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Earth's soft heart

A modern seismological method raises questions about the properties of Earth's inner core Earth's inner core has proven to be a challenging region for geophysicists to investigate, more than 80 years after its discovery (1). It grows slowly from the liquid iron alloy that constitutes the outer core, and its solidification is the result of the planet's cooling over the course of its history. The inner core provides an important part of the energy budget for the geodynamo—the mechanism that generates Earth's magnetic field—as latent heat is released and light elements are preferentially segregated into the fluid outer core. Despite the inner core's importance in the core's energy budget (2), its composition and material properties are difficult to ascertain. On page 329 of this issue, Tkalčić and Phạm (3) employ a new understanding of method that has been successfully used to study the crust and upper mantle to detect energy that has traveled as shear waves through the inner core. The shear-wave velocity (VS) reveals a softer inner core composition than previously thought.