This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. While Mars has been visited by several missions over the past few decades, Venus has seen very little exploratory activity. But in June 2021, three missions to Venus got the green light, two from NASA and one from the European Space Agency. One of the NASA missions is called VERITAS. It will create integrated global maps of the topography, radar reflectivity, gravity field and rock type for the Venusian surface. It is the first mapping mission to Venus since the Magellan mission in the early 1990s.

Sue Smrekar is the principal investigator for the VERITAS mission. She has been part of multiple NASA planetary explorations, including the previous one to Venus, Magellan, and the InSight robotic lander on Mars. Sue Smrekar, welcome to Geology Bites.

Sue Smrekar: Thank you. Thanks for having me.

Oliver Strimpel: It's only a few months since VERITAS got approved. Can you give us an overview of the mission calendar from here on out?

Sue Smrekar: Sure. The timeline we anticipate is to launch in December of '27. It takes about six months to get to Venus, and then we have two phases of aerobraking, where we slow down using the atmosphere: we put our solar arrays into the atmosphere and use them basically as brakes, so aerobraking. So we do that for a period of six months, we interrupt it, and then we do another period of 10 months. In that interim time period, we are in a pretty elliptical orbit, and we can use our spectrometer from that altitude to acquire data from the surface. Once the second phase of aerobraking is complete, which will be in early 2030, we will then start our full science operations, which will continue for three Earth years, and that's basically four Venus cycles. Venus will fully spin underneath us four times during those three Earth years.

Oliver Strimpel: So when you're in this highly elliptical orbit, will you be holding off the aerobraking so that you can use the spectrometer, and then, when that phase of the mission is done, switch the solar panels into their braking position again and go down into your lower circular orbit, right?

Sue Smrekar: During the time when we're taking the spectrometer data, we ease out of the atmosphere a little bit, and that time period is chosen because it covers conjunction, when Venus is on the other side of the Sun from the Earth. So we have a hard time communicating with the spacecraft, so we don't want it to be in any position where it could get itself into trouble. So we pull back a little bit from the aerobraking, so we keep it in a stable configuration, and we take advantage of the fact that we can take good data with our spectrometer during that pause in the aerobraking.

Oliver Strimpel: And then after that phase, when you go into the circular orbit, is the main point to get closer to the surface, so your mapping activities will have higher resolution?

Sue Smrekar: Yes. So the final orbit is a trade-off against having too much atmospheric drag, so that we'd have to be constantly adjusting the spacecraft and keeping it from crashing into the surface, if you will. So we wanted to have a stable orbit, but as low as possible, and that supports the high-resolution data that we acquire from our radar instrument and for our gravity field.

Oliver Strimpel: What are the main scientific questions that VERITAS will seek to answer?

Sue Smrekar: The primary three are, first, understanding how rocky planets evolved. Another is understanding what processes are active on the surface of Venus today. And the third one is looking at the influence of past and present water.

Oliver Strimpel: Before we go into more detail about how the mission will shed light on the scientific questions, let's talk about the instruments on board the spacecraft.

Sue Smrekar: OK. So we have two instruments and a gravity science investigation. So the radar has just one radar wave that we propagate outward, but depending on how we process it, it gives us lots of different types of information. The timing of the return of the radar signal tells us about the topography: we directly measure the timing and get the height of the topography. How the radar wave scatters as it returns tells us about the surface properties. How it's scattered at the surface has to do with the roughness and, to some degree, the composition of the surface at the wavelength of the radar, so around several centimeters. That's what makes up the radar image. So the rougher the surface, the more of the radar signal will be bounced back to the spacecraft, and so it will look brighter. But if it's smooth, the signal might just bounce away, and you won't see anything; it'll look dark. So that's how the surface texture has an impact.

Oliver Strimpel: What about the surface composition? You just mentioned that that can make a difference as well, right?

