This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. To unravel the early history of the solar system, we need to look beyond the Earth, since any evidence of such a distant past has long since been erased by the Earth's active geological processes. Asteroids, and the meteorites they spawn, have been useful in this regard, since they are thought to be unaltered scraps left over from the earliest stages of planet formation. In earlier podcasts, Sara Russell and Harold Connolly introduced us to the asteroids in general, and to recent missions that capture samples from them and return them to Earth. In this episode, we focus on a very special asteroid called Psyche that will be visited by a spacecraft that is being launched in August 2022. Ben Weiss is Deputy Principal Investigator and Magnetometry Investigation Lead on the Psyche mission. He is Professor of Planetary Sciences at MIT. Ben Weiss, welcome to Geology Bites.

Ben Weiss: I'm very happy to be here.

Oliver Strimpel: There are a huge number of asteroids. What's so special about Psyche that made us send a space probe there?

Ben Weiss: Yeah, there are something like 800,000 known asteroids, but Psyche stands out because not only is it one of the largest known asteroids, in fact the 10th most massive asteroid, but it's tied for being the most dense known asteroid, and we have reasons to think it may have a large fraction of its body in the form of metal. We think the asteroids are remnant planetesimals, the building blocks of the planets. So we are interested in Psyche because it is likely a member of this population that records the formation and early evolution of the solar system, while being a unique, perhaps metal-rich world that is fascinating in its own right.

Oliver Strimpel: I've got to ask you how Psyche got its name.

Ben Weiss: So asteroids were discovered for the first time in the 19th century. Psyche's full name is actually 16 Psyche, because it was the 16th asteroid that was discovered. It was discovered by an Italian astronomer named Annibale de Gasparis in 1852. In those days, the convention was to name asteroids after Greek or Roman gods, often female. And so he named it after Psyche, who's the Greek goddess of the soul. She was born mortal, and then she married Eros, the god of love, Cupid to the Romans, and eventually became a goddess.

Oliver Strimpel: Oh, that's quite romantic. Do we have any scenarios as to how a body like Psyche, with so much metal in it, might have formed?

Ben Weiss: So there are two main ideas for how a body could be so dense. Just to put that in context, take the Earth's density. Some of the Earth's density is because its interior is under such high pressure. If you were to take the Earth and uncompress it, and calculate Earth's density in an uncompressed state, you'd get something like 3 grams per cubic centimeter, very like typical rock, maybe 3.5. Psyche is considerably more dense than that; the numbers range between 3.5 and 4 grams per cubic centimeter. That seems like a subtle difference, but it's actually very hard to explain unless Psyche has a significant quantity of metal in it, potentially even twice what a planet like the Earth has.

How you do that is a challenge, because the planets and the asteroids all formed from a primordial cloud of gas and dust, known as the solar nebula, which itself is kind of a fragment of the gas and dust in the local region of our galaxy, in what we call molecular clouds. We think these molecular clouds are the birthplaces of stars throughout the galaxy, and the composition of our solar system, as well as of the local molecular clouds from which other stars are formed, is fairly homogeneous everywhere we look. And you would think that a planet that formed out of that gas would have basically the same composition as the cloud that it's coming from. So if Psyche is as dense as it seems to be, and it's metal-rich as a result of that, and we think that the only cosmically abundant heavy element that could really increase density to that level is iron, then that leaves us with two general ideas for what could have caused the metal ratio to be increased in Psyche.

One is that maybe it did form with a composition equal to the bulk solar system and molecular cloud composition, and then it melted. We think large bodies like the Earth, Mercury, Venus and Mars get hot inside as a result of their formation, and they undergo large-scale melting: heavy stuff sinks to their center and light stuff floats to the surface. That's how we get these radially layered structures that we think of as characteristic of planets: a metallic core made of iron, because, again, iron is the most cosmically abundant heavy element, and a kind of rocky exterior mantle. If that happened on Psyche, it still wouldn't have enhanced its iron abundance. But then you could imagine some other body striking Psyche and stripping off its rocky exterior, leaving an iron-rich center. So that two-stage process could be one explanation: it formed like the Earth, but then had its mantle stripped off by an impactor of some kind.

