Planetary Radio: Space Exploration, Astronomy and Science - Comparing the rivers of Earth, Mars, and Titan

Episode Date: July 12, 2023

Get ready for a journey across the rivers of our Solar System in this week's Planetary Radio. Sam Birch, an assistant professor at Brown University, explores what we know about the alluvial rivers of Earth, Mars, and Saturn’s moon Titan. Stay tuned for the What's Up segment with Bruce Betts and the last question in our on-air space trivia contest. Discover more at: https://www.planetary.org/planetary-radio/2023-rivers-of-earth-mars-and-titan See omnystudio.com/listener for privacy information.

Comparing rivers on Earth, Mars, and Titan, 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. There are only three worlds in our solar system with river systems that we can currently study. Earth, Mars, and Saturn's largest moon, Titan. Our guest this week is Sam Birch, an assistant professor at Brown University in Rhode Island, USA. Sam and his colleagues have used the power of mathematical relations to apply what we know about rivers on Earth to rivers on other worlds. Then Bruce Betts and I will team up for What's Up

and the announcement of our last question in our space trivia contest. Usually this would be the moment where I would share some space headlines, but our Downlink Newsletter team decided that this was the perfect moment to take a little break. We have members of our team in the United States and Canada, so they were celebrating Canada Day and U.S. Independence Day. Sometimes you just need a break to go outside and feel the starlight on your face with friends and family. We'll catch you all up on the exciting news from space next week, but in the meantime,

you can always sign up for our weekly newsletter, The Downlink. Read it or subscribe to have it sent to your inbox for free every Friday at planetary.org slash downlink. Today we're taking a deep dive into the fascinating realm of rivers, but not just any rivers. We're venturing beyond the boundaries of our own planet to delve into the rivers that flow on other worlds, Mars and Titan. The rivers of Mars are long dried up, but Saturn's largest moon, Titan, is the only other world that we've studied that has a whole hydrological cycle. Like Earth, it has clouds, rain, rivers, and lakes. But instead of liquid water, Titan's rivers are filled with liquid hydrocarbons, ethane and methane. Mars and Titan are so strange and yet so familiar. The liquids that shape their surface create everyday landforms,

rivers and deltas, canyons and floodplains. I really wish I could see them myself. But it's tough to study, let alone compare rivers across different planets and moons. Thankfully, with the power of math, we can take what we know about rivers on Earth and apply that to other places to learn more. Our guest today is Dr. Sam Birch, a planetary scientist with a unique approach to studying these otherworldly rivers.

Sam is from Canada, but he discovered his love of landscapes and the patterns that they offer while he was getting his undergraduate degree at UC Berkeley. Sam then went on to earn his PhD from Cornell University, where he spent a good deal of time focusing on the lakes of Titan. But I mean, who wouldn't? They're so cool. He continued his work at MIT as a Huizing Simmons 51 Pegasi B Fellow. And as of this month, he's beginning a new role as an assistant professor at Brown University. His team's new paper called Reconstructing River Flows Remotely on Earth, Titan, and Mars was published on July 10,

2023 in the Proceedings of the National Academy of Sciences. Hi, Sam. Hey. You know, it's not every day that you meet someone who studies rivers, not just on Earth, but like on Titan and Mars. What led you down this path of comparing rivers on different worlds? The project kind of started when we were looking at Titan. It was one of the first things we looked at back when I was starting grad school back in 2014. And we would see these very, very big rivers, so stuff the size of the Mississippi River, and they would drain towards Titan's big seas, which are bigger, so stuff the size of the Mississippi River. And they would drain towards

Titan's big seas, which are bigger than some of the Great Lakes here on Earth. When you look at the end of them, there was no deposit. They just kind of flowed into the sea into nothingness. And it kind of puzzled us back then, and we kind of put it on the back burner, worked on other stuff for my entire PhD. Then we kind of circled back to this. I was like, oh, this could be kind of interesting. What's going on here? Where are the river deltas? One of our hypotheses was maybe the rivers on Titan are just really bad at transporting sediment, really inefficient. And so we said, how do you make predictions of how much sediment a river on Titan can carry? That's kind of how this whole thing started.

And so we went down this path of using Earth, because Earth has rivers. We understand how they transport their sediment and what rates and what controls those rates. And we said, how can we use Earth's rivers and what aspects of Earth's rivers can translate to a place like Titan, where all the sediment is totally different than anything on Earth. You have cryogenic methane and ethane flowing as the fluid. It's completely alien. And so how do you translate what we know on Earth to Titan? We actually didn't come up with the method. It was these mathematical relations developed by Gary Parker in the early 2000s that let. It was this non-dimensional way of saying,

if a river is this wide and this steep, how much stuff is it carrying per unit time? He developed these relations and it lets us then go from planet to planet. And so we tested this on Earth, lots with over 400 rivers from all over the planet, making sure things worked on Earth. If it doesn't work on Earth, then probably shouldn't be using that tool on other planets. And things did actually check out really well. We were able to make estimates of the rates of flow, how long it takes to form river deltas on Earth. And they matched up with the sedimentary record of some river deltas. So it's

like, okay, things are working. Where to next? And we went to Mars because Mars has a lot of the same stuff as Earth, right? It's the same rocks, well, similar rocks. It also had liquid water. And we have actual field measurements, right? We have the Curiosity and Perseverance rovers have imaged measurements, right? We have the Curiosity of Perseverance rovers have imaged river gravels and sediment on fluvial deposits on Mars. So it's like, okay, let's test this on Mars. Can we predict what these rovers have seen? Again, testing to make sure things worked, and they did. It was kind of nice. Mars was a nice testbed for this. And then we went back to Titan finally at the end to make these predictions of the rates of flow. And it turns out Titan's rivers are actually really

