Comparing the rivers of Earth, Mars, and Titan

Sarah Al-Ahmed: 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 and 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 US 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/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 is 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 land forms, 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 other worldly 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 Heising-Simons 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 10th, 2023 in the proceedings of the National Academy of Sciences. Hi, Sam.

Sam Birch: Hey.

Sarah Al-Ahmed: It's not every day that you meet someone who studies rivers, not just on Earth, but on Titan and Mars. What led you down this path of comparing rivers on different worlds?

Sam Birch: The project 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 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 floated into the sea, into nothingness and it puzzled us back then and we put it on the back burner, worked on other stuff for my entire PhD. Then we circled back to this of like, "Oh, this could be 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. So, we said, how do you make predictions of how much sediment a river on Titan can carry? That's how this whole thing started. 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. We said, how can we use Earth rivers and what aspects of Earth 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. 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 two thousands that it was this one-dimensional way of saying, if a river's this wide and this steep, how much stuff is it carrying per unit time say? He developed these relations and it lets us then go from planet to planet. So, we tested this on Earth, plots 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. 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?" We went to Mars because Mars has a lot of the same stuff as Earth, it's the same rocks, well, similar rocks. It also had liquid water and we have actual field measurements. We have the curiosity of Perseverance Rover's 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. They did. It was nice. Mars was a nice test bed for this. Then we went back to Titan finally at the end to make these predictions of the rates of flow. It turns out Titan's rovers 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 on in water. So that didn't end up being the solution for the missing deltas.

Sarah Al-Ahmed: That is so interesting that we can figure that out with the limited data we have about Titan. 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. 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?

Sam Birch: With the Gary Parker's relations that leverages the idea or the fact that ... 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 down slope at its flood level, which occurs every year or two. So, these 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. 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 this 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. So, then we can go to surveyed 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.

Sarah Al-Ahmed: Were there any particular spacecraft that you were using in order to gather the data?

Sam Birch: Yeah, on Mars? We actually used what people had published already. People had looked at channels on the Peace Vallis fan in Gale crater, which curiosity landed at the [inaudible 00:09:09]. So we used their measurements, they already existed. Then we used measurements that were already published from the Jezero Western delta, which Perseverance I think is on top of right now. So, we used 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 is 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 widths of the rivers or at least put a constraint on it because the rivers are quite wide, they're a few kilometers across, like the biggest rivers on Earth. 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 sets that we're missing. So, over those two, we got the slope of the rivers.

Sarah Al-Ahmed: 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 vulcanism 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's very familiar but also so deeply alien.

Sam Birch: I think what's cool about Titan, especially, is that planetary exploration has 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, it's raining somewhere on Titan surface right now as we speak. It's the only other place where you can go study the resulting landscapes from that. 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 in 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.

Sarah Al-Ahmed: 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, just knowing that it's at this point, I feel very lucky that we can study it as an example because we don't have that many and who knows how we can apply what we learn from Titan to say, exoplanets and things like that.

Sam Birch: 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 in planetary thermostat, if you will. 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 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, its exoplanets too.

Sarah Al-Ahmed: It's really cool that you manage to do this level of analysis with just the power of mathematical relations. I think it's important that we explain how this works because you use a type of math that's called dimensionless hydraulic geometry relations. You've described what that means, but what relations are we looking at here with rivers and what can they tell us?

Sam Birch: 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. 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. 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. 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. 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. We looked at his relations again to make sure things were working.

Sarah Al-Ahmed: So far, we've been talking all about 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?

Sam Birch: 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 in, 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. 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 weekly factors in it adjust 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. 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 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 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. Then you also have nitrogen from the atmosphere dissolved in there as a function of the temperature. So the fluids' density can change quite drastically just as it flows across this drainage basin, which is pretty wild and hard to understand what that even means. So, you have to make assumptions on what a representative density is. 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. So, we're trying to estimate minimum timescales because we assume certain properties of Titan's fluids and sediment.

Sarah Al-Ahmed: There's a lot of unknowns there. But this is a great beginning and thankfully, we're going back to Titans, 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?