The other idea is that Psyche somehow formed with a composition very different from the average solar system and molecular cloud composition, and that somehow it managed to accrete from grains that tended to be iron-rich relative to the surroundings. There are a bunch of different ideas for how that could happen. For example, iron grains floating in the nebula may have been attracted to each other by magnetism, like magnets on your refrigerator, and kind of stuck together. And then that can be a runaway effect, because once those things stick together, their magnetic field might get stronger, or their gravity field might eventually get stronger, and the growth rate can increase. You can end up iron-rich from that. There are other ways of sorting out rocky grains and only having metal grains stick together, for example because there was gas in the solar nebula. Today, the planets are more or less orbiting in a vacuum, but in the early solar system, there was a headwind that grains orbiting the Sun experienced, because there was hydrogen gas around, and that headwind would affect less dense materials, like rocky materials, more substantially than something that's denser and can plow through it, like iron. And so there are ways of winnowing out the rocky grains, by having them fall into the Sun at a faster rate than iron grains, since iron grains are more resistant to this headwind. Then, again, that would lead to a local enhancement of iron, producing an iron-rich body. So that's the second idea: that Psyche never ever had lots of rocky material, but actually formed preferentially from iron-rich materials at its location in the protoplanetary disk.

Oliver Strimpel: Wow, that's fascinating. Those are really two radically different formation scenarios. And you said earlier that if we can figure out how Psyche was formed, this might actually have broader implications for the solar system, perhaps as to how the other planets were formed. How do we generalize, if you like, from what we might find on Psyche?

Ben Weiss: We seek to understand how planets form and how they evolve, and one of the most powerful ways we approach this is through a concept called comparative planetology. We examine the diversity of planetary bodies that we see out there, and then we understand, through comparison, what basic physical and chemical principles shape the formation and evolution of planets, just by looking at the diversity of outcomes. And the cool thing about being alive today is that not only do we have the eight planets in our own solar system to play around with, plus 800,000 or so asteroids, but now we have 5,000 known exoplanets; we just crossed the threshold of 5,000 a few weeks ago. So we have a ton of planetary bodies that we can work with to understand the principles of planetary formation and evolution. This includes familiar places like rocky Earth-sized planets, Venus and Earth, and their equivalents around other stars. It includes the gas giant planets, Jupiter and Saturn, which have small rocky cores, probably, and gaseous envelopes. It includes other weirder things that we only see around other stars, like water worlds, or gas giants that are super close to their stars. We've even just found a gas–ice giant the size of Neptune that has an enormous atmosphere, so it's got the density of cotton candy. There's an incredible diversity of bodies we see out there and within our own solar system, and Psyche, as potentially the densest known object in our solar system, is another end-member in that bestiary of planets.

So while weird, it's also representative at the same time. We see other smaller, very dense asteroids. There's one which is actually near the Earth, and there's another asteroid in the main belt, Kalliope. We have Mercury, which is a planet, of course, but it's much denser than the Earth. In fact, we think maybe 80% of the radius of the body is core, unlike the Earth, where it's about 50% of the radius. So basically, Mercury is a core with a little bit of rock slapped on top. And then there have been these incredible discoveries of super-dense planets, ranging from half an Earth mass up to about 10 Earth masses, around other stars, some of which we think can only reasonably be explained by being iron-rich. What's so important about Psyche in all of this is that it's the most accessible of these bodies. It'll be the first iron-rich world, we think, that we can visit up close with a spacecraft. And so as we learn about how Psyche formed and evolved, we will learn about this whole spectrum of dense worlds that we see in our own solar system and beyond.