efficient at transporting sediment, more so than on Earth or Mars, because that alien sediment is very buoyant in its fluid compared to rocks in water. So that didn't end up being the solution for the missing deltas. That is so interesting that we can figure that out with the limited data we have about Titan. And that's what's so interesting about it. Even here on Earth, there are some serious challenges to try to survey all of the rivers. I mean, how do you even begin to do something like that on a planet like Earth where we're living here, let alone get enough data on rivers on Mars and Titan to do this kind of research. With the Gary Parker's relations, it leverages the idea or the fact that, I mean, I like to think of rivers as these conveyor belts of sediment. So the hill slopes in their drainage basin gives them a bunch of sediment of some grain size.

And the climate says you have this much water over this drainage basin, and the river will adjust its width, its depth, and how steep it is, its slope, to make sure it can transport that sediment downslope at its flood level, which occurs every year or two. And so there are these conveyor belt machines that are just constantly adjusting their geometry to do this. On Earth, a lot of rivers are gauged and surveyed in person, especially where people live, because we care about that, right? We care about how often a river floods, how much it floods, and what it's going to do for all sorts of economic reasons, but also people's homes are there. A lot of rivers, say up in the Arctic

or in countries that we can't exactly go to for various reasons, aren't surveyed, so this technique could be used. What we did then is we used that conveyor belt idea and we turned it around. So instead of saying the river's delivered this much sediment and this much fluid, how does it adjust its geometry? We can use spacecraft images that can measure the width. You just need a picture of the river and measure the slope if you have topography data. And then you can make predictions of how much sediment is flowing through it and how much fluid to result in that geometry. And so then we can go to unsurveyed rivers across the Arctic on Earth and make those predictions. We can go to Mars. We have lots of remote sensing pictures and slopes

and on Titan. Were there any particular spacecraft that you were using in order to gather the data? Yeah, on Mars, we actually used what people had published already. People had looked at channels on the Peace Valis fan in Gale Crater, which Curiosity landed at the toe of. So we use their measurements. They already existed. And then we use measurements that were already published from the Jezero Crater,

Western Delta, which Perseverance, I think, is on top of right now. Yeah. And so we use their measurements to make predictions. On Cassini, we used synthetic aperture radar, did a lot of flybys, and it used its synthetic aperture radar to see through the atmosphere because in the visible,

Titan's like this featureless orange blob. You can't see anything. It's a lot like Venus, where you have to use radar to see through the atmosphere. The problem with the radar is that it's very noisy and it's very low resolution. So I think we've imaged about 20% of the surface at less than 350 meters resolution. So that's about three and a half football fields and then about 40% at a kilometer. So it's quite coarse, but we can still use those images to measure the width

of the rivers or at least put a constraint on it because the rivers are quite wide, right? They're a few kilometers across, like the biggest rivers on earth. And then over two of them, we were able to get slope because there's extremely limited topography data on Titan. That's one of the big data datasets that we're kind of missing. And so over those two, we got the slope of the rivers. I'm so impressed with these synthetic aperture radar instruments, because it was only just recently that the data from the Magellan spacecraft that went to Venus told us that there's active volcanism there. I'm beginning to feel like there's not that many worlds that have these thick atmospheres on

them in our solar system. But my goodness, how useful is that? Especially with Titan, that moon is so weird. I understand why that was your focus, because the fact that Titan has this whole hydrologic cycle on it, it's very familiar, but also so deeply alien. I think what's cool about Titan, especially especially is that planetary exploration has kind of showed that stable hydrologic systems are extremely rare. They're not the norm. Mars had one for a little bit. Venus had one for a little bit. But they both went away for various reasons that are still hotly debated.

It's only at Titan and Earth where you've had these stable systems that are active today, right? It's raining somewhere on Titan's surface right now as we speak. And it's the only other place where you can go study the resulting landscapes from that. And things look super similar. If I showed you a picture of a river on Earth and a river on Titan imaged in the same way, I can't pick it out and I don't think most people can. But when you think about it, the sediment is water ice and cryogenic methane. There's so many things that are different about the system. There's no plants, there's no people to interfere with things. So maybe it's a bit simpler to study in a way, but it's a real

opportunity that I'm really excited about, because you can truly use Titan's landscapes to understand other planets too, including our own. And I wonder too, just how lucky we are to see Titan in its current state. I mean, I'm not saying that the atmosphere will go away anytime soon, but it doesn't have a global magnetic field. So, you know, just knowing that it's at this point, I feel very lucky that we can study it as an example. So, we don't have that many, and who knows how we can apply what we learned from Titan to, say, exoplanets and things like that. Exactly. Yeah. So, Titan is tucked within Saturn's magnetic field for most of the time,

but that's one of the biggest outstanding questions in all of Titan sciences. Is this a late veneer of an atmosphere? Did it come around all of a sudden because of something in the Saturn system or inherent to Titan itself? Or has it been around for a very long time? On Earth, you have chemical weathering of rocks that can stabilize our atmosphere for billions of years, the planetary thermostat, if you will. And we use that idea when we study exoplanetary atmospheres, when we're modeling them. That's inherent in how we model their evolution and their habitability.

