This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Following the Apollo missions to the Moon in the late 1960s and early 1970s, it became widely accepted that the Moon formed as a result of a cataclysmic collision when the Earth was forming, about four and a half billion years ago. But a puzzle has remained as to just what sort of collision must have occurred in order to result in the Earth-Moon system we see today. How big was the impacting body? What was its composition? And what happened in the aftermath of the collision?

Sarah Stewart is a professor in the Department of Earth and Planetary Sciences at the University of California, Davis. She conducts lab experiments in conjunction with numerical modeling and out-of-the-box thinking to develop new scenarios for a moon-forming impact and the subsequent dynamical evolution that appears to resolve many of the outstanding difficulties associated with previous solutions. Sarah Stewart, welcome to Geology Bites.

Sarah Stewart: Thank you, Oliver, for inviting me.

Oliver Strimpel: Why do planetary scientists believe it was a giant impact that created the Moon?

Sarah Stewart: It's the best idea to explain the present-day rotation of the Earth and the orbit of the Moon, along with the composition of the Moon. But it wasn't until after the Apollo missions that scientists realized how common giant impacts would have been during planet formation. And right now, we know that giant impacts were very likely events as planets were growing.

Oliver Strimpel: So what was the traditional Moon formation scenario that prevailed before the Apollo missions to the Moon?

Sarah Stewart: Before the Apollo missions, we didn't have a single best idea for how the Moon formed. There were actually multiple competing ideas. The Moon could have been captured, or created alongside the Earth the whole time, or there was even the idea that the Moon spun out of the equator of the Earth, in a process called fission. But the analysis of the Apollo lunar samples changed our ideas. The data from Apollo ruled out all of the ideas that scientists had for the origin of the Moon. The Moon's core was too small to explain the Moon if it had been captured from somewhere else in the inner solar system. The Moon's composition is similar to Earth's mantle, but different enough that it doesn't fit the idea that the Moon just grew alongside the Earth. And fission had so many problems that people really didn't take it seriously. It didn't explain today's length of day or the total angular momentum of the Earth-Moon system.

So the Apollo missions sent everyone back to the drawing board on the question of how the Moon formed. The giant impact idea only emerged in this aftermath, when we were looking for new ideas, and it was at that time that the studies on how all the planets grew together around the Sun realized that there would have been these giant, hugely energetic collisions. Before that, people had really been thinking that the energy of planet accretion involved much smaller bodies colliding.

Oliver Strimpel: So how does the giant impact scenario result in a Moon-Earth system that we see today?

Sarah Stewart: So the way the giant impact was proposed is that the energy of the event would throw part of Earth's mantle into orbit, in a disk around the Earth, and the Moon would grow out of that disk of hot debris. And in the conceptual phase, this could explain why the Moon looked similar to the Earth, but not exactly like the Earth: because it was a hot disk, and the Moon didn't grow with any water or what we call moderately volatile elements, elements like potassium and sodium that vaporize at intermediate temperatures, like 1,000 degrees. And so these distinctions could be explained by a giant impact. In addition, there's the large mass of our Moon. Our Moon is 1.2% the mass of the Earth, and that's much larger by fraction compared to the moons that we see in the outer solar system. And finally, the giant impact gave us a way to explain why we have a 24-hour day today, along with where the Moon is in its orbit. So there are a lot of great features in the idea of the giant impact.

Oliver Strimpel: From our earlier conversation, it seems as if there are three main characteristics of today's Earth-Moon system, all of which a successful model has to be able to replicate. These are the compositional similarity of the Moon and the Earth that you already referred to; the tilt of the Earth's spin with respect to the ecliptic, which is the plane of the Earth's orbit around the Sun; and the inclination of the Moon's orbit. It's been difficult to develop a single model that can reproduce all these characteristics at once. Why is that?

