This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. The basic picture we have of the formation of the solar system is that the Sun and all the objects in the solar system formed out of a giant cloud of gas about 4.6 billion years ago. Something triggered at least a part of the cloud to collapse on itself, with the majority of this material forming the Sun and the remainder forming a disk, which eventually amalgamated into the present-day bodies of the solar system. It was thought that during the collapse of the solar nebula, the material was heated and thoroughly mixed into a uniform composition. This uniform material would then form the Sun and all the other bodies of the solar system. So the variations in the composition of the solar system bodies that we see today would be caused by physical, chemical or other processes that took place during the course of solar system formation, or subsequently. But several decades ago, we detected so-called isotopic anomalies in meteorites, and in tiny grains contained within them. It turns out that these can only be explained by inhomogeneities in the original source material that formed these meteorites and grains. So what can explain this heterogeneity? Can it tell us something about what was happening to the solar nebula when the solar system formed? And does it throw light on how the Earth was formed? Rick Carlson is director of the Earth and Planets Laboratory at Carnegie Science in Washington, D.C. He's an isotope geochemist who studies these isotope anomalies to probe the early history of the solar system. Rick Carlson, welcome to Geology Bites.

Rick Carlson: Thank you, Oliver.

Oliver Strimpel: I introduced you as an isotope geochemist. What exactly is isotope geochemistry?

Rick Carlson: Well, what I do is make measurements of the isotopic composition of elements, for those elements that have more than one isotope. An isotope is the combination of the number of neutrons and protons in an element. The number of protons determines what element it is, but if you have different numbers of neutrons, they're called the isotopes of the element. The advantage of measuring isotopic composition is that it's affected in different ways by different processes. For example, chemical processes can cause mass-dependent fractionation, meaning that the amount of change in the isotopic composition is dependent on the mass of the isotope. Those are subtle changes caused by chemical reactions. But more usefully, at least in my field, there are two other processes that cause isotopic changes. One is the radioactive decay of one isotope into another. This, of course, provides the clocks that we use in the field of geochronology, where we can determine the ages of rocks. And then finally, and I think this is the subject today, the isotopic composition of an element is created when the element is created in a star, and the ways that stars produce elements produce elements with very different isotopic compositions. So by measuring the isotopic compositions of meteorites, we can determine what kind of stellar source they actually came from.

Oliver Strimpel: Do you have a lab of your own in which you can accurately measure such variability in isotopic composition?

Rick Carlson: I do indeed. I have five mass spectrometers of various flavors and a couple of ion probes, which are basically mass spectrometers that sample something by shooting an ion beam at it. So we can work everywhere from relatively low-precision isotope ratios at very high spatial resolution, so submicron spatial resolutions, to very high isotope ratio precision. What's important for the kind of things that we'll talk about today is the high-precision work, which is necessary to resolve very small changes in isotopic composition that are seen in some of the meteorites.

Oliver Strimpel: So as I mentioned in the introduction, we find isotopic anomalies in meteorites and in grains within them. Do we find them in all kinds of meteorites? And what exactly are these grains?

Rick Carlson: Grains are found in many types of meteorites, mostly the more primitive ones. These grains form basically when a star explodes, or when some large stars have winds that blow material out into space. You can imagine an exploding star is a pretty energetic event; it's called a supernova. This blows material out into space, and as it's expanding away from the central star, it'll eventually cool, and as it cools, the refractory elements will condense out as mineral grains. Silicon carbide is one of them; alumina, Al2O3, for example, would be another. So these grains, generally on the order of a micron or smaller in size, are what happens when the gas flows out and gets cold enough as it's blowing away from the star. As it moves out into the galaxy, it just drifts around, and eventually, on occasion, it can get trapped in a higher-density or higher-gravity part of the galaxy, where it becomes part of a molecular cloud. As the cloud collapses and turns into a planetary system, those grains, unless modified, can be preserved in primitive meteorites. So we see them at abundances of a few ppm to tens of ppm in the most primitive meteorites. If the meteorite, though, undergoes any type of thermal processing, like thermal metamorphism, or just gets hot, then, since many of those grains are not in chemical equilibrium with the materials that surround them, the grain will get destroyed. Its isotopic composition might be preserved, but it gets diffused between all the various grains that it turns into. So in high-metamorphic-grade meteorites, meteorites that have melted, you do not find these grains. But in any type of meteorite that has not been melted, you can quite often find them, like I say, in ppm quantities.

