How The Moon Lost Its Magnetism

IRA FLATOW: This is Science Friday. I’m Ira Flatow. You are almost certainly aware of the Earth’s magnetic field. It’s what makes the compass point north. It helps animals navigate home, and protects the upper atmosphere from harmful radiation.

But the moon used to have an atmosphere, too. And scientists still aren’t sure what killed it, or even what started it in the first place. But new research published in Science Advances this week gives us at least one new piece of information. And that is the moon’s magnetic field lasted at least two billion years, a long time for such a small piece of rock.

Here to explain is my guest, Sonia Tikoo, an assistant professor of planetary science at Rutgers University, and co-author of the new research. Welcome to Science Friday.

SONIA TIKOO: Hi. Thanks.

IRA FLATOW: Do we know how the moon got in a magnetic field to begin with?

SONIA TIKOO: Well, within terrestrial or rocky planets, magnetic fields are generated by the churning of liquid metal within the cores of these planets. So basically, there has to be some mechanism that’s causing motion and churning of that liquid metal.

IRA FLATOW: I said atmosphere before. Of course, I meant magnetic field.

SONIA TIKOO: Yeah, I wasn’t sure. I was–

IRA FLATOW: Yeah.

SONIA TIKOO: Yeah.

IRA FLATOW: Sorry about that.

SONIA TIKOO: No worries.

IRA FLATOW: So do we have any idea why it lost its magnetosphere?

SONIA TIKOO: Well, generally the convection, or the motion of liquid metal within the cores, is tied to the cooling of the planet. So you have hot metal rising within the core and cold metal falling as it cools off closer to the mantle. And so the idea is that, on a smaller planetary body like the moon, the moon might cool off more quickly than, say, a larger body like the earth, and that it might run out of energy earlier. And so perhaps the moon’s magnetic field turning off has to do with it running out of the power source.

IRA FLATOW: So as it cooled down, it lost the ability to make the magnetism.

SONIA TIKOO: Probably.

IRA FLATOW: And we know all of this from Apollo samples taken from visits to the moon?

SONIA TIKOO: We know much of this from Apollo samples. So during the Apollo era, the astronauts collected hundreds of lunar samples from the lunar surface. And scientists, kind of like the earlier generation of what I do, they started to take rocks from the moon and measure them in magnetometers. And they found that many moon rocks were magnetized, but the origin of that magnetization was unclear at first.

But more recently, we’ve been able to do more sophisticated experiments and tests to establish that that magnetism, in many cases, came from an internally generated magnetic field. And it’s not just the samples, but there’s also spacecraft that orbited around the moon that have measured ancient magnetic fields preserved in the crust.

IRA FLATOW: That’s interesting. I’m curious about how do you find an ancient magnetic field in a rock sample.

SONIA TIKOO: Well, so whenever rocks form, they create minerals within the rocks that are like little compass needles. And these minerals align with whatever ambient magnetic field is around at the time that the rock is forming. And as the rock cools down, it freezes in that magnetic direction. And there’s also some intensity information that is preserved, too.

So what we can do is we can take the rock samples and measure them in a magnetometer, and get the direction and intensity of the magnetic field in the rock. And then we can convert that to information about how strong the ancient magnetic field was that magnetized the rock initially.

IRA FLATOW: Yeah. Your work was using just one sample from Apollo 15? What made this rock so helpful?

SONIA TIKOO: Yeah, so in this particular paper, we only studied one sample. What was special about this rock was, in previous studies, we learned that many lunar rocks were non-ideal magnetic recorders. They have iron-nickel alloys, which are the magnetic carrying grains.

And sometimes, if those grains are too big– and that can happen in more crystalline rocks, like basalts from lava flows– but in rocks that cooled more quickly, like this impact– this rock formed by a meteorite impact that melted a bunch of rocks on the lunar surface and created a glass. And this glass had these beautiful, small magnetic grains, which were excellent magnetic recorders. And so this was, like, the best sample we could have targeted, basically, because it was the best recorder that we’d identified at that time so far.

IRA FLATOW: If you were an astronaut on the moon, would you know this was an important rock to pick up and take home by looking at it?

SONIA TIKOO: Now we would, but–

[LAUGHTER]

[INTERPOSING VOICES]

IRA FLATOW: Do you know which astronaut picked it up? Is there a record of–

SONIA TIKOO: I don’t know exactly which one. I know the Apollo 15 commander was Dave Scott.

IRA FLATOW: Right.

SONIA TIKOO: He’s a cool guy. I actually got to meet him once. But I don’t know who picked up the specific one.

IRA FLATOW: So what makes this question about the origin of the moon and its magnetic field such an interesting question?

SONIA TIKOO: Yeah. So I mentioned that, on larger planetary bodies, like the Earth and Mercury and, say, one of Jupiter’s moons, Ganymede, you can have long-lived magnetic fields that still exist today. And we know that some planetesimals, like asteroids in the past, at the beginning of the solar system, also had ancient fields that probably died off very quickly.

