The Internal Structures of the Planets

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Summary Prompted by a paper of Sir Edward Bullard (1948), some problems of the internal structures of the planets are discussed. The structures must be inferred from the sparse dynamical data (mass, density, J2) which are all that are available, and must be culled from, analogies with the Earth for the Moon and the terrestrial planets or from theoretical calculations of behaviour of simple materials at high pressures for the outer planets. The importance of the value of the dimensionless moment of inertia, C/Ma2, is emphasized and the validity of applying Bullen's compressibility-pressure hypothesis to other planets than the Earth is considered.