Planetary Radio • Feb 08, 2023

More on Mars’ Watery History

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Tanya Harrison

Mars Expert, Geologist, and Fellow of the Outer Space Institute

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Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society

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Sarah Al-Ahmed

Planetary Radio Host and Producer for The Planetary Society

Mars expert Tanya Harrison shares the details on some of the newest discoveries about Mars’ history, including the discovery of an impact crater thought to be linked to a megatsunami in Mars’ ancient ocean and the discovery of opals, a water-rich gem, in Gale Crater. Stick around for What’s Up as we let you know what to look for this week in the night sky.

Sunset on Mars from Viking Lander 1
Sunset on Mars from Viking Lander 1 This beautiful image of the Viking 1 Lander site was captured about 15 minutes before sunset in Chryse Planitia. The surface is littered with boulders. The diffuse shadows are caused by the sunlight scattered through the dusty atmosphere.Image: NASA/JPL-Caltech
Opal halos around cracks in Gale Crater on Mars
Opal halos around cracks in Gale Crater on Mars This image taken by NASA’s Curiosity Mars Rover in 2015 shows “halos,” pale material around fractures in the bedrock in Gale Crater. The halos are comprised of silica and opal, a water-rich silica-based gem. The presence of opal on Mars suggests long-standing wet conditions in the crater’s past.Image: NASA/JPL-Caltech

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Sarah Al-Ahmed: Uncovering the mysteries of Mars' wet history, this week on Planetary Radio. I'm Sarah Al-Ahmed of The Planetary Society, with more of the human adventure across our solar system and beyond. From fascinating new evidence of possible ancient megatsunamis on Mars, to the discovery of opals in Gale Crater, we're about to delve into some of the latest findings about Mars' watery past with our guest this week. Mars expert Tanya Harrison. Bruce Betts will join us to share more on what's in the night sky in What's Up, and a special Mars Rover prize in this week's space trivia contest. There's some fun news from the Jovian system. According to the Minor Planet Center, Jupiter now has 12 more confirmed moons, bringing the number of known moons orbiting the largest planet in our solar system to 92. That means that Jupiter has finally taken the lead over Saturn's 83 known moons... For now. The James Webb Space Telescope or JWST continues to capture mind-blowing data on objects both near and far, including observations of Chariklo, a 250 kilometer or 160 mile in diameter icy body beyond the orbit of Saturn. When Chariklo passed in front of a star, the telescope was able to detect its rings. The rings were first observed by ground-based telescopes in 2013, and it's thrilling to get even more details on the ring system around this tiny planetary body. JWST also captured images of a developing star system around a red dwarf star 32 light years away called AU Mic. The Space Telescope was able to show the debris disc around the star in two different wavelengths of infrared light. AU Mic already has two known planets, and the disc is the result of collisions between the remaining planet forming materials in the system. You can find these JWST images, info on these stories, and more, in the February 3rd edition of The Downlink, The Planetary Society's weekly newsletter. You can read it or subscribe to have it sent to your inbox for free every Friday at Discovering the history of water on Mars has been a captivating journey for scientists. Over the years, various missions and experiments have helped build a more complete picture of the planet's watery past. From early observations of ice on the surface to more recent findings of subsurface ice, and even the potential for flowing water, each new discovery has added to our understanding of the role that water has played on Mars. In 2016, a NASA funded research project showed that two potential megatsunami events in Mars's watery past could have played a role in forming Martian coastal terrain. It was an exciting finding that was followed up by a recent paper published in December in the Journal of Scientific Reports, saying that a newly discovered impact crater on Mars called Pohl may have caused one of these two megatsunami events. It's estimated that Pohl Crater was created when an asteroid about three to nine kilometers or 1.9 to 5.6 miles across slammed into Mars 3.4 billion years ago. If these findings are correct, the impact created a megatsunami 250 meters tall, that's 820 feet, that flung water and debris up to 1,500 kilometers, 932 miles, from the impact site. Elsewhere on Mars, NASA's Curiosity Rover has discovered opal, a water-rich mineral, in Gale Crater. Our guest to explain these new findings is Dr. Tanya Harrison. She's a Mars expert, geologist and fellow at the Outer Space Institute. She's worked on multiple NASA missions to Mars including Opportunity, Curiosity, Mars Reconnaissance Orbiter, and Perseverance. Hi Tanya, thanks for joining me on Planetary Radio.

Tanya Harrison: Thanks so much for having me.

Sarah Al-Ahmed: I only just recently learned that you were once an intern at The Planetary Society in the past. What was that like?

Tanya Harrison: It was great. It was opportune timing. I was literally sitting in a lab during grad school talking to somebody about how I wasn't sure how I was going to pay my tuition, and I see a post from Emily Lakdawalla popup that says, "I'm looking for an intern," and I immediately messaged her. I was like, "I need a job. I'll do whatever you want," and it worked out perfectly. So I always credit Emily with basically saving my PhD.

Sarah Al-Ahmed: That's so nice to hear. I too, when I first started getting into science communication, like Emily Lakdawalla was one of those persons that I always wanted to emulate, and you used to post Planetary Radio to our website, right?

Tanya Harrison: Yeah. That was one of my weekly tasks. That, and a lot of editing of the blog posts to remove the academies, as it were. Like if professors wrote guest posts, and I fully credit that for honing science communication skills because it was really good practice to sit there and think, okay, if an educated but non-expert wanted to read this, what words would you use to convey this same idea? And so that was another really, really valuable thing that I got out of the internship that I greatly appreciate the chance to have had.