But the moon fell in this really interesting intermediate size range between a body that we know has very long-lived magnetic fields, and very tiny bodies where the fields would have dissipated quickly. So the moon was just, like, a really cool natural laboratory to study intermediate-sized planetary bodies. And that, in turn, tells us about what mechanisms can generate magnetic fields, or what the power sources are within these intermediate-sized bodies, because there might have to be something else just supplementing just the pure cooling of the body.

IRA FLATOW: Mhm. Now we know that Mars does not have an atmosphere because it lost its magnetic field, too, and would helping– you know, would that be helped by understanding what happened to the moon, to understand what happened on Mars?

SONIA TIKOO: Yeah. So to clarify a little bit, Mars still has a very thin atmosphere that is primarily carbon dioxide. But we know that, in the past, Mars had a thicker atmosphere, and that there was also a lot of water on Mars. So there were lakes and seas, potentially. And we think possibly because Mars lost its magnetic field, it lost its shield, and the solar wind was able to strip away much of the water from the atmosphere of Mars and leave it barren.

We’re still not sure exactly why Mars’s magnetic field turned off. There have been a variety of hypotheses about that. But one thing that is out there is just potentially the cooling hit a certain point where the generation of the magnetic field was not feasible anymore. And that is one lesson that maybe the loon can– sorry, the moon can also inform us about.

IRA FLATOW: Now, we know that the solar system is made of a lot of rocky inner planets and a lot of gassy outer planets. Are we the only rocky inner planet that has a hot magnetic field?

SONIA TIKOO: Actually, Mercury has a magnetic field that’s generated by fluid motions in its core. But a mystery about Mercury is actually that its magnetic field is surprisingly weak given the size of its core and the size of the planet. And so that’s another problem people are working on.

IRA FLATOW: So there’s a relationship, possibly, about the size of a body and its magnetic field?

SONIA TIKOO: It has to do a little bit with the size of the core of the body. There’s a formula, it’s like a scaling law.

IRA FLATOW: I’m sure.

SONIA TIKOO: But generally, yeah, there is supposed to be some kind of relationship.

IRA FLATOW: So what do you want to know next? What’s next to find out about the changing magnetic fields, either on the moon or any other place? What would you really like to know?

SONIA TIKOO: What I’d like to know is– well, right now, before this study, we’d shown that the moon had a magnetic field till 3.56 billion years ago. This study, we extend that lifetime to at least 2.5. But we just studied one rock. And there’s a billion-year age gap between 3.5 and 2.5. And then we have, after this rock, we don’t know when the moon’s magnetic field turned off.

So there’s a lot more work to do in terms of figuring out exactly when the magnetic field ceased. There’s a lot of work to do in figuring out how the field declined, because early on in lunar history we think the field was as strong as the Earth’s field is today, about 50 microtesla And the field we retrieved in this rock is about a factor 10, 5 microtesla lower than that. So we really need to learn more about the decline, what could have caused the decline, and whether that means something about different power sources operating in the moon at different times in its history.

IRA FLATOW: Could it tell us anything about the Earth? And are we headed toward a decline ourselves?

SONIA TIKOO: Well, the Earth’s magnetic field is generally generated by convection in our core, but also it’s supplemented by energy released during the crystallization or the solidification of the solid inner core. And so we think that that process might have also been occurring within the moon. The Earth’s magnetic field will probably, you know, turn off at some point, but it’s not scheduled to turn off for billions of years from now. And it’s sort of in competition with the end of the solar system in terms of time scales, when the sun expands and envelops us anyway.

IRA FLATOW: You’ll get back to us on that one.

SONIA TIKOO: Yeah, we’ll have to get back to you.

IRA FLATOW: This is fascinating. I mean, should we go back and collect more rocks from the moon? And would you learn anything if we got more rocks?

SONIA TIKOO: Oh, I would absolutely love it if we collected more rocks from the moon. All the Apollo missions collected rocks from a relatively limited set of regions on the near side of the moon that faces us close to the equator. And there’s a lot of rocks that are missing. There’s a lot of rock ages that are missing from the Apollo sample suite. And in order to really learn about the moon’s history, we have to fill in a lot of these gaps, both in terms of rock types as well as rock ages.

IRA FLATOW: Could we send a rover? Instead of collecting it there, bringing it back, [INAUDIBLE] like we do on Mars, we send a rover to figure that out.

SONIA TIKOO: We could absolutely send a rover to– we could send multiple rovers to the moon, or little hoppers that could collect samples, or even do in-situ analyses on the moon itself. That would be great.

IRA FLATOW: OK. I’m going to put you down for saying we should send rovers to the moon. How about that?

SONIA TIKOO: You can quote me on that.

IRA FLATOW: OK.

SONIA TIKOO: I would love it.

IRA FLATOW: OK. Sonia Tikoo is Assistant Professor of Planetary Science at Rutgers University. I hope you get your wish.

SONIA TIKOO: Me, too.

IRA FLATOW: Have a good weekend.

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