And so it's not a stretch of the imagination to say that there's clays on Mars. We see them. We can see them with our cameras. The story is that we have this type of clay that exists on Earth and on Mars. We can measure it in a lab and see, oh, the numbers here are really high. That makes sense, then, so that we would model it. Model it and we find out that the reflection strength we would get, which is what we measured with the radar, is exactly in the ballpark, within various tight constraints, of what you'd expect to see. What we see in the lab matches very closely to what we do see.

Isaac Smith: And then the last part of it... I was working with a colleague at Purdue University, Briony Horgan. I asked her, "Briony, can you help me find smectites there?" And she found them right on the edge the south pole. Just sticking out there. It's perfect. It's a one, two, three. We showed that this can do it. We do it in the lab. We measure it. And then she goes and finds it. This really great compilation.

Mat Kaplan: I don't want the let the type of clay that you just named go by because it's just a fun name. Smectites. I assume that there are many types of clay, but this one fit the bill.

Isaac Smith: It sure did. A smectite is a type of clay that hasn't been evolved fully. There's different grades of clay and this is one of the earlier grades coming out of volcanic rocks. You see them in places on Earth like Alaska and Costa Rica, where you have volcanoes near the water. Coming from the volcanic rock, it breaks down a little bit and it forms this clay. If you break it down further, you get a different type of clay. And you can break it down even farther. At some point, you'll end up with the kind of clay you'd use in pottery. But this isn't that type. It's closer to the volcanic rock. It's a smectite, but there are many types of clays, even that we see on Mars. And so it was really cool that we see this at the right place. We can measure it. It just answers those questions so nicely.

Mat Kaplan: Jeff Plaut, Isaac Smith, and I are not done with Mars. Also ahead are visits to the moons of Jupiter. All just a minute or so away here on Planetary Radio.

Bruce Betts: Hi again, everyone. It's Bruce. Many of you know that I'm the program manager for The Planetary Society's LightSail program. LightSail 2 made history with its launch and deployment in 2019 and it's still sailing. It will soon be featured in the Smithsonian's new Futures exhibition. Your support made this happen. LightSail still has much to teach us. Will you help us sail on into our extended mission? Your gift will sustain daily operations and help us inform future solar sailing missions like NASA's NEA Scout. When you give today, your contribution will be matched up to $25000 by a generous Society member. Plus, when you give $100 or more, we will send you the official LightSail 2 extended mission patch to wear with pride. Make your contribution to science and history at planetary dot org slash S-A-I-L-O-N. That's Thanks.

Mat Kaplan: I just come back again, Jeff, to this being a great example of exactly how science is supposed to work.

Jeffrey Plaut: Yep. I think we can all agree on that. The ramification of that is the story is not over. Our friends, mostly on the Italian side of the MARSIS experiment, are working very hard, I understand, at analyzing these new results that Isaac and others have published. They will be trying to fold that into their analysis and we'll see what the response is. I think that that kind of give and take... That's very healthy in our science discipline.

Isaac Smith: And so I look forward to that discussion.

Mat Kaplan: Let me ask the two of you as we maybe look away from the polar region and what is hiding underneath that ice at other things that are exciting you. I mean, we learn more and more about just how dynamic and how diverse a place Mars is. That story seems to be far from over. I think of, Jeff, how Mars Odyssey... Going on what? 21 years now? That venerable spacecraft... is still unveiling the planet to us.

Jeffrey Plaut: Well, there's a couple of stories there. One is just the remarkable engineering feat of keeping a robot like that alive and working for over 20 years. It's been over 20 years since we launched Mars Odyssey. In just a few months will be the 20th anniversary of arrival at Mars. Not only the spacecraft, but the science instruments. In particular, the camera system, THEMIS, is working exactly the same as it did when it rolled out of the development lab. I mean, it's working perfectly. And the same goes for MARSIS. MARSIS has been in space since 2003 and shows absolutely no signs of degradation or wear. So, hats off to these engineers who built these instruments. On the science side, we are very demanding of them. We tell them what we want and all the incredible things we want these instruments to do. We don't usually say, oh and by the way, can you make it last 20 years? But that's what they did. So, yeah. Quite remarkable.

