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Oliver Strimpel: This is Geology Bites with Oliver Strimpel. So far, almost all the mapping of Mars has been performed using data from spacecraft in orbit around Mars. While imaging instruments aboard these craft can now obtain an impressive resolution of 25 centimeters, converting the imagery into a geological map requires ground-truthing; that is, making the connection between satellite image features and what they correspond to on the surface. The most reliable way of doing that is to identify an actual corresponding rock sample on the ground. On Earth, we can just organize a field trip to the location in question, look at the rock closely, perhaps using a hand lens, and take it back to a lab, where we can bring to bear powerful chemical analysis and dating techniques. But on Mars, at least so far, we need to land a robotic rover.

Katie Stack is Deputy Project Scientist for Perseverance, a NASA rover that landed in Jezero Crater on Mars in February 2021. A geologist by training, she has been mapping the geology of Mars since the 2000s. Katie Stack, welcome to Geology Bites.

Katie Stack: Thank you. I'm so happy to be here today.

Oliver Strimpel: It's hard enough making a geological map of Earth. How do you go about making such a map of Mars?

Katie Stack: There are some similarities and some differences. Our main data sets, instead of our eyes and walking around in the field as we would do on Earth... it's a photogeologic exercise. So we use orbiter images, and as you mentioned, they're quite good resolution, 25 centimeters per pixel. And we use those images, as well as digital elevation models produced from stereo pairs of those images, to give us topography. It's our job to try to take what we see with our eyes in those photos of the surface of Mars, drape it on top of the topography of Mars, and try to determine what geologic units are present on the surface.

Oliver Strimpel: Before we get to Perseverance, I can't resist asking a kind of broader geological question about Mars. We all know about the volcanoes, and in fact that Olympus Mons is probably the biggest volcano in the solar system, but are there other mountain ranges on Mars, anything resembling the kind of orogenic belts we have on Earth?

Katie Stack: Not really. And that's an interesting thing about Mars, and what makes it different from Earth in a lot of ways. As far as we understand it, Mars didn't have plate tectonics, so we don't get the creation of that kind of topography in those kinds of mountain belts. But we did have structural events occurring in Mars's history. And so while we may not have those big mountain belts, we have vast canyons. Valles Marineris is the best example, the largest canyon system that we know of, about the size of the United States. The other difference we have geologically on Mars is the role and preservation of impact cratering. Impacts have so much shaped the surface of Mars and what it looks like, and we have images of these soaring crater rims and the deposits that form within these craters.

Oliver Strimpel: Let's get back to Perseverance. What instruments does it have that are relevant to the geology on Mars?

Katie Stack: Well, we have a very exciting suite of science and geology instruments on this rover. Perseverance we think of as a roving astrobiologist. It can make those basic, fundamental geological observations, because we have context cameras that can image the landscape from far away, but also very much up close. And we have the next version of the Mars Hand Lens Imager that was on the Curiosity rover; we have a very similar camera on Perseverance. It's called WATSON, and that, of course, assists in building geologic context. But we also have the ability to view the surface of Mars with the Perseverance instrument payload in other parts of the electromagnetic spectrum, at different wavelengths. That ranges from the PIXL instrument, which uses X-rays to study the composition of the surface, to our visible and near-infrared multispectral cameras, which can reveal more about the chemistry and mineralogy of the surface. I should also mention, if we're talking about geology instruments, a brand-new instrument, RIMFAX, which uses ground-penetrating radar to see into the subsurface. That's an exciting new capability, especially from a geology perspective, as we look to build the stratigraphic and depositional context for the rocks that we see at the surface.

Oliver Strimpel: How far can you penetrate?

Katie Stack: It depends on the area, but up to 10 to 20 meters below the surface.

Oliver Strimpel: That's amazing.

Katie Stack: Yep.

Oliver Strimpel: When we spoke before, I think you also mentioned a microphone.

Katie Stack: Yes, we have a microphone riding along with the SuperCam instrument, which is an instrument suite on the mast, the head of the rover. And we can turn that microphone on to hear the sound of the SuperCam laser basically ablating, zapping, the surface of Mars, and we're using that sound to better understand the material properties of the rocks and materials that we're zapping. We can also learn about the atmosphere, as the sound travels through the air to get from the surface to the SuperCam microphone. So we are able to engage another sense for the first time on the surface of Mars. We see, and now we can hear, on the surface.

Oliver Strimpel: So let me see if I understand this. You zap a rock with a laser, it then suffers some kind of explosion or something, which you then pick up with a microphone?

