Short Wave - The Curious Stardust At The Ocean Floor

Episode Date: May 26, 2021

Researchers report in the journal Science that they appear to have some clues about the origin of Earth's plutonium - which has been long debated. Correspondent Nell Greenfieldboyce explains that traces of rare forms of iron and plutonium have been found in extraterrestrial debris that had sunk to the bottom of the Pacific Ocean, hauled up by an oil company, then donated for research. By comparing the iron and the plutonium, scientists found the plutonium was likely forged in a cosmic cataclysm, perhaps a rare kind of supernova, and then rained down on Earth.Ever find yourself existentially musing? Wondering about the state of the cosmos? Drop us a line at [e-mail] and we might jump into a wormhole with you. See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. Hey nerds, Emily Kwong here. Today we are meeting up with NPR science correspondent Nell Greenfield Boyce. Hey there, Nell. Hey, Emily. So I was thinking about Carl Sagan. Do you remember Carl Sagan? Oh, yeah.

You know, beloved astronomer, science popularizer. He had that famous TV show called Cosmos. And there's something he said in that show that was like kind of a famous quote. Do you remember what he used to say about like what we're made of? of. I think I remember. It's like a science classic one-liner. The cosmos is also within us. We're made of star stuff. We are a way of a cosmos to know itself. Right. So when Carl Sagan there said we're made of star stuff, you know, he wasn't just being like metaphorical and poetic. I mean, he meant it literally. He literally meant that, you know, the atoms of nitrogen in our DNA and the calcium in our teeth and the iron in our blood.

it was all forged in the interiors of stars. Yeah, when I first learned this, it went straight to my head. It made me feel so, so special. But those elements that made those stars in the first place, like hydrogen and helium, I thought they were from earlier, like in the big bang that started our universe. Okay, fair enough. But, you know, once the stars were there, the reactions in them started creating most of the elements on the familiar periodic table. Still, there are some elements that it's not so clear exactly where.

where they come from, their origins are a little bit mysterious. Ooh, okay. Which elements are you talking about? So some of the heaviest ones, you know, the kind of fancy ones, like gold, platinum, uranium. But I'm actually here today because researchers just found some new clues about where those came from. And it's a story that involves a rock, a rock at the bottom of the Pacific Ocean, which had some atoms of plutonium. Plutonium. Okay. And it looks like this plutonium was kind of freshly made in outer space fairly recently,

and then it rained down on the earth. Fascinating. Well, today on the show, extraterrestrial plutonium. And what it can tell us about the life and death of stars and how they forge some of the most mysterious elements in the universe. This is Shortwave, the Daily Science podcast from NPR. All right, Nell, let's get into this. We were just talking about the fact that a lot of common elements, the stuff we're made of, gets made inside of stars. We are stardust.

Fact check, true. Right. Okay. And so when a massive star dies, it can actually throw its innards out into space. At least, that's how Brian Fields puts it. He's an astronomer at the University of Illinois. And about 25 years ago, he was thinking about the dramatic deaths. of like massive stars that send lots of elements spewing out. And these are called supernova explosions.

And he wondered, you know, if a supernova went kaboom close to Earth at some point in its history and maybe, you know, even ended up having some impact on Earth's life, how would we even know that? It's not enough to just find dead critters. You want to say, how would you know that there was a supernova near the Earth? And what we realized is if it's close enough to be dangerous, it's also close enough to literally splatter its guts upon the Earth. So all these elements that are made in the supernova get delivered to us and literally rain upon our heads, literally rain down on the Earth. I love how once again we have to stress the literal nature of what we are talking about now. Well, that's because it sounds pretty wild, right?

I mean, supernova explosions, you know, the ones that are how the most massive stars die, they're relatively rare in an entire vast galaxy. You might have like three happen per century or something like that. And mostly, you know, they're very far away. But Fields and others wanted to know, have any supernovas happened near our planet? And so we made some predictions for how much of various things to look for and then hope somebody would do it. And then amazingly, that's actually what happened. And by people, obviously, you know, he means like geologists and, you know, experts who know where stuff gets stashed in our planet. Got it.

So he made some predictions. like a list of what to look for and what was on it? That's the tricky thing, right? Because everything on Earth is made of elements. And we've just talked about the fact that these elements come from stars and supernovas. Yeah. So simply finding those elements on Earth doesn't tell you anything about when and how they got here or if they arrived recently. Right.

So what fields and a couple of colleagues realized is that actually there might be a way to tell. Because in the vast swirling blizzard of reactions, that are going on in supernova explosions. The supernova not only makes stable elements, which were made of, but it also makes all kinds of unstable radioactive elements. And so these forms, these radioactive forms, decay. So that means they only last a certain amount of time. They don't last long.

