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Sarah Stewart

Sarah Stewart

Professor, Department of Earth and Planetary Sciences, University of California at Davis

By David Zierler, Director of the Caltech Heritage Project
August 31, 2022

DAVID ZIERLER: This is David Zierler, Director of the Caltech Heritage Project. It is Wednesday, August 31, 2022. I'm delighted to be here with Professor Sarah T. Stewart. Sarah, it's great to be here with you. Thanks for joining me today.

SARAH T. STEWART: Thank you. I'm excited to remember my experiences at Caltech.

ZIERLER: That's great. Sarah, to start, would you please tell me your current title and institutional affiliation?

STEWART: I'm a Professor in the Department of Earth and Planetary Sciences at the University of California at Davis.

ZIERLER: Now, Earth and Planetary Science—is that a school? Is it a division? A department? What does that look like administratively?

STEWART: This is a department in the College of Letters and Science. It has 21 faculty at present.

ZIERLER: Tell me about what you're working on right now.

STEWART: Aha. [laughs]

ZIERLER: Too much, I know. But what comes to mind?

STEWART: This is very Caltech related. I'm working on two high pressure science centers, one funded by the Department of Energy, one funded by the NSF. They are focused on understanding material properties at extreme pressures, greater than a megabar, in order to understand the insides of planets and stars.

ZIERLER: For context, what is a megabar?

STEWART: A million atmospheres. The core mantle boundary's around 1.35 to 1.36 megabars.

ZIERLER: What aspects of both centers are just purely fundamental research? Are there any applications that you're thinking of?

STEWART: High pressure research, in shock physics in particular, have been used as fundamental exploration of planetary materials to understand: what are the crystal structures in the bottom of the Earth? What are their thermodynamic properties? What are their phase diagrams like? And what does that mean for planet formation and evolution? That's very much basic science of the natural world. It overlaps with applied work related to energy science and the efforts related to fusion driven energy, because the mechanisms that we get to fusion involve using shock waves to help us get there. So, the communities overlap there. How applied you want to be can extend to planetary defense of extraterrestrial asteroids coming at the Earth, to the weapons programs of our government.

ZIERLER: Sarah, looking at your publications, your educational trajectory, there are so many disciplines and fields that you've contributed to and that you represent. What do you see as your home discipline? What are you at the end of the day?

STEWART: I started down this path by a fascination of planets, in a very wide-eyed young girl love of Earth—Star Trek kind of way. It was very much influenced at the time—I graduated in 1991 from high school. We were in the first serious efforts to try and find exoplanets, and only a couple had been announced and refuted. You could tell we were on the cusp. As an undergrad at Harvard, I met people involved in that search. So, planetary science is what I say is the common thread. How I ended up doing high pressure research for that pursuit is very much the accident of going to Caltech. [laughs]

ZIERLER: [laughs]

STEWART: Nobody wakes up one morning and says, "I want a couple of big cannons." [laughs] That's not what happens to little girls.

ZIERLER: Sarah, do you direct any of your research to planet Earth? Or is it all other planets?

STEWART: Actually, much of my work recently has been towards the Earth and the Moon. They're very much a combined problem. But my early career work focused on the outer solar system and icy bodies. That's because at the beginning I thought—well one, I thought I might be an astronomer, an observer though telescopes. But I was interested in these disks that you could see around other stars, and the inference that planets were forming there, even though we hadn't detected them directly yet. And much of the disk was cold, so I was interested in collisions between icy bodies. My initial work was on shock waves through ice and thinking about collisions between icy bodies like the Plutos and Kuiper Belt objects in the outer solar system.

ZIERLER: Tell me about some of the things your graduate students are working on right now, maybe as a window to where the field is headed.

STEWART: [laughs] This isn't where the field is headed. As part of the work of understanding the origin of the Moon, we became very interested in what it takes to vaporize rocks and minerals with a shock wave. We can recreate that experiment at a special facility at Sandia National Lab called the Z machine, which is a big capacitor bank that can discharge a current that's shaped very precisely. That current is used to launch five-centimeter-tall, centimeter-wide aluminum plates to up to 40 kilometers per second, which is [faster than] the orbital velocity of the Earth around the Sun. So, you can reach all of planet formation in these labs. I have students and postdocs who are looking at vaporization of common minerals in the mantle, like olivine and bronzite, and understanding when they melt and vaporize, and we now see new features related to residual strength in the crystal that we're trying to understand. I have students using what we call hydrocodes to model giant impacts between planetary bodies using material models that we've improved with our lab data in order to predict more reliably how much melts, how much vaporizes, and where does the material end up during accretionary growth of the planets.

ZIERLER: That's sort of the overriding theme among your students.

STEWART: Right now, I have two, an incoming student and an early student who's interested in impact cratering—which is where I started, looking at planetary surfaces and impact cratering. The big challenge there is to understand the thermodynamics of the process so that we can better predict where materials melt and where materials end up. That led recently to a direction of research that was not what I expected to be doing, but I'm very happy to be doing, which is to think about the conditions on the early Earth during the rise of life and how impacts, craters at that time affected the surface and upper-mantle environments.

ZIERLER: Sarah, does this get you into astrobiology to some degree?

STEWART: In this way of: How do collisions shape early surface environments, and how is material delivered to Earth-like planets? And that's: How is water incorporated? How are metals incorporated? Specific components that influence the chemistry in the surface environment. So, that's my niche to contribute to this topic, rather than anything that's directly biological.

ZIERLER: To the extent that there is this binary about whether life originated locally or it came from extraterrestrial bodies, what kind of research are you doing that might contribute to those debates for one side or the other?

STEWART: People in my area look at how much is material mechanically and thermally processed when it's transferred between planetary bodies. We have meteorites from Mars that were lofted from Mars by impact craters, and they are launched at sufficient velocities to escape Mars without being melted. So, there's a tricky little regime that is related to our trying to understand the strength and onset of melting in geomaterials that is related to that question. The flip side of that question is my personal view [laughs], which is that we have all the ingredients for life we need on the early Earth, and we don't require any living organisms to be transported to us, and that our environment is as good as any other in the early solar system, probably, for life to be seeded organically. You can get into philosophy pretty quickly when you start asking origin of life.

ZIERLER: How did you get into origins of Moon research?

STEWART: This is not what I thought I'd be doing. This is the story of my life: it's not what I thought I'd be doing. I left Caltech with my PhD focused on the outer solar system, focused on icy bodies, and continued to do so in my early career as a professor at Harvard. I went to meetings, and there were these debates about the origin of the Moon, and I hadn't understood the geochemical problems related to the giant impact model, which I had just accepted as a student.