Katie Stack: Well, I would say it's not quite as exciting, perhaps, as an explosion would make it sound, but we use a technique called LIBS, laser-induced breakdown spectroscopy, where the SuperCam instrument fires the laser at the surface of Mars and we essentially turn that rock or soil into a plasma. Then we have a spectrometer in the SuperCam instrument that analyzes that plasma and looks at its composition, and that's how we figure out what elements are present in the rocks or soils that we're studying. But when we do that zap, when we do that ablation and turn the rock or soil into a plasma, it makes a sound, and we can hear that sound with the SuperCam microphone.

Oliver Strimpel: It's very early days still, I realize, but what have you seen so far with these instruments?

Katie Stack: Yes. Well, we've seen rocks, lots of rocks, and lots of sand, and that's as expected in Jezero Crater, which is now the home of the Perseverance rover. The crater itself is home to a deposit that we think was formed when an ancient river entered Jezero Crater and deposited those sediments, and then we have the crater floor, which is where Perseverance landed. We talked a little bit about geologic mapping, and this is now one of the most mapped craters; any time we send a mission to Mars, that area becomes the focus of many geologic mapping efforts. So we've thought for years about what these rock units represent.

And there was a lot of debate about the crater floor. Some folks thought it might be volcanic. Every time we see a heavily cratered surface on Mars... many folks think of Mars as a volcanic, basaltic planet, and so there's often an assumption that every cratered surface we see is an ancient lava flow. But more recently, especially fed by new observations from the Curiosity rover in Gale Crater, where we have almost exclusively sedimentary rocks, we're learning that maybe that distinction isn't quite as easy to make as we might have thought. So other folks thought that the crater floor could be sedimentary. We are, after all, in an impact crater with a delta and an ancient lake, and so perhaps these rocks are ancient lake sediments. So we went into the landing of the Perseverance rover with this big question: are these rocks volcanic, are they sedimentary, or are they something else, impactites or something like that?

When we landed, we found rocks that appear to be relatively fine-grained, but we are still asking ourselves: are they volcanic, or are they sedimentary? We are still in the process of getting our instruments online and operational. As we bring on some of our instruments that can see the rocks up close and personal, I think we'll make more advances on that question. But it is the big question, and it's one we're still asking, even though we're on the surface right now.

Oliver Strimpel: That is amazing, that with all this incredible technology there, you haven't been able to answer the most basic question about these rocks. If you had a sample of this rock in your hand here on Earth, how hard would it be for you to identify it?

Katie Stack: Yeah, you know, that's a great question, and it almost isn't a fair question, because when we as geologists and humans are looking at a rock, we can bring to bear something as simple as taking the rock and turning it to get a different lighting condition, or changing our point of view on the rock, or moving it closer to our eyes or further away. Those are such simple things for us to do. My guess is that if I was holding this rock in my hand, I don't know, maybe I'd give myself a couple of minutes just to be sure, but then I'd probably have a pretty good sense. It really speaks to the challenges, I think, of doing geology remotely. Every motion, every approach you take to a rock, is something that you have to tell your robotic explorer to do, and lighting isn't always favorable. Sometimes you have shadows in the way, and you have to work around that. And of course, with a fine-grained rock, whether it's a basalt or a fine-grained siltstone or mudstone or fine sandstone, you have to really think what observations, at the limits of resolution that your instruments allow you to see, would allow you to distinguish one from the other. Also, the bar has to be very high for confirming an interpretation, so it's good to have healthy skepticism.

I think what we've seen over the course of the eight and a half years that the Curiosity rover has been on the surface of Mars is that you have this uncertainty in the beginning. You're in a brand-new place, you've never seen these rocks before, and we've only been to a handful of places on Mars to begin with. So it takes a little while to calibrate our eyes and our expectations and build up the geologic context that feeds into interpreting the origin of a rock. But once we get a couple of miles under the wheels and we've been at the landing site for a while, I'm going to guess that we'll look back on these rocks and say, well, of course they were this, we just needed a little bit of time to work through it. It's all part of the natural evolution of doing robotic geology and remote geology.

Oliver Strimpel: Yes. That brings me to my question as to what the overall geological objectives of the mission are.

