A strange green rock discovered in Morocco last year was hailed by the press as the first meteorite from Mercury. But scientists who've been puzzling over the stone ever since say the accumulating evidence may point in a different direction. Maybe, just maybe, they say, the 4.56-billion-year-old rock fell to Earth from the asteroid belt located between Mars and Jupiter.
If that's true, the rock is "still extremely interesting," says Tim McCoy, who curates the Smithsonian Museum of Natural History's collection of 35,000 meteorites. "[It] tells us something about the birth of the solar system, but not the birth of the innermost planet."
The olive green meteorite, flecked with bits of white and brown, first came to scientists' attention last year when a German collector, Stefan Ralew, saw the unusual stone in Morocco and shipped it off for analysis to Tony Irving, a geochemist and meteorite specialist affiliated with the University of Washington in Seattle. Irving routinely receives such packages from all over the world.
"From experience, I knew it was very unlikely to be an Earth rock," Irving says. "It wasn't from Mars, and if it was a meteorite, it was highly unusual." As it turns out, the rock was even weirder than it looked.
"The minerals were very low in iron," he says, "and most meteorites have more iron in their minerals than this." 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.
"I think it was the Messenger data that I had recently studied and just sort of compared it, on a whim almost," Irving says. "[I was] quite amazed to find that some of the chemical features were a pretty close match."
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 Irving needed more evidence, so he packed up samples and sent them to colleagues around the country for further analysis.
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.
"And it's very close to the present magnetic field of Mercury," Irving says. "Putting it all together, I could see that there was a possibility of proposing this Mercury idea."
And that's just what he did. This March, Irving presented his theory at the Lunar and Planetary Science Conference in Texas. But scientists at the meeting were more skeptical. In the audience was Shoshana Weider, a fellow at the Carnegie Institution of Washington, who works on the Messenger mission. Her first thought, when she heard that the rock might be from Mercury, was, "That would be nice." But she's not convinced.
"There was nothing that jumped out and said, 'No, this can't be from Mercury,' " she says, "but there were a few bits that didn't quite match with Mercury." For example, the rock lacks sulfur, while Mercury's surface is covered in it.
She's not the only one with doubts about the green rock. The Smithsonian's Tim McCoy has a problem with the rock's age.
"The meteorite is very, very old — 4.56 billion years old," he says. "So it's essentially formed at the same time as the birth of the planet, whereas Mercury is a huge, hot planet that probably wouldn't have cooled off enough to have solid rock 4.56 billion years ago."
McCoy has his own ideas about the meteorite's origins. He has another shiny metallic rock in his collection that, under the microscope, contains some crystals the same color as the Morocco specimen
That crystal is chromian diopside. It's 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 known to be from that asteroid belt between Mars and Jupiter. McCoy says that maybe the new green meteorite came from the asteroid belt, too.
Irving says he won't be too upset if it turns out the meteorite came from somewhere else. "As far as I'm personally concerned, if this rock turns out to be not from there and we can find an alternative," he says, "that's just fine with me."
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 that he's planning to sell.