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Using several supercomputers, including Expanse at the San Diego Supercomputer Center (SDSC) at UC San Diego, a research team from Florida State University (FSU) and Rice University has recently provided a better estimate of the amount of carbon in the Earth’s outer core. The work has suggested that the core could be the planet’s largest reservoir of carbon, with the chemical element making up 0.3 to 2.0 percent by weight of the planet’s outer core.

“Though the percentage of carbon of the Earth’s outer core is low, it’s still an enormous amount because the area is so large,” said lead author Suraj Bajgain, a postdoctoral researcher at FSU’s Department of Earth, Ocean and Atmospheric Science, who along with his colleagues estimated that the outer core technically contains between 5.5 and 36.8 × 10^24 grams of carbon — an immense number.

There has been a great deal of research over the last decade to determine the carbon budget of the bulk Earth. These earlier studies have used constraints from geochemistry and extraterrestrial matter such as undifferentiated, pristine meteorites. However, the total carbon content of the Earth’s core has continued to remain an open question – owing to the uncertainties in our understanding of the formation of the Earth and the chemistry of the pristine materials that accreted to form Earth and other rocky planets.

The recent FSU and Rice study, published in the journal Communications Earth & Environment, circumvents these uncertainties by providing a direct estimate on the present-day Earth’s outer core’s carbon budget. This will in turn help the geoscience community bracket the possible planetary ingredients and better understand the overall planet formation process.

Why It’s Important

Carbon, oxygen, nitrogen and hydrogen are essential elements for the formation and sustenance of life on Earth. Thus, it is important to know how much of these elements – including carbon – was delivered to the Earth via the planetary building materials. Having a better estimate on the budget of these life-essential elements, including carbon, helps researchers like the FSU and Rice team understand the condition of forming habitable rocky planets. 

Video shows the movement of iron and carbon atoms during the simulation. Dark spheres in the core represent iron and tan spheres represent carbon. Credit: Suraj Bajgain

“Understanding the composition of the Earth’s core is one of the key problems in the solid-earth sciences,” said co-author Mainak Mookherjee, an associate professor of geology at FSU’s Department of Earth, Ocean and Atmospheric Science. “We know the planet’s core is largely iron, but the density of iron is greater than that of the core. There must be lighter elements in the core that reduce its density. Carbon is one consideration, and we are providing better constraints as to how much might be there.”

Previous research has estimated the total amount of carbon on the planet to a range between approximately 990 parts per million to more than 6,400 parts per million. That would mean the core of the Earth — which includes both the outer core and the inner core — could contain 93 to 95 percent of the planet’s carbon.

Because humans can’t access the Earth’s core, they have to use indirect methods to analyze it. The research team compared the known speed of compressional sound waves traveling through the Earth to computer models that simulated different compositions of iron, carbon and other light elements at the pressure and temperature conditions of the Earth’s outer core.

“When the velocity of the sound waves in our simulations matched the observed velocity of sound waves traveling through the Earth, we knew the simulations were matching the actual chemical composition of the outer core,” said Bajgain.

Scientists have attempted to give a range of the amount of carbon in the outer core before. This research narrows that possible range by including other light elements — namely oxygen, sulfur, silicon, hydrogen and nitrogen — in the models estimating the outer core’s composition.

Just like hydrogen and oxygen and other elements, carbon is a life-essential element. It’s part of what makes life possible on Earth.

“It’s a natural question to ask where did this carbon that we are all made of come from and how much carbon was originally supplied when the Earth formed,” Mookherjee said. “Where is the bulk of the carbon residing now? How has it been residing and how has it transferred between different reservoirs? Understanding the total inventory of carbon is what this study gives us insight to.”

How Supercomputers Helped

Thanks to allocations from the Extreme Science and Engineering Discovery Environment (XSEDE), Expanse at SDSC – along with Stampede2 at Texas Advanced Computing Center (TACC) and Bridges2 at Pittsburgh Supercomputing Center – used a simulation process called first-principles molecular dynamics (FPMD), which is computationally expensive.

“Our work would not be feasible without using the computing allocations from XSEDE,” said Bajgain. “In the next steps for our studies, we will use Expanse to explore the effect of multiple light elements – not just carbon – to refine our work.”

Bajgain also commended both XSEDE and SDSC support teams for their help on utilizing the supercomputers. He said that he was especially grateful for the help from Mahidhar Tatineni to validate software licenses and provide direct access to applicable software on the Expanse cluster.

“We are fortunate to have computing time through XSEDE,” he said. “Because of our access to these resources like Expanse, we can produce the results that are publishable in high impact journals.” 

The National Science Foundation and NASA supported this research. Computing resources were allocated by XSEDE (TG-GEO170003). The Research Computing Center at FSU provided additional computing resources for this work.