Katie Stack: Yes. Well, Perseverance is the first step in a potential Mars Sample Return effort to bring samples from Mars back to Earth. The fundamental questions we have about Mars and its history, and whether there was ever life on Mars, really need to be answered here in labs on Earth, using the full set of analytical capabilities that we have here. There's a limit to what you can miniaturize and send to another planet and have it be robust enough to work. It can take some very sensitive instruments to determine whether something is indeed a biosignature, a sign of ancient life, and we see that in our own study of the ancient Earth rock record: it's sometimes quite contentious whether we're looking at evidence for ancient life.

So Perseverance's role is to collect the samples. We have a sampling and caching system that allows us to collect rock and soil samples, about 15 grams per sample tube. We have 43 tubes that we carry with us, some of which are control tubes, witness tubes we call them, but we're aiming to collect about 30 geological samples. And so, because Perseverance has this role in Mars Sample Return, we are looking for a special suite of samples from Mars that will help us address whether there was ancient life on the surface of Mars at some point in the past, and understand the evolution of the planet from a time when it was habitable to today, when it is no longer habitable.

Oliver Strimpel: Let's talk about the logistics of the mission. How do you actually go about achieving these objectives? I guess you're planning from day to day, week to week, month to month, year to year.

Katie Stack: We tackle it at different scales, both spatially and temporally. We had already started before Perseverance even arrived on Mars. We were thinking about our strategic, long-term planning of what the mission would look like, and we started that using the orbital data and doing a lot of the planetary geologic mapping to understand what rock units were present, and even just in selecting the landing site: where did we want to go, and what did we know about these places based on our existing data sets? So before we landed with Perseverance, we had already sketched out possible traverse options with the rover that would ensure that we would visit all of the exciting geological units with high potential for having signs of ancient life. But then, of course, you get on the ground and you have a whole new scale of data to deal with. You can see the surface and see the rocks up close, and that can change things. So you have to build some discovery and exploration margin into your plan. And then, of course, we didn't know exactly where we were going to land. So that's the long-term planning. You're always thinking, OK, where do we want to get to, and what kind of diversity do we need to put into the sample cache? That certainly keeps us motivated to keep moving with the mission.

But day to day, we pull up to a new location and survey the rocks around us. Depending on what our resource allocation is, and we think a lot about power and time every day when we plan for the rover, we'll plan science activities. We'll decide which instruments we want to use today, which rock targets are interesting, and which instrument capabilities would answer the questions we have about the rocks. Sometimes we're using the SuperCam instrument that we've talked about. Sometimes we're using just the cameras. Sometimes we'll say, well, this is really interesting, we have to get the arm of the rover out and use some of our up-close imaging and spectroscopic capabilities. It's dynamic day to day, but you always have to have that higher-level plan in mind; otherwise, you can really go down the rabbit hole at any one outcrop. It's this balance of day to day versus month to month versus year to year. And we've got this follow-on mission to collect the samples, so we're a little different from other missions in that respect: we have a ticking clock, and we have to meet the next mission that's coming.

Oliver Strimpel: Can you give me an example of a kind of opportunistic discovery activity that you didn't know about in advance, but you just had to do?

Katie Stack: Yes. Well, I can give you an example from the Curiosity rover. This was a great example, because we landed right on top of it. When we arrived in Gale Crater with the Curiosity rover, we happened to land on conglomerates, river deposits, and we had no idea they were there in advance, because they were too small to see and resolve in the orbital data. So suddenly we realized that we had plopped ourselves right on top of a major scientific discovery, which was the first confirmed river deposits observed by a rover. And so we had to work into our plan some additional time studying these outcrops, so that we could do some paleohydraulic calculations to figure out the depth of the stream and how fast it was flowing, and what implications that might have for the climate of Mars at the time.

We have another great example from Curiosity. The main focus of that mission is, and always has been, the five-kilometer-high mound of sedimentary rock in the center of the crater. But we realized when we landed that if we went the opposite way, away from the mound, we might have a chance to see some potentially habitable lake deposits. So we took a risk and drove in the wrong direction first, and it paid off. We found our habitable environment, we found those lake deposits, and we were able to check off some of the major high-level objectives of the mission by making a bold decision to go in the other direction. And we have the same thing for Perseverance. We're currently deciding how to get to the delta. We have two main routes in play right now, and we're trying to figure out which way we should go, balancing efficiency with scientific interest.

Oliver Strimpel: Can the rover negotiate all the terrain? There must also be safety considerations about how you get from A to B.