Sarah Stewart: There are so many pieces to the puzzle on the origin of the Moon, and they involve both understanding the physics of collisions, the chemistry of what occurs after those collisions, and the dynamical evolution of the lunar orbit. And the giant impact hypothesis gave us the bulk characteristics, like the mass of the Moon, and the fact that its core was so small, because most of the debris was formed from the rocky mantles of the colliding bodies. But the way the original hypothesis worked out, in terms of where the material came from that went into the moon-forming disk, most of that material came from the body that struck the young Earth, and we gave that body a name. Its name is Theia. So the Moon would look compositionally a lot like this other body. And we know from studying meteorites that no two planets in the solar system are the same; no two asteroids are identical. And so we would expect there to be differences in its chemistry and in the isotopes of the different elements in the body. What we learned from the Apollo samples is that the Moon was identical to the Earth in many ways, and that really called into question the giant impact hypothesis, where the Moon formed mostly from this other body.

We tried to come up with an explanation of how to make the Moon look more like the Earth, and the solution was to mix the two bodies to a greater degree than in the original giant impact. And the big idea that we had at the time was that the moon-forming impact did not lead to our 24-hour day, but actually made the Earth spin much faster. And in giant impacts where the Earth spins faster, the two bodies mix to a greater degree, and you could make the disk out of an equal-proportion mixture as what went into the Earth. And this was one way to explain how the chemistry and the isotopes of each element were more similar between the Earth and the Moon than in the original giant impact model. The catch, though, is that you still have to get to today's 24-hour day, and you must do that during the lunar tidal evolution by transferring angular momentum away, probably through a resonance with the Sun that slows down the spin of the Earth and leads to the path that gets to our 24-hour day.

Oliver Strimpel: So it's, so to speak, allowed to have a high-angular-momentum impact in which the Earth starts out spinning very fast. How does that help us with the question of the same composition in the Earth and Moon, and ending up with the right orbital characteristics today?

Sarah Stewart: Compared to the original giant impact model, these high-angular-momentum giant impacts were also more energetic and led to more vaporization and more complete mixing between the two bodies. However, when they were first proposed in 2012, there were only a few very specific impact scenarios that could lead to exactly the same mixture in the Moon as in the Earth, and the response from the scientific community was to have some skepticism. You know, is that really correct, that you would need perhaps a special-case giant impact to explain the Earth and the Moon? And so there was some pushback, and it seemed like we were still missing something.

Oliver Strimpel: OK, so what did you do next?

Sarah Stewart: It took several years of working on these high-energy, high-angular-momentum giant impacts to try and understand them, and they were just very different from what anyone had studied before, because previously the entire community was focused on explaining the 24-hour day that we have today. But once we were free to explore a larger parameter space, we made giant impacts that had outcomes that we hadn't seen before. And it turns out that giant impacts can transform a planet into a completely different type of astronomical object. And we figured this out around 2016 by looking at the computer calculations of these giant impacts and realizing that the object in the middle just didn't look like a planet anymore.

So there's a very specific definition for a planet that the International Astronomical Union wrote down when we tried to decide if Pluto was still a planet. A planet has to orbit the Sun, it has to clear its zone in its orbit around the Sun, and it needs to have a spherical shape that represents what we call hydrostatic equilibrium. And so any rotating body will have a slightly oblate spheroidal shape. That's a balance between gravitational and pressure forces within the body. And what happens in a giant impact is that the planet becomes so hot that it partially vaporizes and puffs up, and its radius becomes so big that it can't rotate all together anymore and maintain that spheroidal shape. And at that time, it transitions to an object that forms a disk that's constantly changing in shape. So it's no longer hydrostatic, and it no longer satisfies the definition of a planet. And so we had discovered in our computers this entirely new object, and so we had to give it a name. We call these objects synestias, and we derived the word from the goddess Hestia. Hestia is the goddess of the hearth and home. We were studying the origin of the Earth; it was hot. And we attached the prefix syn- to emphasize that the disk that the Moon is forming in was actually part of the Earth. And we proposed that the Moon formed basically inside the Earth, when the Earth had been transformed into this huge object that we call a synestia.

