This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Most of us think of the solar system as the Sun plus a set of planets in orbit around it. But that leaves out the asteroid belt, a region lying between the orbits of Mars and Jupiter, which contains billions and billions of asteroids, ranging in size from pebbles to dwarf planets. It turns out that asteroids can tell us a great deal about the origin of the Earth, in many ways more than we can learn by looking at the other planets, such as Venus and Mars.

Sara Russell is Professor of Planetary Sciences and leader of the Planetary Materials Group at the Natural History Museum in London. Using asteroids as snapshots of the early solar system, her research seeks to unravel how the solar system formed, and to answer questions such as how the Earth got its water and organic materials. She even has an asteroid named after her. Sara Russell, welcome to Geology Bites.

Sara Russell: Thank you very much for inviting me, Oliver.

Oliver Strimpel: What are the asteroids?

Sara Russell: An asteroid is basically a rocky body that is in orbit around our Sun. We can differentiate asteroids from comets, which are the other small bodies that go around the Sun. Comets tend to be much more icy, full of volatile materials, and they originated further out, in the outermost reaches of the solar system, but have eccentric orbits, so they come into the inner solar system, where we can see them. In contrast, asteroids are made almost entirely of rocky materials; that means silica-based minerals and metals and things like that. And we can distinguish both asteroids and comets from meteorites, which are any natural extraterrestrial material that lands on the surface of the Earth.

Oliver Strimpel: So some of the things that land on the Earth, then, are not asteroids at all?

Sara Russell: No, meteorites can be anything. Actually, the vast majority, we think, are pieces of asteroids, which is brilliant for those of us who are interested in asteroids. But some meteorites are pieces of the Moon, some are pieces of Mars, and some we just don't know what they are. So there's a whole variety of stuff that falls from space for us to try to figure out.

Oliver Strimpel: So why are asteroids concentrated just in a region between the orbits of Mars and Jupiter?

Sara Russell: Yeah, we think the answer to that goes right back to the beginning of the solar system. The solar system formed from a disk of dust and gas that was orbiting around the evolving young Sun, and these bits of dust gradually accreted together to make larger and larger pebbles, and then boulders, and larger and larger objects, until they eventually formed the planets that we see today. Jupiter is the biggest planet in our solar system. It formed very early on, and Jupiter was so massive that it caused all sorts of forces in this area between Mars and Jupiter that kept forcing these bodies apart and caused them to be quite energetic and have lots of impacts. That meant that no planet could form in that region. So now we just see the debris of this material from the early solar system, which is now our asteroid belt.

Oliver Strimpel: As I mentioned in the introduction, there are billions of asteroids. Are they concentrated? Are they, like the artists' impressions often show them, pretty close-packed in space?

Sara Russell: No. If you see a sci-fi film where they go through an asteroid belt, you see these pictures of loads of boulders all over the place crashing into the astronauts' spaceship, but actually it's not like that. You would get one boulder, and then the next one would be thousands and thousands of miles away. There's a lot of space out there. So even though there are a lot of asteroids, they're not all that densely packed, and all of the spacecraft that have gone to the outer solar system, like New Horizons and Voyager, have gone through the asteroid belt with absolutely no problem at all. So that shows they're actually not very densely packed at all.

Oliver Strimpel: There are several different types of asteroid.

Sara Russell: Yeah, there are several different types of asteroids.

Oliver Strimpel: So we can investigate this from Earth, looking at the spectral features of the asteroids, I presume. These are features that appear in the spectra of the near-infrared and visible light reflected by the asteroids.

Sara Russell: Yes, and from that kind of work, we know that there's an amazing compositional diversity of asteroids. Some asteroids are very rich in carbon. Some seem to be rocky; some seem to be made of basalt, much like the surface of the Earth. Some are made of metal. We know the largest asteroid in the asteroid belt, Ceres, has got an ice volcano on it. So there's an amazing diversity of different compositions across the asteroid belt. That's one of the fun things about studying asteroids: they're actually much more diverse in composition than the inner planets, for example.

Oliver Strimpel: So what accounts for this diversity?

