New research confirms that the Earth's core contains vast amounts of sulphur, estimated to be up to 8.5 x 10^18 tons. 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 percent 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 2,900 km [1,800 miles] 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.
"Scientists have suspected that there is sulphur in the core for some time, but this is the first time we have solid geochemical evidence to support the idea," said lead researcher Paul Savage from the Department of Earth Sciences at Durham University in the United Kingdom, according to a press release.
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-size 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 analyze 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 Frédéric Moynier from the Institut de Physique du Globe in Paris. "Generally, where there is copper, there is sulphur; copper gives us a proxy measurement for sulphur."
According to Savage: "This study is the first to show clear geochemical evidence that a sulphide liquid must have separated from the mantle early on in Earth's history -- which most likely entered the core. We estimate that the quantity of sulphur in the core is vast, around 8.5 x 10^18 tons, which to give an idea of scale, is around 10 percent of the mass of the Moon. In addition, the work adds weight to the theory that the moon was formed via a collision between the Earth and another body.
"In a way, we can also say that we have life imitating art. For millennia, tales have been told of the underworld being awash with fire and brimstone. Now at least, we can be sure of the brimstone."
"Copper Isotope Evidence for Large-Scale Sulphide Fractionation During Earth's Differentiation," P. S. Savage (1, 2, 3), F. Moynier (1), H. Chen (4), G. Shofner (1), J. Siebert (1), J. Badro (1, 5), I. S. Puchtel (6), Geochemical Perspectives Letters, 2015 July, Vol. 1, No. 1