Sam Birch: I think you can broadly break them down rivers into alluvial rivers and bedrock rivers. So, alluvial rivers have sediment along their bed and their banks and they're flowing through their own sediment and mobilizing it. 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. So, that is what sets the channel width and they'll naturally adjust it. If you increase the discharge, they'll widened and steepen. 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. 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. So, we stick to alluvial rivers because at least there are some theoretical equations for what sets it, which is what we adopt.

Sarah Al-Ahmed: 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 bed load dominated and suspend load dominated rivers. What is the difference there and why is that so important to your research?

Sam Birch: 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. That's another very hot topic right now of what sets that gravel sand transition. 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 and same with sand. 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 not interacting with the bed as much. So, you just change where the load is. On Earth, it's just gravel to sand or bed load to suspended load. 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.

Sarah Al-Ahmed: 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 what 491 rivers. Did you choose those specifically because you had the most accessibility to them and their river type?

Sam Birch: 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 tables and 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 bank full discharge. It's not always reported in the papers. Is that the case? 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 launder and adjust its geometry? So that required some latitude, longitude information so that we can then go look up what these look like in satellite images. So it's quite the adventure over many, many months gathering this data set from dozens and dozens of sources to make sure it was consistent and 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 just, 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.

Sarah Al-Ahmed: Yeah, you never know when future technologies will allow you to do a much more thorough analysis of the data. So, many of the findings that I've been reading through recently are all from data from 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.

Sam Birch: 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 sub-select for vegetation or anything, which I think 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. Same with Titan.

Sarah Al-Ahmed: We'll be right back with the rest of my interview with Sam Birch after this short break.

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Sarah Al-Ahmed: 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?

Sam Birch: So we only picked two. Our initial motivation for this study was again, Titan. We did Mars because we wanted to check our work, 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 Vallis 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. 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?" That was kind of our first off Earth check and it worked, which was nice. I don't want to say surprising. It is nice when you do all this math and all this work and then it does work. It's like, "Oh, ha-ha." Then we could 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.

Sarah Al-Ahmed: So what was the result? What can this tell us about? I'm particularly curious about the Jezero river delta because we're literally pulling samples out of it right now that I cannot wait to get back to Earth.

Sam Birch: 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 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. 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 a while to do that. Little channels will build a big delta in a long time. If it rains, if it's flood every single day and it's full of mud so that you can artificially increase the volume, then it's still tens of thousands of years of constant flow, which seems pretty unrealistic given what we know about the climate. So taking every couple days or a couple of 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 to do. Kept coming back to the same answer.

Sarah Al-Ahmed: 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 and 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 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.

Sam Birch: Yeah, 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. I believe that stuff is getting published or is going to come out in the next couple of years probably, I would assume. 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 helps with some of our assumptions once we know the full stratigraphy of the delta.

Sarah Al-Ahmed: 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 beautiful.

Sam Birch: I think it's one of the most interesting questions in Mars science of what went wrong and what are the processes that occurred to end up in this state? I think it's a really cool question and I think those samples could help with that.

Sarah Al-Ahmed: 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 epoch of water on Mars and they're all very different.

Sam Birch: That's a little further out of my realm of expertise. 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 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 the favorite scenario for the Peace Vallis fan. That's kind of your source of your runoff. So it's a topic that's forever going to be debated. Mars warm and wet or cold and dry? I think it was probably both as it intermixed through time and when it was and the timing of each is a very, very active topic. There 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 nail down this timescales.

Sarah Al-Ahmed: 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.

Sam Birch: 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, fleet on Titan would be a dream.

Sarah Al-Ahmed: 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?

Sam Birch: 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 it can condense the fluid a little bit easier. It does rain across the moon, but it's stable against evaporation up at the poles. So we have at the North Pole we have it's three large seas named after sea monsters. They're Kraken Mare, Ligeia Mare, and Punga Mare. There's another really big lake called Jingpo Lacus. 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. 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 cookie cutters because they look like you've 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 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 like a desert world at the equator and a hydrologic world at the pools today.