For a body the size of Psyche, which is about 200 kilometers or so in diameter, impacts from other bodies are a fundamental part of its life, because there are many other bodies bigger than Psyche, or Psyche-sized, around it. When you're Jupiter, not much can mess with you, but when you're Psyche, there are lots of things that could hit you and do a lot of damage. For those of us living on the Earth, we don't think about impacts shaping our normal geologic processes, or the overall structure of the Earth, on a regular basis. But when you live on an asteroid, the frequency and energy of impacts are a regular part of daily life, at least on the billion-year timescale, and they can play a fundamental role in shaping the planet. So that's actually a really reasonable thing to happen to a small body. The second way I talked about, in which Psyche could form, is that it accreted preferentially from iron-rich materials, more iron-rich than the bulk composition of the galactic neighborhood.

Oliver Strimpel: It seems like we already know quite a bit about Psyche, and it's not very big. So how do we know all that before the probe gets there?

Ben Weiss: In recent years, with the advent of telescopes that can correct for the blurring of the atmosphere, we can actually see Psyche's shape, instead of it just being a point of light, and so we've been able to determine its volume from that. We actually even see its shape: it's like a little egg, maybe flattened slightly, and it looks like it's actually got a big crater at one of its poles that's taken a chunk out of it.

Oliver Strimpel: Wow. We can resolve a crater?

Ben Weiss: Just barely. It's a putative crater, and we'll see when we get there. The reason I'm saying that with such confidence is that similar blurry images were acquired for the asteroid 4 Vesta, and I have to admit, I was skeptical about how much these images were really revealing about the asteroid's structure. And then, when the Dawn spacecraft got there a decade later, sure enough, that big crater was exactly where everyone was saying it was from these blurry images. So this technique of using ground-based telescopes to determine the shape of asteroids has been proven spectacularly a few times.

So we know the volume of Psyche pretty well. What we don't know quite as well, and this is always a challenge for all asteroids, is its mass. You need to know how big it is, in terms of its volume, and how massive it is, in order to get its density, right? The volume part is straightforward; the mass part is not. The reason is that to get the mass of a body like Psyche, which has no known moon, you have to track how other asteroids, which pass by it now and then in their regular orbits around the Sun, get perturbed from their regular motion around the Sun. These are very subtle effects, because none of these bodies pass particularly close. So there's some uncertainty on that mass, and when you put it all together, we get an estimate of the density of somewhere between 3.4 and 4.1 grams per cubic centimeter. So that would be like a really heavy, dense rock. I don't know if anyone's picked up a meteorite, but a heavy rock like that is what it would feel like. If you put that together and assume that there's some metal in there that's causing this heaviness, it could maybe be up to two-thirds metal, or maybe as little as one-third metal, depending on how much void space there is in it, how much porosity. And that's a big unknown, because we don't know whether Psyche has the primordial structure that was established early in the solar system after it formed, or whether, as a result of billions of years of being smacked by other asteroids, it's basically a pile of rubble with big void spaces in between. So that leads to some uncertainty about its composition.

What we see in terms of its size: it's about 140 miles in diameter, so it's about the size of Massachusetts, say. It's got a big crater in one hemisphere that's almost the size of Psyche itself, so it's clear that it got hit at least once so hard that it almost got to the point of being disrupted completely. And from looking at the infrared and visible spectrum of Psyche, there's some evidence that maybe there's some silicate, some rocky material, on the surface, something like a low-calcium, iron-rich silicate.

Oliver Strimpel: OK, let's talk about the Psyche spacecraft a bit. Can you give us a high-level overview of the mission?