Titan very well could have its own thermostat that's operating very differently than Earth's if it has persisted for a very long time. So that's something that when we go back, that's a key question that we want to address because it has huge implications for both Earth, but also, also exoplanets too. It's really cool that you managed to do this level of analysis with just the power of mathematical relations. And I think it's important that we kind of explain how this works, because you use a type of math that's called dimensionless hydraulic geometry relations. You've kind of described what that means, but what relations are we looking at here with rivers and what can they tell us?

here with rivers and what can they tell us? These relations were derived back in 2005 and 2007 by Gary Parker. And what they describe is a series of equations that include how energy is dissipated along a riverbed, how rivers adjust their width to changing flow discharges, and how sediment is mobilized initially along the channel bed. And he came up with these dimensionless relationships that relate how wide a river is to how much fluid and sediment is going through it, how steep it then gets based on how much fluid and sediment is going through it, and then also how deep. And so these rivers, what he found for about 40 or 50 rivers is that they follow this trend,

that if he plotted the dimensionless width versus the dimensionless discharge, he could fit a line through all the data points. And so what we did to check if this was truly working is we gathered data from all sorts of different rivers in different climates, with different rock types, with different vegetation along the banks. And we gathered data from a small little creek in California up to the Mississippi and the Fly River in Papua New Guinea, very huge rivers. And we looked at his relations again to make sure things were working. So far, we've been talking all about kind of rivers on

Earth and how it works with this math, but these other worlds have very different conditions, different gravity, different materials, even different fluid flowing through the rivers on Titan. How does that affect these relationships? It's an assumption that we have to make that things transfer to other worlds. Included in these dimensionless relations are prescriptions for how gravity affects the geometry of rivers. And that was already included back in Gary's original 2007 paper. But the other critical thing is how the fluid density and the sediment density, the buoyancy of a particle factor. And that was

also done by Gary and how we worked it in mathematically. So it's a whole series of equations that include those three factors, the gravity, the fluid density, and the sediment density. And then we're assuming that physics is kind of physics and it transfers from world to world. There's assumptions about how cohesive the banks are and how welded they are together. On Titan, we're assuming that cohesion is similar to that on Earth and also on Mars too, that something is giving some sort of strength to the banks. But on Mars, everything should pretty much work pretty straightforwardly because it is similar rocks and similar fluids. And gravity only kind of weakly factors in, it adjusts things a little bit.

But given the uncertainties on how wide the rivers were when they were flowing and how steep they were, it doesn't really change things that much because on Mars, we have a pretty degraded record. Nothing's happened for a very long time. Fluvially, it is a desert, and there are impacts, but nothing active for a very long time. And so it's not clear exactly how wide the rivers were and how steep they were at the time that they were transporting that sediment. Given those uncertainties, it doesn't affect things too much. On Titan, we just have to assume that things are similar.

And we don't know. We don't know what the surface is made out of. It could be water ice. It could be any sort of organic compound that's synthesized in the atmosphere and falls out. There's a long laundry list of possible materials. We don't know how sticky those things are. We don't know how dense they are. That's the other big unknown of Titan is what are the rocks? What is the rock cycle? You kind of just get a rough estimate of what the density could be based on some lab experiments done on Earth. And what the fluid density is, that's another key one on Titan, is that the fluid density is not just liquid methane or just liquid ethane, it's both.

And then you also have nitrogen from the atmosphere dissolved in there as a function of the temperature. So the fluid's density can change quite drastically just as it flows across its drainage basin, which is pretty wild and hard to kind of understand what that even means. And so you have to kind of make assumptions on what a representative density is. And so we estimate a bunch of bounds. And it turns out in terms of transporting sediment, it doesn't affect things because it's fast. And so we're trying to estimate minimum timescales because we assume certain properties of Titan's fluids and sediment. There's a lot of unknowns there, but this is a great beginning. And thankfully,

we're going back to Titan. So we're going to get more information that we can add into this. I have to ask, though, there are many different types of rivers even here on Earth. And your paper focuses specifically on alluvial rivers. What are those? I think you can broadly break them down, rivers into alluvial rivers. What are those? I think you can broadly break them down, rivers into alluvial rivers and bedrock rivers. And so alluvial rivers have sediment along their bed and their banks, and they're kind of flowing through their own sediment and mobilizing it. And so the width of a river, our assumed theory for what that is, is that it's set by can you mobilize particles at the bank? Because if you can, then the river will widen and vice versa.

And so that kind of is what sets the channel width. And they'll naturally adjust it if you increase the discharge, they'll widen and steepen. And so it's set by sort of a threshold of moving your bed and bank sediment. Bedrock rivers have bedrock on their banks and sometimes they have sediment on their floors but the width of a river is set by the ability to erode rock or detach it from the banks. And we don't consider those ones because it's still a hot topic in earth geomorphology of what sets channel width for alluvial rivers. And it's an even hotter topic for what sets the width of bedrock rivers.