Sarah Al-Ahmed: That's so weird. 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?

Sam Birch: So we took one that drains into Ligeia Mare. It's called Vid Lumina. It's really well imaged and we have the best slope measurements. Casini 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 and comes back. You use the travel time to get the elevation. As it was getting altimetry across the sea, the observation was actually designed to look for waves on Ligeia 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, IE, 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 sea floor and came back and that's how we measured the depth of the seas, which is amazing. You're going at multiple kilometers per second and it was 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 really bright returns over the river network. So, we know the river, the elevation of those liquids very, very well within a few centimeters. We used that slope for our estimates. Then we used the pictures, the [inaudible 00:38:40] 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 Lacus and up the altimeter measured the slope of the plane that that river flows through. So, we assume that the river slope matches the plain slope over long enough distance. It's not the best assumption, but we have almost no reliable topography on Titan, especially to do hydrology. So, this was the best that we had. So those were the only two that had slopes.

Sarah Al-Ahmed: 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?

Sam Birch: So Saraswati Flumen has two lobes near the end of it that we think are river deltas along Ontario Lacus at the south. Those are the only two. Vid Flumina is one of the type examples of huge rivers at the north. I 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. 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.

Sarah Al-Ahmed: Well, 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?

Sam Birch: So for Titan's rivers, the limiting factor doesn't seem to be the transport of sediment. Its rivers transported pretty efficiently. The sediment is more buoyant than on Earth. So for equal flow it can 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're 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 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 it's rivers are always denser than the seas because they're more methane rich and maybe a bit colder. So they'll always plunge. 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 keeps going under the surface as if it doesn't really care that it just hit the sea. So, those are all problems that we're kind of working on in the group now. The other thing that we did is with these relations you can say, "Okay, if a river is delivered this much sediment and this much fluid, how is it 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 river should pretty much be the same. On Titan, we found because the sediment is so buoyant, the rivers don't need a steepen as much to move that sediment down slope when the fluid comes. So, the river can flatten out and in doing so, it can widen out. So, we made this prediction that these alluvial rivers on Titan should be more gently sloping and wider than equivalent ones on Earth. So, when Dragonfly gets there, if it lands near a river and you get a picture of the grain size and then you can measure the width and the slope as it's flying say, you can test this and go back to our initial assumption of what is setting the channel width and test this theory. 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 required to move the bed sediment down slope. On Mars, again, pretty much the same. On Titan, it was two 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. 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 a month straight. Dragonfly is 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 increases the odds that it might detect active sediment transport further down slope because it only takes a small little bit of discharge to move that sediment, which is exciting. If Dragonfly's sitting near a river, it could detect active sediment transport on another world at 10 AU, which would be stunning.

Sarah Al-Ahmed: It is intriguing that we can even accomplish any of this. Now, I'm just thinking about trying to make Dragonfly as methane proof as possible. Can you imagine it? Just hanging out in a rainstorm.

Sam Birch: That would I think be a nightmare, to be honest, they would not fly, I don't think if there's even a possibility of rain. I don't think they would get anywhere near a river if it is rainy. Even though 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. Missions are very risk averse. It's more like a distant observation, if you will.

Sarah Al-Ahmed: 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?

Sam Birch: 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 might be a little lower because it might be a bit porous. So, that was the one that we assumed. So if you assume porous water, ice and liquid methane and ethane and nitrogen, it is much, much more buoyant. I think 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. All of our calculations are pointless at that point because you'd mobilize everything, just be rafting down the rivers. It could also be much denser. 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.

Sarah Al-Ahmed: 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.

Sam Birch: 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 and I think a little higher frequency. 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. Pretty cold obviously. 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.

Sarah Al-Ahmed: We should create that as a VR experience, get some of the people from the Mars microphone on the team, make a whole thing out of it.

Sam Birch: Yeah.

Sarah Al-Ahmed: That'd be so fun. Well, 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. Even with limited data you figured out, while 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.

