An Earthly Origin for Moon Oxygen?
Reporting in Nature Astronomy, Japanese researchers found a regularly occurring pattern in the types of oxygen sampled by the Kaguya orbiter: For five days every month, the moon is bombarded by oxygen ions that are slower and that have less charge than the ones striking it during the rest of the month.
Unlike the faster particles—which spin off from the sun—these slower ions could have come from Earth’s upper atmosphere, say the researchers, noting that the five-day period they correspond to is the time when Earth shields the moon from the solar wind. If that’s true, then the moon’s soil could contain up to 2.5 billion years’ worth of Earth-originating oxygen.
Kevin McKeegan, a cosmologist at the University of California, Los Angeles who’s unaffiliated with the new research, discusses its implications. McKeegan is also the co-author of a new paper that chemically dates the moon, and he explains why it looks like our satellite is nearly as old as the rest of the solar system.
Kevin McKeegan is a professor of geochemistry and cosmochemistry at the University of California-Los Angeles in Los Angeles, California.
IRA FLATOW: And now we’re going to move on to something totally different. We all know the Moon to be an airless, inhospitable place. But in this case, airless does not mean without oxygen. The samples from the Apollo missions have confirmed the presence of oxygen in the Moon’s soil. And while the easiest explanation may identify the source as the flow of particles from the solar wind, Japanese researchers think there’s evidence that the Earth is also donating some of its own oxygen.
Don’t worry. It’s just a small little bit of oxygen. And writing in Nature Astronomy this week, the researchers describe a change in the kind of oxygen that seems to be arriving on the Moon, right, when the Earth is in the prime position to be the culprit. That’s where most of the oxygen when it’s coming.
Kevin McKeegan is professor of geochemistry and cosmochemistry at University of California in Los Angeles. He was not part of this study we’re discussing, but he’s able to talk about it. Welcome to Science Friday.
KEVIN MCKEEGAN: Thank you.
IRA FLATOW: Now–
KEVIN MCKEEGAN: It’s a pleasure to be here.
IRA FLATOW: You’re welcome. We think of the Moon usually as dead and airless. Where is all this oxygen lurking?
KEVIN MCKEEGAN: Well, that’s a very interesting question. The fact that the moon is airless is a really key part of this, because the grains that are on the surface of the Moon are exposed directly to space. So anything that is impinging on the Moon can get trapped in the surfaces of these grains. Whereas, of course, on the Earth, we have this protective layer of our atmosphere that keeps us safe from the bombardments of space.
IRA FLATOW: So it’s coming from the solar wind, or it just happens– where is it– what makes you think, and what does the new research show, that it’s getting sucked off of Earth?
KEVIN MCKEEGAN: Well, the new research is fascinating because it’s based on a discovery from a Japanese spacecraft, which could directly correlate a flow of oxygen ions toward the Moon when the position of the spacecraft and the Moon were such that they were traveling through the magnetic tail of the Earth, so the tail of the Earth’s magnetic field, and therefore concentrating this flow of oxygen onto the surface of the Moon.
IRA FLATOW: Did this oxygen have a special signature to it that you know it’s not coming from the solar wind, for example?
KEVIN MCKEEGAN: Yes, in the case of these measurements, they’re able to determine the energy distribution of the oxygen ions and also their charge states. So it turns out that the Sun oxygen is the third most abundant element in the Sun. And there is a wind that comes off the Sun containing oxygen. But that oxygen is ionized several times over, so it’s at a very high charge state, plus 5 or plus 6 or so.
Whereas this oxygen that was detected only at certain times of the month, when the Moon was passing through the Earth’s magnetic field, this oxygen is only ionized once. And it’s at a lower energy. And therefore they could correlate this with what would be expected to be coming off the upper atmosphere of the Earth.
IRA FLATOW: And so how much oxygen are we talking about that the Earth is losing to the moon?
KEVIN MCKEEGAN: Well, it’s a very small amount, actually. When you put it in terms of number of atoms, it sounds like a great deal. They detect something like 30,000 ions per square centimeter per second, so it sounds like a lot. But in terms of the accumulated mass, it’s not very much. And in terms of the amount of oxygen that the Earth would have lost, because presumably this kind of thing has been going on for billions of years, it’s still not very much. We’re not going to run out of oxygen.
IRA FLATOW: Good to know. This is PRI, Public Radio International. So what happens to the oxygen when it gets to the Moon? Does it get sucked up someplace?
KEVIN MCKEEGAN: Well, these are ions, and they have enough energy so if they encounter a solid material like a mineral grain sitting on the surface of the moon, they will hit that and get implanted slightly below the surface in the crystalline lattice of the mineral grain. And here is where the group wants to make an intriguing but still somewhat speculative connection to previous measurements that were made on lunar samples, lunar soils brought back by the Apollo astronauts.
So because the Moon is generally so poor in oxygen, there is some metal grains, iron metal grains that can exist on the surface of the Moon, whereas of course if you have a small grain of metal on the Earth, it will be rusted away very, very quickly. But that doesn’t happen on the Moon. And, on the other hand, it’s been known now for some time that these grains can have some oxygen implanted into their surface.
And the source of that oxygen has been, I would say, somewhat mysterious or not very well understood. Certainly you mentioned earlier the solar wind. And so the solar wind is striking the surface of the Moon all the time. And it was initially thought that maybe some of this oxygen that was detected in the surface of these metal grains could come from the solar wind. But we now know that that’s not the case.
IRA FLATOW: Yeah, could the lunar oxygen tell us more about the Earth?
KEVIN MCKEEGAN: Yeah, I think that’s the really exciting thing about this study. And the provocative thing, almost, is that this presents a possibility for the preservation of Earth’s atmosphere in grains that had been on the surface of the Moon. And so this is part of a more general point that’s really an important aspect of studying the Moon, is that what happened to the Moon happened to the Earth. And now it’s taken even a step further saying that material from the Earth could be preserved on the Moon.
IRA FLATOW: Wow.
KEVIN MCKEEGAN: And especially gases, right, which wouldn’t stick around on the Earth for very long before getting mixed up. So that’s really something new and something very exciting.
IRA FLATOW: It sounds like you’re making a good case for revisiting the Moon with a new set of eyes, so to speak.
KEVIN MCKEEGAN: I think yes. I think that would be fascinating. We’ve only made the very initial forays into understanding our nearest neighbor in space, and there’s really many compelling scientific reasons for going back to the Moon and studying it in part as a kind of witness plate for what happened on the Earth, and then also in part because it’s possible that ancient Earth materials are actually preserved on the Moon.
So here we have an example of modern atmosphere. But, as I said, this kind of thing has been going on for geologic history. The Moon used to be closer to the Earth. And there’s some really intriguing possibilities there.
IRA FLATOW: Well, I’m ready to go back, yeah.
KEVIN MCKEEGAN: Good, me too. I’ll go with you.
IRA FLATOW: All right, now as it seems, we hear there are private companies that want to go to the Moon and maybe mine it. Is that–
KEVIN MCKEEGAN: Yeah, yeah, that’s a little bit outside my field of expertise. And that raises a whole lot of other interesting questions. And I’m sure at the time that lawyers will find some ways to get involved and make some money.
IRA FLATOW: Well, Dr. McKeegan, I think you’re the first guy named a cosmochemist we’ve ever had on this show.
KEVIN MCKEEGAN: Hey, happy to come back any time, Ira, and talk about cosmochemistry.
IRA FLATOW: All right, thank you. Dr. Kevin McKeegan is UCLA professor of geochemistry and cosmochemistry in Los Angeles. Thank you, and have a good weekend.
KEVIN MCKEEGAN: Thank you. You too.