ZIERLER: What is that model? What does that mean?

STEWART: The only real working model for the origin of the Moon is that it grew from a disk of material around the Earth, and a giant impact would have made that disk of material. Exactly how, we don't understand yet, but the fact that the Moon probably formed from a circum-earth disk is widely accepted based on the available evidence. The physics of it was reproduced fairly well with computer models, meaning you could have a Mars-sized body hit the 90% grown Earth and make a disk that was mostly made from the mantles of the bodies, because the Moon has a very tiny iron core. So, it's one of the things the giant impact explains. The Moon has evidence of forming under hot conditions, so having a hot vaporized disk aided that chemistry. The model was developed with the idea that this impact would have set the angular momentum of the Earth and system and set the length of day today. When you have those constraints and the mass of the Moon—you have to make a disk big enough to make the Moon—you get this grazing Mars-sized body that comes in near the escape velocity of the young Earth.

What I didn't know until I was an early professor is that the geochemists did not like what they saw. They didn't like something very specific that I didn't appreciate. In these computer simulations, most of the disk is made from the body that hit the Earth rather than being ejected from the Earth. When we look around the solar system and measure every meteorite that falls to the Earth, you quickly learn that the relative proportions of isotopes of stable elements like oxygen, titanium, chromium, are different for every planetary body, and these particular elements have different isotopes, because stellar nucleosynthesis makes different proportions of these isotopes, and our solar system was literally seeded by little grains that formed in the atmospheres of other stars that were then recycled into making our star. But every planetary body got a different amount of these presolar grains, and so every planetary body has a different set of isotopes. You can use it like a thumbprint, like a fingerprint to tell what body different materials came from, and you can group meteorites this way and infer a distinct numbers of parent bodies. The puzzle was that the Moon is the same as the Earth in these particular isotope systems, and if the Moon was made mostly by another planetary body striking us in the early history of the solar system, we would have predicted that it would be different. As a young professor, the nuance of the problem escaped me, because I basically was watching two communities. One, the physicists had said, "This is the only answer. This is the only thing that gives us the length of day, the mass of the Moon, the fact that it doesn't have a big iron core." The geochemists were like, "Your answer doesn't work. It just violates [laughs] these principles of chemistry that we're seeing, and you physicists must be wrong." [laughs] This was atypical. You don't usually see such loggerheads where each side is completely convinced they're right, and obviously they both can't be right. The by-chance part was that we hired a postdoc on a prize fellowship who could work on whatever he wanted. This was Matija Ćuk, and he wanted to work on the Moon. But he wanted to work on it for a different reason, which was the Moon's orbit is tilted with respect to the ecliptic plane, and if it were not tilted, we'd have a solar eclipse every month. It's tilted about five degrees, which is weird. I didn't know it was weird until he came along. He convinced me this was a Problem with a capital P. The dynamicists could not explain with this Mars-sized impact theory. Add: the geochemists didn't like it, and now the dynamicists don't like it. He convinced me that we should work on it and look for a different solution, which led to a fast-spinning Earth model that violated the angular momentum constraint, which means you needed something else to get us to today's angular momentum, which Matija proposed, and then I did the giant impact modeling to show that you could find solutions that led to mixing, which would lead to a Moon that is similar to Earth [in isotopes]. This initial foray into the Moon problem, we published in 2012. I've been a professor for almost ten years, and I haven't stopped. The last ten years, we've been going almost nonstop on it, and it's because we were wrong, of course [laughs] and learned—it took us a long time to figure out how we were wrong. But we were wrong in how we interpreted the outcome of these events, because we were studying the system after the impact as if we had a planet and an orbiting disk. It turned out we didn't have a planet anymore, which led to our discovery of this new object that we named a synestia, and that the evolution of getting to understanding that was this very strange road where we didn't have the language or the math to explain it to ourselves. We had to invent a number of things along the way, where we basically understood the high temperature limit for a planet.

One of the other things that happened during my career was, Mike Brown at Caltech, who I overlapped with as a grad student—he was a postdoc and hired as a professor while I was a grad student there—he was just beginning his campaign to observe the outer solar system and look for other Plutos and had a telescope at Palomar dedicated to this search. He found a whole bunch of things eventually, and that led to this definition of a planet, demoting Pluto to a dwarf planet. The definition of a planet also included a hydrostatic shaped object, so a rounded body. We distinguish asteroids and vesta from being a planet, not just because there are other bodies there, but because they're potato shaped. They're small enough and cold enough that their strength can resist the self-gravity, so they're not deformed into oblate spheroids. That's the cold temperature limit of a planet. What we discovered was a high temperature limit to a hydrostatic equilibrium. When the energy of a giant impact is so great, heats the body up so much, that its radius expands, because most—if you take two spheres and collide them, the most common event has some angle to it, so there's always a spin. It's usually quite fast, given growing bodies, so you have a combination of heat and spin that leads to a big oblate spheroid. And when the equator is big enough that it exceeds the point of synchronous rotation for, say, an orbiting satellite, the equatorial mass basically overflows into a disk. And if that material in the disk—there's no hydrostatic solution to calculate a spheroidal shape for a body, and it becomes hydrodynamic, it's always changing, and we violate this definition for a planet on the high temperature end with many, many different kinds of giant impacts. So when we proposed this solution for mixing materials to make the Moon to explain the isotopes, we exceeded this limit, but we didn't know that we did it [laughs]. Nobody told us this was a thing. So we were analyzing our data wrong out of these simulations, because we didn't understand what we had. We since learned that you get pushed into this limit, and one of the funny consequences of making a synestia is that the disk can't fall down onto the planet. Most disks in astrophysics are accretion disks, meaning the disk around the Sun that's mostly hydrogen is flowing onto the stars. It's part of star formation. The disk around Jupiter that made all of its moons was mostly flowing onto Jupiter [laughs], and in the early models of the disk that made the Moon, most of the mass falls onto the Earth. But when the Earth is too hot and spinning so fast, basically the gas pressure is holding the disk up by the gas pressure force—it's a pressure gradient force, and the disk can't fall down until it cools, or until you—exchanging momentum. That takes time, and it takes longer than it takes to grow the Moon, so the Moon is forming in this very different environment. The dynamics are totally different, and we still don't understand it. It's the kind of problem where you go around to fluid dynamicists and you shop for someone who's going to help you work on it and they all turn around and run in the other direction. [both laugh] "That's too hard! Please give me something I can solve. I'm a grad student…"

ZIERLER: Sarah, have you been involved in the Artemis mission? Is there anything that might be interesting to you?