Oliver Strimpel: So do we say that a body has an anomalous isotopic composition because it's different from that of the Earth?

Rick Carlson: Yeah. The trouble with finding these grains is that they're small, but once you find them, they have massive isotopic anomalies. The isotopic measurements are actually pretty straightforward; you just have to find these little submicron grains. You might be familiar with, for example, oxygen isotopic composition. It's a very commonly used tool in many fields in Earth science, but all the variation in oxygen on the Earth is in parts per thousand, in, for example, oxygen-18 to oxygen-16 ratios, which are the two main isotopes of oxygen. Whereas in presolar grains, the ratio of 18 to 16 can vary by five orders of magnitude. So these are not parts per thousand; they're five orders of magnitude. So the isotope variation that's present in these presolar grains is hugely outside of anything that we can get from processes that would occur in our solar system. The only way to get isotopic variation of this magnitude is deep in stars, where these elements are actually being synthesized by nuclear fusion.

Oliver Strimpel: Wow. So when we say anomalous, we mean really anomalous. Can you sketch out the classical picture of the presolar nebula and how the solar system formed out of it?

Rick Carlson: A good starting point is to think of something called a dense molecular cloud. This is an astronomical term, and it's one of those terms that you have to be careful with, in relative wording. In astronomy, a dense molecular cloud is something that's actually about two orders of magnitude better vacuum than the best vacuum we can get in the laboratory, but it's high density compared to the rest of the galaxy. You've seen some beautiful pictures of these from the Hubble telescope, for example. These are density concentrations within the galaxy that come about just from gravity and the way that the spiral arms of the galaxy work. If they get big enough and dense enough, their internal gravity can actually cause them to collapse. So this is one way. As they collapse, basically most of the matter in the molecular cloud will end up in a central star. But since most molecular clouds are rotating, because they're in a rotating galaxy, as they collapse, the conservation of angular momentum makes the central star spin up. So it starts to spin, and as a result, the centrifugal force of the material coming in basically causes it to stop falling into the star and form a disk around the star. And it's that circumstellar disk which is where planets form. So that's kind of a self-imposed formation. The other way, which I find more interesting, is that the collapse can be triggered by, for example, an exploding supernova close by the molecular cloud. Molecular clouds occur in star-forming areas, because molecular clouds are what stars are made out of, eventually. So it's not an unusual association to have a molecular cloud in an area that's got active star formation going on. And when you have large stars forming, well, large stars have very short lives, meaning tens to hundreds of millions of years. So they grow big, they do their nucleosynthetic thing, and then they explode, and they emit a shock wave. And if that shock wave encounters a molecular cloud, just the pressure from that wave, which is also carrying material from the supernova, starts pressing on the molecular cloud. For the most part, it destroys the molecular cloud, but it compresses it to the point where you can get enough concentration in the middle of it that gravity will take over, and it will continue to collapse and form a central star, and often a circumstellar disk around it.

Oliver Strimpel: We know that the mix of elements, as opposed to isotopes, on Earth is different from that, say, in the large gas planets, Jupiter and Saturn. So there's some kind of physical process that separated the elements out according to their masses, perhaps with the heavy elements concentrated closer to the Sun, in the rocky planets, and the lighter elements further out. Is that the kind of physical process that created the variation at the element level?