Sue Smrekar: So if there's a kind of metallic, if you will, type of material at the surface, that also will cause the radar wave to be reflected back more strongly. What we've seen in the Magellan data is that there's something called a snow line on Venus, which sounds a bit crazy, but that term is used because it corresponds to an elevation, which corresponds with temperature. And we've seen evidence of perhaps ferroelectric minerals at the surface, and there's a lot of debate and argument as to what that could actually be. But there seems to be a variation that corresponds to temperature differences that cause the minerals at the surface to be more or less reflective. In some places, the difference in the reflection corresponds very nicely to the elevation. In other places, it seems to not exactly reflect the elevation, suggesting that it's not yet fully in equilibrium. So people have suggested that implies recent activity, so that the chemical reaction has not fully gone to completion. So it could be a place where there's relatively recent volcanism.

Oliver Strimpel: So is Venus a bit cooler as you go further up in elevation, just like the Earth? And are you suggesting that therefore these conductive materials are not stable at the higher temperatures, and kind of have evaporated, and therefore they can only exist as ferroelectric snow, if you like, on top of the mountains?

Sue Smrekar: Yes, exactly. They don't necessarily evaporate, but the near-surface coating will change. So the minerals right at the surface will change slowly as they chemically interact with the atmosphere at a given temperature.

Oliver Strimpel: So Magellan also had a radar instrument. How does the VERITAS instrument compare with that on Magellan, and what kind of different mapping results will we get?

Sue Smrekar: Magellan was an S-band radar; VERITAS is X-band. So we are at a slightly shorter wavelength, and we decided to use that wavelength to focus on producing the highest-resolution topography dataset that we could. It also will give us good radar imaging, but there's a big trade space of what radars will operate on Venus, due to the atmospheric attenuation, and what wavelength is optimal for different types of datasets. And we decided to optimize to produce topography to the best of our ability.

Oliver Strimpel: And what kind of resolution can we expect?

Sue Smrekar: Well, we'll have two orders of magnitude better topographic resolution relative to Magellan. So Magellan is about 15 to 25 kilometers per pixel in topography. VERITAS will have 250-meter pixel size, with 6-meter vertical precision. So if you imagine Hawaii, we have a few dozen pixels at Magellan resolution, so you can tell that there's a topographic high there. With VERITAS, we will be able to see calderas, fracture systems, individual lava flows. So the detail that we'll get in the topography will just be exquisite. Every time we have explored a planet with an order of magnitude better resolution, we have just uncovered features that we had no idea existed there. So I am ready to be fully amazed as to what we may discover on the surface of Venus.

One other type of data that we will acquire with the radar is surface deformation. This is the first time we will search for active surface deformation using radar on another planet. Again, the resolution that we'll be able to get is just incredible. We will have the ability to see vertical deformation on the scale of a couple of centimeters over a kilometer horizontally. So we can see if a caldera above a magma chamber has moved; we can search for active faulting.

Oliver Strimpel: That's fantastic. So it will be on successive passes over the same part of Venus. So the orbit is fixed in space, and the planet is rotating slowly below it, and when it comes back to where it was before, which is 243 days, you'll be able to get another measurement. Is that how it works?

Sue Smrekar: To first order. But to take this kind of data, we need a higher order of precision. So we actually have to adjust the path of our spacecraft very carefully to get in exactly the right spot to reacquire this data, because any differences in the position of the spacecraft could be misinterpreted as deformation. And so we actually have a 160-meter tube, if you will, that we have to fly our spacecraft through, in order to be close enough to the exact same position that we were in previously. So even though, in general, our spacecraft does come around again to the same place, that's on the scale of a kilometer or two.

Oliver Strimpel: Is that a navigational challenge for the rocket engine people?

Sue Smrekar: Yes, yes. Navigators love this kind of challenge.

Oliver Strimpel: I bet. Good old NASA JPL ingenuity at work here. So let's talk about the other instrument, which is called the Venus Emissivity Mapper.

Sue Smrekar: Yes. A couple of decades ago, people did not think you could observe the surface of Venus with a spectrometer through the very thick cloud layer and the dense CO2 atmosphere. However, a couple of missions have shown that that is possible. Galileo flew by Venus with a spectrometer that included the bands near 1 micron, and was able to get the first hint of the surface beneath the clouds using spectral measurements. And that was followed up by ESA's Venus Express mission. Although their spectrometer was not designed for that purpose, they were also able to take observations of the clouds, which we need to do part of the data processing, and in a relatively narrow band near 1 micron, 1.02 microns, which is in the infrared portion of the spectrum, they showed that they could map variations at that wavelength on the surface, related primarily to composition. And our spectrometer, the Venus Emissivity Mapper, is the first one that is fully designed to optimize observing the surface of Venus.

