Gas and Ice Giant Interiors

Abstract

The interiors of the gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune) are discussed. Emphasis is on the basic physical principles rather than the details of specific models. This article covers the nature and properties of the materials that comprise these planets and the basic principles of their behavior at high temperature and pressure, leading to the predominance of metallic hydrogen in the interiors of Jupiter and Saturn but no first order phase transition between molecular and metallic states. The consequences of hydrostatic equilibrium are assessed, and the resulting dependence of radius on mass and dependence of internal temperature on pressure are described. Thermal evolution of planets is described through a complete explanation of the consequences of the virial theorem, and from this, cooling times for these planets are estimated, leading to the implication that Jupiter is primarily cooling throughout whereas Saturn depends in part on an additional energy source (helium rainout) while Uranus and Neptune are storing part of their energy of formation. The method for using gravity data to constrain models, in particular moments of inertia, is outlined but not developed in detail. A summary is provided of current interior modeling and how it relates to the observed magnetic fields. The general discussion also allows an appreciation of the nature of the many giant exoplanets being found, but these bodies are not explicitly described.