Mat Kaplan: Isaac, as somebody who I'm sure eagerly awaits every bit of new data from spacecraft like Mars Odyssey and the rest of that flotilla that we have at the Red Planet, what are you most excited about right now? I know that you study ices and surfaces all over the place.

Isaac Smith: I do. My main focus is on Mars and I'm really interested in surface-atmosphere interactions, especially with ice, and the poles are really great places to study that. They're actually really great analogs to much of the solar system in that places as far out as Tritan, one of Neptune's moons, and Pluto have surface-atmosphere interactions with ice. There might be evidence for bed forms like sand dunes or ripples. There might be evidence... There's definitely evidence on Tritan for seasonal processes. There's these jets that come out, I think, even kilometers from the surface. Just like we see on Mars to a smaller extent. Mars is a great analog for all these things that are happening in the solar system. It's a dry place, so it's a better analog than Earth is. It's got two different ices, carbon dioxide and water ice, that behave these ways. They go from solid to gas with no liquid phase. So, I'm very excited about thinking how Mars can teach us about the rest of the solar system. I'm always looking at pictures of Mars. Every day. I have a great group of students who I work with who are looking at pictures and modeling and doing wonderful things. It's a really fun field to be in and it's great to see all the discoveries that come out all the time.

Mat Kaplan: Jeff, what are you most excited about as we continue to study the Red Planet?

Jeffrey Plaut: Well, right now, I am following very closely the Perseverance sample collection mission. I don't know about... if we'll ever get those samples back to the Earth. That's the plan. But it's got such a powerful suite of instruments that I think we're going to learn some amazing things through the course of that mission. It was very exciting to see the first drill core acquisition turn out in a way that nobody expected, which was basically it vaporized under the drill. It just shows you that there's all kinds of surprises looking out there. But I think that mission is really going to tell us a lot about this question about was Mars habitable? Did it, in fact, host life forms? Is it possible it still does today?

Mat Kaplan: I sure hope that... I know you do too... that we get those samples back some day. But it's going to be a long time. Curiosity is still going strong as well, of course. Before we go, Jeff, I want you to have a chance to talk about some other stuff you have going on with some other worlds around our solar system. Specifically, tell us RIME and REASON.

Jeffrey Plaut: RIME and REASON, yeah. They're like a brother and sister. They're two more radars. Not at all unlike MARSIS. MARSIS also has its sibling, SHARAD. Those are the two radar sounders that are currently in orbit around Mars. Really, thanks to the spectacular success of those two experiments, space agencies around the world... not only NASA, but also European space agencies... were very eager to send similar instruments to the moons of Jupiter. These are the icy satellites, the Galilean moons. In particular, Europa and Ganymede. The instrument RIME is flying on ESA's JUICE mission and will focus primarily on Ganymede and the instrument REASON is flying on NASA's Europa Clipper mission focusing on Europa. In both cases, we expect to see all kinds of things below the icy surface of these moons. We know they're predominantly water ice and, in the case of Europa, there's a lot of good evidence that the liquid water that is in the interior of Europa occasionally punches through and reaches the surface and there may be pockets of liquid water at shallow depths. Those are some very exciting programs that are going to be taking place later on this decade.

Mat Kaplan: Can't wait. And as I mentioned, both of you, just our previous show last week, we talked with Al Cangahuala about the progress of Europa Clipper toward its 2024 launch. Isaac, I'm sure all this sounds great to you. You must be looking forward to getting data from these new spacecraft.

Isaac Smith: Absolutely. Radar is a great way to study ice and look for water, as we all know, and so it's going to be fascinating to see the results. We've never seen anything like that on an icy moon before. Can't wait.

Mat Kaplan: Isaac, do you have other work coming up in your lab at York to follow up on what you've already found?

Isaac Smith: We do. Some of the experiments we did, we did at low temperature, but not quite as low as we'd expect at the south pole of Mars. And so the goal is to upgrade the current chamber we have that studies ice at the south pole. Upgrade it so that we can run these experiments in there. Currently, we don't have all of the fittings that we need to go between the walls. It's really cold and really low pressure and so we have to upgrade to these fittings. It shouldn't take too long. But the goal would be to go to even lower temperatures than we did before and measure the radar properties of these clays in order to continue this discussion that we're having between us and the other team who thinks there might be water.

Mat Kaplan: Well, stay warm.