STEWART: People are quite interested in "What is the Moon really made out of?" Because we have very limited samples from the near side, and some would argue heavily biased samples from a singular event: the Imbrium Basin and its ejecta on the near side of the Moon. There's interest in obtaining samples from the far side, or deeper, or anything that's just not crust—finding mantle samples. Whether or not an astronaut would bring those back, or there are now many robotic missions, private and government driven, some hoping to bring samples back. Those would be very interesting on understanding, "What is the Moon really made out of?" Because the geochemists still have gaps even though they're very certain about some things. But I'm not personally involved in manned—crewed missions.

ZIERLER: Sarah, what do you see is the extrapolatability of Moon research to other satellites in our solar system? Does it tell us about the origin of other moons?

STEWART: This is a question that we have just about—are synestias something that happen a lot? Or was it a quirk for our Earth? Just from a probability point of view, as you grow planetary bodies and the domain of their collisions, synestias are quite a common event. Maybe toward the late stage of giant impacts, maybe more than half would make synestias, so in that sense, I think of them as a general object that we haven't learned what changes yet with that. For moon formation, it's not clear. We're going to learn from the Japanese mission to Phobos about how similar the Martian moons are to Mars. That might help with understanding those moons, which people do think formed from a disk, but probably a colder and not synestia disk, compared to Earth. Venus and Mercury could have both been synestias that lost their moons. Mercury, because it's close to the Sun. Venus, possibly because the Moon spiraled in from tides that net move the Moon in rather than out because of its retrograde rotation. Those are questions we may never have any observational data for, because they're lost. In terms of, "How special is Earth?" I am pursuing a line of thought on how much did this impact influence the early chemical conditions on the Earth, the heat budget of the Earth, and therefore its early evolution leading to our habitable planet. If we decide that some pieces are important for making our environment, then you would want to look for that as a factor in looking for other Earth-like bodies. Most of the studies of Earth's later evolution assume a clean slate at the end of planet formation, without much memory of the building part of Earth. As people have taken more measurements of Earth's mantle, we actually see evidence of materials that are preserved from before the Moon forming giant impact, and my husband has worked on some of that, or most of that for noble gases. He was also a student in the Caltech department, and that is where we met. So we joined with Ken Farley's graduate student. He also was a case of not expecting to do what he thought—noble gases [laughs]—was a career change, a direction change. It has led to remarkable evidence that the Moon-forming giant impact did not homogenize the Earth.

ZIERLER: Sarah, I wonder if you can talk about how far computational modeling gets us beyond theory into something that can become accepted—the new view, the new orthodoxy in the field.

STEWART: The not so dirty secret is that we have very poor numerical models of something as complicated as two planets colliding. They're poor in the sense that much of the discriminating data are chemical, and the models don't do chemistry. They're physical models—conserving energy, momentum, [laughs] with gravity forces—but not running any chemical reactions within them. That is an area that needs development, because it's the discriminating factor when we try to understand what range of events could have made the Earth and Moon or any of the other bodies that we see. In some ways, the codes are just fine. So, the question of, "What is a synestia?"—part of our work on convincing ourselves and others that we understood what we were talking about was to run simple hydrodynamic calculations with bodies where we just kept increasing their temperature, and increasing their rotation, and demonstrating that you get into a threshold where the body's shape changes from a nice oblate spheroid to something that looks more like a flying saucer. In that sense, they're still powerful—the codes, but one of the tricky parts of planetary science versus astrophysics is that astrophysics can take all of the high-Z elements and lump them into a corner, and planetary scientists care very much about species [laughs] in those high-Zs, and that becomes incredibly complex.

ZIERLER: Sarah, besides being such a huge honor, did the MacArthur Fellowship allow you to do things that otherwise might not have been possible?

STEWART: [laughs] The COVID thing kind of happened in the middle of all this and put a pin in some things. I've been fortunate to be involved in these research collaborations like these high-pressure centers that I mentioned, where we have had great license in our applications of our high-pressure work to planetary problems. That work has been supported the whole time through standard means, and part of my MacArthur hat has been lately directed to more nonstandard problems, like changes in the way that we do science these days and how to change with the times, but also how to direct science interactions in a more productive way. I'm calling it the "scientist Dunbar number." Are you familiar with the Dunbar number? Strong relationships with about 150 people. We're approaching a hundred-year anniversary for the Department, and my career has been about a quarter century. In that time, I have noticed tremendous changes in the volume of science output, the numbers of people, and the narrowing of what I can keep up with. I can only talk to so many people. I can only read so many people's papers. Then the hours in the day are done. When I was a student—and particularly with the Caltech faculty, who were all broad-minded people, even if their direct specialty was focused, some not focused, some very focused—but they were very knowledgeable outside their own particular specialty. That was one of the main benefits that I inherited from that environment, was the idea that this was important and beneficial. For problems like the Moon where I had to learn to talk to those isotope geochemists and dynamicists and link these different areas for this complex problem, that really benefited my ability to work on bigger problems. But over time, the capacity to do that is diminished by the volume, and this is a societal problem in that there's just too much information, and it leads to anxieties or difficulties in finding the right piece of knowledge that already exists somewhere. Science as a whole has gone back and forth on individuals versus teams working on things: consortia versus PIs, and direct mentoring relationships versus broader-scale teaching. We're in another one of these transitions, so that's some of my effort, is to work on the impact community particularly in some of the generational turnover in knowledge, as well as this issue of hyper-specialization versus needing broad knowledge across different areas.

ZIERLER: Sarah, with everything going on in our solar system, do you have the bandwidth to have a hand in exoplanet research?

STEWART: Exoplanets. I just described—no, there's no bandwidth left [laughs]. Certainly, my work on planet formation, not just the Moon stuff, but the general outcomes of collisional growth was work with Zoë Leinhardt, my first postdoc, and ongoing work on the generality of when two bodies hit each other, what happens? We have a lot of work on the general question, so those are all applicable to studying the growth of planets around other stars. There are specific oddities that are seen where people want to invoke giant impacts, the way that people use giant impacts in our solar system to explain oddities that we see. So, those have come across my desk as things to think about. Then, this habitability question is probably the one that's the most serious for generalities, but there's so many papers on exoplanets that you have to really be deep in the field to keep up with it all.

ZIERLER: Sarah, let's go back now in history for you. At Harvard, was the kind of physics you wanted to pursue—is there that intellectual seed there of geophysics and planetary science right from the beginning?

