Moon Rocks Collected In 1972 Reveal New Secrets

FLORA LICHTMAN: This is Science Friday. I’m Flora Lichtman. A bit later in the hour, why AI chatbots like ChatGPT could be telling you debunked, racist medical misinformation and how that could affect the care patients receive. But first, it’s hard to imagine but the moon that we know and love hasn’t always been in the sky.

Like all of us, it has an age. And that age has, for a while, been estimated to be about 4 billion years old. But recent research on lunar crystals from the Apollo 17 mission has helped us pinpoint a more specific age for the moon and it turns out it’s about 40 million years older than we thought.

OK, I know. That might sound like a drop in the bucket when you’re 4 billion years old. But my next guest says it’s actually a really big deal because it tells us about what the solar system was like when it was just a baby. Joining me now to talk about this is the study’s lead author, Dr. Jennika Greer, Postdoctoral Researcher at the University of Glasgow in Scotland. Welcome to Science Friday.

JENNIKA GREER: Hi. Thanks for having me.

FLORA LICHTMAN: OK, let’s start with this. Why is it exciting that the moon is 40 million years older than we thought? Or is it exciting?

JENNIKA GREER: Yeah. I think it’s at least important, because 40 million years may be, like you said, a drop in the bucket compared to our solar system’s entire history, but you can put some pretty important bookmarks on either side that really put that 40 million years into context. So the earliest solar system solids formed around 4.56 billion years ago. And the oldest terrestrial solids that we know of formed around 4.4 billion years ago.

FLORA LICHTMAN: So we have space rocks forming like 4.56 billion years ago. And then when you say terrestrial, do you mean Earth rock?

JENNIKA GREER: Yes. So we have Earth zircons.

FLORA LICHTMAN: And that’s forming later?

JENNIKA GREER: Yes, at 4.4 billion years ago. So the time between those two, when you have the very first solids in the solar system, the stuff that’s forming right after the sun formed to when you have a planet with crustal processes, with surface processes, is about 160 million years. So 40 million years is pretty significant when you compare it to that. People have been using computational models to try to figure out how moon formation looked and some of those models put the time to form the moon after the giant impact at around 100 years.

FLORA LICHTMAN: Whoa, very, very specific?

JENNIKA GREER: Yeah. 40 million years is nothing to sniff at when it comes to early solar system processes.

FLORA LICHTMAN: It sounds like this is a very high drama part of the solar system’s history, too.

JENNIKA GREER: Absolutely. A lot of really important stuff happened really quickly. And then you have this kind of quiescence almost where you have the planets doing their own thing. But you can also look at an object, like the moon where you have a lot of important stuff happening. And right now, we look at it and pretty much the only geologic process that’s active on the moon right now is impact events.

FLORA LICHTMAN: Remind me how the moon was born.

JENNIKA GREER: So the leading theory for how the moon was formed, and this is a theory that basically has to explain a bunch of different characteristics of the Earth-Moon system, is that there was a large Mars-sized impactor that hit the proto-Earth and mixed a bunch of material. And some of the material that flew off later coalesced to form the moon.

FLORA LICHTMAN: What did the moon look like in the early days? Would it be recognizable to us now?

JENNIKA GREER: So when people look at the moon, the things that we readily identify are the dark patches. And those are lava flows that only developed later in the moon’s history. If you use a pair of binoculars to look at the moon, you might see craters, and those were definitely not there 4.4 billion years ago.

FLORA LICHTMAN: Let’s talk about how you figured this out. Where did the moon samples come from?

JENNIKA GREER: So these were samples that were picked up by Apollo 17 astronauts. The Apollo 17 mission was unique among the Apollo program in that one of the astronauts that landed on the moon was actually a trained geologist. These rocks were brought back by the astronauts, characterized by scientists, and eventually, as these materials were requested by the broader scientific community, they get analyzed for all sorts of different things by so many different people. And eventually, the sample gets allocated to my co-author, Baodong Zhang, and he uses a technique called NanoSIMS to get a uranium lead age out of this crystal.

FLORA LICHTMAN: NanoSIMS. That sounds technical.