Isaac Smith: Okay, yeah. Thanks. Yeah, you saw that picture. I had scarf and gloves and hat and sweater and everything on. It's cold in the lab. Especially when you're running nitrogen through your hands.

Mat Kaplan: Isaac, I'm a little disappointed. Here we are at the end of the conversation. We haven't heard from the baby.

Isaac Smith: Yeah, the baby's in the room right next door. I think his mom is doing a really good job of helping him fall asleep.

Mat Kaplan: Well, extend our gratitude to both of them and maybe we'll get to meet your youngin another time. Thank you both. Isaac Smith and Jeff Plaut. Keep up the great work. Even when it's disappointing. We know there are many more surprises to come.

Isaac Smith: Thanks very much for having me. I really appreciate it.

Jeffrey Plaut: Yep. It was a lot of fun to talk to you guys. Thank you.

Mat Kaplan: Jeff Plaut is the senior research scientist at JPL. The project scientist for the Mars Odyssey orbiter and the co-principal investigator on the Mars Express MARSIS radar, among other things. Isaac Smith is an assistant professor at York University, where he holds Canada research chair in planetary science. Time for What's Up on Planetary Radio. Here is the chief scientist at The Planetary Society. That's Bruce Betts. Well, he's here, but I want to read you something first. This is from Russell Vernon in California. "Great show. Even the jokes are mostly funny."

Bruce Betts: Thanks? I think. That's probably an accurate assessment. I'll give you that one.

Mat Kaplan: I use that only as a sample. There are so many people who are saying lovely things about the show and I am so far behind in responding to those lovely people that this is my way of apologizing. But, yeah, Russell. We'll take it. Thank you very much. We'll take the night sky, too.

Bruce Betts: Nice segue. So, we've got Venus looking super bright as always. Low in the west in the early evening. If you check out Venus on the ninth of September, the crescent moon will be nearby. It will make a lovely view. And then about a week later... A week later, on the 16th, it will be... the moon, no longer a crescent, will be hanging out near Saturn, and the 17th, will be hanging out near Jupiter. Both of which are high in the east in the early evening.

Bruce Betts: We move on to This Week in Space History. It was this week that, in 1976, which... carry the three... 45 years ago... Viking 2 successfully landed on Mars, following Viking 1's landing a few weeks earlier. The following year, this week, Voyager 1 launched into space.

Mat Kaplan: Still trucking on. I wasn't there for the Viking 2 landing, but I was there for Viking 1. One of the greatest days in my life. For a lot of other people as well.

Bruce Betts: You were hanging out at JPL, interviewing people, way back when.

Mat Kaplan: I was. For my college radio station. My two buddies and I were standing with Ray Bradbury among other people.

Bruce Betts: No wonder it was the greatest day of your life.

Mat Kaplan: Well, that was good, but I saw Ray pretty frequently, actually.

Bruce Betts: Yeah, that's true.

Mat Kaplan: It was this landing on Mars thing that kind of got us excited.

Bruce Betts: Well, yeah. It doesn't happen just every day. Particularly in 1976. It was the first one. Successful. I like it. You know what else I like? Random space facts!

Mat Kaplan: I guess you do like it.

Bruce Betts: I do. Of course, most of the planets in our solar system, in English, are referred to by as Roman god names. In Greek, modern Greek, in Greece, the planets are called by the Greek god counterparts. For example, Neptune is called Poseidon.

Mat Kaplan: Why didn't I know that? I should've known that. So, Jupiter is Zeus?

Bruce Betts: Yes. Jupiter is Zeus. It's pronounced somewhat differently that I can't recreate, but it is Zeus.

Mat Kaplan: Dr. Zeus?

Bruce Betts: Yes. They've given him a Ph.D. Or is it an M.D.? I always get confused. I think Zeus can have whatever he wants. And Saturn is Cronos. Uranus is just confusing because it's a Greek version of a Roman... It's a Roman version of a Greek... Anyway. Uranus is a mess. But yeah. And Mars is Aris. Venus is Aphrodite. I think we've tidied them all up now. All right. We move on, then, to the trivia contest. I asked you how many orbits of Saturn did the Cassini spacecraft complete?

Mat Kaplan: Here is an answer from our poet laureate, Dave Fairchild, in Kansas. Cassini circled Saturn in an orbit underscored by going around a lot of times. I think 294. [inaudible 00:37:02] and Titan got their data all derived and, in the end, Cassini took that final deep, deep dive.

