This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. For quite some time, we've known that the Moon had a strong magnetic field between about 4.25 and 3.5 billion years ago. In the past few years, we've discovered that a weaker field was operating as recently as about one and a half billion years ago. After that, the Moon's magnetic field vanished entirely. Several theories have been put forward to explain the early magnetic field, but they have trouble explaining why the field was so strong. It has also been challenging to come up with a mechanism for the subsequent, longer-lived, weaker magnetic field. Certainly, no mechanism can explain both the high-field early epoch and the long, weak-field tail. Sonia Tikoo studies the history of magnetic fields on the Moon and other small solar system bodies, using paleomagnetism and fundamental rock magnetism. She's an assistant professor in the Department of Geophysics at Stanford University. Sonia Tikoo, welcome to Geology Bites.

Sonia Tikoo: Thank you for having me.

Oliver Strimpel: Can you fill in the picture of what we know about the history of the Moon's magnetic field? When was the field strongest, and how strong was it compared to Earth's today?

Sonia Tikoo: Yes. We believe that the Moon's magnetic field turned on sometime, perhaps, within the first 100 million years after the Moon's formation, when the crust was initially forming and solidifying. And we have evidence that the field was strongest during the period between about 3.9 billion years ago and 3.5 billion years ago. That's a period when the strength of the field was actually comparable to the strength of the Earth's magnetic field today, so somewhere between, let's say, 30 and 70 microtesla for comparison, depending on whether you're at the equator or the poles, where the highest values exist. After three and a half billion years ago, we believe that the strength of the Moon's magnetic field began to decline over time. The pace of that decline, whether it was sharp or gradual, is a bit debated, but we know that it continued to decrease, and we know that the Moon's magnetic field turned off sometime before 1 billion years ago. And so since 1 billion years ago, we have not had an actively generated field on the Moon.

Oliver Strimpel: How do we know this history?

Sonia Tikoo: We know the Moon's magnetic history from a combination of different approaches. First of all, to study the earliest magnetic field records on the Moon, we have spacecraft that have been orbiting the Moon in the past, collecting magnetic field measurements using magnetometer and electron reflectometer instruments. Based on that data set, we can see that the crust of the Moon is magnetized, and the ages of the magnetized crust tend to be around 3.8 billion years old. That's the age range that we can probe using crustal magnetism. Now, the issue with a lot of lunar rocks that are younger is that they have lower iron contents than, let's say, some of the regions on the Moon that have the strongest magnetic anomalies, which are usually related to impactor-added metal that comes from exogenic sources and is incorporated in ejecta or in impact melt sheets. So when we're looking at younger igneous rocks, they may not have a detectable magnetic signature from orbit.

And so to understand this later period in lunar history, when there were fewer impact effects, we have to study rock samples that are returned to the Earth by spacecraft missions. In the United States and other countries, we've been working with the Apollo samples returned by the astronauts, and more recently, we have additional samples that were returned by the Chang'e missions by China. We can take those rock samples and bring them into what are called paleomagnetism labs, and we can measure the magnetization that is recorded within the rocks. What we try to do is understand how strong the magnetization in the rocks is, and what that means about how strong the ancient field was at the time that those rocks formed. And while we do this, we also have to do tests to determine whether the magnetization in the rocks is truly from an ancient global field on the Moon, or whether it can come from some other, secondary contamination. So we do a lot of tests to rule out secondary effects and try to establish that the magnetization we observe, if we observe it, is primary in origin.

Oliver Strimpel: That reminds me very much of the episode I did recently with Alec Brenner about paleomagnetism in the Archean here on Earth, and he talked quite a bit about those tests, perhaps different tests, but nonetheless tests to establish the origin of the magnetic field, especially as we're dealing with such huge periods of time.

Sonia Tikoo: Yeah. The older the rocks that you're studying, the more time those rocks have had to experience things later in their history that could reset their records. So it's always very crucial to make sure that you are measuring the original magnetic signal.

Oliver Strimpel: And in the case of the Moon, the resetting is most likely to be due to an impact. Is that right?