Oliver Strimpel: Can you describe what exactly a synestia is?

Sarah Stewart: When you think of a planet, you think of a spherical body that rotates around all together, and the shape of that body is spherical by a balance of forces that we call hydrostatic equilibrium. But if you make the body larger, by heating it up and vaporizing the outer layers, and making it spin a little faster, that increases the size of the equator of that object. And there's a certain distance from the center where the body can no longer rotate all together, and any material outside of that special distance rotates more slowly, in a disk-like region around the central mass. That transition we call the corotation limit. For a planet, exactly what that critical distance is depends on what the planet is made out of and how massive it is. But once it's exceeded, the shape of the object is no longer a nice spheroid, but instead makes a thin disk that's connected to the interior mass, and that object is called a synestia.

Oliver Strimpel: But is the synestia just the bit beyond the rigid part?

Sarah Stewart: When a body exceeds the corotation limit, there's no distinction between what is on the inside of that distance and on the outside. All of the material is connected and interacting in the same fluid system. And so the name synestia represents the entire system, all of the mass, because there is no distinction between a planet and a disk anymore. It's all one body.

Oliver Strimpel: So there'd be no surface to stand on.

Sarah Stewart: When a rocky planet like the Earth is turned into a synestia by a giant impact, it's so hot that the gas in the outer layer becomes a high-pressure gas, and a supercritical fluid as you go deeper and deeper, to higher and higher pressures. And there's no liquid surface layer present at these extreme energy conditions. So that's right: there would be nowhere to float about.

Oliver Strimpel: I can see that this would give you a framework where you would get an awful lot of mixing, which would solve the problem of the Moon having the same composition as the Earth. But does it result in the orbital characteristics that we see in the Earth-Moon system today?

Sarah Stewart: There are still big requirements on what happens after the Moon accretes from that debris before we can get to the present day. Initially, any giant impact makes a disk in the equator of the new post-impact body, and so we expect the Moon's orbit to be in the equator of the young Earth. But as the Moon tidally migrates away from the Earth, it transitions to having its orbit dominated by interactions with the Sun and the ecliptic plane of the solar system that the Sun defines. And in that transition, we expect the Moon's inclination to end up perfectly in that plane. And if the Moon were in that same plane, then we would have a solar eclipse every month: the Moon would line up perfectly with the Sun. But we don't, and we don't because the Moon is tilted by 5 degrees from the ecliptic plane. And that's not what we expect if the Moon formed in a giant impact. So there's still another major puzzle to be solved.

Oliver Strimpel: So how did you go about resolving that?

Sarah Stewart: We went back again to the idea of a high-angular-momentum giant impact. It turns out that there are many ways to make a synestia. Most giant impacts have a lot of angular momentum, and many different kinds of giant impacts can make a synestia. And so we now have more choices for the kind of giant impact. But we proposed a path that could lead to the 5-degree inclination that involves a lot of angular momentum, more than two, two and a half times what we have today, and in which the giant impact tilted the axis of the new Earth over very far, maybe 65, 70 degrees from celestial north. Today we're at a 23-degree tilt, and a giant impact has equal probability of the Earth's spin axis pointing anywhere on the sky after the giant impact. So it is plausible that the Earth's spin axis was tilted over. But then something has to happen to change the tilt of the Earth, and that's part of the tidal migration of the Moon.

If the Earth starts tilted over, in the process of the Moon migrating away from the Earth, there's an interaction with the Sun that leads to the Earth's tilt being restored to where it is today, but leaves the Moon in a high-inclination orbit, tilted away from the plane of the solar system, high enough that during its tidal evolution it doesn't reach zero today. So this is a brand-new idea that my colleagues and I have published as a way of explaining this 5-degree tilt, and it's just been launched into the community to try and work on. But currently it is at least a self-consistent way to make our flavor of Moon and its peculiar orbit.