STEWART: No, I didn't know anything about rocks. [laughs] Caltech had to teach me all about rocks. At Harvard, I went in expecting to be a physics major, and they had this astrophysics and physics concentration—they call them a concentration, it's not majors—and that looked very attractive. The Physics Department was a little cool to undergrads, in that "you don't know enough to do many things yet" way, whereas the Center for Astrophysics—the largest collection of astronomers in the world, and the Astronomy Department is embedded within it—everybody up and down the hallway has more data than they can analyze from something or another, so as an undergrad, you can walk in with a little bit of math and programming skills and be helpful. So, that was how I ended up finding the early exoplanet people, who were at the time finding mostly binary stars but were working their way down to smaller and smaller stars that would eventually become planets. Then there were planet formation people who got me into working on planets in my senior thesis project which was on planet formation.

ZIERLER: Sarah, were there any faculty, either in the department or at the center, that you became close with at Harvard?

STEWART: My advisors for the thesis were Jane Luu and Scott Kenyon. Jane Luu is famous for being the co-discoverer of the first Kuiper belt object as part of her thesis [laughs]. So, that was my link and my entry into the outer solar system interest, which were these icy bodies in the outer solar system. We saw these dusty disks around other stars, and I knew there were collisions that were making this dust, and that led me on the early trajectory of studying icy collisions as part of understanding planet formation before we had the direct planets. That was a great, just a tremendously satisfying student experience. The Center for Astrophysics was very welcoming to young people. We had a good cohort of astrophysics majors as well. But there was an Earth and Planetary Science Department that was the more geologic focus. Planetary climates and atmospheres were being studied there, but I was intimidated by the intro sequence since I didn't know anything about rocks. [laughs] I didn't take it; instead I took the astrophysics intro sequence.

ZIERLER: Sarah, what advice did you get for graduate programs? Did anybody put in your mind that you should start thinking about rocks?

STEWART: At the time I thought I would be an observer. Jane's background was observations. I was doing a modeling project just as a student, but I thought maybe I would end up observing these disks and then inferring something about planets with them. Jane gave me a list of graduate programs to look at. I probably applied to eight or so different places. I'll admit I was not a perfect undergraduate student. I was the president of the drama club, and I probably spent more than half my time in the theater as an undergrad [laughs]. So, my grades were acceptable [laughs]. I didn't get in where I applied, because I didn't have the stellar grades for some of the places. But I had great interests and good advisors who helped overcome some of my portfolio's deficiencies. I wasn't sure what to do, but Jane basically said something along the lines of, "Of course you should go to Caltech. They have all the different things that you'll need" for the things I said that I wanted to do, and that really helped the decision making.

ZIERLER: Why ultimately Caltech?

STEWART: What did I like about this?

ZIERLER: Did you visit campus?

STEWART: I did visit a few places, and it was not the best time to be wanting to go into planetary science. It was after Mars Observer blew up.


STEWART: When you visit graduate students whose theses had just been reconfigured to be theoretical, they're not a happy bunch of people, so it was a transition time when I was making that choice.

ZIERLER: But it was GPS that you applied to, not PMA?

STEWART: I applied to GPS, that's right. Because the planetary science was in GPS, and that was before Shri Kulkarni and Astronomy had jumped into planets. That year I moved, 1995, was the year Didier Queloz had published that first 51 peg for the hot Jupiters, and everyone went, "What the heck are hot Jupiters?" It was just happening right at that time, and no one had really figured out what to make of it. There were just these whispers at conferences of very weird things that people were finding. The focus was on solar system exploration. Arden Albee was in the Department, and he was the PI on Mars Global Surveyor, which was the big mission following the big explosion of Mars Observer. It arrived 1996-ish, I think, and it was phenomenal. It was the beginning of this golden age that we've had in planetary science ever since, from a mission exploration point of view. The temporary depression that planetary science was under was quickly overturned. In what were the tipping point aspects—I went to Hawaii to check them out versus Caltech, because Hawaii had all the biggest telescopes—20% of the time, all the telescopes at Mauna Kea. Caltech was bought into various telescopes including Keck and had a whole lot of observational time, but I did like Caltech's environment better than the Hawaii one, which felt very isolated. Their Institute for Astronomy is separated from the rest of campus, and that felt a little weird. Caltech was just hopping in a big idea kind of way that I liked. It had the telescopes. I hadn't met Tom Ahrens to learn about shock wave experiments, so it was primarily a mainland place with big telescopes and people who seemed to be right in the thick of it.

ZIERLER: Sarah, what was it like determining who would be your graduate advisor?

STEWART: At Caltech, you needed to choose starter projects in the GPS division, so I did one that was observational that was related to Jupiter, because the Galileo mission was just arriving, and they had a probe that went in the Jupiter atmosphere, and there were a bunch of people there at JPL involved in that mission. One of my projects was related to ground-based observations in Hawaii at the IRTF for observing Jupiter simultaneously with that event and trying to understand the cloud structure, particularly when that probe went into a cloud-free zone when everyone had wanted it to go into a cloudy zone. The second one was with Tom Ahrens who was a co-I on a now, soon to be canceled comet lander mission. The comet thing was interesting to me, because that was a part of the undergrad interest in the outer solar system and icy bodies colliding, that there was a mission plan. It was with the French [lander]; it was called Champollion. Tom had this instrument on the lander that was a penetrator. The lander would land, grip, and then fire these rods into the ground with little thermometers and measure the heat flow and therefore the thermal conductivity of the comets which were porous and icy, and that was very interesting, because I wanted to know what these icy bodies were like. We set up lab experiments to just try and understand the physics of heat flow through not-very-conducting objects and tinkered around with that.

He had this gun lab. That wasn't the starter project, but as I was learning about looking for planets and disks, I basically inferred that collisions were making the dust. We didn't actually understand what was happening with the collisions, so I started doing experiments that would break up fine-grained porous bodies in the lab. We learned that we didn't know a lot, so that was very interesting. I basically was making a decision between going the observational astronomer route—and Mike Brown was just hired, so I was looking at working with him, and Tom Ahrens with the collisions, trying to understand the physics of collisions route. In trying to decide, I wasn't enjoying very much staying up all night at the telescope. [laughs] Which seems like a really weird thing, but nowadays everything is remote. You don't necessarily have to be there, but some things can be automated. Most astronomers go and are there through the night, through the weather decisions that you need to make, et cetera. But at the time, I didn't know I actually had an undiagnosed illness that gave me sleep apnea, and I was chronically sleep-deprived. So, I actually was physically—it was not going to work to stay up all night. That, I wasn't diagnosed with until much later when I was trying to have children. I have acromegaly, which is a pituitary disorder that leads to too much growth hormone, and it's causing all sorts of problems, which now are medically cured, at least.