And so we stick to alluvial rivers because at least there are some theoretical equations for what sets it, which is what we adopt. Not to get too far into the weeds on this, but I'm not a geologist. So as I was going through your paper, I had to look up a lot of terms. And as I was doing so, I encountered the terms bedload-dominated and suspend-load-dominated rivers. And what is the difference there? And why is that so important to your research? On earth, you have gravel rivers, and then you have sand rivers. And as you walk down a river, it's gravel along the banks, gravel, gravel, gravel. And then all of a

sudden, over a very short distance, it becomes a sandy river and you have sand along the bed. distance, it becomes a sandy river and you have sand along the bed. And that's another very hot topic right now of what sets that gravel-sand transition. And it's not clear what's driving that on Earth, but it's a very consistent pattern that it goes from gravel to sand quite quickly. On Mars and Titan, we use the terminology bed load and suspended load because gravel on Titan is not exactly the same size as gravel on Earth. So gravel has a very specific size range of x millimeters to x millimeters, the same with sand. And the basic idea of that is if you have a gravel river, sediment is moving by bouncing along the bed, and it's bed load. But once you get to the sandy part, most of the transport, it's suspended through the column, and it's bed load. But once you get to the sandy part, most of the transport,

it's suspended through the column and it's not interacting with the bed as much. And so you just change where the load is. On earth, it's just gravel to sand or bed load to suspended load. And because gravel technically is not gravel on Titan, because the sediment will be bigger for equal flow because it's more buoyant. We have to be a little more agnostic in our terminology. Thank you. That clarifies it. I always love when I get into a science paper and I end up just going down the rabbit hole of terminology. And by the end of reading your paper, I felt like I knew so much more about rivers than I ever did before. So thank you for challenging me. So let's start with earth because that's where we figured out that these relations

work. Your sample size was 491 rivers. Did you choose those specifically because you had the most accessibility to them and their river type? It was, I think, the longest part of the project. We didn't actually go out and take field measurements ourselves. People have been surveying rivers for decades. And so what we did is we went to past work, past compilations, and gathered this data from all sorts of tables and stuff from a bunch of papers. stuff from a bunch of papers. It's nice in modern science that we have requirements on making your data available and accessible. 30, 40 years ago, that wasn't necessarily the case, and it wasn't consistent or well described. So it was quite the adventure going through these papers trying to figure out what are these measurements, where were they taken, are they representative, because we wanted to make sure we had a consistent data set. We wanted to make sure that the river width, depth, slope, bed grain size, and flow discharge were measured at the flood stage, or what's called the bankful discharge. It's not always

reported in the papers, is that the case? And then we also wanted to know what the bed and banks were like. Are they confined in a valley, in which case the width is set by the valley, not by the river itself? Or is it free to kind of launder and adjust its geometry? So that required some sort of latitude-longitude information so that we can then go look up what these look like in satellite images. So it was quite the adventure over many, many months, gathering this data set from dozens and dozens of sources to make sure it was consistent and kind of a clean data set for this work. It was quite, I don't want to say a mess, because they did a lot of good work. It's good that we have modern standards for data reporting and requirements for that,

because it's a good lesson for also future research to make your data available, because people might not use it this year or next year, but 30 years from now, having a good data set is like gold. Yeah, you never know when future technologies will allow you to do a much more thorough analysis of the data. And so many of the findings that I've been reading through recently are all from data from like 30 plus years ago. And that's phenomenal. I think for me, what would be really interesting about trying to study rivers on Earth is that they're impacted by humans and other life on this planet. And you have to take that into account when you're studying them. Not so much on Mars.

Exactly. So we had to throw out any data that's upstream or downstream of a dam or has a canal or something. We didn't subselect for vegetation or anything, which I think kind of shows how universal these relations are, is that it added some scatter to the data, but the same trend was observed regardless of if you had big trees along the banks or grass. Either way, that didn't really affect things, but we did to be careful throughout anything that could even be thought of as being impacted by humans. On Mars, yes, it's nice and easy. There's no humans or plants. The same with Titan. We'll be right back with the rest of my interview with Sam Birch after this short break. Greetings, planetary defenders. Bill Nye here.

At the Planetary Society, we work to prevent the Earth from getting hit with an asteroid or comet. Such an impact would have devastating effects, but we can keep it from happening. The Planetary Society supports near-Earth object research through our Shoemaker-Neo grants. These grants provide funding for astronomers around the world to upgrade their observational facilities. Right now, there are astronomers out there finding, tracking, and characterizing potentially dangerous asteroids. Our grant winners really make a difference by providing lots of observations of the asteroid so we can figure out if it's going to hit Earth. Asteroids big enough to destroy entire cities still go completely undetected, which is why the work that

these astronomers are doing is so critical. Your support could directly prevent us from getting hit with an asteroid. Right now, your gift in support of our grant program will be matched dollar for dollar up to $25,000. With your support, working together, we can save the world. Thank you. Of course, the rivers on Mars are all dried up, so that poses a whole other host of questions. So which ancient rivers on Mars did you study? So we only picked two. Our initial motivation for this study was, again, Titan.

We did Mars because we wanted to check our work, kind of checking our homework, if you will. And we picked the two places where rovers had been. So we picked two systems of rivers that were on two river deposits. So we picked channels on the Peace Valis fan and Gale Crater. And we picked channels on the Jezero Delta and the Jezero Crater. And those are being explored by Curiosity and Perseverance. So we had pictures of the bed grain size, we had high-rise images of the widths and high-rise topography data of the slopes, and so we

can use the width and the slope to make predictions of the bed grain size and then say does it match with what we see. And that was kind of our first off-earth check and it worked which was nice. I don't want to say surprising. It is kind of nice when you do all this math and all this work, and then it does work. It's like, oh, haha. And then we can make predictions after that of how much fluid was going through those channels and how much sediment. And if you know how much sediment is going through the channels per unit time and you know the total volume of sediment in the deposit you can estimate how long or how much time it took to form that deposit which is what we then did so what was the result like what can this tell us about i mean i'm particularly

curious about the jezero you know river, because we're literally pulling samples out of it right now that I cannot wait to get back to Earth. Yeah, it was a bit of an adventure of which slope and which width do you choose. But when you take the best estimates we have from orbit, the timescale, it depends on how frequently it rained or how frequently snow melted to give runoff. So you kind of have to assume what the climate was like. If you assume it's like arid rivers on earth, it's hundreds of thousands or millions of years. It takes quite a while to form that delta. And the main reason for that is it's a pretty big delta given the size of the channels feeding it. And so it's always going to take kind of a while to do that. Little channels will build a big delta in a long time.

