Oxygen a bit player in Earth’s outer core

Sulfur and silicon may be more abundant in the planet’s heart

Like a virtual journey to the center of the Earth, laboratory experiments are helping resolve a decades-old mystery that even Jules Verne couldn’t explain: what Earth’s core is made of.

LAYERED Scientists know that Earth has inner and outer cores made largely of iron, but the abundance of lighter elements in the outer core is unknown. NASA

In particular, oxygen makes up a smaller percentage of the core than scientists had thought, suggests a study in the Nov. 24 Nature. Knowing the core’s contents helps researchers better understand how Earth clumped together 4.5 billion years ago, says coauthor Yingwei Fei, a geochemist at the Carnegie Institution for Science in Washington, D.C.

Arguments about the core’s chemical makeup have raged for nearly 60 years, says geophysicist Thomas Duffy of Princeton University. But “the work reported here suggests that a solution to the problem may finally be at hand,” he writes in a commentary accompanying the study.

In earlier experiments that tracked how seismic waves ping through the planet’s innards, changing speed and direction when they hit a major transition, geophysicists worked out that the core is made of two layers: a solid inner core and a liquid outer core. Both are made mostly of iron with a dash of nickel, but about 10 percent of the outer core’s mass must come from other stuff — “light” elements such as sulfur, oxygen, silicon, carbon and hydrogen. Some researchers argue that oxygen must be the most common of these light elements, while other scientists favor sulfur.

To help settle the question, Fei and his colleagues, including Haijun Huang of the Wuhan University of Technology in China, turned to the lab. They put together two samples consisting mostly of iron. One sample contained 8 percent oxygen and 2 percent sulfur by weight, and the other 2.2 percent oxygen and 5.3 percent sulfur. The researchers then used a high-pressure machine to send shockwaves coursing through the samples, momentarily subjecting them to the high pressures and temperatures experienced deep within the Earth.

The sample with 2.2 percent oxygen turned out to better match the properties observed deep within the Earth, as measured by the density of the sample material, as well as how quickly sound waves traveled through it. Further calculations suggested that having just 0.5 percent oxygen in the outer core is probably an even better match with reality.

The findings mesh well with a recent theoretical study suggesting the core could be made of 8 percent silicon, 2 percent sulfur and 0.5 percent oxygen, Fei says. Still, 8 percent silicon may be too high to explain other characteristics of the core, such as the difference in densities between the inner and outer cores. “I am working on models with different silicon/sulfur ratios,” he says. “More experimental data are needed to find the combination that is consistent with both geophysical and geochemical constraints.”

Having so little oxygen in the outer core implies there was far less oxygen around when Earth was born as pieces of rock and dust glommed together in the early solar system.

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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