Rick Carlson: Actually, the biggest process that segregates elements at that level is volatility, especially in our solar system. The gas giants, Jupiter and Saturn, Neptune, Uranus, are way out there, and they're far enough out in the disk that that portion of the disk probably remained cold, and by cold I mean cold enough that water ice could form, for example. If you think about the composition of the Sun, it's mostly hydrogen and helium. Hydrogen is six orders of magnitude more abundant than silicon, for example. So if you can collect the hydrogen, you're going to end up as a big planet. Something like Jupiter could have collected it because water ice was forming out beyond what we colloquially refer to as the snow line. So you had water ice condensing, and it accumulated with all the other dust, the same way that the Earth formed. But the Earth was far enough inside that it was simply too hot for water ice to form. So the water, the hydrogen, stayed in the gas form, and then probably something nasty went on with the star. Stars in their formation do all sorts of things, outflows, stellar winds, various things, and it probably blew off the gas from the inner solar system before the water ice had a chance to end up on the planet. Once a rocky planet gets big enough, it can attract hydrogen and water just by its own gravity, but the fact that the Earth doesn't have a big atmosphere suggests that it never got big enough while there was gas around to collect it. Instead, Jupiter probably got it because it collected all the ice and got big enough. And once you get big enough, you have enough gravity that you can just suck in the gas that's in the circumstellar disk. So you end up with a gas giant like Jupiter or Saturn.

Oliver Strimpel: Okay, so given that we expect to see all these variations in the abundance of the elements as we look at different parts of the solar system, why could we not also explain these recently discovered isotopic anomalies in that way as well, through some physical or chemical process?

Rick Carlson: Well, we probably can't, in that many of these isotopic anomalies are found in elements that are very refractory, for example titanium, chromium, rare earth elements like neodymium and samarium; barium, for example, is another one. These are elements that would condense even at pretty high temperatures. They are abundant elements in the Earth, so we know that they weren't particularly separated. For refractory elements in particular, the barium, neodymium and titanium concentrations in the Earth are more or less the same as their abundances in the Sun relative to some other element, for example silicon. So we know that those elements were not fractionated in the inner solar system, and they shouldn't have been, because they condensed at such a high temperature. They were part of the dust that eventually accumulated to form the planets. So the physical mechanism that I described, which separated water and hydrogen between the inner and the outer solar system, just won't work for an element like titanium. And yet what we see is that the titanium, the chromium, the barium isotopic compositions are different between different planets. Of the terrestrial planets, we only have samples of two, Mars and the Earth, but the meteorites also show quite a wide spectrum in these elements. Some of the meteorites probably come from asteroids that are further out in the solar system. None of them come from beyond Jupiter, but they may have been beyond Jupiter when they formed. So what we're seeing is that even in these elements, which we know should not have been fractionated by the physical processes that made Jupiter on the one hand and the Earth on the other, they are different between materials that are in the inner solar system versus the outer solar system.

Oliver Strimpel: So how did we manage to decipher what actually did cause these anomalies?

Rick Carlson: Well, nucleosynthesis has some very distinctive fingerprints in the way that it makes isotopes. We know this for a few reasons. One is the theory of stellar nucleosynthesis, worked out in actually a magnificent paper from the '50s by Burbidge, Burbidge, Fowler and Hoyle. It was just a marvelous treatise on this, before we even knew how elements were made. They did a great job on that. But it turns out that the presolar grains we measure, which are actually the grains that are outflows from stars, also give us clues as to the mechanisms, and the various isotopic compositions that can be created in different stars. The way a star makes elements depends on the pressure and temperature in the star, which in turn depends on its mass, for example, and where in the star the nucleosynthesis is happening. So each star makes a pretty wide range of isotopic compositions that are going to be different from star to star. So we know what to expect when we look for isotopic variations caused by nucleosynthesis. I mentioned that other processes cause them as well, for example radioactive decay. But with radioactive decay, it's basically one isotope decaying to another isotope, so if you don't see a correlation between the parent isotope and the daughter isotope, that's not the cause of it. Chemistry can cause isotope changes as well, but they're usually mass-dependent; not always, but often. But the real key is that we know, particularly from presolar grains and from nucleosynthetic theory, that some nucleosynthetic processes produce these very strong fingerprints on the elements that they produce. We see those clearly in the presolar grains, like I said, in a big way: five orders of magnitude of oxygen isotope variation, for example. But the other place you see them is in elements that have a lot of isotopes. So for example, barium and neodymium both have seven isotopes; samarium has seven isotopes. And what often happens in nucleosynthesis is that you see what are called odd–even effects, where nuclei with an odd number of neutrons plus protons are made in a different way than those that have even numbers. It's not quite that simple, but you basically see these effects that affect odd-numbered or even-numbered nuclei differently. No other process does that quite so well. So that's what we see when we measure barium and neodymium in meteorites, and it matches very well what we've seen in presolar grains, and it matches nucleosynthetic theory. So by far the strongest argument is that these are nucleosynthetic anomalies that are tracking nucleosynthetic heterogeneity that was present in the initial molecular cloud.