Sarah Al-Ahmed: It's so funny how that process of learning how to say the same thing with just more accessible language can bring in a whole new audience. I've had to work on that over the past, so it's nice to hear. There are just so many new discoveries going on on Mars right now, particularly about Mars' watery past. So I'm really glad that I have you on to talk about this. I've been following your career for ages, and it's wonderful to finally meet you.

Tanya Harrison: Oh, thanks so much.

Sarah Al-Ahmed: Ages and ages ago, it seems, humans used to look at Mars and they used to imagine that maybe it was a watery world, and of course we got there and we realized not so much. It's a dusty ball of sand and rock, but that makes it all the more exciting to me that as we explore more and we learn more, it turns out that there's a wealth of evidence that Mars used to be covered in oceans, and lakes, and rivers of water. I'm glad to have you here to put it all in context, because the journey from how we started to learn about this to where we are now is just mind blowing, given how short the time is. Back in 1976, NASA's Viking I lander touchdown on Mars. And scientists looked at those images, and it was just a little baffling at the time because they found this terrain that was just littered with boulders, and I don't think that's what they expected. Why was that so surprising?

Tanya Harrison: So they chose the landing site for that lander based on looking at images from orbit that showed that this should have been the mouth of a gigantic outflow channel. We're talking volumes of water flowing through carving this channel larger than anything we've seen on the Earth. They expected to land and see all of this evidence of flowing liquid water and instead they ended up in this boulder field, and not full of boulders that look like they were smoothed by water like you would expect. They're very angular, very jagged, probably the kind of thing that would've been shot at all over the surface by giant impacts, which is probably not surprising. Mars is covered in craters, but they didn't find that evidence of all of the water that maybe should have been there, or we would've guessed would be there based on all of these outflow channels emptying into that area where the lander landed.

Sarah Al-Ahmed: This goes back to the imagery that we've gotten from space of Mars. When we looked at it with Mars Global Surveyor and Mars Odyssey and Mars Reconnaissance Orbiter, which you worked on, the Martian coastlines just weren't what we expected, and there was all this evidence of just a strange history going on on Mars. What did we expect to see from space on these coastlines, and why did the data throw everyone for a loop?

Tanya Harrison: I feel like the shorelines have been this will they won't they kind of thing since the Viking era, basically. It was really easy to look at Mars at the global scale and say you have all these huge outflow channels, they flow into the northern plains, which is a lower elevation than the rest of the planet, generally speaking. So it just made sense to think, well, this water emptied out, it had to have pooled there and formed this ocean. But then when you looked along that, what we call the dichotomy boundary between the Northern and the Southern Hemisphere, where you might expect the edges of that ocean to be, at Viking resolutions, there wasn't really a ton of evidence for shorelines. There might be a couple scientists that would argue with that, but in general, it was not widely accepted in the community that there was indeed this ocean that took over the whole Northern Hemisphere. Then moving forward with Mars Global Surveyor, you get this huge increase in image resolution. With Viking, the highest resolution images are generally something like 10-ish meters per pixel, maybe some that are a little bit higher than that. Mars Global Surveyor got us down into the one to six meter resolution range, and so they not only mapped the whole planet at a lower resolution, but they were getting these postage stamp views along where we thought the shoreline should be. And again, the features that you would expect to see because we know what shorelines look like thanks to having them all over the Earth from our oceans here, we weren't seeing any morphological evidence of that, and so it was really confusing. We said, "Okay, well where did all this water go then if you had these channels that are emptying into this large basin?" Mars Reconnaissance Orbiter got us down to sub-meter resolution imagery so we could get an even closer look at these areas and we still weren't seeing a ton of widespread evidence for shorelines. There were some ideas that maybe there were some places that we saw deformed evidence of shorelines, and maybe that's why we didn't pick it up before. There was a few groups proposing that maybe there were these megatsunamis based on some data from Mars Reconnaissance Orbiter. Again, a little bit controversial depending on how you look at the images, and the way that they interpreted them. So we're still left at this point where, well, morphologically it looks like there could have been an ocean there. Climate-wise, in terms of climate models, there could have been an ocean there in the distant past. But the observations and the models and everything aren't really coming together to cleanly say, yes, there was an ocean here, it lasted for X amount of time. Where are all the sediments that we should be seeing if there was an ocean? Where is that shoreline? It's still a little bit muddy, no pun intended.

Sarah Al-Ahmed: Yeah, I remember back in 2016 reading those first articles about potentially there being megatsunamis on Mars and being baffled by that. Because as you said, Mars might have had water there in the past, but a megatsunami evokes this idea of just a massive, massive wave, so much bigger than we would expect here in modern times on Earth. I remember reading that it was like they thought there were two separate tsunamis that had happened. How did the data lead them to that conclusion?

Tanya Harrison: I think it was because of the shape of the things that they had mapped out as tsunami deposits. It wasn't really consistent with all of that coming from a single point source, so they've been trying to track down with their research what craters might have been the initiation points for these tsunamis. And so they think that they've tracked it down based on backtracking where those deposits came from.

Sarah Al-Ahmed: I read that recently too, that they think that one of the two tsunamis, the older one, they think they found the crater that it came out of, which is Pohl Crater, which is northeast of where Viking I landed, but it's quite a distance. It's almost a thousand kilometers. They talk about these boulders from the original Viking discovery in the paper that they wrote about this subject, and I'm wondering, is their idea that this megatsunami potentially literally carried boulders over there, or this blast launched these boulders in that direction?