Sonia Tikoo: Yeah. The strongest hypothesis that has been proposed as an alternative to an ancient lunar dynamo origin for the magnetization of lunar rocks is that in some cases their magnetizations may have been acquired by shock waves propagating through the rocks in the presence of a transient magnetic field that is generated by the motion of plasmas that form during impact events. So basically, you have an impact, you're vaporizing the lunar crust as well as the impactor, and you're causing this conductive plasma to expand and move. That process could generate a temporary magnetic field that, depending on the size of the impact, could last from less than a second all the way up to maybe hours, for a large basin-forming impact.

Oliver Strimpel: OK, let's talk about how we think these fields were generated. Let's start with the strong early field. Are we talking about some kind of fluid in the outer core acting like a dynamo, as we think is operating today in the Earth's core?

Sonia Tikoo: Yeah. So magnetic field generation within rocky planets is thought to largely occur via what's called the dynamo process, where the organized motion of conductive liquid metal in the cores of planetary bodies can generate a magnetic field. It's essentially like a current loop, if you want to think about it in a very simplified form, where you have moving charges going around, and you can thread that current loop with a magnetic field that is generated. But imagine that you scale that up to the size of a planetary core, with lots of individual fluid flows and lots of current loops, which are in turn getting organized by the overall rotation of the planetary body. If you back away from that, you're going to have a larger magnetic field structure emerge that encompasses the entire planet. We think that for most terrestrial bodies that have had magnetic fields generated in their history, this is the most likely mechanism, because we know that cores have iron, iron–nickel, which is metallic and therefore highly electrically conductive. It's sort of the Occam's razor explanation for generating a global magnetic field on a planetary body.

There have been some discussions about a different layer in the Moon perhaps generating a magnetic field. The reason that we have this discussion is that the Moon's core is very small. The Moon itself is a very small body. For reference, the Earth's radius is about 6,378 kilometers; the Moon's radius is 1,730-something kilometers, so it's much smaller. The Earth's core is about half the radius of our overall planet; the Moon's core is about a quarter of the radius of the Moon, at most. And scaling laws have been developed for the intensity of magnetic fields that you'd predict for bodies of different sizes and in different regimes, based on observing the Earth's dynamo and stellar dynamos. These laws predict that the larger the core radius is compared to the size of the overall body, and also the more efficiently that core can lose heat out of its interior, the stronger the magnetic field you're going to get. The problem is that the Earth is a large body, but it also has plate tectonics and a very dynamically convecting mantle that facilitates heat loss, and its core is pretty big. In contrast, the Moon has kind of a solid, one-plate crust; it's sort of in a stagnant-lid regime, and the core is very small, and that fraction of core radius to overall body radius is also very small. So the scaling laws for thermal core dynamos predict that the Moon's magnetic field intensity should not be more than just a few single-digit microtesla, if not just one microtesla or lower, for that body. And so the fact that we see Earth-strength paleointensities in the lunar magnetic record is very confounding from that perspective.

And so there have actually been some workers who have proposed that maybe it's not the core at all on the Moon that is the conductive, convecting layer. Some have proposed that the Moon may have a basal magma ocean, a molten layer that overlies the core, a silicate layer that may contain titanium and some iron too, in quantities that are sufficient to create conductivity in that layer. If you have heat loss and convection in a basal magma ocean, then perhaps that could generate a dynamo as well, and that would be closer to the Moon's surface, and therefore would solve this kind of one-over-r-cubed problem that we see with the scaling of how magnetic fields drop off with distance. So that's another mechanism that's been proposed for the Moon. It's also been proposed for the Earth in various studies as well, not as the sole source of our magnetic field, but perhaps as a contributor. But I think that the Moon's magnetic history is inspiring these other discussions of alternative dynamo mechanisms.

Oliver Strimpel: So you said that the scaling laws suggest that the Moon's field shouldn't be anything like as strong as we've seen that it was in the past. What is the underlying physical mechanism for that? Is that all to do with the cooling rate and the amount of conductor that can move and the speed at which it can move?

Sonia Tikoo: Yeah, all of that comes into play. The main idea is that you need to have heat transfer out of the core, and to have a stronger magnetic field, you have to have more efficient heat transfer out of the core, such that you are crossing what is called the adiabatic threshold. If you have mantle convection atop the core that's very efficient, you can more easily transfer heat out of the core. And so if that convection is associated with a lower mantle viscosity, you can have higher magnetic field intensities. Likewise, if the viscosity of the core is also very low, that facilitates convection, and you could have a higher magnetic field intensity. So it comes down to several factors that really get into the material properties of a planet's interior, both in terms of its mantle properties and its core properties. And these are things that we honestly don't know very well; we can just take an educated guess at them from geophysical measurements right now.