Oliver Strimpel: I understand you figured this out by running some numerical simulations on a computer. But do you have a qualitative understanding of how this inclination becomes reduced while the Earth's tilt gets back to what it is today, and we settled down into this stable orbit that we have now?

Sarah Stewart: During the lunar tidal migration outwards, there are a couple of dynamical resonances that occur, and you can think of these resonances as being times where the orbit is a bit unstable and very sensitive to large changes. And so if people intuitively want to understand what is happening with the Moon's orbit and the Earth's tilt, the first major process has to do with that transition from the Moon being in an orbit around the Earth's equator, dominated by interaction with the tilt of the Earth, to the Moon precessing around the ecliptic plane. And that transition is called the Laplace plane transition. And when the Earth begins tilted over, there's a big instability during that transition that changes the tilt of the Earth and changes the length of day of the Earth, to get us on a path to today, and leaves the Moon in a high-inclination orbit. And then, as the Moon continues to migrate away from the Earth, it feels special tides with the Earth at high inclination that lower the inclination, but not all the way to zero, which is what we want.

Oliver Strimpel: It doesn't seem intuitive at all to me, but I'm sure the computer models are correct. I can see that this is something that was surprising to the community. Did you expect to get this result out?

Sarah Stewart: No. I have been surprised at pretty much every stage of my work on the origin of the Moon. And it's partly because we live in a world where our textbooks have given us an explanation of a Mars-sized object called Theia that struck the Earth and gave us the angular momentum that leads to 24 hours. But the details were missing, and there were some serious observations that couldn't be explained by this specific giant impact event. And what has happened with the angular momentum work and these new tidal models is that there's actually a lot of new ideas, a lot of changes. The dust has not settled at all on trying to explain the lunar orbit and the isotopes. So it's a very exciting time in trying to understand the origin of the Earth and Moon.

Oliver Strimpel: As I mentioned in the introduction, you also conduct lab experiments. Can you tell us about those?

Sarah Stewart: The experiments are amazing, because we can now reach in the lab all of the pressures and temperatures that are generated during planet formation, even in these humongous giant impacts. And what's hard to appreciate is how much power is in a giant impact. If you calculate the total kinetic energy of the two colliding bodies over the timescale of the event, the power of a giant impact is like all of the energy coming out of the Sun. And so all of that energy going into a planet leads to nearly complete melting and partial vaporization. And we can recreate that thermodynamic path in the lab.

Oliver Strimpel: How on Earth could you do that? No pun intended.

Sarah Stewart: We do what are called shock wave experiments in the lab, and we can generate these huge pulses of pressure that lead to temperatures of tens of thousands of degrees, by using lasers, or huge capacitors that can launch metal plates at up to 20, 30, 40 kilometers per second, and that's close to the speed that the Earth is orbiting around the Sun. So these facilities that can reach these extreme conditions were built by the Department of Energy. They're housed at Lawrence Livermore National Lab and Sandia National Lab. They were built to try and achieve nuclear fusion by generating intense pressures. And so we use the same platforms to study natural processes like giant impacts, and understand how planetary materials respond to very strong shock waves.

Oliver Strimpel: So you managed to get some time on these giant laser fusion machines.

Sarah Stewart: These world-class facilities that were built by the Department of Energy all have basic science programs, and so scientists from all over the world can apply to conduct experiments on these facilities, and we compete with each other for time. And so we have successfully gotten precious days on these massive machines to make giant impacts in the lab.

Oliver Strimpel: So what do the results of your experiments tell you?

Sarah Stewart: Because we could reach the exact pressures obtained during a giant impact, millions of times more than atmospheric pressure on the Earth, we could determine how hot geologic materials got during an impact event. And we developed a special technique to find the boiling points of rocks and metals that are reached during giant impacts. And so we could calculate, for the Earth and for the bodies that collided with us, what fraction became gas and what fraction melted into magma oceans.