Tanya Harrison: It sounds like their proposal was that the waves generated from the impact could transport a lot of this material. This is where it gets to be a little bit controversial, in that there are folks that have tried to model what would happen if an impact occurred into an ocean here on Earth. Galen Gisler, I apologize if I'm pronouncing his last name incorrectly, I've only ever seen it written, but he's been doing a lot of models to see, would you actually get a tsunami if you had an asteroid enter the water? The models that he's ran for years say no. The way that the physics works with the shockwaves that you get from the interaction of this asteroid coming through the atmosphere and entering the water are not the same as the dynamics that you get when you actually have something like the movement of the sea floor, which is generally what triggers a tsunami. When you have something like an earthquake or a volcanic eruption, you actually have something that is displacing an entire column of water in the ocean, and that means that it can propagate through the ocean way more efficiently and much farther than something like a point source, an impact like an asteroid entering the water. That's more like throwing a pebble in the water. You see ripples come away from that, but they dissipate very quickly, and so we might be talking apples and oranges in terms of how you actually get these things dispersed. And one big difference here though is that if these models are being run at Los Alamos for the Earth, obviously our oceans are incredibly deep. On Mars, looking at these tsunami models, it looks like their assumption is that this asteroid, the impact would actually make it through the water column and impact into the sea floor, and that's how it's transferring a lot of this momentum into the water, and generating these waves, maybe throwing stuff along the sea floor, also being transferred by these waves to be littered across the surface. That's where you'd really have to dig into the inputs of the models, and that gets a little bit beyond my area of expertise, but I would be really curious to see a comparison of the inputs and the outputs of both of those models side by side. If you took the model from Earth and then adjusted it for things like Martian gravity, Martian ocean depths, Martian temperatures, any of that stuff, would you get something that is reasonable with, or lines up really well with, the interpretations of these Martian tsunami papers?

Sarah Al-Ahmed: What I've read is that these impacts, if they happened, were about 3.4 and 3 billion years ago, and I'm wondering about what was occurring on Mars at that time with water. My understanding is that a lot of the water on Mars when it existed was already starting to dry up at that point, so what ocean was it that they smashed into?

Tanya Harrison: There've been two major oceans proposed on Mars. There's a Noachian ocean. The history of Mars is basically split into three geologic eras, the Noachian, the Hesperian, and the Amazonian. Much easier to remember than the plethora of geologic eras that we have here on the Earth, so that's a very convenient. So the Noachian ocean we're not really talking about in this particular model that would've like three and a half, 4 billion years ago, when Mars was much warmer and wetter, theoretically, with its thicker atmosphere. When you look at this second ocean, what we call the late Hesperian ocean, this is when Mars is starting to turn into this more cold polar desert, that's a lot more akin to what we see today. And so there's been a lot of controversy over whether, well one, did that ocean exist? Two, if it did exist, was it around for very long? Was it actually an ocean that was capped by ice? Was it a transient ocean where you'd have a little bit of water and then you'd go through this temperature fluctuation and it would freeze over, and then maybe some of it actually is still retained underground in the form of ice or water that's actually bound up in minerals? There are two separate ideas of a huge time period in between this ancient, ancient ocean on Mars, and this slightly less ancient ocean on Mars.

Sarah Al-Ahmed: Yeah, I know I've read some people think that maybe there were underground aquifers that burst open and let all this water out into Mars, and so it was more of a shallow ocean, which in my brain makes a little more sense how that asteroid that potentially created this megatsunami actually made it all the way to the ocean floor. Maybe it wasn't as deep as say oceans here on Earth. You would never make it to the bottom.

Tanya Harrison: Right. Especially if you have these little shallow transient oceans, it might be something that's more when you're talking about the scale of the whole Northern Hemisphere or at least the scale of Chryse Planitia, like the little basin where Viking landed and where you see all of these outflow channels emptying into, it's almost a mud puddle. There's a lot of debate over how deep that ocean might have been, and they did look at different parameters there when they were running their models, but that would definitely have an impact, again, no pun intended, on how your shockwaves would propagate in a situation like this.

Sarah Al-Ahmed: You were talking about the idea that some of this water on Mars could have been bound up in minerals, and that brings me to another discovery that was announced recently that literally made me double take, which was that Curiosity, the Curiosity Rover on Mars may have found evidence of opals in Gale Crater. How did that happen? How did it find opals on Mars?

Tanya Harrison: I'm glad the Curiosity is back in the news. I feel like anytime you get a shiny new rover, the older rovers take a little bit of a backseat, but it's still trucking along in Gale Crater, and still making really important discoveries like this. This opal that Curiosity found is concentrated in what we call veins in the rocks, so they are these little cracks where you tend to have minerals that will form. You can see this if you look at rocks in places on the Earth. If you see lighter colored stuff or darker colored stuff moving throughout a rock, you can pick it up pretty easily. This is something where usually you have water flowing through these cracks, and then you'll have stuff that was in that water precipitate out, or that water will interact with stuff in the rocks that it's moving through, and it can cause stuff to crystallize in there. And so you had water moving through these cracks in Gale and creating this silica. Usually on Earth when we see this, it tends to be in hydrothermal settings, so you have some kind of volcanic activity that's heating water as it's moving through these cracks. That's obviously a big deal for Mars, because water plus heat on Earth is a really good environment for life, so maybe this is something where, when the silica was forming, it could have been a habitable environment. We know that Gale Crater used to be a lake. It was probably around for tens of thousands to maybe even hundreds of thousands of years. If it was warm, plus we know it had all the chemical components that life needs to survive, maybe this was a great little place for little microbes to be hanging out three and a half billion years ago. If we look at it separate from the idea of past life on Mars, opal's also a really great resource for maybe future humans on Mars, because it has water in its mineral structure, and it's easy to, in the grand scheme of things, easy to remove that water. And so it could be potentially a resource for humans living on Mars if we want them to be at these lower latitudes like Gale. Gale's pretty close to the equator. Usually when we're talking about things like buried ice on Mars or even surface ice, you're at latitudes that are not at all hospitable for humans to have an extended presence. The weather in the higher latitudes is really bad, and it's very, very cold up there. But if you're at the equator in summer, you can get temperatures akin to a nice sunny day here on the Earth, so we would love to keep people at the lower latitudes if possible, but you got to find water for them to survive. So maybe we'll find more of this opal in other places that we can harvest for the astronauts.