ZIERLER: Sarah, is there something about just the way GPS is set up, where looking back, it was kind of inevitable that you would get involved with rocks?

STEWART: Well, yes. It is a geological department, and it would have been hard to ignore the depth of knowledge that was held by the materials side of the problem. There are now people who are dynamicists, pure dynamicists, pure exoplanet observers that don't need it as closely, but these questions that I was interested in, about how you physically grow planets, the material properties are key to understanding the outcomes. Then it really was a tremendously good environment to have that level of expertise, the breadth of expertise in the one environment, because you could walk down the hall—and Tom Ahrens was a special person in that he interacted extremely widely with the mineralogists by looking at crystal structure changes with the geophysicists; and seismologists, by trying to understand what was going on in the deep Earth; with the planet mission people, by wanting to go on land on comet; with the accretion people, because he was writing the first papers about how to deliver water with impacts in the young Earth. His personal breadth was astounding, and his productivity was a little nonhuman. It meant I was in this environment where it looked natural to put it all together. It wasn't a question. He just did it somehow, and I was like, "Well, that must be normal," because that's what you think is normal, because when your advisor's doing it—only later did I realize this was exceptionally [laughs] not the typical fusion of disciplines to go after a problem.

ZIERLER: Did you get pulled into seismology as a result of being Tom's student?

STEWART: I took the seismology class that Hiroo Kanamori taught. It was sort of the textbook "everybody needs to know these things." I appreciate having done that, because so much of what we're trying to infer about the high pressure mineralogy is based on the seismology, so that part was valuable. The Seismo Lab culture of interacting and talking across these disciplines was influential in ways you don't appreciate until much later.


STEWART: In some ways, Tom was a little bit of an oddball, like in the universe, but in Seismo Lab, he was one of the very first people who said, "I can use impact experiments to understand the mantle." Then they had these great debates about whether or not things you made in microseconds actually mattered for the deep Earth, or were they just these transient things? A lot of work was done to convince everyone that it did matter, that you could use these data that were made in a dynamic very short-lived experiment to get to high pressures and temperatures and apply them in the deep Earth to what the seismologists were seeing today. That was the foundation of a lot of what happened since, with many of his students that went on to work in high pressure, many in a new field that followed, which was diamond anvil experiments to get to high pressures more easily than with a big gun. But then you sacrifice the sample size, and then you need sort of a synchrotron kind of probe to make measurements. The techniques are quite complementary—dynamic compression and static compression—they've been used together to try and solve a number of problems. Tom was sort of in the thick of all of that and developed with a long-standing collaborator, John A. O'Keefe, applications to impacts. John A. O'Keefe did most of the modeling, and Tom understood the thermodynamics and could get lab data that went with trying to interpret impacts in the solar system. I was sitting in the planetary science option; that's what I was admitted to. During that time period where first-years are looking for their two projects, usually you're very clear on the first one. It's what you thought you were going to do. That ended up being the observing project. Tom had a habit of walking up and down the hallways and peeking in people's doors and pulling them out and saying, "Let me show you something in the lab," as a way of recruiting students. [both laugh] I was in planetary, so in Seismo Lab, there was a little warning to grad students that he does this, which is not a bad thing. They were all…

ZIERLER: I'm sure it was charming.

STEWART: Charming. They're all difficult and brilliant people in their own way, the faculty in the Department. Tom hooked me with this comet experiment, coming down and talking to me about it and dragging me into his lab. It didn't take much for me to appreciate that he had the capabilities to do experiments that you couldn't do anywhere else in the world.

ZIERLER: Why, Sarah? What was unique that he was doing that you couldn't do elsewhere?

STEWART: This was the best compliment that I heard from a certain person, which was Gerry Wasserburg. Gerry was a professor there, and he made a comment at some point to me along the lines of, "Tom Ahrens is someone who invents things in the lab—invents equipment, invents types of measurements—and does things for the first time." He invented ways to melt rocks, to make them molten before the shock wave experiment occurs so you can study molten rocks. He invented ways of understanding this thermodynamic path of shock and release to look at phase changes, and then made these temperature measurements that were—there are very few people in the whole world that were doing these, trying to make shock temperature measurements. He did things like sample recovery. He had four different guns. [laughs] It was just this nonstop—it was like every thesis involved building something totally novel and new. That was an incredible learning experience. He had great lab staff that were there for decades that just held the whole thing together by just depth of knowledge. At some point, I had this "I've got to make a decision" conversation with him, and we were overthinking everything as grad students might do, and basically asked a rather straightforward question of the two things that were really special about GPS that I could leverage for my interests. One was the telescopes, the access to telescopes. The other was this shock wave facility that didn't exist in most places. We talked through what kinds of things we could do, and I wrote a grant, essentially, to fund work on ices, studying ices, and other things I think we didn't get to. [laughs]

ZIERLER: That was my question: why ices? Why did you settle on ice?

STEWART: Because we didn't understand shock waves and ice. There was just not much data; it's a pain in the butt to work with.

ZIERLER: Why is it a potentially fascinating topic? At the conceptual phase.

STEWART: At the conceptual phase, we wanted to know when it melted. This is the part of the whole astrobiological context of things. When is there water? We know it's ice during planet formation, but these collisions do lead to melting. The icy bodies' mechanical response that were porous ice, we—snowballs are hard physics to understand. And for collision work, there just wasn't a lot of data on ice. So, we, thinking comets, thinking disks, I see disks—this was key data to have to put in models. In this GPS department, we had Barclay Kamb, a glaciologist, who had a cold room on top of South Mudd, and it had an outer room that was not so cold, and an inner room that was -40° Celsius that he kept his Antarctica cores in, and he let me go up there to prep ice targets. I was up there slicing and polishing ice.

ZIERLER: What does an ice target mean?

STEWART: For my experiments, I needed to make little hockey puck disks that I would then stack with gauges between them to build a target that we hung in front of a gun. So, we built a little contraption to keep it cold, with liquid nitrogen spraying onto the targets. I literally, as a student, just made them, made ice pucks. They had to be very flat and parallel, so we devised a way to polish them to be flat and parallel. The way that you would do a rock, or a thin section, is just—I was doing it in ice up in Barclay Kamb's cold room. I would never have been able to do this thesis without that cold room. The idea of doing this like in a glove box or something would have been crazy. It was this magic combination, and I could walk outside under the roof of South Mudd when I was done and just warm up in the California sun [laughs] and just get back into the real world.

