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EARTH SCIENCE
A fine marriage between seismic data and laboratory experiments carried out at the extreme conditions of Earth's deep interior indicates that the planet's liquid outer core is poor in oxygen. See Letter p.513
Earth's core accounts for one-third of the planet's mass and has a central role in Earth's overall energy budget and dynamics. Although the core has long been known to be composed mainly of iron, together with some nickel1,2, the identity of the lighter elements that make up about 8% of the core's mass has been an enigma for nearly 60 years. Work reported in this issue by Huang et al.3 (page 513) combines the results of laboratory experiments with geophysical data for the core to address this long-standing mystery. Better knowledge of the core's main light element (or elements) will shed light on heat flow in Earth's deep interior, on the origin and growth of the core's solid inner region, and on the generation and evolution of Earth's magnetic field4.
But which element is it? Here we can bring out the line-up of usual suspects: sulphur, oxygen, silicon, carbon and hydrogen. Each is geologically abundant and can dissolve in liquid iron under the appropriate range of pressure and temperature conditions. Geochemical arguments for and against each can be constructed5. Each also has its own implications for core formation and evolution of the early Earth6. For example, an oxygen-dominated core would imply that oxidizing conditions were present during much of core formation, whereas a silicon-dominated one would require mostly reducing conditions. Hence, unmasking the identity and abundance of the core's main light elements will also be a major step...