Ben Weiss: We have a spacecraft which is supposed to launch on August 1st on a SpaceX Falcon 9 from Cape Canaveral. It is carrying three instruments: a magnetometer for measuring Psyche's magnetic field; a gamma-ray and neutron spectrometer, which will measure the composition of Psyche; and an imager for imaging the surface in the visible and near-infrared. And it has a really cool, sort of Star Trek-y propulsion system, something known as electric propulsion, where it shoots out of its tail end ionized xenon, kind of a blue glowing plasma, and propels itself forward continuously over many years by the reaction force, just by conservation of momentum. It's basically shooting out this tenuous, low-density gas that's moving at very high velocity. After a three-and-a-half-year cruise, flying by Mars in the middle of that, in 2023, to get a gravity assist to speed up the spacecraft, we're supposed to arrive at Psyche in 2026, and then we'll go into orbit for 21 months. We'll progressively lower the spacecraft from higher altitudes. We'll start at about 700 kilometers, and eventually the plan is to get it to about 85 kilometers altitude above the asteroid. We do that because we don't know the gravity field very well; there are a lot of lumps and bumps that we're likely to experience, and we need to progressively map it to higher accuracy in order to stay in a stable orbit.

Oliver Strimpel: You're in charge of the magnetometer studies. What exactly will the magnetometer be able to measure, and how will that bear on the formation scenarios that we talked about earlier?

Ben Weiss: The magnetometer will measure the magnetic field around Psyche. We don't think that Psyche is generating a magnetic field today in the way that the Earth is generating its field. The Earth generates its own field by the churning of its liquid metallic center, its core. Because the material that's moving is an electrically conducting metal, currents flow through that liquid metal, and we see evidence for that process by looking at the large-scale magnetic field of the Earth that is generated by these currents. We call that process the dynamo process, and we see it ongoing in Mercury today as well. All the giant planets, Jupiter, Saturn, Uranus and Neptune, are also generating dynamo magnetic fields. The Sun's magnetic field is a dynamo from its own churning, as are those of most stars. We think Mars did it in the past, as well as the Moon, and we see evidence for this encoded in meteorites as well.

In the case of Mars, the Moon and these meteorites, which are not generating a magnetic field today, we see what's called remanent magnetization. That's like a fossil magnetism from an earlier epoch, when these bodies are thought to have been generating a magnetic field. What happens is that if they were generating a magnetic field by this dynamo process, materials on the surfaces of these bodies that cooled below a certain temperature, which we call the Curie temperature, could lock in a record, in the same way that a tape could record music, for example. So we see on Mars, for example, that although Mars does not have a large-scale magnetic field today, the rocks on the surface are strongly magnetized. Likewise, we see this on the Moon, and we see it in certain meteorites. We don't know exactly what bodies they came from, but we infer that they came from asteroids, and they look like ancient lavas that formed on the surfaces of these bodies. We also see iron meteorites, which look like they were samples of certain asteroidal cores, and some of them, too, are magnetized. And so we infer that maybe some ancient asteroids generated magnetic fields by the dynamo process in their metallic cores.

However, we've never seen a magnetic field around an asteroid so far with any spacecraft observation. So that's what we're hoping to do at Psyche: see if it has a magnetic field or not. That would be this fossil, or echo, of an ancient dynamo magnetic field. If we find that Psyche does have a present-day fossil magnetic field, then that would support the hypothesis that Psyche is indeed formed from the core of a mantle-stripped early body that was differentiated, underwent large-scale melting and formed a metallic core. Now, if we don't find that Psyche has a magnetic field, which is entirely possible, then we're going to have to do other things to try to test whether Psyche is a core, because it's possible that Psyche was a core but didn't actually generate a magnetic field. There are many ways that can happen. Or maybe it did generate a magnetic field by the dynamo process, but there were no materials that were sufficiently cool to lock in a record of it that could be preserved until today.

Oliver Strimpel: Let's talk about the other instruments. What kind of information will we learn from what they capture?

Ben Weiss: The gamma-ray and neutron spectrometer measures the composition of a body by a really fascinating process. Space is full of cosmic rays, very high-energy particles. They originate largely outside the solar system, as well as from the Sun. And when these high-energy particles strike atoms, they actually cause them to emit characteristic gamma rays. Each element emits its own characteristic gamma rays when it's struck by one of these cosmic rays, and by measuring those gamma rays and their energy, we can actually figure out the elements that produced them. It turns out neutrons are also emitted in this process, and you can measure them as well. They're a little less discriminating in terms of elemental composition, but they can tell us the ratio of metal to rock, which is, of course, a very important ratio that we care about in this mission. So gamma-ray and neutron spectrometry together can tell us whether Psyche has a metal-rich surface or not.