Katie Stack: Yes, there are certainly safety considerations, but we have capability on Perseverance that allows us to navigate terrain, I think, more efficiently than previous missions. The rover has an updated computer processor that allows it to do autonavigation, so it can image the surface as it's driving and basically process those images to pick a safe route as it's driving along. Previous rovers have had this capability too; Curiosity has it, but with a slower computer it is very time-intensive to do this kind of autonavigation. With Perseverance, we can do that autonavigation much more efficiently, so we should be able to drive upwards of 150 meters per day on good terrain, whereas previous rovers have averaged something closer to maybe 30 to 50 meters per day.

Oliver Strimpel: What's the long-term destination? You mentioned checking out the delta deposits. How far away is that, and where do you expect the rover to wind up at the end of its mission?

Katie Stack: We landed about two kilometers from the main delta deposit, although we have a couple of what we think are remnants of a formerly more extensive delta deposit, so we might have the ability to preview these delta deposits before we get to the main, contiguous delta. But of course, the delta is one of our main exploration targets in Jezero Crater, as well as some deposits around the inner margin of the crater, essentially at what we think might have been the shoreline of the ancient lake that once existed here in Jezero. We're very excited to explore those marginal deposits, as we call them. Our goal is to have completed our exploration of Jezero at about the qualified lifetime of the rover, which is about one and a half Mars years, or about three Earth years, and to have deposited a cache of samples on the surface.

But then after that, if the rover is healthy and doing well, we have a decision. We can choose to stay in Jezero and continue our exploration within the crater, but we've also talked about venturing outside the crater. We often talk here on Earth about source to sink. Since we landed in the crater, we're probably doing the sink first, but we might have the opportunity to go and explore the source of the sediments that were brought into the crater and that we've explored in the delta. It would be really exciting to have the chance to see these rocks in place, where they were originally deposited. And outside the crater, we also have some very ancient Martian crust, 3.9 to 4 billion years old and older. That period is a part of solar system history that is not well preserved in rocks here on Earth, because of the constant recycling of our own crust.

Oliver Strimpel: I'm curious about the team logistics and how you operate. What is your responsibility as Deputy Project Scientist?

Katie Stack: Yes. I have the great honor to be part of the science leadership team, leading a team of about 450 scientists from around the world who participate in the Perseverance rover mission. There are three of us in the project science box: our Project Scientist and then two deputies, of which I am one. It's our job to lead the science team, and also to advise and consult with the engineers who are operating the rover. Oftentimes we are consensus builders, and getting those 450 people on the same page to operate a single robotic explorer is sometimes no small feat.

Oliver Strimpel: What's a typical day like for you at the moment? Are you somehow tied to the Martian diurnal cycle?

Katie Stack: Yes, it's a tradition for Mars rover missions to be on what we call Mars time in the early days of the mission. The Mars day is about 40 minutes longer than the Earth day, so sometimes that means that Earth and Mars time are pretty well aligned, but at other times we're completely out of phase with Mars. For the first 90 days or so of the mission, we have the scientists and engineers living on Mars time. I did a modified Mars time, because I have young kids here at home with me, and young kids don't respect the Mars time schedule. So I did my best, putting in a full day and then staying up later to help operate Perseverance. But fortunately, we are getting to a period where we are now out of those overnight shifts, at least for the West Coast of the US, where I am. So we are transitioning back to Earth time. With that, though, comes a loss of efficiency in how you operate the rover, because there are times now when we'll be sleeping but the rover could be awake doing things, and we're just not awake to program the rover to do it. But of course, it comes with the advantage of giving us all a little bit of a break.

Oliver Strimpel: At what point will you... Have you already pulled out your suite of astrobiology experiments?

Katie Stack: We haven't done it yet, because those instruments are still coming online, but we anticipate that in probably about one or two months, we'll have the full instrument suite ready to go. We're very excited to pull out our astrobiology instruments for the first time and to do that really detailed mapping of the surface of these rocks, to look for these combined signals of elemental composition, mineralogy, and the morphological textures that we can see, as well as the distribution of organics. By putting all of those things together, that's how we hope to make a case for a possible biosignature.

Oliver Strimpel: Far and away the biggest prize of this mission would be to find a biosignature on Mars. Can you speculate as to what combination of discoveries with the instruments on Perseverance, if we are so lucky as to find a biosignature, is likely to at least initially clinch the issue?

Katie Stack: Yes, that's a great question. What would a biosignature look like on Mars, and what are we looking for? Based on our previous exploration with other rovers and landers on Mars, and on our understanding of how Mars has evolved over time, we're not necessarily expecting to find evidence of ancient complex life forms on Mars. We joke about the proverbial dinosaur bone. Our expectations are that if there was once life on Mars, it probably did not progress past the microbial stage. So we're looking for evidence in the rocks of past microbial life on Mars, and we have some great analogs for that here in the early Earth rock record.