If it rains, like if it's flood every single day and it's full of mud so that you can like artificially increase the volume, then it's still tens of thousands of years of like constant flow, which seems pretty unrealistic, given what we know about the climate. So kind of taking every couple days, or a couple days, every year, it takes hundreds of thousands, if not millions of years, we struggled to get that timescale shorter. We were trying to see what conditions can it be as short as possible. And it was kind of hard to do. Kept coming back to the same answer.

Yeah, that is a challenge because you're having to make assumptions about the climate in order to figure out the timing. But if you knew more about all the riverbeds and everything involved, maybe it could even help you learn more about the climate or make some kinds of estimations about what was going on there. It's all kind of feeding into each other was just challenging. But how cool is that, that we can even at all estimate the timescales on which these river deltas formed on a planet that's now a dry rock? That's amazing. Yeah.

And I think so we did all these calculations, this project started before Perseverance even launched from Earth. So when we did all these calculations, we didn't know what the stratigraphy of the delta is like. And I believe that stuff is getting published or is going to come out in the next couple years, probably, I would assume. And once we know that, we can learn about the intermittency and the duration of flows, how much mud there was in the deposit, things like that that can help us narrow down exactly. It kind of helps with some of our assumptions once we know the full stratigraphy of the delta. And I, you know, it'll get even better if and when we get these samples back from the Mars

sample return mission. I know there are some challenges right there with the budget going on with that mission right now, but we're trying to mobilize as many people as we can in the United States, at least to go to Washington, DC this next September and personally advocate for this mission and others, because this might just be a personal thing, but I just really want to know as much as I can about that. Mars captures the imagination because it used to be so much like Earth and the fact that it's not anymore is frightening, but also kind of beautiful. I think it's one of the most interesting questions in Mars science of kind of what went wrong and what are the processes that occurred to kind of end up in this state?

I think it's a really cool question. And I think those samples could help with that. We know that the data from these rovers is consistent with what you guys have been predicting for gravel size and things like that. But does this in any way help us get a timeline for when this river delta formed? So there have been many epochs of water on mars and they're all very different that's a little further out of my realm of expertise um i'm new to mars if you will i believe though that the way that you can date these is with crater counting and i think

my understanding is that the jezero delta is quite old. And that's kind of reflected that it looks kind of beat up with craters. And it's quite degraded, which made some of our estimates a bit harder to do. Whereas Gale Crater was a little more recent. And so Mars kind of went through these fluctuations as it was drying. It might have been warmer and wetter very early, and then a series of glacial interglacial cycles where most of the fluid was actually from melting snow up in the drainage basins. That seems, I believe, to be kind of the favorite scenario for the Peace Palace fan. And that's kind of your source of your runoff. So it's a topic that's forever going to be debated. It was Mars warm and wet or cold and dry,

I think it was probably both as it kind of intermixed through time. It was Mars warm and wet or cold and dry. I think it was probably both as it kind of intermixed through time. And when it was and the timing of each is a very, very active topic that has been for decades and will continue to be. So we need more orbiters. Maybe not more orbiters, but we definitely need

more landed assets like more capable helicopters or rovers to go look at the stratigraphy of all these different deposits to kind of nail down this time scales. Yeah, I'm really glad that Mars Sample Return is going to be taking more little copters with it, because Ingenuity has just proven itself to be super useful. And I hope we have just a fleet of tiny copters on every world that has even a vague atmosphere in the future. I think it's opened up the door to be much more mobile.

So you can look at more things more rapidly. And Dragonfly is going to take it to a whole nother level once it gets to Titan. So I agree. Having a fleet of these on Mars, a fleet on Titan would be a dream. You've touched on this a little bit in that Titan isn't covered in liquid water the way that we have rivers and oceans here on Earth. It's covered in liquid hydrocarbons, ethane, methane, enriched by this nitrogen atmosphere. What is that hydrological cycle like? So Titan, all of its fluids today are located at the poles.

And so Titan, it orbits Saturn. Saturn has an obliquity just like Earth. And so we have seasons on Saturn and Titan by definition. Presently, all the liquids are at the poles because it's a little bit colder and you can condense the fluid a little bit easier. It does rain across the moon, but it's stable against evaporation up at the poles. And so we have at the North Pole, we have three large seas named after sea monsters. They're Kraken Mare, Lygia Mare, and Punga Mare. There's another

really big lake called Jingpo Lacus. And then at the South Pole, there's one big lake called Ontario Lacus. Those are kind of the big seas, if you will. And into them drain these huge rivers that we know are actively flowing on Titan today. And they're just, they're enormous. They would be some of the biggest rivers on this planet as well. On top of that, there's all these small lakes, and there's over 600 of them on Titan. And we like to call them like cookie cutters because they kind of look like you've kind of used a cookie cutter to cut out dough. They're really confusing on what they are. There is no good explanation. I spent quite a long time during my thesis trying to figure this out.

And we're kind of at a bit of a roadblock. Just because we need a bit more data. There are some theories out there, but no theory can explain everything that we see. There's hundreds, I think 600-something, and they're at both poles, and they're both filled and empty in today's climate. At the equator, it's a giant global desert. You have these dunes that wrap all the way around the moon. What they're made out of is one of the key objectives of Dragonfly, actually.

