Mystery of missing carbon cracked

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Earth's mantle seems to be depleted in carbon, but chemical processes might explain why.

A new model may explain why the Earth's mantle is missing carbon. Credit: NASA

Mysteriously, Earth has much less carbon in its rocks than would be expected from the amounts of carbon available in the planet-forming regions of our Galaxy. But a new model suggests that chemical reactions between carbon grains and oxygen could be the explanation.

Planets form from the disks of gas and dust that coalesce around stars. The gas and dust in these disks make up the interstellar medium that forms the space between stars in galaxies, with the dust containing carbon-rich and silicate-rich grains.

But despite the green, carbon-rich surface of our planet, Earth's mantle is remarkably poor in carbon compared with the amount in the interstellar medium. Meteorites, thought to be the building blocks of our planet, also have missing carbon, whereas comets, which are formed farther away from the Sun, do not. Conversely, silicon seems to make it from the interstellar medium, from which planets form, into the bulk material of the planet. Astronomers have struggled to fully account for the carbon shortfall in Earth's mantle and in meteorites.

Now Ted Bergin at the University of Michigan, Ann Arbor, has come up with a model that could explain what happened to the carbon. He presented his ideas at the International Union of Pure and Applied Chemistry congress in Glasgow, UK, last week.

Grain drain

Previous theories to explain why all the interstellar-medium carbon didn't make it into the material that formed Earth include the evaporation of primordial carbon-rich grains from the disk. But for this model to work, temperatures would have to have been at least 1,000 kelvin, and at Earth's distance from the Sun, those temperatures are not reached.

“The reaction is oxygen hitting the carbon grain and sputtering carbon off. Ted Bergin , University of Michigan”

Bergin, together with Jeong-Eun Lee of Sejong University in South Korea, and their colleagues, modelled the chemical processes that could have been occurring in the disk, to see how hot the oxygen there might have been, and where in the disk reactions might have been taking place.

Bergin says that the surface of the disk would have been hot, although nowhere near as hot as the 1,200 kelvin needed to evaporate carbon. Here, oxygen atoms exist, he says, that react with the tiny carbon grains but not with silicates. These grains are about one-tenth of a micrometre in diameter. Because of this reaction, the middle part of the disk, where planets are formed, would become depleted in carbon.

"The reaction is oxygen hitting the carbon grain and sputtering carbon off," says Bergin. And this can happen at around 500 kelvin, he says. Farther out from the heat source, the reaction between oxygen and carbon would have been much slower, which explains why planets such as Mars don't seem to have the carbon deficit.

Life-saver

Mike Jura at the University of California, Los Angeles, says that the model is very plausible. "In random scoops of interstellar matter one would expect a lot of carbon," he says. Jura agrees that the model suggesting evaporation of carbon is flawed, "We're nowhere near 1,200 kelvin. That takes a lot of heating, he says." Bergin's chemical model helps to take away the need for that heat, he says.

If the new model is correct, one would expect to see lots of extra carbon in the gassy part of the disk from which planets are formed. He's now working out how this carbon might be detected astronomically, and hopes eventually to be able to test this. And all the extra gas-phase carbon that the model releases might have helped life to form, Jura suggests.

"It's abundantly clear that if the Earth had got all of the carbon that was available, I guess that would have presented a bit of a problem for forming life," Bergin says. "We would have had way too much of a greenhouse and the water would have boiled."

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Sanderson, K. Mystery of missing carbon cracked. Nature (2009) doi:10.1038/news.2009.816

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