Oliver Strimpel: Okay. So if we see these clear signatures of a given progenitor star, if you like, does that mean that each different anomaly represents a different source star, for each different grain, or each different meteorite, or for the Earth, or for any particular body?

Rick Carlson: Yeah, basically it does. If you think of it, a molecular cloud is basically just a garbage dump in the galaxy, so it's collecting the outflow from stars all around it. Our planet formed about 10 billion years into the history of the universe, so there was plenty of time to make elements ahead of it. So the molecular cloud that our solar system formed from had 10 billion years to accumulate stuff from exploding stars. So there could be thousands or millions of stars whose outflows contributed to the materials that make up our solar system. Molecular clouds in particular are at tens of degrees Kelvin, 10, 20, 30 degrees Kelvin, so they're very cold, and they basically just sit there until something exciting happens, like they collapse and start forming a planetary system. So their starting point is basically zillions of little bitty grains that came from different stars, which have unique isotopic compositions depending on the star they came from. If you average all of that, though, the average that you get will be the composition of our Sun, because the Sun contains 99.8 percent of the mass of the solar system. So the average that's provided by the Sun represents the average of all those contributions. But when you get down to an individual grain level, you're looking at the individual outflow from individual stars over the 10 billion years before our solar system formed.

Oliver Strimpel: So the fact that we see these anomalies, is that then pointing to the idea that maybe the presolar nebula wasn't as well mixed as we once thought?

Rick Carlson: What it is showing us is sort of what we expected: that the molecular cloud contained these individual components. But the traditional wisdom was that with planet formation, as molecular clouds collapse, the inner disk is hot enough that basically everything is evaporated; even refractory materials are evaporated. So once you put everything in the gas phase, as that circumstellar disk is orbiting the star, it gets mixed up pretty well, and that process would obliterate the individual stellar components that are present in the molecular cloud. So that was the traditional wisdom when I was a student, quite a long time ago. There were no nucleosynthetic anomalies in materials that we could find, because we didn't have the precision to measure them. So that's what you'd expect if the circumstellar disk got hot enough, turned everything to gas and mixed it up really well. And then, as the star consolidated and started to cool down, you would start condensing new grains out of that gas. Those new grains are going to have the average isotopic composition of everything in that circumstellar disk, and so you end up with stuff that we call solar composition. Again, the Sun has almost all the mass in the solar system, so the closer you get to the average composition of the molecular cloud, the closer you're going to be to the composition of the Sun. So that's kind of the background: what we're made out of, what the Earth and all the planets are made out of, is what we would call solar. But as we got into it and started looking, the first thing to come out was really weird: finding these presolar grains. You found these grains through rather abusive techniques for meteorites. You basically dissolve everything away, and there are some grains, like silicon carbide in particular, that don't dissolve in most of the acids that we use to dissolve meteorites. So you dissolve away everything, you end up with this pile of silicon carbide grains, and then you measure them, and they're isotopically all over the place. So these are grains that were never evaporated. They are the individual grains that survived the collapse of the molecular cloud and the formation of the circumstellar disk. They're rare, but they're the extreme examples. Where we're at now, though, is that we can actually measure nucleosynthetic isotope differences at the scale of planets. So we know, for example, that Mars and the Earth are isotopically distinct in terms of nucleosynthesis. We also know that different types of meteorites seem to break down into two families. Now, with the bad terminology we always use, one is carbonaceous chondrites, because they've got lots of carbon, and the other one is non-carbonaceous chondrites, because they don't have a lot of carbon. But those are the two families, and it turns out that they're quite isotopically distinct. So what that means is that they somehow preserved these nucleosynthetic signatures; neither one of them got to the average composition of the Sun. So they are preserving, to a smaller degree, the heterogeneity that was present in the molecular cloud.

