GEODYNAMIC ESTIMATES OF THE VISCOSITY OF THE EARTHS INNER-CORE

Recent seismological studies(1,2) have suggested that the inner core i s rotating relative to the bulk of the Earth, a situation which (accor ding to numerical simulations(3)) may be sustained by convective flow in the liquid outer core. On the other hand, large gravitational force s due to the heterogeneous distribution of mass in the Earth's mantle should be sufficient to keep the inner core aligned with the mantle(4) . Here I show that the differential rotation of the inner core can be reconciled with these strong gravitational forces by allowing the shap e of the inner core to adjust as it rotates, so permitting an estimate of the effective viscosity of this innermost region of the Earth. The inferred rotation rate constrains the viscosity of the inner core to be less than 10(16) Pa s or greater than 10(20) Pa s, as two different dynamical regimes are possible. The viscosity estimates for these two regimes have very different implications for the origin of seismic an isotropy in the inner core.