Introduction

The models of the planets which have been adopted so far depend explicitly on the assumption that the planet is in the hydrostatic state, so that the density is a function only of radial distance (in a generalized sense when the planet is flattened by spin). That may be an appropriate first assumption to provide a starting point for further developments, but it is clearly not adequate: the gravity fields of the Earth, the Moon and Mars contain harmonic components that would be absent if the internal state were hydrostatic; the irregular surface features of the terrestrial planets are inconsistent with strict hydrostatic equilibrium; and the structure seen in the atmosphere of Jupiter reveals internal motions, if only superficial. A density distribution not in hydrostatic equilibrium requires a stress system to support it that departs from the simple normal pressure to which hydrostatic equilibrium corresponds. Such a stress system may be developed in two ways: statically, through strains of the planet, or dynamically, through movements of the material. According to which mode is effective, so the planet may be considered to be cold or hot (though, as has already been argued, no planet is hot in relation to the effect on the equation of state). If the planet is cold, the materials within it have high strengths, can support large stresses and so maintain statically non-hydrostatic distributions of density. If the planet is hot, then parts will be molten, as is the core of the Earth, or will be sufficiently hot to creep steadily under applied stress.