A new geological study is suggesting that what we know about the lower mantle of the Earth may have to be reevaluated. Since we are unable to very sample the Earth at those depths, scientists rely on the use of seismic waves to study the lower reaches of the Earth. This new study suggests that material in the lower mantle has unusual characteristics that make sound move more slowly, suggesting a softer makeup than long ago thought. "What's most important for seismology is the acoustic properties--the propagation of sound. We determined the elasticity of ferropericlase through the pressure-induced high-spin to low-spin transition. We did this by measuring the velocity of acoustic waves propagating in different directions in a single crystal of the material and found that over an extended strain range (from about 395,000 to 590,000 atmospheres) the material became 'softer'--that is, the waves slowed down more than expected from previous work. Thus, at high temperature corresponding distributions will become very broad, which will result in a wide range of depth having subtly anomalous echo that perhaps extend through most of the lower mantle."

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