ZIERLER: And what was the gun? What did it look like?

STEWART: The guns look like cannons. Have you been in it?

ZIERLER: I have not.

STEWART: It's still there. Paul Asimow now runs the gun lab. He was a graduate student who was finishing early in my graduate career, so we overlapped—that they hired back. He was Ed Stolper's student as a petrologist. A similar interest was the deep mantle, so when he came back, Tom got him doing lab experiments, and he took over the lab. The guns come in a couple of flavors. The standard cannon is a big launch tube, and he has a 40 millimeter gun, which is where I was doing these ice experiments with a high pressure breach at the end. You put the projectile in the breach end, and it's basically a plastic cylinder that holds a metal plate, or usually if you want to get to high pressures, in my particular experiments it was just plastic because of the type of data we were getting. Behind it is the high-pressure vessel. Using either compressed gas, like a compressed helium, or gun powder, you push the projectile down the barrel with a high-pressure gas or this burning propellant. That particular gun could get up to 2.6 kilometers per second, which in a rock can get you to the lower mantle. This was what Tom first set up when the Seismo Lab was in Arroyo. He had a little shed with a little gun that could get to these lower mantle pressures, and when they moved to the South Mudd building, they built in this gun lab for him in the basement of the building, which was an amazing feat, because this tall ceiling, it had the big crane, this drive-up loading bay. It was just a beautiful lab. And he added this big two-stage gun which is now two barrels. So, just like a rocket uses multiple stages to get into space, because you have to sort of increase the energy density as you keep going, a two stage gun has a launch tube just like a single stage gun. You put your projectile in the back of the launch tube, and there's this coupling breach to another tube that's filled with a light gas—usually hydrogen if you want to go very fast, and that gas is compressed with a piston that's driven by gunpowder. So, put in an initial load of low pressure hydrogen, like 100 PSI, compress it with the piston to generate much higher pressures that burst the diaphragm that then pushes—the hydrogen gas expands down the launch tube and pushes the projectile toward the target, and that can reach seven kilometers per second, and get to core pressures of the Earth, so hundreds of gigapascals, GPa, or a couple megabars. For most of our experiments, the Tom Ahrens shock physics style of experiments, we're studying planar shock waves through a sample, and that simplifies the math in interpreting what we're doing, and it gives us very much a material properties measurement. We did make craters. We did bust apart many asteroid analogues. We did those things as well. But the novel, the really difficult, more unique experiments that he did were these shock physics, shock thermodynamic experiments. In those cases the target is hung very close to the mouth of the gun, so that when the projectile just exits, it immediately hits so there's not tilting, because these are not aerodynamically shaped objects. These are just cylinders, and you're just trying to make a flat plane wave to make a measurement. Many of these are developed using streak cameras, very fast sweeping cameras across a film and then later CCDs that let you watch a line across a target and measure when the shock waves reach different points, different planes. Then you could infer shot velocity and other things as you develop them. He also used optics to collect—fibers to collect the thermal emission to make temperature measurements. I did these planar shock experiments in these stacked ice hockey puck targets with in-material gauges that let me measure the shape of the shock front. And ice has a gazillion crystal structures. It's one of the most annoying materials in the universe [laughs], because it's got such a bendy molecule that it can assemble in many, many different configurations. At low pressures, you see multiple different densities being made along the—when you shock to different pressures, which correspond to different ice phases that are being made. Then the system melts and you get into high-pressure melts. I did the first measurement in ice in this regime where we could clearly identify these phase transitions and understand the calculated temperature rise and the criteria for melting and then published these new criteria for melting in the solar system, which are pretty low pressures. You almost very easily get to some melt occurring in most planetary impacts. It's fun!

ZIERLER: Sarah, maybe an obvious question, but what were the safety considerations in the lab?

STEWART: I tell people that chemistry labs are much more dangerous, because there was HF being used in the other building, and the emergency exhaust was up to the roof where that ice—where the cold lab was. So, I'd be out there warming up, and there would be this, "Death could come out of this tube from this chemistry lab next to me!" But in the gun lab, if you're careful, the normal hazard is shrapnel. We have windows into the target chamber. The target chambers are big cylinders, which are basically large volumes to capture the propellant gases and the debris. The idea is that the volume's large enough so that it's not over-pressurized so you can contain it all and let it cool and pump out the gases and bring in room air and open it up. Because we used windows to observe velocities and the target itself in the shock wave, there's always some brittle material that's part of the tank. Every once in a while, shrapnel gets through, but not very often. Once you've done it a few times, you know where to put windows and where not to.

They built this lab for him. It's in the sub-basement. You were on the concrete slab. He had these sliding metal doors that went across the regular lab doors. Right next to the 40 millimeter gun that I was using, in the sliding door, there was this big dent, because he had done this metal on metal experiment and there was a little corner jet, like a little jet of molten metal that just punctured through the window and hit the door and made this big dent in the door. He showed new students that, and everyone agreed that no one's in the room during an experiment. Everyone is in a control room. When you fire the guns, it's a little anticlimactic, because you don't hear anything. Everything's contained in these vacuum chambers, so if you do hear anything, it's because you broke a window and that was bad. Normally you hear a thud and ringing of shrapnel around these target tanks. The lights sway a little, because there's a little seismic wave that goes into the ground that the building can feel. The mechanical hazard is the routine hazard. The not-routine hazard that's the bigger hazard is igniting the hydrogen. If you are not careful with your order of operations, you could make the mistake of introducing oxygen and hydrogen in the presence of a spark and ignite the hydrogen, which is a pretty big exothermic reaction that would—it does a little bomb-poof thing, a little bit bigger than the Martian. [laughs] It happened at Lawrence Livermore Lab a while ago. They made a mistake and ignited their hydrogen, and it blew off the roof, blew out a door, and injured a person's hearing, but not much more than hearing. So, it is a dangerous place. The shrapnel hazard occasionally escapes the building, but I only know one case where that happened. [laughs]

ZIERLER: Sarah, to get a sense of Tom's style as a mentor, how close was your ultimate thesis topic to what he originally pulled you in to show you from that first time?

STEWART: He hooked me with this mission that was canceled. Even though we were just starting—Mars Global Surveyor was a success at the time—and Galileo—there was still a lot of worry amongst grad students about doing mission-based thesis work. So, when that was canceled, I did make a conscious decision to do lab work for my thesis instead, and that led to that proposal for ice experiments. It did end up working out pretty closely to where we started, and he was quite experienced. I was there toward the end of his career. He had a good sense that it was a doable thesis. Even though of course we were surprised in any number of ways, it was still doable. [laughs]

ZIERLER: Sarah, were there any theories that provided intellectual guideposts for the experiments?

