Planetary Geoscience

The interior structures of the Earth and Moon are determined from seismic data. The existence and sizes of cores in other planets are inferred from observations of planetary sizes, masses, and shapes, which constrain their uncompressed mean densities and moment of inertia factors. Mantle and crust thicknesses can also be estimated from gravity data obtained by orbiting spacecraft. Successful models of planetary interiors constructed from compositional data must be consistent with observed densities and moments of inertia. High-pressure laboratory experiments can constrain the mineralogy of mantles and cores and the partitioning of elements between silicate and metal in the terrestrial planets. The interiors of the giant planets are not well understood, because of uncertainties in their compositions and internal temperatures and pressures. The states of hydrogen and helium in the interiors of Jupiter and Saturn, and the crystalline forms of ices in Uranus, Neptune, and icy satellites, are inferred from experimentally determined or calculated phase diagrams. The giant planets may have small rocky cores, with successive layers of either metallic hydrogen (Jupiter and Saturn) or ices (Uranus and Neptune), and molecular hydrogen. Planetary mantles and cores evolve over geologic time, through cooling and extraction (or reintroduction, in the case of Earth) of crustal components.