In particular, they can tell us whether that metal is concentrated in one region and rock in another region, or whether they're intimately mixed at a fine scale. A metal-rich body that has metal concentrated in large regions would be consistent with formation as a core, whereas a body that doesn't have metal concentrated in one region, but where it's intimately mixed, would be more like a primordial, unmelted body, because the metal wouldn't have been able to segregate spatially from the rock. It's also possible that we will find that Psyche isn't even that metal-rich, maybe at the lower end of our estimates, like 30 percent iron. We'll be able to test that, again, with the gamma-ray and neutron spectrometer, in which case we would just infer a normal, low-porosity, primordial rocky body, like we typically see in the asteroid belt.

The imager can further distinguish between these two origin hypotheses for Psyche by establishing whether, again, there are rocky regions that are spatially segregated from metal regions. The imager is designed to acquire much higher-resolution images than the gamma-ray spectrometer, but it doesn't directly measure composition. It's got certain bands, kind of colors, that it measures, which can allow us to potentially distinguish between various types of rocky materials versus metal. And so, again, we can look for whether there has been some large-scale segregation of metal and rock.

The other thing that the imager is going to do is look for craters and map them across the surface. That's a very powerful technique that we use throughout the solar system: we can determine the age of a surface by counting how many craters there are on it. The older the surface, the more craters, just because it's had longer for impactors to strike it. So we'll be able to determine the relative ages of different locations on the surface, and it's even possible that we'll be able to distinguish the population of bodies that was striking Psyche. If Psyche just formed like an asteroid, iron-rich, and never had its mantle, its outside, stripped off, then the distribution of sizes and numbers of craters may look like what we see on the Moon or Mercury. Whereas, if Psyche were a body that had its whole outer part stripped off in a catastrophic late impact, we might see fewer craters on the surface, because the early cratered surface would have been stripped off. And then, even if some of that material reaccreted later, after it was blasted off, it would have accreted at low velocities, because it was in the cosmic neighborhood of Psyche, and so, again, we'd see a more featureless surface.

Oliver Strimpel: The spacecraft will also be conducting gravity measurements. How does it do that, and what will that tell us?

Ben Weiss: Yeah, we will be measuring Psyche's gravity field using radio science. The cool thing about this technique is that you don't need to carry a special instrument to do it. All spacecraft can effectively measure a body's gravity field, almost passively, just because we have very precise methods that we use to track the spacecraft's position. That's, of course, how we know how to communicate with it and how we keep it in a stable orbit. And so, just by the very process of keeping track of the spacecraft, which we would do anyway even if it had no science instruments, you can effectively measure the velocity of the spacecraft as it's in orbit, by looking at the wavelength of the radio waves that it emits as part of its normal communication with the Earth. We can see the Doppler effect in the radio waves; we can detect the change in wavelength and use that as a way of inferring the velocity of the spacecraft. The velocity is important because it tells us how strong the local gravity field is, and so we can infer the mass distribution simply by seeing how the spacecraft subtly changes its velocity. Really cool.

This is really important, because when we measure Psyche's density, we want to turn that density into an estimate of its composition. Is it really true that Psyche is made out of mostly metal? Or is it a primordial pile of rubble, produced by asteroid impacts over the eons, with large amounts of porosity in its interior? If we don't know how much void space there is inside, then we won't be able to do that calculation, right? So if we measure some density, and then we find out from the gravity measurements that there's lots of void space in the interior, then that would lead us to conclude that the non-void-space material must be very, very dense material, like iron.

Oliver Strimpel: That reminds me of one of the earlier podcast episodes, with Dan McKenzie, who talked about using exactly the same Doppler effect on the orbit of the Magellan spacecraft around Venus, to try and infer what was going on and whether there was dynamic topography there. But I think that was on a much larger scale, with a much higher orbit than you're talking about here. To be able to figure out porosities, I imagine you'd have to be in a fairly low orbit to get that kind of resolution of the internal structure.