Our instrument has six bands right around 1 micron. Those are the only ones where you can see through the CO2 atmosphere. Fortunately for us, that area is very active for iron mineralogy, iron 2+ and iron 3+. That's where the absorption features are for various iron minerals. And although we don't have such a broad part of the spectrum that we can say, OK, this is a given mineral, what we can do is get at the overall iron content, and that's just really important for a lot of fundamental questions about the rock type on the surface of Venus. You can also use it to address this question of whether or not there are recent volcanic flows on the surface. Venus Express saw six or 10 different locations where there's high emissivity of the surface, and that's consistent with relatively fresh basalt on the surface that has not fully interacted chemically with the atmosphere. And we'll also be looking for thermal flows, things that are actually incandescent on the surface, recently erupted. We'd have to be very lucky to catch active flows with our spectrometer, because as soon as a flow comes out of the surface, it cools at the surface, forming a crust, and so the time period during which a typical flow has an incandescent glow is relatively limited, on the order of weeks, typically. The Venus Emissivity Mapper, unlike the radar, is a passive instrument, and it's just examining the surface, looking for its emission in this near-infrared band around 1 micron.

Oliver Strimpel: So you're going to basically get out of this the global near-infrared map, I suppose. What spatial resolution will that map have?

Sue Smrekar: Well, it's quite low resolution compared to most spectrometers. It's limited to 50, 60 kilometers, because that is the height of the cloud layer, and there is absorption of the signal in the cloud layer, and we have to do data analysis to take out the effects of the clouds. But we're limited by that scale of the cloud height. But it will be the first-ever global look at composition.

Oliver Strimpel: So you also mentioned a gravity experiment, and that reminds me of an earlier podcast in which Dan McKenzie talked about using accurate measurements of the orbital velocity of Magellan to measure the gravity field, and he used that, coupled with a topographic map, to determine something about the interior of Venus. Will you be doing a similar experiment with VERITAS?

Sue Smrekar: Yes, very similar. We will be improving on the resolution for gravity by a factor of two or three. That basically gets us down to 150-ish-kilometer resolution, and that's very important for understanding the elastic thickness, which is the topic you were speaking to Dan McKenzie about. So we will be able to see much more precisely the deformation of that elastic lithosphere, reflected in the gravity and topography. So I'm very excited for that measurement as well.

Oliver Strimpel: OK, so now that we've discussed the instruments and the gravity experiment and the kinds of data that you'll be collecting, let's talk about how these data bear on the scientific questions that you mentioned earlier. And let's start with the processes that shape rocky planet evolution.

Sue Smrekar: We are basically going to be studying the tectonic and volcanic processes expressed on the surface of Venus, and those processes give us a window into what's going on inside the planet. They give us a sense of how the interior shapes the surface, and how that has contributed to the evolution of the atmosphere over time. For example, one question is: did Venus catastrophically resurface, or has it been steadily resurfacing over time? And by resurface, what I'm really talking about is what happened to the impact craters on the surface of Venus. If you look at the Moon or Mars or Mercury, they're littered with impact craters. Venus has only about 1,000. That tells us that it is a young surface. How young is a matter of debate. It could be on average as little as 150 million years; at the far end, it could be as much as close to a billion years on average. But perhaps the bigger question is, what happened to all the prior impacts?

