Tracing Carbon

By Aimee Balfe
Feb 28, 2014 Bookmark and Share

Astronomy Professor Ted Bergin’s new grant draws interdisciplinary expertise to trace the origins and fate of Earth’s carbon.

Bergin and his collaborators aim to determine which carbon molecules arrived to Earth via rocks and comets, and what stage our planet was in at the time.
Image Credit: Tim Wetherell, Australian National University. 


Astronomy Professor Ted Bergin has spent a good portion of his career tracking molecules necessary for life on Earth. He’s made great progress explaining how water likely arrived and managed to remain here. Now, thanks to a new NSF INSPIRE grant, he hopes to do the same for another key building block of life – carbon.

“What’s surprising is that Earth is actually a carbon-poor world,” says Bergin. “We think of the Earth as having carbon everywhere, but it turns out if you look at how much is in the Earth’s mantle, it’s many orders of magnitude below what was present in the primordial materials.”

These primordial materials, from interstellar space to the gas cloud that gave birth to our sun, have vastly more carbon relative to the benchmark element silicon than we see on Earth – though, of course, we don’t know how much carbon is inside the Earth’s core. There is also more carbon in these primordial materials than inside the primitive meteorites thought to be the building blocks of our planet.          

“If the Earth and these meteorites are made out of interstellar materials like everything is, we should be sitting on a carbon rock, but we’re not,” says Bergin. “Both of these things are made mostly of silicon. So something happened in the system as the planets were being born that destroyed the rocks made mostly of carbon, left the silicon ones behind, yet still gave Earth enough carbon to allow for life.”

Just how that process happened is what this grant aims to explore.

Bergin has already helped explain some of the missing carbon. About half of the carbon in interstellar space – and all of the silicon – is in the form of a rocky dust. He’s worked on models that show how the sun’s radiation, in the presence of oxygen, could burn up these carbon grains in the inner reaches of a protoplanetary disk. The products of this process – gases such as carbon monoxide – would then be transported to the disk’s outer reaches to form ices, often becoming part of asteroids and comets. The silicon-based rocks would avoid this fate.

But this is just the start of the story. To fill in the rest, Bergin has teamed up with longtime collaborator Geoffrey Blake, an astrochemist from Caltech, and two geochemists – Marc Hirschmann from the University of Minnesota and Jackie Li from U-M. While the astronomers identify what forms of carbon would have made their way to Earth, the geologists will clarify how these compounds might have been affected by the conditions on our still-developing planet.

If the local rocky carbon that should have comprised our planet was burnt off, much of our carbon must have come to us as volatile forms trapped in rocks and comets that were “degassed,” or released into the atmosphere, upon impact with our planet. But which carbon-containing molecules arrived and when they arrived would make all the difference in how much carbon would be available to our budding biosphere.

“If the carbon arrives late, when the Earth is solid rock, most of it would stay at the surface and available for life,” says Bergin. “But if it arrives sooner, while the Earth is molten, it could have a different fate. Particularly if it arrives in hydrogen-rich forms, much of it could be dissolved into the planet’s ocean of magma and be sequestered into its core.”

To work out a plausible scenario, the astronomers will identify the primary carbon carriers in various regions of young disk systems using the Atacama Large Millimeter Array (ALMA) and the Keck Telescope. Then the geologists will conduct experiments to determine how soluble the various carbon species would be in the Earth’s magma and how carbon might be partitioned into the planet’s atmosphere, mantle, and core. The team would then combine their findings into models that would help determine when the various forms of carbon likely arrived and how they were processed on our developing planet.

Ultimately, the team hopes they can illuminate not only how the Earth got its carbon, but how it might become available to life on other planets, as well.

This work is supported by a three-year NSF INSPIRE Grant beginning on January 1, 2014.