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Now, a story about a mysterious green rock that fell from space. Several fragments of it were discovered last year in Morocco. And earlier this month, a researcher made headlines when he proposed a striking theory: The rock could the first meteorite ever from the planet Mercury.

But NPR's Geoff Brumfiel reports, other scientists are not so sure.

GEOFF BRUMFIEL, BYLINE: If a rock falls from the sky then there's a pretty good chance a piece of it will eventually end up here, at the Smithsonian Museum of Natural History in Washington.

TIM MCCOY: We have more than 35,000 meteorites in our collection. So we keep something like 5,000 of them here and the rest are offsite.

BRUMFIEL: Tim McCoy is curator-in-charge of the national meteorite collection. He says when a new meteorite turns up, you check to see if it looks like something you've seen before. He hands me a small rock about the size of a ping-pong ball. It's gray and spotty. It looks kind of like granite.

I mean, it does look a bit like a countertop to me, or like somebody chipped the corner of a countertop.

MCCOY: Yeah, it'd be a pretty expensive countertop.

BRUMFIEL: It turns out this meteorite came from the Moon. It's easy to tell because it looks just like the rocks brought astronauts back by on the Apollo missions.

But the rock from Morocco doesn't look like anything scientists have seen before. It's a beautiful shade of olive green, with flecks of white and brown.

TONY IRVING: From experience, I immediately knew that it was very unlikely to be an Earth rock. It wasn't from Mars. And if it was a meteorite, it was something highly unusual.

BRUMFIEL: Tony Irving is an affiliate professor at the University of Washington in Seattle. Collectors from all over the world sent him samples. And a sample of this green meteorite turned up one day in a package from Morocco. A German collector had a chance to buy it from local and asked Irving to check it out. So he did. And it was even weirder than it looked.

IRVING: The minerals were very low in iron and most meteorites have more iron in them - in their minerals than this.

BRUMFIEL: Irving's mind turned to the planet Mercury. New data had been coming in from NASA's Messenger spacecraft, which is orbiting Mercury. It revealed that Mercury's surface lacked iron.

IRVING: I think it was the Messenger data that I had recently at the time studied and just sort of compared it, on a whim almost, and was quite amazed to find that some of the chemical features were a pretty close match.

BRUMFIEL: He started to get excited. Every now and then, something big strikes a planet and knocks off a few chunks. Experts predict that some chunks of Mercury may have already made the 57 million mile trip to Earth, but none have been found. This could be a piece. But he needed more evidence, so he packed up samples and sent them to colleagues around the country for further analysis.

IRVING: Then, you know, I'm waiting for the emails and wondering where it's going to fall and what the data's going to show.

BRUMFIEL: One of the most exciting findings came from a colleague who had measured the magnetic field of one of the pieces. It was smaller than almost anything yet seen in the solar system.

IRVING: And it's very close to the present magnetic field of Mercury. Putting it all together, I could see that there was a possibility of proposing this Mercury idea.

BRUMFIEL: And that's just what he did. This March, Irving presented his theory at the Lunar and Planetary Science Conference in Texas. And soon it was on CNN.

(SOUNDBITE OF A CNN CLIP)

UNIDENTIFIED MAN: They know it's not from Mars because, you know, men are from Mars and women are from Venus. And this is a green rock that they believe is from Mercury.

BRUMFIEL: But scientists at the meeting were more skeptical. In the audience was Shoshana Weider, who works on the Messenger mission. Her first thought when she heard that the rock might be from Mercury?

SHOSHANNA WEIDER: That would be nice.

(LAUGHTER)

BRUMFIEL: But she's not convinced.

WEIDER: There was nothing that jumped out and said no, this can't be from Mercury. But there were a few bits that didn't quite match with Mercury.

BRUMFIEL: And she's not only the one with doubts about the green rock. Tim McCoy at the national meteorite collection has a problem with its age.

MCCOY: The meteorite is very, very old - 4.56 billion years old. So it's essentially formed at the same time as the birth of the solar system. Whereas Mercury is a huge, hot planet that probably wouldn't have cooled off enough to have solid rock 4.56 billion years old.

BRUMFIEL: He has his own ideas about its origins. He takes a shiny, metallic rock out of a drawer and puts it under the microscope.

MCCOY: OK, can you see the green crystal in there?

BRUMFIEL: Yeah, maybe. Maybe, it's a little hard to say. It's a little hard to say. It's sort of - I don't know, it looks like moss or something?

MCCOY: Yeah, a little bit. Yeah, kind of a dull green crystal.

BRUMFIEL: That crystal is chromium dioxide, the same mineral that gives Irving's meteorite its distinctive green color. And just like Irving's rock, this one is 4.56 billion years old. But it's not from Mercury; it's from the asteroid belt between Mars and Jupiter. McCoy says that maybe the new, green meteorite came from the asteroid belt, too.

MCCOY: I think the evidence suggests that it might be a different kind of meteorite - still extremely interesting; something that actually tells us about the birth of the solar system, but maybe not the birth of the innermost planet.

BRUMFIEL: Irving says he won't be too upset if it turns out the meteorite came from somewhere else.

IRVING: As far as I'm personally concerned, if this rock turns out to be not from there, and we can find an alternative, that's just fine with me.

BRUMFIEL: In the meantime, admirers of the little green rock will soon be able to own a piece. The German collector, who sent Irving that first sample, has several other fragments which he he's planning to sell.

Geoff Brumfiel, NPR News.

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