Ben Weiss: The magnetic field and the gravity field get sharper as you get closer. That's why we progressively lower the orbit through the mission. Our best, highest-spatial-resolution data will come when we are closest to the body, but that's also the most challenging place to maintain a stable orbit.

Oliver Strimpel: How low will the spacecraft actually be able to go over Psyche?

Ben Weiss: It's going to start out at 700 kilometers altitude and eventually get down to 85 kilometers, which is very close.

Oliver Strimpel: You mentioned earlier that we're finding thousands and thousands of exoplanets, some of which seem to be these giant, heavy Earths. Does that population cast any light on what we might think is the more likely scenario for the origin of Psyche? I mean, if we see these things, does that suggest that they all might have been created by your first scenario, in which the body differentiated and then the rocky outside was blown away, or could it equally well be explained by the alternative hypothesis?

Ben Weiss: Our understanding of dense exoplanets is changing rapidly, by the week, but we can already see that the largest of these bodies probably could not be produced by the mantle-stripping mechanism, just because the bigger you get, the easier it is to hold on to your rocks. It's almost impossible to strip off the exterior of a very massive planet, just because its gravity is so strong. That's where other exotic mechanisms have been proposed, like formation from an iron-rich substrate, or having a super-compressed rocky core. Those seem to be a little more attractive for the largest super-Earths. As for the smaller bodies, maybe Earth-sized, and we've now even detected bodies that are half Earth-sized that are inferred to have densities approaching that of pure iron, those bodies seem to be decent candidates for formation by mantle stripping, the way we think Psyche may have formed.

Oliver Strimpel: I probably shouldn't ask you to speculate, but I'd love you to. Do you have a hunch as to what we'll find when we get to Psyche, and which scenario we're going to be favoring at the end of the day?

Ben Weiss: Yeah, I'm probably going to regret this, but I would speculate that Psyche is a mantle-stripped body. We see tens of thousands of meteorites, and these, we think, are derived from something like 120 or 130 parent bodies, different asteroids. Iron meteorites represent much of the diversity of known asteroids, and yet we've never found a single iron asteroid. In fact, that's a conundrum by itself, a fundamental scientific mystery: why there seems to be this disconnect. So that's why I have this sneaking suspicion that maybe this one is an iron meteorite parent body. They've got to be somewhere. If it turns out not to be, that's just going to deepen the mystery as to where these things are coming from.

Oliver Strimpel: If the destination of NASA's next exploratory space probe were up to you, what would it be?

Ben Weiss: For me, the most compelling destination would be one of the ice giants, like Uranus or Neptune. These two bodies represent an end-member of planetary evolution that's essentially unexplored, with the sole exception of a several-day flyby of each by Voyager 2 in the late 1980s. And yet they are fascinating, mysterious worlds that offer us a huge range of unanswered questions. For example, Uranus has a spectacular moon and ring system, with 27 moons, five large moons that are icy and show evidence of recent resurfacing, maybe hiding oceans underneath their surfaces, and a magnetic field that's like spaghetti; it doesn't have a nice north and south pole like we have for the Earth. The planet's spin pole is tilted on its side. Its composition, like Neptune's, is fundamentally different from the terrestrial planets in the inner solar system and from the gas giants, Jupiter and Saturn. And these bodies are closer in mass to the most common type of planet we observe in the universe, which are these planets larger than Earth but a little bit smaller than Neptune. We call them sub-Neptunes or super-Earths, and they're just basically terra incognita. So I would vote for going to one of them.

Oliver Strimpel: I'm going to book a slot for you on Geology Bites in 2026, to see how this all plays out.

Ben Weiss: Awesome.

Oliver Strimpel: Ben Weiss, thank you very much.

Ben Weiss: Thanks so much. That was a lot of fun.

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