The Origins of Planets

It is amazing to think about this, but Earth is actually a carbon- and nitrogen-poor world.
We think of the planet as having these elements everywhere, as they are essential for life as we know it. However, it turns out that if you look at how much is in the Earth’s mantle, it’s many orders of magnitude below what was present in its primordial materials.

These materials, from interstellar space to the gas cloud that gave birth to our sun, had vastly more carbon and nitrogen relative to the benchmark element silicon (think dirt or sand) than we see on Earth. In fact, compared to what is available, the Earth’s surface received fewer than one in 1,000 carbon atoms that were present and one in 100,000 nitrogen atoms. Yet we as carbon-based life forms are here. We do not know how much carbon is locked deep inside the Earth in its core, but we also see more carbon in the primordial materials than inside primitive meteorites thought to be the building blocks of our planet. So, if Earth and these meteorites are made out of interstellar materials as everything is, we should be sitting on a carbon rock. Instead we find that these bodies are made mostly of silicon. This suggests that something happened in the system as the planets were being born that destroyed the rocks made mostly of carbon, left the silicon ones behind and yet still gave Earth enough carbon to allow for life.

Just how that process happened is what I aim to explore.

I have 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. I’ve 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 parts 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, I have 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 we astronomers work to identify what forms of carbon would have made their way to Earth, the geochemists 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 arrived late, when Earth was solid rock, most of it would stay at the surface and be available for life. But if it arrived sooner, while Earth was in a molten state, it would have experienced a different fate – particularly if it arrived in oxygen-rich forms. Then much of it could be dissolved into the planet’s ocean of magma and be sequestered in its core.

To work out a plausible scenario, we 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 Earth’s magma, and how carbon might be partitioned into the planet’s atmosphere, mantle and core. The team will then combine their findings into models to 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.