A new analysis of the way in which sound waves are propagated by the Earth’s molten interior suggests that it may be softer than previously thought.

According to a paper published in Science, iron-rich material in part of the lower mantle, which extends from 660 to 2,900 kilometres below the Earth’s surface, has unusual characteristics that make sound waves propagate through it more slowly than previous work had shown. Because geologists use the speed at which such waves travel through the Earth to determine the composition and density of its various layers, the work may force a rethink of models of the planet’s interior.

Molten material in the lower mantle is subjected to intense pressure – up to 1.35 million times the atmospheric pressure at sea level – and temperatures of 1,540–3,700°C. The scientists recreated these conditions in a laboratory and found that the extreme pressures changed the way in which iron atoms interact with each other. This, in turn, affects the way in which the material propagates sound waves, making it appear ‘softer’ than it actually is.

Jonathan Crowhurst of Lawrence Livermore National Laboratory in California, lead author of the paper, said: ‘The lower mantle constitutes a large fraction of the entire planet and profoundly influences many terrestrial phenomena, including several that are of direct relevance to us.’