You know, if you look at the surface of the Earth, you don't see very many impact craters. Now, we have very different processes, in the sense that erosion is a dominant process and sedimentation is a dominant process on Earth, not so much on Venus. So many impact craters are simply eroded away. But still, the surface of the Earth, especially the oceanic lithosphere, is quite young: at most 200 million years, on average about 50 million years. And so the age of the surface of Venus is more similar to that age, which really says it's got to be an active planet. So what wiped out the impact craters, and when? There are two competing models. One is the so-called catastrophic model, and that says there was some massive event. Let's call it volcanism, flood volcanism. The surface would have been covered in a kilometer-thick layer at least of volcanism, globally, the whole planet. So you can imagine that that is an entirely different geodynamic regime than anything we've seen on the surface of the Earth. That's just mind-boggling, and both geodynamicists and climate scientists love this idea. It blows your mind. How would you do that? What would it have done to the climate? It suggests that something was happening in the interior that was just quite unlike the Earth, and it would have produced very massive changes in the climate: hundreds of degrees, rapidly, in the atmospheric temperature.

Another model shows that if you have small patches of resurfacing (again, let's call it volcanic, because that kind of makes sense for Venus), with volcanism covering up impact craters on scales of a few hundred to a thousand-ish kilometers, that fits the data just as well. It's not quite as dramatic, but it fits the data just as well. And you can take into account other factors, such as the distribution of extended ejecta, the very fine-grained material that gets projected into the atmosphere when a meteor hits the surface. Those features are huge on Venus. They can be as long as 2,000 kilometers; the dust and small particles get carried downwind. We can see those in the Magellan data. So if you take into account how those have been disappearing, that also points towards a more steady process of removal of craters. And there's all kinds of new data suggesting that there has been recent activity on the surface of Venus: the Venus Express data I mentioned. People have also looked again at the Magellan snow line information that I mentioned, and there are also variations in the composition of the atmosphere, the concentration of sulfur dioxide, on a very short timescale of years, a decade. So that could be coming from volcanoes on the surface. So I think this paradigm of Venus having catastrophically resurfaced, the thing that you typically read in textbooks, is really changing. So that's a question that we'll very precisely address, by looking at the distribution of volcanism, particularly in and around impact craters.

Oliver Strimpel: With the infrared data, mainly?

Sue Smrekar: Well, a combination. One of the big questions that we'll address is: what's the dark stuff, the radar-dark stuff, at the bottom of 80% of impact craters on Venus? It's dark, so it's smooth. Is that material that's filling those impact craters volcanism, or is it aeolian material that's gotten trapped inside those impact craters? If you look at impact craters on Mars, they're just full of dunes, because the dust gets in there and it can't get out, and that could be the case for Venus. But some preliminary studies suggest that the dark floors on Venus are actually volcanism. And that's based on the fact that if it's volcanism, you would expect to see not only that the impact crater floors would be shallower, but outside of the rims, you would also expect the volcanism to be flooding around the outside of the craters. And it's just hard to be completely definitive about that with the data that we have now. So not only will we be able to use the image data to see, does it look like a dune, does it look like a lava flow, but we'll also be able to see how the impact craters are flooded, if it is volcanism.

Oliver Strimpel: When you mentioned the idea of a catastrophic resurfacing of Venus, I was reminded of a previous podcast with Peter Cawood, in which he talked about a possible catastrophic overturn of a stagnant lid on Earth sometime before plates formed. Are you saying that the evidence suggests that such an event probably did not occur on Venus?

Sue Smrekar: Yeah. What type of geodynamic process you'd have is a very high-level question. I said volcanic resurfacing, but another idea that people have had is that the entire lithosphere somehow became gravitationally unstable and sank into the mantle: so, lithospheric overturn. A recent study looked at the relationship between the gravity and topography and suggested that that really didn't make sense, because if you had massive amounts of the cold lithosphere having sunk through the mantle to the core-mantle boundary, that would produce a signature in the gravity that we don't see. So could that have happened billions of years ago? Maybe, but that would have been a different event than the one that is purported to have wiped out impact craters. So that's another reason to suggest that this catastrophic idea doesn't fit the data well.

But in terms of a stagnant lid, yeah, it's a huge question. Geodynamic models have tended to focus on whether or not there are plates moving around at the surface. That's a first-order question for understanding the Earth: how did these plates come about? What are the conditions that allowed that to happen? One thing I'm super excited about studying is features that appear to be subduction on Venus. Dan McKenzie and others who were working on the Magellan data initially looked at many of these features and said, yeah, these are very much like some of the subduction features we see on the Earth. They seemed to suggest a similar strength of lithosphere, and they have similar morphology. That was a big debate, again. However, others said, wait, these features are circular, semicircular; they look like mantle plumes. So these are two entirely different processes, right?

