Viscosity of high-pressure ice VI and evolution and dynamics of Ganymede

Abstract

Current models^1–5 of the evolution of the jovian satellite, Ganymede, assume that it was formed by homogeneous accretion of water ice and silicate particles (about 50 mass%) with chondritic abundance of the radioactive elements U, Th and K. As the satellite reached its final size it would probably have been composed of a mixture of silicate particles and high-pressure phases of ice, from the centre outwards: ice VIII, ice VI, ice II and finally, near the surface, ice I⁴. Due to radioactive decay, the temperature would rise in the interior, presumably transforming ice VIII into ice VII, and bringing ice VII and VI close to their melting point. We have measured the viscosity of the high-pressure ice VI at room temperature and pressures of 1.1–1.2 GPa in a sapphire anvil cell; fine particles were used to visualize the flow of ice down the radial pressure gradient. The low value we found for the viscosity (η≃10¹⁴ P) suggests that solid state convection might have taken place during the early evolution of Ganymede, thus preventing melting and differentiation.