Oliver Strimpel: Does this mean you're exploring new parts of the phase diagram of Earth materials at high temperatures and pressures?

Sarah Stewart: That's right. We can't reach these conditions in static experiments. When we think of phase diagrams: when does a material melt, and when does it turn into a gas? For many materials, like water, we can reach all of those conditions in the lab by controlling the temperature and pressure. But for the main minerals in planets, the temperatures are just too high for us to be able to hold the material at those temperatures and study them. So the only way to get to the boiling points of many of the phases in the Earth is to do it transiently, during a shock wave experiment. And then we can generate a little parcel of rock that's boiling and study it, and the whole experiment could last tens of nanoseconds.

Oliver Strimpel: Over what sort of scale? How big a piece can you experiment on? And is it a piece of olivine you stick inside these machines?

Sarah Stewart: We've been studying the major minerals in planets. That includes olivine and bridgmanite-type compositions (bridgmanite is the main mineral in the lower mantle), and our samples are tiny, because we can only take a small volume to these high pressures in a controlled way, so that we have time to study it. So the sample thicknesses are only a few hundred microns.

Oliver Strimpel: So how do you use the results of your lab experiments to try and figure out what happened during this collision?

Sarah Stewart: We use our lab data to extract the thermodynamic properties of the major minerals in the Earth, and we construct a model for that material that is called an equation of state, and that model is fed into the computer codes that calculate the giant impacts. And so because of our lab data, we could believe the temperatures being calculated in the codes, and trust that the amount of vapor that was being calculated was real. And that's when we realized that these huge vaporous planets were as large as the codes were calculating, that this was an accurate result, and that lent support to the idea that this new object called a synestia was being created.

Oliver Strimpel: So if we adopt your synestia model and the results of your dynamical simulations for the subsequent evolution of the Earth and Moon orbits, does that manage to replicate all the compositional and orbital characteristics we observe in the Earth and Moon, without positing a contrived, low-probability scenario for the nature of the moon-forming impact?

Sarah Stewart: Everything about planet formation that we know today tells us there were lots of giant impacts, and most of them are energetic and have a lot of angular momentum, and many of them will make synestias. And so I think the idea of a synestia is not a low-probability event. There's a lot of work, though, that still needs to be done in order to fill in all of the puzzle pieces for what we see today. And one of the biggest challenges that we have is linking our physical model of a giant impact, where usually we're only calculating the first 24 hours or the first 48 hours, to the data that we have on the Moon from four and a half billion years later. And in between, a lot has happened. There's core formation on the Earth and the Moon; exactly how the Moon formed out of the disk; the tidal evolution of the Moon; volcanism on the Moon. And all of these affect the chemical signatures that we're using to guide us. And so the challenge that we have is to make specific chemical predictions from our physical model, and to date, that's been really difficult. We've only just gotten the temperatures right. And so now we're going to go another step, to understand the chemistry.

Oliver Strimpel: All right. The number and diversity of exoplanetary systems has exploded of late. Does your work provide some new predictions about what we might expect to see in these systems?

Sarah Stewart: All of our calculations to date indicate that synestias are a common outcome of giant impacts. And what we know about planet formation is that every star forms with a disk that generates planets, and we think that all of those systems had giant impacts. So the prediction at the moment is that almost every star in the universe has had synestias, one or more, around it. But the catch is that they didn't last very long. They cool off very quickly, because in most planetary systems it's not hot enough to sustain rock vapor; that would only occur very close to the star. So most would cool off quickly, and the gas would rain down onto the planet to form a magma ocean, and we would have to be very lucky to see that in action. Now, we've seen the debris from giant impacts around other stars, and so astronomers are looking for other odd features around young stars during the planet formation stage, and we have made predictions for what a synestia should look like around another star, but we haven't found one yet.

Oliver Strimpel: Sarah Stewart, thank you very much.

Sarah Stewart: Thank you for having me.

Oliver Strimpel: For illustrations that support this podcast, you can go to geologybites.com.