Oliver Strimpel: You said earlier that no physical process other than nucleosynthesis could create the observed anomalies. But now you're actually suggesting that there's this physical process, which is to do with how refractory the silicon carbide presolar grains are, that results in a certain kind of physical or chemical fractionation that gives us this different pattern of isotopes in different parts. How does that square with the idea that the origin of these anomalies is purely nucleosynthetic?

Rick Carlson: I mentioned that if you mix everything perfectly, you'll lose all that history of the presolar materials. But if you don't do that, then you can retain components of it. The example I like to use, because I like cookies, is chocolate chip cookie dough. When you make chocolate chip cookie dough, the dough itself is a mixture of flour and sugar and butter, and you mix that up to the point where, once you've got the dough, you're never going to go into the dough and pull out a grain of butter, for example, or a grain of sugar. So it's very well mixed, and everywhere you sample it, you're going to get the same material. But then you have the chocolate chips, and you mix them in, but they don't dissolve in the dough. So what you've got then is a mixture where you could go in and, for example, take out some of the batter without the chocolate chips, and you'd end up with one component of it. That, for example, could be the carbonaceous meteorites. And then you go in with another spoon and take out some that has both batter and chocolate chips, and that could be the other type. So when you think about physical processes, the only physical process that creates the anomaly is nucleosynthesis, but there are many physical processes that can destroy it. For example, like I said, stirring the batter together destroys the difference between the flour and the sugar, so that you can't separate them after that; you have the average of the mixture. But you can imagine that if you took out a spoonful for each chocolate chip cookie, the different chocolate chip cookies would be pretty close to the same composition, but they would probably have a different ratio of chocolate chips to batter in them. And that's the kind of scale that we're seeing between, for example, planets, and between the different meteorites. Whereas with the presolar grains, what we're doing would be going in and pulling out a chocolate chip all by itself. So then the question you ask is, what's the physical process that would do that? There really are two. One of them is heat. I mentioned that if you evaporate all the presolar grains, they turn into a gas; they're going to get mixed up more efficiently when they're a gas, and when it cools down again, they'll turn back into mineral grains, but they'll turn back into mineral grains that have the average isotopic composition of everything they mixed with. The other one, though, is if you think about the fact that these anomalies are carried by different grains: different minerals will have different evaporation temperatures. So depending on where you are in the disk, the mineral grains that evaporate at low temperatures will evaporate and get mixed into the average composition, whereas the higher-temperature ones won't. So basically, it's an issue of mixing away what you start with, and the way that you keep the components segregated is by keeping them away from one another, either because your temperature doesn't get hot enough to completely evaporate and homogenize them, or because you put a barrier in the way. And one of the interesting theories that's out now that explains the dichotomy between the carbonaceous and the non-carbonaceous chondrites is that Jupiter formed very early in the circumstellar disk. And one thing that we see now, with these beautiful images that are returned from ALMA, which is a large radio telescope in Chile, is circumstellar disks around other stars that are forming planets. You look at these disks, and they're a nice disk around the star, but they have gaps in them. And a gap is a place where there's a planet forming, and it's basically clearing out all the material that's in the orbit of that planet. So if Jupiter formed early, you can imagine that Jupiter formed a gap in the disk, so that material in the outer solar system couldn't get into the inner solar system, because Jupiter would have collected it, and it would be part of Jupiter now. So there are physical mechanisms, both temperature and early planet formation, like Jupiter forming early, that could stop the mixing in the process of evaporation, leaving the disk heterogeneous.

Oliver Strimpel: Okay, let's suppose, then, that the solar nebula was thoroughly mixed and homogenized when the Sun formed, and then Jupiter formed early on and blocked subsequent transport between the outer parts of the disk and the inner parts. What might then later have changed the composition of the outer parts of the disk, beyond Jupiter?