One of the key questions that people studying plate tectonics, 50 years after the original hypothesis, are asking today is: how did it start? And one idea is that mantle plumes helped initiate subduction, and this idea of plume-induced subduction appears to be what's going on on the surface of Venus. So we have these curved, semicircular arcs of subduction, and that's what's produced when you have plume-induced subduction. The lithosphere of Venus is very hot because of the surface temperatures today, and many people have pointed out that that makes it a good analog for the early Earth, when the planet hadn't cooled down very much. So we can go to Venus and look at this process of plume-induced subduction, and attempt to better understand the elastic thickness of the lithosphere, whether or not it's happening today, maybe even see it forming in action. So we can really study in detail a process that probably hasn't happened on the Earth for billions of years. Or if it has happened, it's been in the context of a planet that already has plates. So we can look at it in the context of a planet that does not seem to have plates, and try to understand its potential role in starting the process of plate tectonics.

Oliver Strimpel: So does Venus have a stagnant lid?

Sue Smrekar: It doesn't seem to have any evidence for plate motion, but it may have an entirely different system. It may have a system that's going to eventually evolve into plate tectonics. Those are all questions we really want to address.

Oliver Strimpel: Let's move on to the geological processes that are currently active on Venus.

Sue Smrekar: Sure. In the Venus Express data, the locations that we've seen that appear to have recent volcanism are all in areas where there's additional evidence for mantle plumes. In the gravity data, there's evidence of low-density material at depth. The topography is uplifted in a broad swell, like the Hawaiian topographic swell. There are volcanoes. So far, what we've seen is that the recent volcanism is concentrated above mantle plumes. So is that a sampling question? Did we only happen to see that in the Venus Express data, or are there other types of volcanic processes? That's a really important question, because where and how volcanoes form gives us a window into what's going on in the interior: not only the temperatures in the interior, but also the volatile content. The more volatiles you have in a magma, the lower the temperature that it erupts at. So it's important for understanding the temperature and the viscosity of the mantle.

Also, a fundamental question that we're going to answer, or at least attempt to answer, is: is water still being released from volcanoes and coming out of the interior? Many people have proposed that the primary difference between Venus and Earth is that Venus is dry. Well, the atmosphere is definitely dry. The crust is very hot and definitely dry. But we don't know what's going on in the mantle. Some data even suggest that there may be more volatiles retained in the interior of Venus than of the Earth. Water has such an important effect on the viscosity of the mantle, the viscosity of the lithosphere and the processes of atmospheric evolution. It's an absolutely first-order question: is there a lot of water still in the mantle?

Oliver Strimpel: How would the data give us any clues on the answer to that question?

Sue Smrekar: One of the channels of our spectrometer is designed to see water in the atmosphere, close to the surface, in the lower 10 kilometers or so. Of course, we'd have to be lucky to see that. You have to have significant quantities of water, a few weight percent, shall we say, in the magma, which is observed in some magmas on the Earth. But you need that in order to have enough buoyancy for the gases to escape under the very dense atmosphere at the surface of Venus. So we'll be looking for that.

Oliver Strimpel: Is there any evidence of past water on the surface of Venus?

Sue Smrekar: Some people think there may be evidence of actual fluvial erosion features on the surface of Venus. I think that's still quite controversial, but I believe we'll be able to answer that question much more fully with VERITAS. But what a number of people see as evidence for past water is chemical evidence, basically a chemical fingerprint of past water. There are very large plateaus on the surface of Venus. There's about half a dozen. They are on the scale of one to two thousand kilometers. There are a variety of reasons to think they may be analogs of Earth's continents. The gravity data suggest that they could have crustal roots, just like our continents do, so they're so-called isostatically supported. So it's just this big pile of relatively low-density material, and they're highly deformed. VERITAS will be able to see all of these plateaus, and we'll be able to get at the iron content. And we think that they may be so-called felsic crust, granitic crust. Our continents are much lower in iron and higher in silica. That's a fingerprint of past water, because to produce that massive volume of felsic crust, what you need is to melt tons of basalt, more iron-rich rock, in the presence of water. You can't get continent-scale volumes of felsic rock without water. So these would really point to the presence of water at or near the surface on the timescale of the average age of the surface of Venus.