Sarah Al-Ahmed: It hurts me inside just thinking about them crushing up precious Mars opals to get water, but I mean any water on Mars is amazing and it's already very difficult for to conceive of a way to land something at the Poles, let alone humans, so I guess that's a good option. How much water could you actually get out of crushed opals?

Tanya Harrison: I'm not sure what the volume yield would be. I imagine it's not an incredibly efficient ratio and we're not seeing massive quantities of silica in these veins, so you would have to find some pretty sizable deposits if you were going to try to do this for a city size of human presence there. We have found silica in other places before. Spirit was actually the first surface mission to find silica by accident. When one of the wheels broke, and it was dragging its wheel behind it, it dug this trench in the very soft dirt, and it exposed this bright white material just buried deeply enough underneath the top layer of more red, dusty stuff that we didn't see it before the wheel broke, and turned the rover around, pointed it, discovered that it was opal and silica. So there could be a lot more of this that's not just in these veins, but is buried on Mars, and we just haven't been able to see it, because we are only able to see what's exposed at the surface, and the very few places that we've managed to dig through, or drill holes, or brush off some dust with little brushes.

Sarah Al-Ahmed: I mean that's an interesting idea that maybe just under the surface there's all this water just trapped up in minerals. We're only finding it in this situation because there are these fractured cracks, and these networks of cracks around along the bottom of this crater floor. How did those cracks form? Was it part of the lake drying out, or was it some other process?

Tanya Harrison: You see a lot of cracks in these sedimentary rocks, so the rocks that Curiosity's been driving over are these layered sediments that were left behind as the lake in Gale Crater was evaporating away. You could also get cracks from things like impact events, but we're probably not looking at those kind of cracks in this situation. So as the water's evaporating, that also helps to concentrate it in these little veins in some cases, so these might have been places where there was water that persisted for a longer period of time than overall lake setting. When the rest of it was a mud flat, you might have still had some little bits of liquid water hanging out in these veins.

Sarah Al-Ahmed: That's cool to think about, because if there was once life on Mars and it migrated into these wetter places as the planet dried out, who knows what might be captured in those deposits and those cracks? I mean, I'm not saying we're going to find that mosquito in amber situation, but I cannot wait until we get some samples of that. That's just going to be wild.

Tanya Harrison: Yeah, that's going to be so exciting.

Sarah Al-Ahmed: Right? I know, too, that from the images that Curiosity has taken of these cracks, the reason they suspected there was something weird going on here was that you've got these cracks in the ground, and then around it there's this lighter material, these, they're calling them halos. When Curiosity looked closer, it realized this is opal, but it had been finding evidence of this all along the way. So why is it that now we're hearing that it's found opals on Mars, but it took us a while to get to this conclusion?

Tanya Harrison: That's a good question. I'm not sure why they weren't able to get... It might have been something where they just didn't get close enough to be able to take a measurement. I know that this discovery they made with the DAN instrument, which is a neutron spectrometer. We've been using this on Mars, we've had a different type of this in orbit. There's one on Curiosity specifically to look for these hydrated minerals, so it might have been something where they just needed to be able to get in a good position to be able to use DAN, because I believe it's mounted under the belly of the rover, if I recall correctly. I worked on the cameras, and I feel like you get hyper-focused on the thing that you have to work on, because you don't have a lot of time and energy to pay attention to the other instruments on board, unless it's coming down to resource allocation. Who wants to use this number of watts in this time slot on a given day? But I would imagine that that was probably a huge constraint in why they weren't able to necessarily do this analysis. Or just getting enough data points to be able to definitively say, okay, yes, we know this is silica, and we can extrapolate based on we know what it looks like here, so we can say that it's probably the stuff we also saw back here that we didn't take measurements of.

Sarah Al-Ahmed: Do you think these Martian opals are in any way visually similar to what we find here on Earth? When I think of opals, I think of these beautiful fire opals from Australia and stuff like that. Do you think they're going to look the same or is it going to be very different because it's from a different planet?

Tanya Harrison: I would love to think that they would look as beautiful as the ones we find on the Earth. Generally, if you walk up and pull a hunk of opal out of a cave wall in Australia or something like that, it's obviously not going to look exactly like what you might have in a ring setting, or a polished tumbled version at a gem show, but you do see that iridescent look to it. It's a very distinctive thing about opal, and you'll peep it in different spots, but you can also have opal that doesn't have that flash in it, so it's much less visually appealing. I would imagine since these are such small deposits in these little veins, you probably wouldn't see that, unfortunately, but I would really love to be wrong because it would be pretty cool to have a ring made out of Martian opal someday.

Sarah Al-Ahmed: Right. I'm sure out there right now is someone writing that sci-fi book about the person that goes to Mars and starts their business on Mars from scratch, just mining out that Martian opal. I would love to have a ring or a necklace or something. That would be amazing. We'll be right back with the rest of our interview with Tanya Harrison, after a short message from Ambre Trujillo.

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Sarah Al-Ahmed: As a geologist, are there any other kinds of gemstones you think might be on Mars that we just haven't discovered yet?