Bruce Betts: Nice.

Mat Kaplan: The poet says 294. Is that what you were looking for?

Bruce Betts: That is what I was looking for. Now, as some listeners pointed out, you could make different assumptions and maybe edge it to 293, so I'll accept either. I was looking for the official JPL value of 294.

Mat Kaplan: Almost everybody responded with that number. There were a few who gave us the 293 as well, some of whom said, "Hey, I found it on your website." Well, okay. We'll take care of that. But at least it's only off by one. And it is, as Bruce said, open to interpretation. Here's our winner. A first-time winner but he has entered many, many times. Christopher Lowe. Christopher, you are going to be the first to receive those Chop Shop store robotic spacecraft posters... the poster, I should say. Singular... of your choice. You can choose from among a group including the three new posters for Juno, Pioneer, and Viking that are currently in development. That Kickstarter campaign, if you're catching this show early, is still underway at We thank Thomas Romer, the guy behind Chop Shop, for allowing us to give these away this time around. He's also the guy who comes up with the Planetary Radio t-shirts and other stuff we give away now and then because all The Planetary Society swag is at

Bruce Betts: I find it surprising, actually, that it's only 294 orbits considering it orbited Saturn for roughly 13 years. They were long orbits as they figured out all the clever, clever orbits they used to study different latitudes of Saturn and different moons and et cetera, et cetera.

Mat Kaplan: A number of people pointed out the same thing. They were surprised that it was so few. But they certainly got a lot out of every one of them. Here's more. Mel Powell in California. "What a coincidence! I think 294 is also the number of times has not picked me."

Bruce Betts: Could be.

Mat Kaplan: "Still grateful for the one time it did," he says. [inaudible 00:39:23] Zimmer in Germany. "That's also roughly the number of curses Cassini spewed when it finally realized what was going on."

Bruce Betts: Fortunately, spacecraft, so far, are not sentient. Or alive.

Mat Kaplan: Yeah. They're working on that. Laura Dodd in California. "453048 images across those two decades in space." 13 years at Saturn, as you said. But she says that if you extrapolate from one of her two-week vacations, she beats it. She just wishes her photos could be as glorious. Finally, this from another poet. [Gene Lewin 00:40:02] up in Washington. "Some differences of opinion exist. 293 or 94. Cassini traveled around and around, completing its Saturn tour. Much like we folks we see at the mall circling around the parking lot, about 7.9 billion clicks to finally find a parking spot." I like that one, too. Clever. We're ready for another one of these.

Bruce Betts: All right. As of the day this episode comes out, September 1st, 2021, how many spacecraft are docked or visiting the International Space Station? We're not counting CubeSats that are hanging out on board. We're not counting the International Space Station. We're counting anything that carries cargo or crew back and forth to the space station. How many spacecraft are docked at the Space Station as of September 1st? Go to

Mat Kaplan: Thank you for that clarification and for that question. You have until September 8th... Wednesday, September 8th at 8:00 Pacific Time... to get us the answer for this one and win yourself a CubeSat. No.

Bruce Betts: No! No!

Mat Kaplan: We're giving them away. How about a rubber asteroid? That is to say a Planetary Society Kick Asteroid r-r-r-rubber asteroid. Somebody asked me why we don't roll the R in asteroid either. You want to try it?

Bruce Betts: Yeah. R-r-r-rubber aster-r-r-oid.

Mat Kaplan: That sounds very cosmopolitan somehow. Anyway, that'll be yours if you're the one chosen by With that, we are done for yet another week.

Bruce Betts: All right, everybody. Go out there. Look up at the night sky and think of your favorite wor-r-rd to roll your R's in. Thank you and goodnight.

Mat Kaplan: I'm r-r-r-really very grateful to Dr. Bruce Betts, the chief scientist of The Planetary Society... darn, no R's in there... who joins us every week here for What's Up?

Bruce Betts: R-r-r-r-r.

Mat Kaplan: Planetary Radio is produced by The Planetary Society in Pasadena, California and is made possibly by it's members. R-r-roll on up to them at Mark Hilverda and Jason Davis are our associate producers. Josh Doyle composed our theme, which is arranged and performed by Pieter Schlosser. Ad astra.