It's going to land in the dunes and sample it and tell us what they're made out of. They're probably organic solids that form high up in the atmosphere and fall down to the surface slowly. What exactly they are will await Dragonfly's results. But yeah, it's kind of like a desert world at the equator and a hydrologic world at the pools today that's so weird i mean life as we know it doesn't form in flowing methane but just the fact that it literally rains organic compounds there and has any kind of hydrologic cycle at all just begs the question could life exist in that kind of scenario we must know more which rivers did you end up studying on titan so we took one that drains into ligeomare it's called vidflumina

it's really well imaged and we have the best slope measurements. Cassini had an altimeter as part of its radar actually and all that does is it sends a radar echo down to the surface, bounces off the surface, it comes back, use the travel time to get the elevation. And as it was getting altimetry across the sea, the observation was actually designed to look for waves on Lygia and that it would look straight down and then off to the side and then straight down and for waves on lygia and that it would look straight down and then off to the side and then straight down and then off to the side when it looked off to the side if it got a bright return it meant that there was parts of the sea that were oriented towards the spacecraft i.e not flat which is waves it didn't see any waves but it did measure

the bathymetry of the sea and that the radar signal went all the way through the sea, off the seafloor and came back. And that's how we measured the depth of the seas, which is amazing. You're going at like multiple kilometers per second, and it was kind of an accidental observation. And you're using it as the sounder. It's pretty stunning. Before it did that though, it went over a network, a river network, and it got over a network, a river network, and it got really bright returns over the river network. And so we know the river, the elevation of those liquids very, very well, within a few centimeters.

And we use that slope for our estimates. And then we use the pictures, the SAR images for the widths. The other slope was in the south, and a river that was draining towards the only known delta on Titan, which is along the shoreline of Ontario-Laucus. And the altimeter measured the slope of the plane that that river flows through. And so we assume that the river slope matches the plane slope over long enough distance. It's not the best assumption, but we have almost no reliable topography on Titan, especially to do hydrology.

And so this was kind of the best that we had. So those were the only two that had slopes. This whole thing started because you're trying to figure out why there weren't these robust river deltas coming out of some of these lakes. Were either of the rivers that you looked at an example of this? Yeah, so Saraswati Flumen has two lobes near the end of it that we think are river deltas along Ontario Lockers at the south. Those are the only two.

Vid Flumina is one of the type examples of huge rivers at the north that have absolutely nothing at the end of it. It just goes into open ocean immediately. Not all rivers on earth have river deltas, but most do. On Titan it's kind of the opposite, most don't. And we looked and we tried. I really wanted to see them because then you can use their sizes and their morphology to understand about the climate. It's much easier to do things that way. So we're trying to learn about what the climate was like now based on the lack of deltas. There's still some things that we can predict about rivers on Titan based on these

relations that you put together. So what can you pretty confidently say about the rivers on Titan? So for Titan's rivers, the limiting factor doesn't seem to be the transport of sediment. Its rivers transport it pretty efficiently. The sediment is more buoyant than on Earth, so for an equal flow it could carry a lot more sediment. It doesn't rain as often on Titan though, so the overall time is a bit longer. Other factors could be affecting the lack of deltas. At the north, the landscape is very flooded. There's rising sea levels that are filling up the surrounding landscape, so

maybe you've just flooded the landscape at the north, at the south, sea level is dropping, so it's a little more noticeable to form a delta that way. Tides and wind can move sediment along the coast. That's another factor. Not all deltas on earth are really obvious. They don't all have this like perfect lobe shape. Tides and wind can reshape that sediment, which changes what it looks like. So maybe we're just not recognizing them as well because of these factors. It could also be that because of Titan's fluids, on Earth, freshwater is always going to be less dense than saltwater. So rivers will be buoyant, the fluid itself, and whether they plunge when they hit the sea depends on how much

sediment you have in the river. On Titan, it's likely that its rivers are always denser than the seas because they're more methane rich and maybe a bit colder. So they'll always plunge and then if you add sediment, it'll only plunge more. So another solution to this could be that when the river hits the sea, it just kind of keeps going under the surface as if it doesn't really care that it just hit the sea. And so those are all problems that we're kind of working on in the group now. if a river is delivered this much sediment and this much fluid, how is its geometry going to be different on these different worlds, given the differences in the densities and the gravity?

On Mars, we found, because the rocks and the fluids are the same and gravity doesn't really affect things, that the geometry of rivers should pretty much be the same. On Titan, we we found because the sediment is so buoyant, the rivers don't need to steepen as much to move that sediment downslope when the fluid comes, and so the river can flatten out, and in doing so it can widen out. And so we made this prediction that these alluvial rivers on Titan should be more gently sloping and wider than equivalent ones on Earth. And so when Dragonfly gets there, it lands near a river, and you get a picture of the grain size, of what is setting the channel width and test this theory. And then the final thing that we did is we said for a river of a given geometry, so with a set width, slope, and depth, how much discharge is it required to move the bed sediment downslope?

And on Mars, again, pretty much the same. On Titan, it was like 2% to 6% of the discharge. On Titan, it doesn't rain a lot, and it might trickle. There might be a small little rain frequently, and then every few Titan years, once or twice a Titan year, which is every seven years on Earth, you get huge storms where it rains for like a month straight. Dragonflies going to a place on Titan where that's not expected to be the case, but maybe there's a small rainstorm up on the crater rim, and it kind of increases the

odds that it might detect active sediment transport further downslope, because it only takes a small little bit of discharge to move that sediment which is exciting if dragonfly is sitting near a river it could detect active sediment transport on another world at 10 au which would be stunning it is intriguing that we can even accomplish any of this and now i'm just thinking about trying to make dragonfly as like methane proof as possible. Can you imagine it just hanging out in a rainstorm? That would kind of, I think, be a nightmare.

