By Jason Matthews

We are made of stardust, but some of it came from much further away than we thought. Scientists had a look at Earth's mysterious carbon deficit.

Image Credit: Paul Campbell via iStock/Getty Images

The condensation model, part of the theory of planet formation, has been the accepted explanation of how all our protoplanetary disc elements came together to form the Earth as well as other planets for decades. However, one element, carbon, does not fit well into this theory. Once the protoplanetary disc heated up, before cooling and condensing into planets, carbon would have vapourised. Carbon does not condense back into a solid state like other elements. Instead, it bonds with other volatile elements to create larger molecules.

Li, a professor in the U-M Department of Earth and Environmental Sciences and lead author of the study, explains, "The condensation model has been widely used for decades. It assumes that during the formation of the sun, all the planet's elements got vaporized, and as the disk cooled, some of these gases condensed and supplied chemical ingredients to solid bodies. But that doesn't work for carbon."

For Earth to become habitable, carbon had to be present in an exact amount. Too much carbon and Earth would have a runaway greenhouse effect like Venus, with temperatures exceeding 400 degrees centigrade. Not enough carbon and carbon-based life would never have developed. So, where did the Earth get its carbon?

To better understand how the Earth acquired its carbon, the research team analyzed seismic data to determine how much carbon the Earth contains. Professor Bergin, chair of the U-M Department of Astronomy, described their approach, "We asked a different question: We asked how much carbon could you stuff in the Earth's core and still be consistent with all the constraints. There's uncertainty here. Let's embrace the uncertainty to ask what are the true upper bounds for how much carbon is very deep in the Earth, and that will tell us the true landscape we're within."

Bergin continued, "The planet needs carbon to regulate its climate and allow life to exist, but it's a very delicate thing. You don't want to have too little, but you don't want to have too much."

Their research has led them to believe carbon arrived on Earth within one million years of the protoplanetary disc condensing into the planets we have today, from interstellar space. Small planetesimals (the building blocks of larger bodies) would have lost most of their carbon as they heated up to form the planet's core. The solar system must have collected carbon as it moved through interstellar space, delivering it to the early Earth and avoiding vaporization.

An artist’s impression of a protoplanetary disc - Image Credit: ESO/L. Calçada via Wikimedia Commons

Professor Hirschmann of Earth and environmental sciences and lead of a second study conducted by the same team said, "Most models have the carbon and other life-essential materials such as water and nitrogen going from the nebula into primitive rocky bodies, and these are then delivered to growing planets such as Earth or Mars. But this skips a key step, in which the planetesimals lose much of their carbon before they accrete to the planets."

Professor Ciesla, another member of the research team, said, "Answering whether or not Earth-like planets exist elsewhere can only be achieved by working at the intersection of disciplines like astronomy and geochemistry. While approaches and the specific questions that researchers work to answer differ across the fields, building a coherent story requires identifying topics of mutual interest and finding ways to bridge the intellectual gaps between them. Doing so is challenging, but the effort is both stimulating and rewarding."

If you are interested in the subject and would like a more detailed look at the study covered in this article be sure to check out the paper published in the journal: Science Advances listed below.

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