Sara Russell: We don't know in detail, but it could be that in the earliest times in the solar system, the giant planets, Jupiter and Saturn, actually migrated. They moved in and out of the protoplanetary disk, and that could have disrupted small bodies that were forming in other regions. It could cause, for example, bodies that were rich in volatiles that formed in the outer solar system to be thrown into the inner solar system. So we end up with this mishmash of loads and loads of different bodies that formed in different parts of the solar system. And also, depending on how big they are, they have very different histories. A lot of asteroids seem to have not changed at all since the beginning of the solar system, but others have undergone some geology, and so they've changed since they first formed.

Oliver Strimpel: How can asteroids actually, then, tell us anything about the Earth?

Sara Russell: In lots of ways, Oliver. In some ways, the Earth has kind of forgotten about its early history, because on Earth there's so much geology going on. Everything's been mushed up and mixed and melted. But many of these asteroids have not changed since the beginning of the solar system, and so they tell us something about the primordial material that all the planets, including the Earth, came from. The majority of asteroids and meteorites that we have to study on Earth, when we look at them in detail, have never melted, and they're made up of particles of different sizes all mixed up together. Some of these particles formed at very high temperatures, and some of them formed at low temperatures. So that's telling us that the solar system formed from this dust cloud where there were lots of different environments, and lots of different things got mixed up together. And this was probably the primordial material that made the Earth.

Looking at these asteroids, or meteorites, we can learn how old the Earth is, because we think they formed at the same time as the Earth, and they can tell us that the Earth is about four and a half billion years old. We can even unpick the components and look at how long these particles existed in the dusty protoplanetary disk. We get these ages from radiometric dating methods, most commonly using the decay of uranium to lead, just as we do for Earth rocks.

Oliver Strimpel: So you said that among the different kinds of asteroid, there are the kinds that are rocky and the kinds that are more metallic. In the ones that seem to be more metallic, what happened to the rocky stuff? Did they just never have it to begin with, or did it vaporize?

Sara Russell: Yes. As I said, most asteroids, we think, never melted at all, and they're just made up of this primordial stuff that the solar system was made of. But there are some asteroids that seem to be made mostly of metal, and we see these in our meteorite collections. We have meteorites that are made of iron-nickel alloy, and we think that these formed from asteroids that became big enough, and had enough heat in them, that they melted. When they melted, the iron in them sank down to form a core, so they had a core of metal, and then that was surrounded by igneous, rocky material. And actually, in our meteorite collections, we also have samples of this differentiated igneous rocky material that made up the mantle or crust of these asteroids.

But one of the kind of weird things in meteorite science is that we have basaltic meteorites, which would have been the crust of these asteroids, but we don't really have so much of the mantle material. So we have this missing mantle, and we don't really know what happened to it. It could have been, as you say, that it was vaporized during impacts in the asteroid belt, and we've just lost that record. But the iron-rich cores are more resilient to impacts, so maybe they survived better, and maybe that's why we see so many of them. The iron-rich meteorites are interesting, because they might provide an analog for the Earth's core. We know that the Earth has gone through this process of heating and melting and forming a metal core, but we can't access our own Earth's core to look at what its composition is. We can use these iron meteorites as some kind of analogy for what might have happened during Earth's differentiation.

Oliver Strimpel: That's interesting. So the metallic ones are like a naked core that we can look at.

Sara Russell: Yeah. Well, that's what we're thinking. Actually, there's a space mission called Psyche that's going to visit one of these metal-rich asteroids, and that should provide some clues about their origin and show whether that is the case.

Oliver Strimpel: So if the Earth was hot enough to melt metal and have it sink down to form a core, how did it get to be covered with water?

Sara Russell: Well, we can also thank asteroids for that, because many asteroids contain quite a lot of water, and they contain a lot of organic material as well. The Earth has been continuously bombarded with asteroids, or meteorites, since its formation, and in the early stages of the solar system, the impact rate would have been even higher, when the solar system was a more energetic and chaotic kind of place. And so after the Earth had differentiated, the water that created the Earth's oceans could have been brought there by meteorite impacts.

Oliver Strimpel: That's a hell of a lot of meteorites.

Sara Russell: Well, I don't think it is. I mean, we look at the Earth from space, and it looks like a watery planet, right? Two-thirds of the Earth is covered in water, and from a human perspective, these look like really deep oceans. But from a planetary perspective, this is a very, very thin skin on the surface of the planet. So from a planetary bulk-composition perspective, there's not that much water, whereas meteorites can contain loads of water. Carbonaceous chondrites, which seem to be the predominant flux of material, at least for very tiny particles, can contain up to 20% water, in the form of clay minerals. So they contain these clays that can trap water in their structure, and so they can actually deliver huge amounts of water to the Earth that way.