Rick Carlson: The main one that you would think of that way is that the collapse of a molecular cloud and formation of a star system probably occurs over millions of years. That sounds long, but it's clearly a very short time geologically. But it's millions of years, whereas, depending on how you form Jupiter, it could have formed much quicker than that. So one way that you could stop the homogenizing evaporation and mixing of everything together again is if you form Jupiter early enough, while material is still flowing in from the molecular cloud. So if the molecular cloud was initially heterogeneous, and the early material that flowed in and formed the star was different from the later material that came in, and Jupiter blocked that, then that would be one way the outer solar system might end up a little distinct from the inner solar system. And there's an interesting additional factor with that. I mentioned at the beginning that one of the ways that we think the molecular cloud could start collapsing is if a supernova shock wave started it. These shock waves are what they sound like, but they're also basically the exploding wind from when the explosion occurs. So they're not only shocks, but they're bringing material in with them, and that material is newly synthesized. It's got an isotopic composition that's very distinct, and it's very unique to the supernova that exploded to create it. So one of the options is that basically you started our solar system with a shove from a supernova shock wave, and the early material, the material that was in the molecular cloud to begin with, collapsed, and that's what the Sun and most of the inner solar system formed from. But the material that's coming in from the supernova was coming in very late, and it may have been preferentially concentrated in the outer regions of the molecular cloud, so it was the last stuff to get sucked into the star. So if during that process you form Jupiter, and you cut off that inflow because Jupiter formed and was sweeping up everything, then the outer solar system might have more of the supernova material than the inner solar system would have. So these are the questions. I present these like we know what we're talking about, but these are the possibilities. And the fact that we have these nucleosynthetic anomalies now allows us to ask these questions, because if you don't have the anomalies, you have no way to track the heterogeneity that was present in the solar disk. Now we have heterogeneities, and now we have to figure out what they mean.

Oliver Strimpel: Do you have a personal preference for one of the explanations of the heterogeneity?

Rick Carlson: Well, I like the supernova trigger. I just like the idea of some exploding star, you know, creating our solar system; I find that intriguing. I think the Jupiter barrier idea has a lot of merit. I think it will tell us a lot about how Jupiter formed. We've now seen thousands of exoplanets around other stars, and one thing that we know is that our solar system is very different from other solar systems in its architecture: where its gas giants are, how many planets there are and how big the planets are. So there clearly were different mechanisms that worked in ours; probably the same physical mechanisms, but with very different results. So this combination, of being able now to map the heterogeneity, to think about a supernova that contributed to it, to think about early formation of Jupiter, just gives us so much more observational data that we can plug into the theory of planet formation and see how well it matches our system. The other nice thing about a supernova trigger is that supernovas make stable isotopes, but they also make lots of radioactive isotopes, and my main field is radioactive geochronology. So the supernova is blowing a bunch of clocks into our solar system. So, you know, once we can discern those, it's very useful. One of them, for example, is aluminum-26, which has a very short half-life, of 700,000 years. We know that it was present when meteorites were forming, and we're using it for chronologies in meteorites, where, in the best of cases, we can determine the ages of meteorites to precisions of about 90,000 to 100,000 years, in a rock that's four and a half billion years old. So, you know, 100,000-year age precision doesn't sound great, but if you think about it as relative precision, it would mean knowing basically the minute of your birth rather than just the day of your birth. So it's not too bad. So that's the information that we can get out of this in terms of the early history of the solar system. What we do know is that most of the exciting things, in terms of forming our solar system, happened in the first 1 to 5 million years, and then the planets sort of slowed things down and took tens of millions of years. When I was a student, our geochronology was limited to 20- to 30-million-year precisions, and, you know, everything was the same age, because we couldn't resolve anything. But now getting million-year precisions is relatively straightforward, and with million-year precisions, you can actually pull apart the individual events that were happening while the planets were forming. And now that we have nucleosynthetic anomalies, we can tie that into compositional differences in the disk and really understand the physics of how that disk turned into a bunch of planets.

