Science Success Story
Multiple Supercomputers Used to Simulate Carbon in Earth's Outer Core
Researchers use advanced simulation techniques to set constraints on carbon content
By Kimberly Mann Bruch, SDSC External Relations; Bill Wellock, Florida State University Communications
The internal structure of the Earth is layered: The topmost layer is the crust, followed by the rocky mantle, with the innermost layer being the metallic core. The core, primarily composed of iron, is divided into the inner and the outer core, which is mostly liquid iron. Dark circles in the core represent iron, and tan circles represent carbon atoms. Tan lines show the paths taken by carbon atoms during the simulation. Credit: Suraj Bajgain
Using several supercomputers, allocations from the XSEDE were used by a research team from Florida State University (FSU) and Rice University to provide 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 – immense numbers that are comparable to celestial bodies. For example, the mass of Pluto falls within this range, while the upper estimate is about half the mass of our moon.
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 of 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 – were delivered to the Earth via the planetary building materials. Having a better estimate of 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.
"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."
Video shows the movement of iron and carbon atoms during the FPMD simulation. These simulations can be used to predict the density and compression wave velocities of binary and ternary systems with varying compositions. Dark spheres in the core represent iron and gray spheres represent carbon. Credit: Suraj Bajgain
Previous research has estimated the total amount of carbon on the planet to a range from 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% 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, 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 XSEDE Helped
Thanks to allocations from XSEDE, Expanse at SDSC – along with Stampede2 at Texas Advanced Computing Center and Bridges-2 at the Pittsburgh Supercomputing Center – used a simulation process called first-principles molecular dynamics (FPMD), which is computationally expensive.
"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." – Suraj Bajgain, postdoctoral researcher at FSU's Department of Earth, Ocean and Atmospheric Science
"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 in 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.
At a Glance
Florida State University and Rice University researchers have published a study regarding carbon in the Earth's outer core.
The research 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.