Tanya Harrison: That's a good question. A lot of the more interesting things that we tend to get on Earth in terms of gemstones, you have to have processes that don't necessarily occur on Mars. We don't have plate tectonics, so you don't get a lot of granites and certain types of metamorphic rocks. You would probably find some really beautiful peridot here and there, because you have lots of olivine on Mars, so that could be pretty cool. That's my birthstone, so another option, that would be much more likely than finding really nice opal in these veins. That's probably the most widespread, but if you're talking about something like would we find diamonds on Mars? Probably not. We've had a hard time finding much carbon on Mars at all. This was another one of these great mysteries in terms of the idea of an ocean, was that if you had an ocean, you should have carbonates everywhere. We have not found very many carbonates on Mars compared to what we expected, from thinking that there might have been an ocean across the whole Northern Hemisphere. So unfortunately, meteorologically speaking, Mars is not super exciting and doesn't appear to be super diverse compared to the things you would find on the Earth.

Sarah Al-Ahmed: That's all the more reason why these opals are so special, but that actually reminds me, we do have some evidence that both Curiosity and Perseverance have potentially found some more organic compounds, and organic compounds just by definition take a lot of carbon chains. And is that why that's so exciting? Because finding those organic compounds doesn't necessarily tell us that there was life on Mars in the past. It just tells us that the components for creating that life were there, which is special considering how little carbon we're finding on that planet.

Tanya Harrison: Exactly, and I think people hear carbon or they hear organic material and they immediately equate that with life. Obviously it's not the case. We found organic material on meteorites, for example. You find the building blocks of life in these places, so that's important. If we didn't find any organic material on Mars anywhere, that would definitely be one strike against Mars ever having had life in the past. But there's a lot of issues with trying to find the carbon in the first place because you have such a harsh radiation environment at the surface of Mars that it would break down a lot of organic material. So one of the things that scientists look for when we're picking landing sites for these rovers tends to be not only ancient areas where life would've been more likely to have been hanging around, compared to a surface that's maybe a 10 million year old lava flow, probably no bacteria there, but you also want something that is recently exposed. So if you had a surface that was four billion years old, but it's been sitting out on the surface of Mars for four billion years, you've probably destroyed any organic material that was there. But if you have something like Gale Crater was buried and then exhumed, again, another huge mystery, there's gigantic portions of Mars that were completely buried and then exhumed. Some places you wouldn't even know that this had happened, except you have little bits here and there of some of the craters that are still partially buried. Or in the case of Gale, the mountain of this sedimentary material in the crater is taller than the crater itself, so it had to have been completely filled, buried, and then exhumed to the point that the mountain looks like it does today.

Sarah Al-Ahmed: What could possibly exhume a mountain? Was it just the winds on Mars eroding away all of the dust around it? How does that happen?

Tanya Harrison: Yeah, it's probably just wind, I guess you can't see my air quotes here. We don't have a lot of places on Earth where wind is the only source of erosion. Even in deserts, there is some amount of rain, generally speaking, so we don't really have a good analog on Earth for what does a surface look like if the only thing acting on it for the vast majority is wind? Maybe it is really efficient at doing that, and especially with a crater, if you have a mountain in the middle, you're creating this little vortex of wind that's skirting around the edge of it. So it promotes the formation of these layered mounts inside craters, and we see these in lots of places around Gale and over in Arabia Terra on Mars. That's the primo spot of these buried and exhumed craters that are either partly buried or partly filled with this layered stuff. But the amount of material that you have to transport in this process is mind boggling. I mean, Gale is something like what, 220 kilometers in diameter, and five and a half kilometers deep. That's a lot of sediment. I live in Seattle, so I usually make the comparison that Mount Sharp and Gale Crater is taller than Mount Rainier. And if you look at Mount Rainier and you're like, if that was all layered sediments in a crater, that's really hard to grasp. So yeah, how did all that get deposited? How was it eroded away, and then where did it go? The where did it go is the other major question. That's a lot of stuff to transport out of these craters. I think it's a less exciting question than where was the water and what happened to it? So I don't think it's as much... It definitely doesn't get as much attention.

Sarah Al-Ahmed: I bet you all of that mountain is just blowing around in those giant dust storms.

Tanya Harrison: That's really the source of all of that dust. It was just all Gale Crater.

Sarah Al-Ahmed: It was all Gale Crater. And just imagine what might be buried in those sediments. Every time I see these images of just the sedimentary layers, or even more recently with the images from Perseverance, you can see clearly these sedimentary layers as it's climbing up this river delta. And it's so strange and a little melancholy in my heart to think that there's just all of this old evidence of this watery world just left behind, and all of these things just trapped in these rock layers that we just can't get to yet. It makes me feel happy that Earth is so special and watery, but how many of these watery worlds lose their oceans over time? It's weird to think about.

Tanya Harrison: We're just poking at the surface of all of these places. We can only take tiny cores with Perseverance. We're going to find things like that buried evidence of life preserved in these layers of the delta, or preserved in the layers of Gale Crater. My hope and my fear is that it's there, but we just can't get to it with what the rovers are able to do. So I think as soon as we send the first human mission that is able to drill cores like we do in Greenland or Antarctica, I think it's going to completely revolutionize our understanding of Mars. We're going to realize a lot of stuff we thought before was wrong. We're just going to get so much more information that we just didn't have capability to do, and this is really where the rovers are wonderful. The orbiters are really wonderful too. I feel like a lot of times the orbiters don't get enough credit for what they have contributed to the science, because it's more exciting for the general public to talk about the rovers. But a lot of what we've learned really at the global scale comes from the satellites, and then the rovers help fill in more granular information in terms of, okay, we know from space that there was a lake here, now from the ground, what was the chemical composition of that lake? What was the temperature? How long did that lake last? So it's a really synergistic relationship, but it can only do so much, and we can't replace the ability of humans, yet, at least, with the robots, so I really, really can't wait. I feel like even just the first human that steps off a spaceship and picks up one rock, can turn it 360, look at it and then look around them at the context of their landscape, literally within the first five minutes of being on the surface of Mars, they're going to be like, "Oh man, we got X, Y, Z all wrong." I think it will change that quickly.