JENNIKA GREER: Yeah. Yeah it stands for nanoscale secondary ionization mass spectrometry. The uranium-lead system in zircon is kind of the gold standard for geochronology because you have uranium that’s readily incorporated into these zircon crystals. It decays through radioactive decay to lead. And because the zircon hopefully didn’t incorporate any lead when it crystallized, all of the lead that’s present, should be the result of the uranium that was there.

So you can use the uranium-lead ratio to figure out how old it is because we know the half life of uranium. Because of the importance of this age and because it’s so ancient, we have to look at the structure at the nanoscale of this crystal to make sure that it hasn’t been altered since its formation. And that’s what we were able to do in this study. So we needed to double check that the age that my co-author measured was the correct age. Because a lot of stuff has happened to the moon’s surface since these rocks first formed.

FLORA LICHTMAN: OK, got it. So your co-author calculated an age, and you guys were like, these grains are super old. We got to double check.

JENNIKA GREER: Right.

FLORA LICHTMAN: And you used this other technique.

JENNIKA GREER: Yeah. And it’s an example of how as we’ve progressed in technology, we can start using these techniques to look at the nanoscale and try to better understand these samples. I mean, I would say that now, if you’re going to present the scientific community with an age this ancient, you will then be expected to provide evidence at the nanoscale of what’s been going on.

FLORA LICHTMAN: That’s really cool. And kind of mind blowing, actually.

JENNIKA GREER: I mean, this is a realm that we haven’t had access to. This is not something that would have been possible when these rocks were first returned to Earth.

FLORA LICHTMAN: Right. Just in the last 50 years, this technology evolved, right?

JENNIKA GREER: Right. So the atom probe is maybe around 50 years old. Rocks like this couldn’t have been analyzed until the laser was first introduced to the atom probe, and that was about 20 years ago. But there was a study done in 2014 that basically did this for a terrestrial zircon, the oldest terrestrial zircon. And since then, it’s been kind of not expected, but it’s considered due diligence at this point.

FLORA LICHTMAN: Do you have a dream material that you want to analyze?

JENNIKA GREER: Well actually, that’s kind of interesting. This technique, atom probe, is now being applied to biologic samples. So people are doing human tissue. People are now doing fluids by freezing them out. There’s a huge space in atom probe for trying to analyze these really unusual samples because people are interested in the distribution of atoms at the atomic scale.

FLORA LICHTMAN: Does it tell you something other than how old a material is, the distribution of atoms?

JENNIKA GREER: Well, sure. Another project that I’ve been working on is looking at space weathering products from the moon. So the moon is an airless body. It doesn’t have a protective atmosphere. And it’s being constantly bombarded by the solar wind and micrometeorites.

So the sun is throwing off waves of particles all the time and it’s impacting the surfaces of these grains and it’s altering them. And something really interesting I’ve been working on is you have these rocks, this lunar soil rich in oxygen, because oxygen is a mineral-building element, and you’ve got hydrogen, the most abundant element in the sun constantly being blown off.

So when you’ve got hydrogen and you’ve got oxygen, you can form water. And even though that hydrogen only really impacts the top about 100 nanometers of the soil, it could be that the space-weathered grains could be a potential resource for water for future astronauts.

FLORA LICHTMAN: Let me ask you this. What is it like to actually hold these sort of ancient– I know it’s just like dust, but what’s it like to hold these sort of ancient grains in your hand? What’s it like to hold the moon in your hands?

JENNIKA GREER: I’m not the best person to ask this because during my PhD, I was a resident graduate student at the Field Museum, and the Field Museum has the world’s largest private meteorite collection. So I was always handling space rocks. So it’s almost routine.

I would say that the more daunting aspect of that is knowing that you’re holding something that decades of scientific research has gone into. And so many people have analyzed this and gotten the body of knowledge that you have access to, and now you’re given the sample. You do not want to mess that up. You do not want to drop it.

FLORA LICHTMAN: Any close calls.

JENNIKA GREER: No. And you know that is for the benefit of NASA. Please keep allocating me samples. Yeah.

FLORA LICHTMAN: That’s about all the time we have for now. I’d like to Thank my guest, Dr. Jennika Greer, a Postdoctoral Researcher at the University of Glasgow in Scotland.

JENNIKA GREER: Thanks for having me.

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