Oliver Strimpel: We're actually in a very exciting time from an asteroid point of view, because there are two missions currently underway to fetch samples from the surface of two different asteroids and return them to us. And you're a member of the science team for both of them.

Sara Russell: Yes. There are two missions currently in flight. There's a Japanese one called Hayabusa2, which has already collected a sample from its asteroid, Ryugu, and it's on its way back to Earth now. It's going to return to Earth in December 2020. The other mission is a NASA mission called OSIRIS-REx, which is currently in orbit around the asteroid Bennu. It's going to collect a sample next month, in October 2020, and then it's due back to Earth in 2023.

Oliver Strimpel: How big are these asteroids?

Sara Russell: They're not very big at all. Ryugu, the target for Hayabusa2, is about a kilometer across, and Bennu, the target for OSIRIS-REx, is about 500 meters across, so it's about the size of a skyscraper, really. It's not very big in the grand scheme of things, but that's what makes it really exciting, because the smaller it is, the more likely it is to have this primordial material and not have had too much geology happen to it. So it's more likely to be a pristine sample of the early solar system.

Oliver Strimpel: And do these things have enough gravity to hold a space probe in orbit around them?

Sara Russell: Well, yeah. They only have a teeny, tiny fraction of the gravity of Earth, because they're such tiny bodies. OSIRIS-REx, for example, is in orbit around Bennu, but to be in orbit around Bennu, it has to be super, super close. It's only about a kilometer away from the center of the asteroid, so it actually has the world record for being the closest orbiting satellite around a body. It's an incredible feat by the rocket scientists and the engineers to have actually managed to successfully put the spacecraft in orbit around this tiny rock.

Oliver Strimpel: And then it's actually going to go down to the surface to scoop up a little bit.

Sara Russell: Yes. This is perhaps the most dramatic moment of the mission. It's going to go down to the surface, and it has a kind of reverse vacuum cleaner. A blast of gas will be pointed at the surface of Bennu, and that will disrupt the rocks and fragments on the surface of the asteroid, which will then get captured in the collection capsule, and then hopefully brought back to Earth for us to take a look at.

Oliver Strimpel: So what are we hoping to learn from these samples that will be returned with great difficulty to Earth?

Sara Russell: I've spent my career studying meteorites, mostly meteorites that come from asteroids, and these missions are like the dream for scientists like me. We love our meteorites, but the problem with meteorites is that they fall on Earth, and we don't really know where they've come from. We don't have a chance to do fieldwork like terrestrial geologists do. We can't choose the samples that we get; we just get whatever lands on Earth. But these missions are the chance for us to look at a sample of a primordial asteroid, a body that we think came from the very earliest times in the solar system, that will tell us about the origins of our solar system. And it will tell us that with a global geological perspective, from all the mapping that these missions have been doing at the same time. So it will really give us some geological and astronomical context for these samples, which is so important for learning about their origins.

Oliver Strimpel: Will it be like the so-called ground truth that we have with Earth missions, where we have lots of pictures of the Earth, and to be able to correlate the two, you actually have to go down to the bit that you're looking at and grab a bit of it? Is that going to enable you to link what we get back from Bennu with some of the meteorites that are in your collection, so that you can say, oh, well, those definitely came from asteroids?

Sara Russell: Absolutely. The Hayabusa2 mission is actually, as you might expect from the name, the second asteroid sample return mission that the Japanese have done. The first asteroid return mission, Hayabusa, went to a type of asteroid called S-type asteroids, which we'd always suspected were similar to a type of meteorite in our collections called ordinary chondrites. We didn't know for sure, and it was only when we had that sample returned that we could make that connection definitively, and that then enables us to understand our meteorite collections much, much better. So yeah, one of the important things about these missions is that they'll help us to understand not just the meteorites that look identical to these rocks, but our whole meteorite collection.

And the other thing it will help us do is understand the diversity of asteroids that we can observe from Earth, because it will ground-truth our spectral data. It will give us that one connection: we have a sample on Earth, and we can also see it in space. That will help us to calibrate when we look at all the asteroids and small bodies in our solar system.

Oliver Strimpel: How did we select Bennu out of all the thousands of asteroids we could have picked?