Sarah Al-Ahmed: I bet that's true. It makes me feel a little uncomfortable thinking that we have to send a person there, and stuff from Earth that just might bring a whole new just biome of all these little bacteria and stuff along with it. We'll still, if there was life on Mars, be able to find some stuff in the rocks, but I worry about contaminating the world when we're trying to find evidence of life. That could complicate things.

Tanya Harrison: Definitely. Humans are inherently dirty, and I think no matter what you try to do, once there's... We can sterilize the robots, but once you send a human, planetary protection is not entirely out the door, but there's only so much you can do. We have a lot of stuff that lives in us and on us.

Sarah Al-Ahmed: Yeah. But thankfully we have some plans to actually bring some of these samples back to earth and test them, which is interesting that we're even at that phase of Martian exploration. Just the idea of going to another world. I know we did it with the moon, but this is a whole different thing. As you said, it's not like we have humans there picking up the samples and bringing them home. These are robots we're controlling from afar, pulling samples out of Mars and sending them back home. Gosh, what do you think we're going to find in those samples?

Tanya Harrison: I think those samples will be that first step in that, oh man, we had X, Y, Z all wrong, because there's going to be so much we can do in laboratories here on Earth that we just can't miniaturize everything to do it on Mars. You have stuff that needs really large equipment. You need some stuff that uses consumable reagents and stuff like that, that we can't send to Mars. So I'm hoping that we are able to answer some really critical questions. The other thing is that these are the first samples that we're going to have where we know where they came from. We have meteorites from Mars, and so those have been extremely valuable in that we know that they have hydrated minerals in them. We've found some of these building blocks of life in these meteorites, but we have no idea where on Mars they came from. So it's like the equivalent of if I just walked outside here, picked up a random rock and handed it to you and said, tell me the entire history of Earth from this one rock, which is something planetary scientists tend to do. They're like, I'm going to look at this one thing, and from that tell you the history of the whole planet. We would never try to do that on the Earth, so I don't know why we try to do that on Mars. It makes no sense, but now we're going to have samples where we know this came from this particular layer of the delta in Jezero, or this particular rock X meters away from the delta, and so we'll actually be able to firmly put together this story in a way that we've never been able to do before.

Sarah Al-Ahmed: That's really exciting. I wish we didn't have to wait so long for all these samples to come back.

Tanya Harrison: I know.

Sarah Al-Ahmed: I'm so happy we're here, but at the same time, I don't have the patience. I need those samples yesterday.

Tanya Harrison: I feel for the people that have been working on both the science and the engineering side of this since... I mean initially sample return was going to be on Curiosity and Curiosity was going to launch in 2009, and all of it was going to be with Curiosity, the sample caching... I don't know if the return to Earth was ever a single mission or if that was going to come later, but a lot of this was supposed to be done in a single mission. And then they realized, oh, this is really complicated. We should probably break this up so that it's not, don't have all your eggs in one basket. So this is 20 plus years in the making, and people devote their entire careers to working on this stuff. I cannot wait for those people that have been working on this since they were in grad school or something like that to get those samples back, and hold them in their hands. We did it. We actually did it.

Sarah Al-Ahmed: Do you think they're actually going to let people physically hold these things in their hands, or are they going to be behind several layers of glass with these giant gloves?

Tanya Harrison: That's true, I guess in their hands in a figurative sense, but separated by many layers of protection.

Sarah Al-Ahmed: Yeah. Even just to be in the same room with one of those, I've touched a Martian meteorite before, and it's cool just being that close to another world. Just the saga of all of these different Mars robots and where it's led us to. I would cry. I would cry being in a room with one of those Mars rocks. I definitely would.

Tanya Harrison: I think that's fair. I think there'll be a lot of tears when they come back, tears of joy.

Sarah Al-Ahmed: There's just so much left to learn about this world, and I'm glad that we're at this point. We know so much more than we used to, but there is just decades of discovery awaiting us ahead. What are you working on now? What are you most excited about what we're learning on Mars?

Tanya Harrison: Oh gosh. I just love in general how every new piece of data that we get from Mars shows us that it's more complicated than we realized. It's been this roller coaster of, we thought before we got there that Mars was this maybe lush planet with aliens that had built this complex canal network to bring water from the poles down to the deserts at the equator, and water their farms. And then we got there and the first images we got from early Mariner missions, it just looked like the moon. We didn't fly past any of the cool stuff like the fluvial features, the channels. We didn't fly over any of the volcanoes, so it felt like this major letdown like, oh man, it's just another moon that just happens to be red out there. And then finally with all these missions and their increasing camera resolutions, and adding new instruments on board, and then being able to land more precisely. To be able to land inside of craters and have mobility systems where we can drive for... I mean, Curiosity's been on the surface for more than 10 years now, and it's still driving. I mean, I guess that's not quite as impressive as Opportunity yet, but it's still cool. We just keep finding out that we thought Mars was really complicated, and then we thought it was really simple because we got our hearts broken, and we didn't have all the information, and now it's like, oh. I don't think we fully appreciated that Mars is probably just as complex as the Earth was, in a very different way. We've had two very different histories on these worlds, but just because Mars doesn't have life, and doesn't have oceans and rivers, and all that stuff today, doesn't mean that it wasn't vibrant and complex in its own way as its own planet.