One thing that we point to often is a texture called stromatolites. Those are fossilized microbial mats, and they form when growing mats interact with the sediment, competing with the sediment to reach the light and get to the water. Here on Earth, we see in the rock record that they form all kinds of interesting morphologies, conical shapes and mound shapes. They're the kind of feature that, when you find them in the field, you look at them and think, well, that's odd, it doesn't seem like physics would just naturally form these weird shapes. But again, on Mars we have to have a really high bar for what could be formed biologically or abiotically, and work has shown here that you can even get stromatolite-like shapes without life being present. That's always a tricky thing. But that's exactly what we're looking for: interesting textures in the rock that make us pause and scratch our heads and think, well, how would you have formed this without life, without microbial metabolism?

And then we combine those textures with the geochemical and mineralogical side, knowing that when life is present and preserved in rocks, you'll often see evidence of that microbial metabolism. You'll see the elements and the minerals that life likes to use, preserved and concentrated in ways that would be unexpected if life wasn't present. Good examples of that are concentrations of silica, fine silica laminae, or iron concentrations, or certain minerals that life might be taking advantage of. And then, of course, we have the SHERLOC instrument, which can detect and map the distribution of organics. So we'd be looking for concentrations of organic molecules, perhaps in these laminae, suggesting that it's not just a random distribution of organic molecules that may be raining down onto the surface of Mars, but concentrations of organics suggestive of past microbial life, or of microbial mats even growing there.

Oliver Strimpel: Short of a potential biosignature, what would you regard as the most significant finding that the rover might be able to make where it is now?

Katie Stack: One of the things is to learn more about the ancient lake that we find ourselves in: understanding the evolution of that system, the interaction of the rivers coming into Jezero Crater, the history of water in this crater, and how that system evolved in its habitability and composition, and how long it was there. I think we can make progress on that, and on learning more about this delta, using it as a representative of other ancient crater lakes that we know are present on the surface of Mars.

One of the exciting things about Jezero Crater compared to other places on Mars is the presence of carbonates, carbonate minerals that we've seen from orbit. There's been a long-standing question about the evolution of the atmosphere of Mars. We think that Mars once had a thicker atmosphere, and the atmosphere on Mars today is mostly CO2. But then there's been this question: if Mars had a thicker atmosphere, where did all that CO2 go? One idea is that it could have been taken up into carbonate minerals, but carbonates are relatively rare on the surface of Mars, and Jezero is one of the places where we have confirmed carbonate deposits. So I think by studying those deposits in Jezero, we have an opportunity to make some great progress in understanding the evolution of the atmosphere and surface environments on Mars.

Oliver Strimpel: If you could design your ideal rover for the next Martian exploration mission, what would it look like, and where would you send it?

Katie Stack: Oh, this is a great question. It's kind of like picking your favorite child or something like that, because there are so many great places on Mars to explore. One thing that I think would be so valuable for a Mars rover mission is to have absolute age-dating capabilities on the rover. That's one of the things we are most looking forward to with Mars Sample Return, because while we have constructed a relative geologic history for Mars, we're missing the absolute time scale. We have to rely on geologic relationships: we think we see this unit on top of that one, but we don't know how old they are. So being able to put absolute ages on the rocks that we explore in situ on the surface of Mars would, I think, be a great advance.

Then the question would be, where would I send it? I tend to like the sedimentary rocks on Mars; that's what I study and what I'm most interested in. One of the places that I'm very intrigued by is Valles Marineris, the big canyon system. What's really neat about Valles Marineris is that you have in one place this combination of major tectonic, structural events on Mars coupled with sedimentary processes. We have deposits within Valles Marineris that we think are probably sedimentary; we have sublacustrine fans, and we also think there is evidence for volcanic deposits there. So in this canyon system, kind of like the Grand Canyon here in the western US, you have the chance to see a vast record of geologic history in one place, while also studying the confluence of structural, tectonic, sedimentary and volcanic, and maybe even impact, processes on the surface of Mars. So I'm very intrigued by Valles Marineris. We've never sent a robotic explorer there, so I might choose that place.

Oliver Strimpel: Katie Stack, thank you very much.

Katie Stack: All right. Well, thank you so much for having me.

Oliver Strimpel: For more about Geology Bites, as well as pictures and illustrations that support this podcast, you can go to geologybites.com.