STEWART: There were analogous data on iron, actually. Because iron undergoes these phase transitions in shock waves in a manner that—you could put up two shock wave traces, and you wouldn't know if one was from iron or one was from ice. They have a lot of analogies to them. Iron was used as the example material in early high-pressure research to convince the world that you could learn something from these lab experiments. So, Birch at Harvard, and Bridgman in early high-pressure experiments—iron was an example material and one of the first ones where shock waves were demonstrated to be useful. That was a guide for helping me interpret the data, and then the thermodynamics part was really Tom's way of doing things. They were just the methods he had developed to analyze data.

ZIERLER: When you were deep in the experiments, what surprised you? What stands out in your memory?

STEWART: [laughs] Ice is hard. [laughs]

ZIERLER: You mean difficult to understand?

STEWART: No, it was hard to work with, because our temperature control system was not very sophisticated. We did better later at Harvard, but at the time, it was a jerry-rigged first attempt thing, and it was hard to control. So, it would sometimes take hours to get to the temperature that we wanted to, or we'd get stuck somewhere we didn't want to be. It was really—[laughs] trying to get the target temperature we were trying to get with our imperfect control systems was a frustration. We had many late, late evening shots as a result of not having a more proactive control system. We broke gauges because of cold. It took us a while to figure out that we had broken gauges because of cold. They had become really brittle. It just didn't conduct anymore during the experiments, so it just took us a while to figure out that that had happened in some number of experiments, but we did figure that out. At that time, Tom was a manager professor, and the lab staff didn't like him being there along the lines of walking in and breaking something. [laughs] But he's actually, really an exceptionally good experimentalist even if the lab staff didn't want him to touch anything anymore.

ZIERLER: [laughs]

STEWART: That was the way it was run at that time, and sort of how I run now. I'm not there supervising every shot; my lab staff is. So, we did the intellectual before-and-after with Tom.

ZIERLER: Sarah, how did you know when you had enough to defend? When did it feel like a complete project? Or at least one that you could present?

STEWART: Well, uh, my husband was done. [laughs]

ZIERLER: That works.

STEWART: So, I needed to be done. [pause] When do you know? I had an unusual grad student experience. Partly because my advisor had some difficulties at the time. I was sort of forced to be a little more independent by circumstance in a way that after time, you're very anxious, but afterwards was sort of confidence building. The "how do you know you're ready to be done" question—I was sort of at the bit on wanting to do my own problems. For some reason, I thought it would be a good idea to go and set up another… [laughs]. Which was nuts, to be frank. In the end I would say it worked out, but it was by no means a high-probability-success endeavor.

ZIERLER: Sarah, notwithstanding Ed Stolper's assurance that you'd be fine, were you fine? Was that okay for you?

STEWART: It did. It was not a Tom Ahrens experience for many reasons. It's a different era, the funding model is different. You couldn't work the lab techs the same way. They were running six days a week in Tom's early career. A program manager showing up and giving him money, telling him to do things. So, it was a little bit of a different time. Safety—well, no, I don't think safety was a big issue; it was just slow. But I also was a split person with wanting to model as well as doing experiments, whereas Tom really was in the lab for his early career. It was different, but it was successful in that I got new and interesting data, and that very much helped me, early career, before I then pivoted to this really high-energy stuff that you can't do in-house; you have to go to these national facilities to get to. I think the training that I got at GPS—I don't know, I just felt I was very well prepared for that—go on and find your own problems and tackle them your own way. Tom gave me advice. He was actually a very good mentor to his many, many students in helping them in their early careers. Tom would get out of the way of science topics for his students. If he knew that one of his students, especially early on, was going to go and pursue something, he would stay away from the topic. He knew I was running in the outer solar system working on ice and some more other things, and he went and did other things. He told me to try and be—he called it "one-stop shopping." It was really an encouragement to be as integrative as he was, where I could define a problem, define some experiments, either analytically or computationally apply them to some planetary problem, and do it all in-house within my group. It gives you a full loop on a problem that's really good for students: to be able to see all the pieces all together, without relying upon something else. He really encouraged that I was conscious about that, even as I was keen to have other collaborations that wouldn't be in that mold. Then some years later, I guess one of the last real career advice moments before he passed away, he looked at me and said something like, "When are you gonna work on the Earth, lady?"

ZIERLER: [laughs]

STEWART: Like, "Come on! Quit screwing away on this really big problem in front of you!" I had avoided it, and it was partly he was doing it, and I didn't need it to overlap with Tom too much. But then I was more established, and competent, and learning about what some of the open questions were, and that led to the Moon work. It was amazing. It really was a kick in the pants—"When are you gonna work on the Earth, lady?" [laughs]

ZIERLER: Besides Tom, who else was on your committee?

STEWART: Andy Ingersoll and Mike Brown, I believe. I think those were the committee. Tom—there may have been another seismo person; now I can't remember.

ZIERLER: Did you have a ready-made speech about your contributions, your findings, what was significant about the research?

STEWART: Did I have a ready-made?

ZIERLER: Going into the defense—"This is what it is." Did you think about that?

STEWART: Tom—that was one of the good things about Caltech. They were good at teaching students to suss out the punchlines.

ZIERLER: Which were what? What were those punchlines?

STEWART: I had defined the melting points of water at different pressures and temperatures in the solar system and demonstrated its prevalence during planet formation. That was the good, hard chapter. The other chapter was, using that knowledge, I could say that during impact cratering into the Martian permafrost that water would have been produced and that it should have influenced the formation of ejecta blankets on Mars. There was a little bit of modeling support for that, and taking some of the water results. This was now the weakness of the modeling, was how robust were some of my interpretations, which Andy Ingersoll rightly coached me about during my thesis defense, about how strong could I make statements about the modeling results. So, it was a good learning experience, if stressful. But in the broader community, being able to speak about the thermodynamics of what was happening on things like the Martian surface in regards to water was something that the broader planetary community really appreciated. Even if they didn't really know much about or care much about shock waves and high-pressure techniques, they really appreciated the applications of melting in the solar system. So, that was the focus as I was on the job market at the end of Caltech. [laughs]

ZIERLER: Sarah, looking back: your subsequent work in the high-energy area, were there any seeds that were planted at Caltech that you can trace back to?