Yeah. To be honest. They would not fly, I don't think, if there's even a possibility of rain. And I don't think they would get anywhere near a river if it is raining. a river if it is raining even though if it i think if you're sitting in a river the sediment is imparting not a lot of momentum in the flow so it wouldn't damage anything but they would never ever ever do that you don't want to risk anything yeah emissions are very risk averse it's more like a distant observation if you will how is it that we know that the gravel in these rivers is more buoyant is that an assumption we're making off of what we think the materials are

involved yeah so there are some lab and theoretical estimations of how dense the fluid is on titan and then the sediment density for water ice is just the density of ice. It might be a little lower because it might be a bit porous. And so that was the one that we assumed. So if you assume porous water ice in liquid methane and ethane and nitrogen, it is much, much more buoyant. I think like two and a half times more buoyant. Organic solids is the big unknown.

Some of them can be almost as dense as the fluid, which means they'd float. And all of our calculations are kind of pointless at that point, because you'd mobilize everything, just be rafting down the rivers. It could also be much denser. And so there's a huge range in these experiments. Most of them converge around the density of water ice, though. So that's why we picked that density in our study. I know that we're going to get Dragonfly to go there for us, but I wish that we could personally survive the trip there because I feel like taking a vacation by the lakes and rivers of Titan is exactly the kind of on-brand way that I would personally go out. I could see that happening. I think there's a few of us when we were on Cassini that had our vacation spots picked out around the lakes and which spot would be the best for our cabin. It would be amazing. Everything would be a bit louder if you could hear it because the atmosphere is a bit denser.

I think a little higher frequency. And so if you're sitting next to a river and you're hearing the flowing of a creek, it would be a bit sharper and maybe like a bit prettier to hear. And it would all look very, very similar. It would be pretty hazy, pretty overcast, if you will, and pretty cold, obviously. And there would be no plants, but everything else would look and sound very familiar. So yeah, I have my spot picked out of where I would want my cabin. We should create that as a VR experience, you know, get some of the people from the Mars microphone on the team, make a whole thing out of it.

Yeah. That'd be so fun. I love that we are able to even begin to do this analysis of what's going on on Titan. There's so much left to learn. And even with limited data, you figured out, well, you and your team have figured out quite a lot about this. And that's just startling. And I'm glad you did it because I've always been curious about this. Titan's amazing.

It has something for everyone there. It has this global subsurface ocean that could be in contact with the surface. And you have all these interesting organic molecules in a dense atmosphere. It might have some sort of tectonics, cryovolcanism, fluvial erosion. It has deserts, impacts. It's a place, I think, for everyone. Dragonfly is an amazing first start to go back and study this prebiotic chemistry, tell us what the rocks are made out of. But going back again, even with more missions, like an orbiter would be, that's my personal dream, to get real images of these coastlines. You can watch them evolve a lot like how Landsat does here on Earth. It would be kind of a geologist's dream going back with such a mission. Hopefully we do it

sometime soon. I wish we had funding to put just a bajillion spacecraft around every world and especially now that we've made better synthetic aperture radar because we've been preparing for things like VERITAS. Who even knows what we could learn if we went back with an orbiter? Well, thanks for joining me, Sam, and for explaining all of this and for this awesome science, because we're all going to have to be patient for the next missions to Titan. But in the meantime, now we can dream about the loud and very chonky rivers that they have over there. Yeah, thanks for having me and letting me talk about all this and Titan too. It's been quite a lot of fun.

And there's so much more between now and Dragonfly to do. We're kind of just barely scratching the surface of Titan. And it's exciting to see more and more people get into it and start studying it too. So looking forward to the next few years and decades. It's a real shame that we can't survive the surface of Titan, but knowing that its gravel buoyantly floats down into wide rivers of hydrocarbons is enough to get my imagination going. If you'd like to dive into Sam's team's paper,

I'll link to it on this week's Planetary Radio page at planetary.org slash radio. Now let's check in with Bruce Betts, the chief scientist of the Planetary Society, for What's Up. Hey, Bruce. Hey, Sarah. Is that like the clog in your throat from all the firework smoke? Oh, but it was worth it. Totally worth it.

Hey, Sarah, how are you doing? I'm doing well. And as much as I love fireworks exploding in the sky, obviously here in the United States, we just celebrated our Independence Day on July 4th. So lots of fun explosions in the sky. But as soon as that smoke clears, we can finally see the stars again. So that's what I'm actually looking forward to. Well, take heart. There are plenty of stars still up there if you don't have things in your way when you're looking like clouds and smoke and the like. And, of course, planets. And I'm a little redundant week after week,

but Venus will be going away in the next few weeks and resting before it comes back in the morning sky. So continue to check out super bright venus in the low in the west getting lower over the days and weeks to come and mars looking reddish much dimmer up above it on july 20th you can check out the crescent moon joining in and near reddish mars and then also hanging out near them is the somewhat reddish but similar brightness star Regulus, the brightest star in Leo. All of that over in the evening west in the pre-dawn sky. You've got Saturn already high in the sky looking yellowish and Jupiter low.