Sarah Al-Ahmed: If only we had a time machine, what that would do for geology.

Tanya Harrison: That would be so amazing.

Sarah Al-Ahmed: Where would you go? If you could go into the past on Mars, what would be the first place you'd go to?

Tanya Harrison: Oh man. This feels cheesy, I guess, but I think I would go to Gale when the lake was there. It would be very cool to stand on the shores of this massive lake, seeing the mountain popping out in the middle, Crater Lake style, and look down and see, was there anything there? Are they fish popping out? Is there kelp floating around, or something like that? I think that's the most heartbreaking thing of all of this, is even if we put together the pieces, it's like, man, it would've been really cool to see it when it was there.

Sarah Al-Ahmed: It's true. All this information just underscores how precious our planet is, and all the things that we should do to protect Earth while it's still beautiful and vibrant and has all those things. And you can always go to a beautiful Crater Lake here on planet Earth and just think about Mars in the past.

Tanya Harrison: There you go. It's almost as cool.

Sarah Al-Ahmed: Almost as cool. Well, thanks Tanya for sharing so much about the history of Mars and its watery past. There is so much left to learn, and I hope in the future you'll come back on the show when we learn even more things. Maybe when Mars Sample Return brings all those samples back.

Tanya Harrison: That would be amazing. I'll talk to you in 2030.

Sarah Al-Ahmed: Awesome. We'll put it on the calendar. All right. Thanks so much, Tanya.

Tanya Harrison: Thanks, Sarah.

Sarah Al-Ahmed: It was an absolute pleasure having Tanya Harrison on Planetary Radio. The idea of a thriving, watery Mars is captivating, and I can't wait to learn what other secrets the red planet has in store for us. And now it's time to turn our attention to the night sky with our favorite segment, What's Up with Bruce Betts. Hi, Bruce.

Bruce Betts: Hi Sarah. How are you doing?

Sarah Al-Ahmed: Doing pretty good. And I'm sure after that last interview with Tanya Harrison, people are going to want to know how they can go outside and look up at the sky, and hopefully see Mars. So is Mars up this week? I mean, what's up in the night sky?

Bruce Betts: Beautiful segue. Mars is beautiful and easy. I like to start you with super bright Venus. I mean it's Venus, but it's really, really bright, and it's low in the west in the early evening. And then you go up to Jupiter up above it, and they're getting closer together and Jupiter's still very bright. And then you follow a line, follow a line to way up in the sky for most latitudes, and then you'll see a reddish star, and another reddish star nearby. Mars is still the brighter one compared to Aldebaran and Taurus, so that's how you can see Mars. We've got a comet up that is tough to see, at least unless you're at a really dark site or have some nice binoculars or telescope, then it's really pretty. But Comet ZTF C 2022 E3 is going to be between Mars and Capella, so you got Mars looking really reddish and bright, and Capella another bright star on February 8th. It'll be hanging out near Mars. You can catch the two together, but probably only if you have a telescope by that point. But they will be hanging out near each other on February 10th, and it is the Green Comet, so-called, that they're people are getting beautiful pictures of. But it's tough to see with just your eyes. We'll let that be the rundown for this time around.

Sarah Al-Ahmed: We did actually have some people try to get an image of the comet, some more successfully than others, but I really appreciate everyone sending in these pictures. It's amazing. And this sounds like a really good opportunity to try to get a picture of the comet, and potentially Mars, all at once.

Bruce Betts: Yeah, I got a picture of the comet last night. Of course, I had to actually come in on my computer and blow it up and then see, oh, there is a little fuzzy thing and it's right where it's supposed to be. Yes, that's glorious. But then I live in the LA metropolis, so it's rather bright skies.

Sarah Al-Ahmed: I always think about how funny it is that we wanted to teach people astronomy, and so we ended up living in a city where the light pollution is so bad you can't even see anything but planets. But that's okay.

Bruce Betts: But I became a planetary scientist, and some of the planets are some of the brightest objects. So there are three really easy to see even from a metropolis, except when it's cloudy and raining, which strangely it has been recently in LA. Let us move on to a couple oddballs in this week in space history. 1990, Galileo spacecraft flew by Venus on its way to Jupiter, one of the first of the, "Hey, let's go in towards the sun, get a gravity assist from Venus and then head out to Jupiter." In a counterintuitive move, it would do several other flybys of Venus and Earth, and then head out to Jupiter and do a wonderful tour there. And then in 2001, NEAR Shoemaker, the spacecraft that orbited Eros, well, they were out of fuel just about, so they decided, "Hey, let's try to land on an asteroid even though we don't have a lander." And it worked, and that was cool. So the first landing on an asteroid was by a spacecraft not designed to land on an asteroid, and it actually returned some data after doing so.

Sarah Al-Ahmed: That's interesting. I actually haven't heard this story. How did it accomplish that? Did it just try to land really softly, or...

Bruce Betts: Yeah, I mean the good news, it's big by near Earth asteroid standards, but it's still an asteroid, so the gravity's fairly low and they had a fluffy regolith. But still quite an engineering accomplishment to guide it in. And you got some benefit from doing that, because things like some of the fields instruments and things are basically dependent on resolution on how close you get. So if you snuggle the surface and they got pictures of the surface. So yeah, that's this weekend's space history. We move on to Random Space Fact!

Sarah Al-Ahmed: Nice musical one. I like that.