Oliver Strimpel: I understand that you've proposed that the early magnetic field may have been episodic rather than continuous, with peaks of high intensity rather than a steady strong field. What made you propose this, and would such behavior be consistent with the experimental measurements that we've made on the lunar samples?

Sonia Tikoo: Yes. We proposed this idea because if we assume that the Moon is constantly cooling at a steady state, which is what the scaling laws that we discussed earlier assume, basically the Moon runs out of energy, and it doesn't have the ability to generate sustained high magnetic fields. There's only so much energy trapped in the lunar core from the time of its formation. And so one way to get around that problem is if the magnetic field was operating at a weaker baseline, and there were transient events that could temporarily increase the convection of the core and the heat flux out of the core, with an associated spike in magnetic field intensity. If we can achieve that, we can explain some of the higher fields that we infer from the lunar sample record.

Now, whether that's consistent with the Apollo lunar sample record: I would say yes. I came up with this idea of an intermittently high field, along with collaborators and co-authors, because as a paleomagnetist, I was aware of a sampling bias issue, actually. When we were initially studying the lunar paleomagnetic record in the modern, post-Apollo-era generation of studies, we were really concerned with trying to determine whether the Moon had a magnetic field or not. It was really a very binary problem, yes or no, at first. So what we did was go through the original Apollo-era paleomagnetic studies, and we preferentially selected samples that during the Apollo era had been found to have strong magnetizations, because we were trying to see if there was a field there. A lot of those studies focused specifically on Apollo 11 and Apollo 17 mare basalt samples, and therefore we had these initial, consistently positive results of strong, Earth-strength magnetic fields.

And it was because I knew that this was the way the problem was being approached that I realized that there were a lot of studies, on a lot of samples, that had not been done yet. So what I wanted to do was go back to samples from different Apollo missions, or even Apollo 11 and 17 samples that had not been studied before, or samples that had slightly different ages, and revisit the problem to see if this was really just a sampling bias, or whether the field was actually continuously strong. In my research group, I have a great former student, now a postdoc, Ji-In Jung, who has done a lot of work on this problem. She measured Apollo samples that had not been heavily scrutinized during the initial wave of paleomagnetic studies, and we actually found evidence for a weak field, an order of magnitude weaker, you know, a single-digit-microtesla magnetic field, in some samples in her work. And this actually matches some other studies that other people in the field were doing, where they found samples that did not appear to be magnetized, even from the so-called high-field era.

The question is, do those other samples not appear magnetized because there was no dynamo at the time that those samples formed? Or are they not looking magnetized because they are poor magnetic recorders, so that the magnetic minerals in these rocks are just not good at retaining magnetic fields? There's also a third possibility: that impact shock events that occurred hundreds of millions of years or billions of years after the samples initially formed could have demagnetized those rock samples. In our group's work, we actually showed that one sample that we looked at that did not appear to be magnetized did actually have petrographic evidence of shock. There were deformation features in the minerals of that sample that were associated with shock, and there was no magnetic record in that sample. So that might support that hypothesis, at least in a limited subset of cases.

We've also done additional work that looks at the magnetic recording properties of lunar rocks, and we found that most of them are actually pretty poor magnetic recorders, specifically slowly cooled samples, which have a lot of time for the iron grains in the sample to grow large into what we call multidomain grains in magnetism. Those are less efficient magnetic recorders, and they're also easily remagnetized or demagnetized by later events. So we have possible explanations for the absence of magnetization in the samples that we see. But even for those samples that aren't the best magnetic recorders, we still think there's a minimum field such that, if the Moon's dynamo was at that level or higher, we would have retrieved some magnetization from the sample. We can figure out what that number is for each rock, and based on that, we can assign what we call upper limits to the paleofield intensity for those particular samples at those particular moments in time. So when you take all of these data in aggregate, samples that have high paleointensity, samples that have low paleointensity, samples that are not magnetized but are poor recorders, we can infer that the Moon's magnetic field was probably fluctuating by an order of magnitude, even during this so-called high-field period between 3.9 and 3.5 billion years ago.

