Shear softening of Earth's inner core indicated by its high Poisson's ratio and elastic anisotropy

Earth's inner core exhibits an unusually high Poisson's ratio and noticeable elastic anisotropy. The mechanisms responsible for these features are critical for understanding the evolution of the Earth but remain unclear. This study indicates that once the correct formula for the shear modulus is used, shear softening can simultaneously explain the high Poisson's ratio and strong anisotropy of the inner core. Body-centred-cubic (bcc) iron shows shear instability at the pressures found in the inner-core and can be dynamically stabilized by temperature and light elements. It is very likely that some combinations of light elements stabilize the bcc iron alloy under inner-core conditions. Such a bcc phase would exhibit significant shear softening and match the geophysical constraints of the inner core. Identifying which light elements and what concentrations of these elements stabilize the bcc phase will provide critical information on the light elements of the inner core.