Bruce Betts: Oh, thank you. I was trying to come up with a constellations based fact, and the ones I came up with, mostly I'd actually done before. And then I realized something odd that I'd never had pointed out. So truly random, of the 88 IAU defined constellations, 22, or one quarter of them, start with the letter C, which is also for cookie.

Sarah Al-Ahmed: But there's no cookie constellation. Yeah, that'd make a little difficult to memorize them in alphabetical order if you ever tried, but dare you.

Bruce Betts: There are other clumps of several that have the same letter, but C wins by landslide. Let's move on to the trivia question. I asked you, speaking of odd orbital maneuvers that seem counterintuitive, what's the only mission to fly to Jupiter and then go inwards rather than outwards in the solar system? How did we do?

Sarah Al-Ahmed: People definitely got this one, which is interesting because it's a bit of a quirky orbit there, but it was the Ulysses spacecraft. It was a joint mission between NASA and the European Space Agency to go study the sun. But in order to get into the right orbit around the sun, in 1992, it had to perform a gravity assist maneuver around Jupiter and then slingshot itself back into the inner solar system. So pretty cool, kind of the opposite of what Galileo accomplished, what you were talking about earlier.

Bruce Betts: Yeah, they were using it to mostly to change the inclination of the orbit so they could go and see the polar regions more.

Sarah Al-Ahmed: Our winner this week is Jason Manning from Rockford, Illinois, USA. And Jason, you've won a Planetary Society kick asteroid rubber asteroid. And it's funny, I got a lot of messages and I didn't expect this, but people were concerned that I didn't roll the R on that rubber the last time we were talking about it. So my apologies.

Bruce Betts: Matt? He built up quite an expectation for the rolling Rs.

Sarah Al-Ahmed: Oh, and this is a great opportunity to share a poem from our Planetary Society poet laureate. We haven't gotten to do this for a little bit, but Dave Fairchild from Shawnee, Kansas, USA wrote us a little bit of a verse about this trivia question. Ulysses was launched on a shuttle you see, delayed 1990s debut. Its heliocentric velocity change was nothing a rocket could do. Instead, it's sent to a Jovian world, where 80 degrees was the plan. Then zipped heading inwards, and once at the sun, it did some fast latitude scans.

Bruce Betts: Amazing the continuing of the poet laureate's rhymes with oddball words.

Sarah Al-Ahmed: I just love it though. Fun fact about me, I was the president of my high school creative writing club, but none of my poetry stacks up with what everyone else is sending us. It's mostly just angsty teen poetry about how no one appreciates my love of space.

Bruce Betts: Well, we should probably do that as a trivia question sometime, but not today

Sarah Al-Ahmed: Yeah, maybe in a few years. Have you written any poetry, Bruce?

Bruce Betts: I can neither confirm nor deny that I've written poetry. I've written the most basic of poetry, haiku's, as part of my job in the distant past.

Sarah Al-Ahmed: That counts.

Bruce Betts: Little known fact that can be buried.

Sarah Al-Ahmed: Oh, I also wanted to share another great comment that we got from Nathan Hunter, from Vancouver, Washington, USA, who wrote in to say that if they win another rubber asteroid, all they need is one more to be able to juggle them. So unfortunately they didn't win this week, but continue on Nathan. One of these days you'll get one more, and please send us the video of you juggling.

Bruce Betts: I have juggled them. It is challenging because the rubber likes to bounce, and it's light and doesn't set... Anyway, more than you need to know about rubber asteroid juggling. I'm going to give you another trivia question for the host because she brings to us the most, huh?

Sarah Al-Ahmed: Thank you.

Bruce Betts: Okay, here we go. New question. What astronaut included his two rescue dogs in his official NASA photo? I know you know it, Sarah. Shh, don't say.

Sarah Al-Ahmed: I won't say, but it's one of my favorite astronaut photos of all time.

Bruce Betts: Go to

Sarah Al-Ahmed: And you have until Wednesday, February 15th at 8:00 AM Pacific time to get us the answer. And we'll be selecting one lucky winner to receive a Good Night Oppy 12 ounce thermal mug, which is kind of topical because our guest, Tanya Harrison once worked on the Opportunity Rover. I had a part of our conversation that wasn't recorded, was actually about how she saw herself in that documentary and was a little surprised, because she didn't know that they were all being recorded in those moments of excitement when Opportunity was landing on Mars. So that was awesome.

Bruce Betts: That's cool. Does the thermal mug for Opportunity come with a little radioactive heat sources like Opportunity had?

Sarah Al-Ahmed: Yeah, just to keep your coffee warm. No, no, it does not.

Bruce Betts: Just checking.

Sarah Al-Ahmed: Yeah, a little RTG attached to the cup.

Bruce Betts: Well, I was just going with the smaller heating units they used in the electronics box. But yeah, an RTG, Curiosity or Perseverance, you could keep that puppy boiling. It's probably not a good idea. Everybody go out there and look out on the night sky, and think about your favorite form of maniacal laughter.

Sarah Al-Ahmed: Awesome. Thanks Bruce. That was Bruce Betts, the Chief Scientist of The Planetary Society. We've reached the end of this week's episode of Planetary Radio, but we'll be back next week to speak with Jean Luke Margot and Megan Lee from UCL about the release of their Planetary Society step grant funded Citizen Science project, to search for extraterrestrial Intelligence. Planetary Radio is produced by The Planetary Society in Pasadena, California, and is made possible by our Red Planet Researching Members. You can join us to help support future Martian missions at Mark Hilverda and Rae Paoletta are our associate producers. Andrew Lucas is our audio editor. Josh Doyle composed our theme, which was arranged and performed by Pieter Schlosser. And until next week, ad astra.