So Fields and his colleagues came up with a list of elements in forms that only last for millions of years. If they live for millions of years, that's long enough that there's time enough for the newly formed elements to make it to Earth and be ready for us to find, but not so long live that there'd be any left, if any, were present at the birth of the Earth, which was billions of years ago. I get it. So identifying these radioactive elements, basically if you find any of these relatively short-lived

forms of the elements, these radioactive forms, you know that they had to have arrived recently and that they are, in fact, starguts spewed onto the earth. Exactly. So one of the things on their list of what to look for is a radioactive form of iron called iron 60. And over the years, people have looked and they have indeed found it in deep sea rocks, in samples from the moon, even in snow, in an Arctic snow. And it looks like the bulk of it arrived about three million years ago. So that means that some supernova explosion happened nearby in the last few million years.

years and I guess it littered our planet with radioactive iron? That is exactly right, exactly. Okay. And radioactive iron is cool. No, don't get me wrong. But it's not the radioactive metal you promised me. Where is my plutonium? So here's the thing. Over the years, people have been looking for plutonium, too, you know, a kind of short-lived form of plutonium that would also have to be a recent arrival on Earth. And Fields told me, you know, that one has just been a lot harder to detect. But now there actually is a study in the journal Science that is clearly for real. An international group of researchers say they have found some of this telltale plutonium in a rock from nearly a mile beneath the Pacific Ocean.

Why are they looking at the bottom of the Pacific Ocean? Actually, it was an oil exploration company that hauled up this rock like years and years ago. They were looking for oil, not rocks, obviously. That makes sense. So how did the researchers end up with this rock? Well, the oil company donated it to science decades ago. So, you know, a university had it in their collection. And when the researchers who did this study went looking for a rock with some special properties

that they thought would make it a likely collector of any extraterrestrial atoms coming from some exploding star, they ended up finding it. They found this rock. So what's so special about this kind of rock that made them pull it from the archive and take a closer look? So it's a kind of rock that sort of likes to grab atoms. of iron and other certain elements out of the seawater, and then it kind of deposits them in layers. So, you know, that means that it lets you look at time, sort of like you would look at time in tree rings, right?

There's a sort of record. The layers of this rock go back in time. And that means if stardust hits the earth and settles down to the bottom of this ocean, it can end up in a rock like this. And Anton Volner is a physicist with the Australian National University and also the Helmholt center in Germany. He told me there's a history of like 10 million years in one section of this rock that's just like a few centimeters thick. The idea is that we have samples which collected this extraterrestrial particles over the last thousands or millions of years.

And let me guess there was plutonium in there. Yep, they found plutoniums. I mean, they also found iron, you know, which was similar to what's been seen before. They saw a pulse of iron that was delivered about three million years ago. So, you know, that seems like maybe a supernova nearby. And, you know, this rock they looked at actually had another dose of iron about three million years or so before that. You know, so maybe that's from another nearby supernova. Because, you know, I think most astronomers agree that supernovas can make iron. I mean, that's pretty clear.

The plutonium is a different story. I mean, it's plutonium is one of those very heavy elements, you know, that scientists just can't agree on where it's from exactly. And, you know, Volner told me that whatever made it had to be more than just a plain old supernova. Because if you look at the relative amounts of iron in plutonium that they found in the rock layers and you sort of compare that ratio to what you'd expect to see based on like the theoretical models they've been making, you know, about how a supernova generates elements, like what they see just doesn't match up with what you would expect if it was just, you know, supernova. Got it. Okay. So it wasn't the same supernova that spread that radioactive iron you mentioned earlier. And it sounds like it wasn't a supernova really at all. So what did create this plutonium? Well, I mean, it could be something exotic, like two neutron stars smashing together. Wow. Metal.

Neutron stars are these super dense stars. They're like the size of the city, but they've got more mass than the sun. So, you know, imagine those like ramming into each other. And for a while now, some astronomers have thought that, you know, a regular old supernova isn't intense enough to generate heavy elements like plutonium. They've thought, you know, you need something exotic, like a neutron star merger or something like that. And Volner told me, you know, these new findings would support that. It might be that both scenarios are necessary. It's both. It's supernova explosions that produce a part of these heavy elements, but also neutron star mergers or any other rare events. So what do other researchers think about this theory? Do they buy it? Well, I talked with Hendrik Shats at Michigan State University, and he wasn't part of the research team.

And, you know, he told me, yeah. Like, it looks like supernova explosions would not be enough to explain what they saw. I mean, you know, supernovas are not the only source here. It could be a very rare form of a supernova. But it's not going to be, at least not predominantly sort of your average supernova. He thought it was really great that they'd found this plutonium. I mean, people have been looking for a while. And Fields told me, you know, it's just wild. that they can find this stuff from beyond our solar system at all. It's like very strange for astronomy. Because usually astronomers are looking at telescopes to see far away things, like, you know, many, many millions of miles away.

And yet here, instead, you've basically got star guts embedded in rocks. I mean, speaking of exploding stars, do we have to worry about any of this radioactive star dust? No, no, no, no, no, no. I mean, Brian Fields told me, you know, we know where massive stars are. the kinds of things that might go supernova. So, like, there's nothing nearby that's going to go, you know, whammo and pose any danger to Earth. Well, that is a relief. Let me thank you once again for reminding us that we are made of Star Stuff.

Indeed, we are. Although, you know, not plutonium. This episode was produced by Rebecca Ramirez, edited by Giselle Grayson and backtracked by Rasha Auredi. The audio engineer for this episode was Josephine Neonai. I'm Emily Kwong, and this is... Shortwave, the Daily Science podcast from NPR.