New research shows Earth’s core contains 90% of Earth’s sulphur (equivalent to 10% mass of the Moon)

Schematic evolution of copper concentration and isotopic composition of Earth’s mantle as modelled in this contribution. Earth accretes as a mixture of chondrites such that the bulk Earth Cu isotope composition is δ⁶⁵CuCBE. Core formation sequesters ~60 % of Earth’s Cu in the metal phase, which is enriched in the heavy isotope, driving Earth’s mantle to a lighter composition. The formation of a Fe-O-S layer, the “Hadean Matte”, sequesters isotopically light Cu, driving Earth’s mantle to its present day composition (δ⁶⁵CuBSE)

So perhaps there is some truth in the old legends of the underworld reeking of brimstone (or sulphur, as it is now called)? New research confirms that the Earth’s core does in fact contain vast amounts of sulphur, estimated to be up to 8.5 × 10¹⁸ tonnes. This is about 10 times the amount of sulphur in the rest of the Earth, based on the most recent estimates (and for comparison, around 10% of the total mass of the Moon). This is the first time that scientists have conclusive geochemical evidence for sulphur in the Earth’s core, lending weight to the theory that the Moon was formed by a planet-sized body colliding with the Earth.

The Earth’s core begins 2900 km beneath our feet, so it is impossible to investigate directly. However, an international group of researchers have been able to develop indirect geochemical methods to show core composition.

For a long time it has been known that the Earth’s core is too light to be made only of iron and nickel, and it had been assumed that the core contained other lighter elements, such as sulphur, silicon, oxygen and carbon. However, given the depth of the core, this has been impossible to confirm directly. Fortunately, a cataclysmic event in the distant past – when the Earth collided with a large, planet-sized body, tearing off the part which became our Moon – left a fingerprint, which has been used to confirm the core content.

The researchers believe that the impact of the collision melted the Earth’s mantle, allowing a sulphur-rich liquid to form in Earth’s mantle, the vast middle layer between the core and the crust; some was probably lost into space, but some remained and sunk into the core. The key to confirming this lay in measuring the isotope ratios of elements (isotopes are atoms of the same element with slightly different masses) in the mantle, and comparing these to certain meteorites, which are believed to be the best match to the Earth’s original composition.

Because of variability in mantle composition, it is difficult to draw firm conclusions from measuring sulphur directly, so the researchers chose to analyse copper from the Earth’s mantle and crust – copper is often bound to sulphur. “We chose copper, because it is a chalcophile element, which means it prefers to be in sulphide-rich material – so is a good element to trace the fate of sulphur on Earth,” said senior author Professor Frédéric Moynier (Institut de Physique du Globe, Paris). “Generally, where there is copper, there is sulphur; copper gives us a proxy measurement for sulphur.”

The work comprised 3 distinct stages:

• Firstly, the researchers had to estimate the isotopic composition of copper in the Earth’s mantle and crust.
• Secondly, they had to estimate the isotopic composition of copper in the Earth before it formed a core, and was bombarded by giant impactors. Direct measurement is of course impossible, so they used meteorites, which are regarded as the best analogue.
• Finally, they had to simulate which copper isotopic signature would be generated by the removal of sulphur-rich liquid after the ‘giant impact’.

Using the state-of-the-art mass spectroscopes at the Washington University in St. Louis and the Institut de Physique du Globe, Paris, they were able to confirm that there was a difference of 0.025% in the copper isotopic ratios between the Earth mantle samples, and the meteorite samples. Because the isotopes of copper divide unevenly between a sulphur-rich liquid and the rest of Earth’s mantle, this shows that a large amount of sulphur must have been removed from the mantle.