Universal viscosity growth in melted metals at megabar pressures: the vitreous state of the Earth's inner core

Experimental data on and theoretical models for the viscosity of various ty pes of liquids and melts under pressure are reviewed. Experimentally, the l east studied melts are those of metals, whose viscosity is considered to be virtually constant along the melting curve. The authors' new approach to t he viscosity of melts involves the measurement of the grain size in solidif ied samples. Measurements on liquid metals at pressures up to 10 GPa using this method show, contrary to the empirical approach, that the melt viscosi ty grows considerably along the melting curves. Based on the experimental d ata and on the critical analysis of current theories, a hypothesis of a uni versal viscosity behavior is introduced for liquids under pressure. Extrapo lating the liquid iron results to the Earth's core pressures and temperatur es reveals that the Earth's outer core is a very viscous melt with viscosit y values ranging from 10(2) Pa s to 10(11) Pa s depending on the depth. The inner Earth core is presumably an ultraviscous (> 10(11) Pa s) glass-like liquid - in disagreement with today's idea of a crystal inner core. The not ion of the highly viscous interior of celestial bodies sheds light on many mysteries of planetary geophysics and astronomy. From the analysis of the p ressure variation of the melting and vitrification temperatures, an entirel y new concept of a stable metallic vitreous state arises, calling for furth er experimental and theoretical study.