Sam Birch: 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 tectonics, cryo volcanism, 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 could watch them evolve a lot like how Lancet does here on Earth. It would be a geologist dream going back with such a mission. Hopefully, we do it sometime soon.

Sarah Al-Ahmed: I wish we had funding to put just a bajillion spacecraft around every world. 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 chunky rivers that they have over there.

Sam Birch: Yeah, thanks for having me and letting me talk about all this in 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 at 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.

Sarah Al-Ahmed: It's a real shame that we can't survive the surface of Titan, but knowing that it's 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, a link to it on this week's Planetary Radio page at planetary.org/radio. Now, let's check in with Bruce Betts. The chief scientist of The Planetary Society for what's up. Hey, Bruce.

Bruce Betts: Hey, Sarah.

Sarah Al-Ahmed: Is that the clog in your throat from all of the firework smoke?

Bruce Betts: Oh, but it was worth it. Totally worth it. Hey Sarah, how you doing?

Sarah Al-Ahmed: 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.

Bruce Betts: Well, all right, 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. 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 skies. 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. 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 predawn 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 getting pretty high in the East in the early morning and Jupiter and Saturn will be playing with us for the next few months.

Sarah Al-Ahmed: It's always nice to have them there after Venus goes away. I always love that walk home from work when you can see Venus just shining brightly as you're walking home.

Bruce Betts: It's pretty awesome. Onto this week's space history, a couple of big spacecraft flybys separated by many a year. In 1965, Mariner 4 first successful flyby of Mars giving us our first glimpse at the Red planet up close. A few decades later in 2015, New Horizons flew by Pluto and the Pluto system giving us all sorts of fun surprises.

Sarah Al-Ahmed: It's throwing me back to ... I don't remember what age of the internet everyone got back into Gregorian chants, but I was there for that.

Bruce Betts: It was a dark, dark day. All right, so got an oddball one here. 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 teams as they call it at the time of the Mariner 10, which was the first to flyby 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.

Sarah Al-Ahmed: 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.

Bruce Betts: Well, 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 how'd we do?

Sarah Al-Ahmed: We did really well, although I must guess that most people had to Google this because their units were very, very precise. But when I first learned this, it actually blew my mind because I expected that it was going to have to be pretty thick to guard this spacecraft from the sun and make sure that one side was 70 degrees Fahrenheit cold enough to not melt everything. But that heat shield is only about 4.5 inches thick. That's 115 millimeters.

Bruce Betts: Yes indeed. It's pretty amazing.

Sarah Al-Ahmed: We have two winers 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 Hane Woo Cheng from Seoul, South Korea. So you'll be getting awesome posters. We've got some great comments this week specifically about the heat shield because this is just so strange. 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 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." 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. 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, "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. 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. 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 [email protected]. 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.

Bruce Betts: It is an honor indeed.

Sarah Al-Ahmed: So what's our trivia question for next week? This is our last space trivia question on the show. So everyone, get ready.

Bruce Betts: Yeah, I hope to want to do better, but this is what I've got. I just made it personal with things near and dear to my heart. In my PhD thesis, I quoted the great musical group Warrant. Yes, that's right, "metal."

Sarah Al-Ahmed: Metal.

Bruce Betts: I quoted from at 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/radiocontest for the last time.

Sarah Al-Ahmed: That's a deep cut and people will have a harder time googling this one. So I like this. This is going to be a challenge, and you have until Wednesday, July 19th at 8:00 AM Pacific time to get us your answer. 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 sales stuff, and one of our last rubber asteroids. So, I'm excited. I'm excited for this one.

Bruce Betts: Cool, cool, cool. Do some digging. All right. Everybody go out there. Look up the night sky and think about your shadow and how you can make it look like funny little animals. Thank you and good night.

Sarah Al-Ahmed: 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 [email protected]/ join. Mark Hilverda and Rae Paoletta are our associate producers. Andrew Lucas is our audio editor. Josh Doyle composed our theme, which is arranged and performed by Pieter Schlosser. Until next week, ad astra.