Oliver Strimpel: Do you have a mechanism that would cause this fluctuation? I understand the energy budget situation is partially or completely resolved that way, but how would you get these flare-ups and then these quiescent periods?

Sonia Tikoo: Yes. Several different mechanisms have been proposed to cause intermittently intense magnetic fields on the Moon. One of them was actually proposed back in, like, 2011 or so, when a group of authors talked about how large basin-forming impacts hitting the Moon could actually kind of shake the lunar mantle and cause these librations, where the Moon is sort of recovering after each basin-forming impact and wobbling back and forth. That wobbling can actually mechanically stir the lunar core and create temporarily intense magnetic fields.

More recently, there have been mechanisms to create intense temporary magnetic fields that are actually an outcome of the Moon's original formation. There's this idea that the ancient Moon formed by a large impact of a Mars-sized, or maybe even a little larger, planetary body hitting the proto-Earth, and the debris from that impact basically came together and recoalesced to form the Moon. As a result of that process, the initial Moon would have been extremely hot and molten, and the Moon would have cooled down over time. The lunar mantle would have been made out of what we call a magma ocean. And as this magma ocean cooled, what would have happened, based on what we know about the pressure and temperature conditions in the lunar interior, is that somewhere in the middle of the magma ocean, you would start to solidify what we think of as canonical mantle minerals, like olivine and pyroxene, and these are dense, and they sink to the bottom of the Moon's mantle; they go to the core–mantle boundary. But at some point, you start to crystallize the feldspathic mineral anorthite, and that is buoyant with respect to the melt, and it goes up to the top and forms this beautiful white crust that we see when we look at the Moon.

And in between is this layer of residual melt that gets increasingly enriched in all the junk that doesn't want to go into the olivine layer that's sinking, or the anorthite layer that's floating up to the top. This junk fluid layer has a lot of titanium in it. So when it finally crystallizes and forms this layer between the lunar crust and the mantle, just by a quirk, due to this high titanium and also some iron content, it's denser than the stuff that's underneath it. And because of that gravitational instability, that layer overturns and goes down to the lunar core–mantle boundary. There's been some thought that this layer, when it's down at the core–mantle boundary, is blanketing the core, and at some point, due to radiogenic heating, or maybe heat from the core, that layer melts. And when that layer melts, it can create mantle plumes, basically, that rise up away from the core–mantle boundary. The buoyancy and heat transfer associated with these plumes can actually cause temporarily increased convection in the core and a temporarily enhanced dynamo. That mechanism has been advanced in different flavors by several different groups, some work that I was involved with and some that I was not. But in general, there seems to be a convergence on this idea as a possible way to generate intermittently intense fields on the Moon.

Oliver Strimpel: OK, let's move on to the period when the field began to decline in strength. Why can't we apply the same theories to explain this weaker field as well as the strong one?

Sonia Tikoo: Yes. The issue with explaining the late history of the Moon's magnetic field with these other mechanisms is that we think the drivers for these other mechanisms don't exist anymore past a certain point in lunar history. If we want to talk about the mechanically induced dynamo driven by large basin-forming impacts on the Moon, those stopped happening about 3.8 billion years ago or so, maybe a little bit later, depending on uncertainties in basin ages. So we don't have that energy source anymore. Likewise, consider the mechanism of this titanium-rich layer around the core melting and sending plumes up through the mantle. Well, those plumes, because they were titanium-rich, led to the eruption of the high-titanium basalts, or ilmenite basalts, that we have collected from the Apollo missions and that exist in various places on the lunar surface. The thing is that that high-titanium basalt volcanism also petered out. From the Apollo sample suite, the youngest ilmenite basalts that we have are around 3.5 billion years old. It's possible there might be some that are a bit younger, but the point is that later in lunar history, when we're talking about 2 billion years ago or 1 billion years ago, we don't have evidence of that composition occurring in our sample suites. And so that mechanism is not feasible at that later stage in lunar history.