Oliver Strimpel: You mentioned the gravity experiment and how we'll be able to get a much better handle on the elastic strength of the surface. But what about the core, looking deep into Venus?

Sue Smrekar: Absolutely. Every rocky body has a core, mantle and crust. It's just a fact of a planet forming very hot and melting and separating into its different density layers. But Venus is a challenge to look at the core using spacecraft methods, because what we are essentially doing is looking at the wobble of a planet. So now, the classic experiment is a raw egg and a cooked egg. You put them on the desk and you spin them, and they wobble differently because of the distribution of mass inside those eggs. Same thing with a planet. However, Venus is kind of a billiard ball, because it rotates so slowly. It doesn't have the hydrostatic flattening that most planets have. So we need to pay attention to the wobble. It does still have a wobble, but it's harder to measure than in other places. We're going to be able to do that well enough to get the core size to plus or minus 50 to 100 kilometers, depending on mantle viscosity and other things. That will tell us not only something about the composition of the core, but we'll also be able to describe the motion of Venus well enough to determine whether it has a solid core as well as a liquid core. We don't know that currently. That's really important not only for understanding the thermal evolution of Venus, but for a little bit of insight into why Venus lacks a dynamo today. Now, all kinds of crazy bodies in our solar system have a dynamo that we never expected. Mercury has a dynamo; some of the icy moons have dynamos. Dynamos are crazy beasts that are hard to model, and it's very perplexing as to why Venus lacks a dynamo today.

Oliver Strimpel: We've now discovered getting on for 5,000 exoplanets. Will a better understanding of Venus help us understand what it takes to make a rocky planet habitable, and therefore enable us to refine what it is we should be looking for in our search for exobiology out there?

Sue Smrekar: Yes. I absolutely believe we're going to get important insights into what makes a rocky planet habitable. Well, if you were in another galaxy looking at our solar system, trying to look for planets around our star, you would have a hard time distinguishing Venus and Earth. They, of course, have different distances from the Sun, so you would know that Venus is getting more energy from the Sun, but you would never predict that it has this intense greenhouse. Based on distance from the Sun, it should be 50, 100 degrees hotter, instead of the 900 degrees hotter it is. So they look very much identical from a first-order exoplanet standpoint. Predicting habitability for a planet is a complex question, and one of the things that I find very intriguing and compelling is that people are trying to look at the first-order properties of a planet and say what matters in creating a habitable long-term environment.

And there have been a number of studies trying to get at Earth's evolution and why it created habitable environments. What are the important factors? So a number of studies point to plate tectonics, the formation of continents, in creating the right environment at the surface of the Earth, or in the oceans in particular. And there's one hypothesis that says that the formation of continents created a different chemical composition in the atmosphere: basically, erosion of continents dumped the elements that life likes into the ocean, the nitrogen, the phosphorus, the oxygen, and the formation of more continents coincides with the Great Oxygenation Event on the Earth, when life flourished in the oceans and changed the composition of our atmosphere to be more oxygen-rich. And Venus is just a fantastic place to contrast with the Earth and try to understand what the variables are that really matter.

Oliver Strimpel: I noticed you got very excited about subduction zones. Is there a potential discovery, or answer to a particular scientific question, that you're anticipating the most?

Sue Smrekar: I think we're really going to just learn so much. You know, on Earth, subduction zones carry carbon sediments back into the interior, and that's part of the overall climate cycle, and it may be a reason that plate tectonics started on the Earth. It could be a place where there's massive amounts of volcanism being produced today, which could have an effect on Venus's climate. So yeah, that's the kind of problem that sits at the intersection of many different scientific questions and datasets. And so yes, that's one that's definitely very near and dear to my heart.

Oliver Strimpel: Sue Smrekar, thank you very much.

Sue Smrekar: It's been a great pleasure.

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