Oliver Strimpel: So with our present-day knowledge of the detailed compositional and isotopic differences between the different types of body in the solar system, particularly the meteorites, can we identify one or more types that match the isotopic signature of the Earth, which might therefore be progenitors of the Earth?

Rick Carlson: We can. In fact, that's been one of the goals of this work, but it's coming up with a most disquieting result: it turns out that the group of meteorites that's most similar to the Earth in isotopic composition is a group called enstatite chondrites. These are rather unusual rocks, in that they're very reduced. All the iron is metal; there's no iron oxide. Things like silicon are in the metal. There are sulfides like calcium sulfide, which you'll never see on the Earth. So they're very reduced material. It's also a type of meteorite that's got quite a lot of silicon in it, so it's got a high silicon-to-magnesium ratio. So if the Earth formed out of enstatite chondrites, it would explain the Earth's isotopic composition, at least on a nucleosynthetic basis, but we'd end up with a very strange Earth composition, where we have a lot more granite than we have. We know the mantle composition pretty well, and it doesn't match the magnesium–silicon ratio of enstatite chondrites. So the alternative is that the enstatite chondrites are sampling what the Earth formed from, but they're sampling it late in the story, and in fact the Earth swept up most of what it's made of. So, you know, in our meteorite collection, we have what nature delivers to us, which is not necessarily what was there when the planets were forming. In particular, most of the meteorites that we have probably came from the asteroid belt, which is outside of Mars, so it's not necessarily the same feeding zone as that of the Earth. So the answer is that we have this collection of meteorites that spans the compositional spectrum. In some of these elements, the Earth is in the middle of it, so it'd be easy to combine these meteorites and make the Earth in terms of isotopic composition. In other ones, the Earth is at one end member, and in those cases you've got to worry that you're not really seeing the end members, that there's something beyond the Earth. And there's actually some evidence of that now, in ruthenium isotopes, for example: there may be things that are more extreme than any of the meteorites, or the Earth. But basically, we're using this to type the building blocks of the planets, and we just have to sort out why enstatite chondrites fit the isotopic model so well and fit the chemical model so poorly.

Oliver Strimpel: Not surprisingly, there are still lots of outstanding questions about the early evolution of the solar system and the origin of the Earth. If you had unlimited resources, how would you go about finding the answers?

Rick Carlson: The first thing I would do, just because of my expertise, is build a mass spectrometer that had isotope ratio precisions better than a part per million. At the precisions we have now, we're resolving anomalies that are about 5 to 10 parts per million. And what that's showing us is that these anomalies exist, which is a discovery of its own, and it's very nice, but the ratio of signal to noise is so low that basically all you can say is that they exist. So if we could get our precisions down to the point where it was very clear that these anomalies existed, we could begin to ask what kind of anomalies there are. What stars contribute to them? Are there differences beyond the ones that we've already seen? That would tell us more detail in terms of, for example, looking for the supernova component. The other way, though, to spend a lot of money is sample return missions. In your last question, we talked about the fact that the meteorite collections are not necessarily representative of everything out there. What we've learned from sample return missions, the OSIRIS-REx mission to Bennu, for example, asteroid Bennu, has been very useful in terms of finding out about new materials that are out there that we don't have on Earth. There are all sorts of things out there that we could go and get. There are these things called the Jupiter Trojans, which are a group of asteroids that are locked in the orbit of Jupiter, in front of it and behind it, at a gravitational point where they're quite happy being. But they probably got trapped there early on, and so they could be quite different from the stuff that we have in the asteroid belt. So we have such a narrow sampling of the materials that are in our solar system. If you want to think about planets, we have meteorites from Mars, we have meteorites from the Moon, we have lots of samples from the Moon. We have nothing from Venus. So the planet that's most like us, we don't have any samples of. So to me, sample return and having better mass spectrometers would be my two unlimited-funding requests.

Oliver Strimpel: Rick Carlson, thank you very much.

Rick Carlson: Thank you, Oliver.

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