Sara Russell: One big reason that it was picked is that it's what we call a C-type asteroid, which we think contains organic material. One of the main aims of the mission is to learn about organics in the solar system, how they may have come to Earth and seeded life on Earth, and to tell us about our origins as earthlings: how we got our water, and what it can tell us about the formation of the Earth. So the type of asteroid was very important. And the other thing that was very important was where it was. Bennu is a near-Earth asteroid, so it's not in the asteroid belt between Mars and Jupiter. It probably was once, but it's been knocked into an orbit that's much closer to the Earth's orbit around the Sun, so that makes it much more accessible, and much easier for us to go there and sample it.

Oliver Strimpel: There's something else that I think you mentioned to me when we were talking earlier: that by studying the details of the albedo, that is, the fraction of the incident radiation that is reflected by the surface of the asteroid, we can tell in detail what its future orbit is going to be. And we really care about that, because it has a finite chance of colliding with the Earth.

Sara Russell: Oh, yes, of course. That's another important reason for looking at Bennu, and near-Earth asteroids in general. We know on Earth that we do get hit by asteroids. Sometimes they're big asteroids and can do a huge amount of damage on Earth, and we want to understand that risk as well as we can. One of the ways that we can do that is to study asteroids like Bennu in detail. Bennu is a potentially hazardous asteroid, in that it's got a tiny probability of hitting the Earth in about 200 years' time, but what we want to do is make our calculations about whether it's likely to hit or not a bit more precise. We can do that by making very precise measurements of its albedo, and the heterogeneity of the asteroid, and so on, and that will help us to understand what's called the Yarkovsky effect, which is an effect of solar radiation on small bodies, pushing them with a very, very gentle force. But it's something that we need to understand to be able to have accurate models of how the orbits of these objects are going to evolve over the next centuries.

Oliver Strimpel: This Yarkovsky effect that you mentioned is to do with solar radiation pressure, which depends in turn on the albedo, which is how much of the incident solar radiation gets reflected, and therefore how much it pushes on the asteroid and changes its orbit. So if Bennu did wind up hitting the Earth in some catastrophe, how would that compare with the asteroid that we think hit the Earth 66 million years ago and caused the extinction of the dinosaurs and much of life?

Sara Russell: Yeah, well, I was going to say a great thing about asteroids, but another interesting thing about asteroids is that they have this capacity to cause global destruction. The last time that happened, of course, was about 66 million years ago. An asteroid impacted off the coast of Mexico, at Chicxulub, and we think this impact caused massive devastation and mass extinctions of the dinosaurs and other animals. But then that led to the rise of the mammals, and eventually to humans. So these catastrophic events do change the course of evolution, but they can sometimes change it in potentially interesting ways.

We think that if an asteroid about a kilometer in size, about the size of Ryugu, hit the Earth, it would cause global devastation; it would have catastrophic global effects. So that's the threshold for the very worst level of impact, and we think such a globally destructive impact might happen about once every hundred thousand to a million years or so. So it's not something that's likely to happen on our watch, but it can happen, and it probably will happen over the four or five billion years that the Earth has got remaining. So that's something that we're interested in: mapping which near-Earth asteroids might be potentially hazardous, and how hazardous they might be.

Oliver Strimpel: So for my last question, I want to ask you: if you had unlimited resources for your research, how would you use them?

Sara Russell: Right, I love that question. I love sample return missions, for all the reasons that we've talked about. Actually going into space and bringing a sample back that we can study in the lab tells us so much about the origins and age and characteristics of the object. So it would have to be a sample return mission that will bring stuff back to Earth for us to have in our collections. And the place that I would love to get a sample from is one of these interstellar interlopers. A couple of years ago, the first interstellar asteroid or comet was observed coming into our solar system, called ʻOumuamua. We don't know where it came from. It came into our solar system, and then it disappeared off again. But having a sample of something that is from beyond our solar system could tell us so much about our place in the galaxy: whether there's anything peculiar about our solar system that led to the evolution of life, and ultimately us, or whether we're very common. That is the one thing that we could do so much science on, to learn about not only our solar system, but the universe beyond us.

Oliver Strimpel: Sara Russell, thank you very much.

Sara Russell: Oliver, thank you so much. I really enjoyed talking to you, and thanks very much for including me in your podcast.

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