Introduction

Leaving aside the special case of the Moon, the properties of planets that can at present be determined are certain gross quantities descriptive of a planet as a whole; these are the size, the spin angular velocity, the mass and mean density, the moments of inertia and the coefficients in a spherical harmonic expansion of the gravitational potential, together with some features of the magnetic field and possibly electromagnetic induction in the planet. The Moon alone is open to the study of the variation of properties with depth by seismology. The investigation of the internal state of a planet depends on what can be inferred from the measured gross properties, and fails unless those properties can be measured with precision. Given only integral properties, a wide range of internal distributions of density is consistent with the data, but the more precisely the integral properties are known the more restricted the range of possible distributions.

The various dynamical properties of a planet are not independent, for all are determined by three factors: the spin, the chemical composition and the temperature. Suppose the spin acceleration at the surface at the equator (where it has its greatest value) to be small compared with the acceleration of the self-gravitational attraction. Then composition and temperature together determine the equation of state, the latter mainly by its control of the occurrence of any polymorphic phase changes.