So we have to rely on something else to explain the late history of the Moon. The easiest mechanism to invoke is actually the mechanism that we think is generating the Earth's magnetic field today, which is core crystallization coupled with heat loss from convection. We know, for example, that the Earth has a liquid outer core, and due to the cooling of the Earth, there is a solid inner core that is solidifying. So you have liquid iron in the outer core, and as it cools down, the iron preferentially freezes out and sinks to form the growing inner core. That process releases latent heat. And the residual liquid in the core becomes increasingly rich in light elements that do not freeze out. That could be sulfur, carbon, you know, other things. And those light elements also provide the outer core with chemical buoyancy that can help stir the core as well. We think it's possible that the lunar core also started to crystallize at some point in its history, and that process could actually sustain a magnetic field until about 1.5 or so billion years ago, which, coincidentally, is about the age of the youngest magnetized lunar sample.

Oliver Strimpel: So those crystallization mechanisms are a way of generating motion in the outer core, albeit weaker than the kind of motion we were looking at before.

Sonia Tikoo: That's right. It doesn't have the same intensity as some of the other mechanisms that have been proposed for earlier in lunar history. In the studies that explore this, again, similar to some of the scaling laws that we've talked about earlier in this conversation, we only expect to see maybe single-digit-microtesla magnetic fields from core crystallization, if not weaker magnetic fields. And the sample that's 1.5 billion years old and is magnetized has a paleointensity of somewhere between 2 and 5 microtesla. So even that is higher than what is predicted by the canonical scaling laws. But it's also possible that, for whatever reason, those scaling laws don't perfectly apply to the lunar setting, and that requires a lot more work in the dynamo modeling space to try to figure out where this slight discrepancy is coming from. So it's not hard to produce a weak, long-lived field on the Moon, but, again, bridging that paleointensity gap is an active field of research.

Oliver Strimpel: So those mechanisms that we described all eventually petered out, about 1.5 billion or certainly by 1 billion years ago. But the Moon's magnetic record didn't completely disappear with that. Some regions of the lunar crust still carry strong remanent magnetization today, and at the surface above them, we see striking patterns of light and dark. These are the so-called lunar swirls. What do we think they are, and what's the connection to the underlying magnetism?

Sonia Tikoo: Lunar swirls are very enigmatic albedo anomalies that we observe at the lunar surface. It almost looks like a child spilled a glass of milk, or you spray-painted the lunar surface with white paint in a very random way, and you get these squiggles there. What's really interesting about them is that there doesn't seem to be an obvious compositional difference between the light areas and the dark areas in terms of the rock types that are covered by these lunar swirls. One thing that is interesting about the lunar swirls is that they are associated with some of the most strongly magnetized regions in the lunar crust. Why that's the case, and what the relationship between the albedo anomaly and the magnetization is, is a bit of a mystery, but some hypotheses have been proposed to explain this relationship.

Probably the leading hypothesis is that the magnetic field emanated by whatever magnetized source body is in the crust is so strong that it may be deflecting the solar wind. The Sun is constantly sending charged particles outwards, and as those particles interact with lunar crustal magnetic fields, they're funneled by the magnetic field lines, such that in areas where the magnetic field is more vertical, those charged particles can follow the field lines and interact with the lunar surface. The reaction of those particles with the lunar surface is called space weathering, and that can cause the formation of nanophase iron at the very surface, and that can darken the lunar surface in appearance. Whereas in areas where the magnetic field might be more horizontal and is shielding the surface from space weathering effects, you retain more of that lighter-colored, pristine surface on the Moon. So that's probably the most discussed hypothesis. There are others, including the idea that maybe the magnetic fields contribute to electromagnetic sorting of dust particles that are lofted above the lunar surface, and that can again create these lanes of dark and light channels. And there's even a hypothesis that maybe grazing cometary impacts could scour regions of the lunar surface, and that the compression of the coma of the comet as it engages with the surface could cause a locally compressed, high magnetic field that magnetizes the crust. So in the coming years, hopefully, we can visit some of these lunar swirls and measure their magnetic anomalies, get a better sense of the composition at the surface, measure the incoming solar wind environment and charged particle flux at the surface, and try to disentangle the various proposed hypotheses.

Oliver Strimpel: Sonia Tikoo, thank you very much.

Sonia Tikoo: Thank you so much for having me. It was great chatting with you today.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you'll also find a subject matter index of all the episodes. There you can also give me feedback, which I welcome, as well as sign up to get my emails about new episodes.