Well, it's actually not that low anymore. I hear from those who are up at those hours that it's up getting pretty high in the east in the early morning. And Jupiter and Saturn will be playing with us for the next few months. Yeah, it's always nice to have them there after Venus kind of goes away. I mean, I always love that walk home from work when you can see Venus just shining brightly as you're walking home. It's pretty awesome. On to this week in space history a couple of big spacecraft flybys uh separated by many a year in 1965 mariner 4 the first successful flyby of mars

giving us our first glimpse at the red planet up close and a few decades later in 2015, New Horizons flew by Pluto and the Pluto system, giving us all sorts of fun surprises. On to... Let's throw me back to... I don't remember what age of the internet everyone got back into Gregorian chants, but I was there for that. It was a dark, dark day. All right. So I got kind of an oddball one here. I just, I was, I was talking about Mariner 4 and then I was thinking about my thesis as you'll find out. And I was thinking about my thesis advisor, Bruce Murray, who was one of the co-founders of the Planetary Society. So here's your random Bruce Murray space fact,

which is he was on all of the Mars Mariner imaging teams before he then headed the imaging team, or TV team, as they called it at the time, of the Mariner 10, which was the first to fly by two planets, both Venus and Mercury, and the first and only one to see Mercury from up close until 2008 and Messenger. So there you go. Man, what a career. When people have careers like that, I really do hope that they get a moment to reflect and really think about how cool that is, because I've been thinking about that recently with this job. So many awesome things have happened

in my life since I took this job, and it's hard to take the time to slow down and actually reflect on how cool that is. The moments I get to have talking with people on the show and all the wonderful messages people have sent me, the cool events I get to go to, that's not just me. That's everybody else involved making my life cooler, including you, Bruce. Why, yes. Yes, I am. Let's move on to the trivia contest. I asked you, approximately, how thick is the Parker Solar Probe's protective shield? Protecting it from those close flybys to the sun, or certainly closer than anything else we've ever hurled up there. And how'd we do? We did really well.

Although I must guess that most people had to Google this because their units were very, very precise. But, you know, when I first learned this, it actually kind of blew my mind because I expected that it was going to have to be pretty thick to guard the spacecraft from the sun and make sure that one side was like 70 degrees Fahrenheit, cold enough to not melt everything. But that heat shield is only about 4.5 inches thick. That's like 115 millimeters. Yes, indeed. It's pretty amazing. We have two winners this week because I was going wild last time. But we announced this question right around Asteroid Day. So our two winners are going to be winning some Psyche mission posters, Psyche being the metallic asteroid that we're all looking forward to getting that mission to. Our winners this week are Christopher Lowe from Escondido, California, and Hain Woo Chang from

Seoul, South Korea. So you'll be getting awesome posters. And we got some great comments this week specifically about the heat shield, because is just so so strange uh one person robert laporta from avon connecticut wrote in to say that technically it's a little thicker than 4.5 inches because that's how thick the carbon foam core is but there's actually this this layering on the outside that makes it a little thicker than that so if we're going to be specific it's a little thicker than 4.5 inches. And this actually cracked me up a little bit. Elijah Marshall from Australia wrote in to say, funny thing, I heard that the Parker Solar Probe wasn't vegan, as it contained a coating made out of

powdered animal bones. And it turns out that's true. So it's not actually vegan. I also wanted to say that I really appreciate a lot of people have sent me some really kind messages this week. And I don't always like to read the, you know, yay Sarah messages that feels a little weird. But I want everyone to know that I read every message that comes into the show. And I really appreciate all the love you have for me and Bruce, but also for this show. Everyone clearly loves Planetary Radio so much. And it's an honor to work on something that so many people care so deeply about. And I just want to reiterate that I know it's going to be a little difficult for all of us as we make changes to Planetary Radio.

And a lot of people love this trivia contest, but we are moving it into our member community. So again, I want to encourage everyone to continue sending your messages to us at our email, which is planetary radio at planetary.org. We read all of them and I'm still hoping to share your poetry and your messages and maybe whatever questions you have for me and Bruce. So please continue to send those to us. It is an honor indeed. So what's our trivia question for next week? And this is, this is our last space trivia question on the show so everyone get ready yeah i hope to uh want to do better but i this is what i've got i just made it kind of personal uh with things near and dear to my heart in my phd thesis i quoted the great musical group warrant yes that's right. Metal! Metal!

And I quoted from the beginning of one of my chapters saying, dancing with my shadow and letting my shadow lead. My question for you is, what shadow was I referring to? Go to planetary.org slash radio contest for the last time.

That's a deep cut and people will have a harder time googling this one so i like this this is this is going to be a challenge and you have until wednesday july 19th at 8 a.m pacific time to get us your answer and i'm again giving away a whole bunch of prizes i'm going to throw together a grab bag of awesome posters and patches, some light sail stuff, and one of our last rubber asteroids. Dun, dun, dun. I'm excited. I'm excited for this one. Cool, cool, cool.

Do some digging. Alright, everybody, go out there, look up at the night sky, and think about your shadow and how you can make it look like funny little animals. Thank you, and good night. We've reached the end of this week's episode of Planetary Radio, but we'll be back next week to learn more about the history of Mars.

Planetary Radio is produced by the Planetary Society in Pasadena, California, and is made possible by our river-loving members. You can join us as we advocate for missions like Mars Sample Return and Dragonfly at planetary.org slash join. Mark Hilverda and Ray Paoletta are our associate producers. Andrew Lucas is our audio editor. Josh Doyle composed our theme,

which is arranged and performed by Peter Schlosser. And until next week, Ad Astra.