STEWART: Yeah. One of the other advices that I got from Tom was to diversify my funding portfolio. He had had long-standing support from NSF and NASA for the core high-pressure mineral physics part of his program, but he also had funding from DOE, for various things, from fracture mechanics, penetration. All of that work had overlapping interests with DOE. He had grown up in the time of nuclear bomb testing. It was imperative that they understood after shock waves to interpret what was happening during these experiments. After testing, the whole world leaned a bit more heavily on modeling, but then used these lab experiments to help fill in the development of the codes. He had some DOD money as well. Over time, I did consciously think about how my work could intersect with different constituencies. It really has been quite rich and felt very natural at this intersection.

ZIERLER: When it was time to go on the market, were you looking at postdocs and faculty positions?

STEWART: My class—that included Emily Brodsky, who you spoke with—we entered with 18 people. It was on the large end, at the time. I think 12 of us became professors. It's a truly astounding fraction. It was one of those cases of our peers really bootstrapping each other up. I went to Harvard as an undergrad. The premed stories of mutual sabotage were all over the place, and this was the opposite experience where we really were bootstrapping each other to greater success. We were all simultaneously on the job market [laughs]. It was really scary. Emily was at the leading edge. She got out of there pretty quick. Being on the job market at the same time was a little stressful, but we were still helping each other with our written materials, with our applications, with our job talks. That was key. It was cream on the whole process. I and my husband, because we were the same class, were all on the job market at the same time. We were stupendously lucky in timing in that we needed two jobs, and the department at Harvard had managed to go down a few positions with retirements and were trying to do multiple hires. They hired three people the year we applied. We were two of them, so that worked out okay. We did take postdocs. We went to Carnegie Institution of Washington which is now renamed Carnegie Institute of Science. The Broad Branch campus which had the geophysical lab where I was and the department for terrestrial magnetism where Sujoy went—they had a presolar grains isotope cosmochemist there that Sujoy worked with and I worked with diamond anvil high pressure people in the geophysical lab. That group has been merged and renamed the Earth and Planets Laboratory.

ZIERLER: What kind of word did you get about tenure prospects at Harvard?

STEWART: That it was tough.

ZIERLER: You were aware of the reputation; that was not a surprise to you.

STEWART: No, everyone warns you before you go there, not to expect to get tenure. Because I was an undergrad—and Jane Luu left before she went up for tenure; she may have gotten finger to the wind kind of thing, some advice, anyway. I knew that it was hard on her, that level of scrutiny. When we spoke to people who had been there and left, they described living under a microscope and that stress that goes with it. I think I went in partly with the mindset that the pros should outweigh the cons.

ZIERLER: Plus you solved the two-body problem, at least in the short term.

STEWART: Yeah, it put us on a path to keeping it, too. Because once you start, it's usually yours to keep. I won't pretend that it wasn't really tough [laughs]. Life lessons, I guess. I still think, at least for me, the time was very valuable, and did lead to a lot of very interesting things.

ZIERLER: Fast-forwarding to 2014 or thereabouts, why the move?

STEWART: This was about not getting tenure. That instigated the massive job search for a second time, which was interesting, because we were both well-known at that point. There was a lot of interest, which partially made up for the trauma of a forced move. That did lead to many opportunities at University of California, particularly with this high energy density science.

ZIERLER: Yeah, it's worked out pretty well.

STEWART: That part has been quite fun, yeah.

ZIERLER: Sarah, have you remained connected with Caltech? Either in terms of collaborations or just as an alum?

STEWART: I see them at meetings. Given that it's one of the other few gun labs still in existence, Paul Asimow and I will commiserate about all the troubles trying to keep gun labs going and give each other tips about how to do things. Yeah, I've been back. I was on a visiting committee one year, as well, for them. We have been connected in science. I went back as the first Thomas and Earleen Ahrens Lecturer for the division.

ZIERLER: Oh, that's nice. That's great.

STEWART: That was very fun to do.

ZIERLER: Sarah, for the last part of our talk, a few retrospective questions, then we'll end looking to the future. So first, what has stayed with you from Caltech? Both in terms of the way you approach science, and also just being a member of a scientific community and being a collaborator.

STEWART: This business of 12 of your classmates going and becoming professors was just this instant cross-country club that we were members of. Also, the breadth of the Department, it meant we knew somebody who was an active researcher in all these different areas. That network really just has paid off over and over again, knowing people. One of the most grounding things from Caltech was another Ed Stolper quote. This was made part of a decision about where to go and what to do after Caltech. He said something along the lines of, "The good problems don't go away. They're not solved instantly. In general, someone may do a piece of it, but it won't be done." It was that idea that the really meaty, fundamental problems are career problems. They're not one-off papers. I don't have to worry about one-off scoops if my eye is on something that's a much bigger, bigger goal. That helps tremendously, because scientists are people. [laughs] Things do happen that are discouraging parts of interacting with people, but in the long run, I really held on to that. The specialness of Caltech, in some ways, was how incredibly research-focused it was. It's a small institution. They're dedicated to educating this sort of special population of people who fit the Caltech environment. But there was so much action at the grad student, postdoc, professor level, that you really—I was there as a grad student. I did have a sense it was the heart. Research was really the grounding of the whole enterprise. So, you appreciated just the value on it, the focus. It's very clear eyed. The history—I hadn't realized it was actually coming up—the history of how that department has participated in inventing the field of planetary science is really tremendous.

ZIERLER: Sarah, exactly on that point, for my last question, thinking about this quote from Ed Stolper about the fundamental problems—they're career problems; it's the big goal—so what is that? How do you define that for however long your career may be?

STEWART: [pause] Well, I got into this whole shebang from wanting to explore. This is the childhood tricky part. This is amazing! You drive up to a new planet [laughs] and just one after another—new, new, new things. I've tried to articulate, "What are the things that I keep coming back to, that I can't let go?" This "Why are we Earth?" problem is one I keep coming back to in different pieces over time. Now, more the center of the Earth than the Moon part of it necessarily, but what's going on in the deepest part of the Earth is the latest focus of my attention in the general problem of "Why are we this special planet?" Then, there's this little child-with-new-toy aspect of these large-scale experimental facilities that we have access to that are constantly generating new capabilities with new instruments and new upgrades that let us get to parts of planetary processes that literally were just no man's land before. I alternate between, "I get to go to this new place and do something that no one's done before," and then coming back to our origins question to keep us grounded at home. That guides a lot, those two things.

ZIERLER: Sarah, this has been a fantastic conversation. I'm so glad we connected for this. Thank you so much.