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Encore: Freshly made plutonium from outer space found on ocean floor

ASMA KHALID, HOST:

Last year, scientists announced they'd found a rare type of plutonium at the bottom of the Pacific Ocean. This plutonium is believed to have come from an exploding star. NPR's Nell Greenfieldboyce reminds us why researchers went looking for it in the first place.

NELL GREENFIELDBOYCE, BYLINE: Astronomer Carl Sagan used to say, we are made of star stuff. He meant it literally. The burning cores of stars forged elements like carbon, nitrogen and oxygen. But what about the heaviest elements on the periodic table - ones like gold, platinum, uranium, plutonium?

ANTON WALLNER: These are the elements where we are still in a mystery, and we do not know exactly where they are produced and how much is produced in different sites.

GREENFIELDBOYCE: Anton Wallner is a physicist with the Australian National University. He led an international team that's been trying to get clues about how these elements are made by looking for specimens that arrived on Earth relatively recently. He says a massive star explosion or collision will spew out a slew of elements. Those can find their way to our planet and get incorporated into certain rocks.

WALLNER: So the idea is that we have samples which collected these extraterrestrial particles over the last thousands or millions of years.

GREENFIELDBOYCE: The sample in this study was a rock from nearly a mile beneath the Pacific Ocean. It was hauled up by an oil exploration company. In the journal Science, researchers say they found tiny amounts of iron and plutonium in telltale unstable forms that don't last long.

Brian Fields is an astronomer at the University of Illinois who wasn't part of the research team. He says this find is fantastic.

BRIAN FIELDS: Now, we only have tiny amounts of material. After all, we're talking about hundreds of atoms here. But, you know, we should be grateful for that because they're freshly made from exploding stars.

GREENFIELDBOYCE: What kind of star explosion isn't clear. What the researchers saw in the rock had to have been produced by something more than just the typical death throes of massive stars, which are known as supernova explosions. It looks like more rare events must be involved, too, like maybe collisions between neutron stars. Those are super-dense stars as small as a city, yet with more mass than our sun.

FIELDS: Knowing that there's plutonium there is amazing, even though it opens new questions. I mean, that's what you want - is to open new questions. And now we have a chance to really learn more about this.

GREENFIELDBOYCE: Fields says to help sort this out, it would be great to find other rare forms of elements that were delivered to Earth as stardust.

Nell Greenfieldboyce, NPR News.

(SOUNDBITE OF ANIMALS AS LEADERS' "ANOTHER YEAR") Transcript provided by NPR, Copyright NPR.

Nell Greenfieldboyce is a NPR science correspondent.

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