Summary of Chapter

In order to develop our knowledge about the interior of a planetary body, a model has to be constructed and adjusted until its predictions of external observations match the actual observations as closely as possible.

A model has as its basic features a specification of the composition, temperature, pressure, and density, at all points within the interior. For planetary bodies, variations versus radius from the centre are dominant.

Details of the gravitational field of the body, plus other data such as mass, radius, and rotation period, constrain the density at all points in the interior.

If the body has a large magnetic dipole moment then this is thought to be due to electrically conducting liquid layers that are convecting. If this explanation is correct, then the location of these layers can be deduced.

Seismic waves reveal the presence of liquid layers, the presence of regions that are plastic, and changes in composition. Seismic waves also constrain the composition, particularly if combined with other data.

The composition is also constrained by the composition and properties of accessible materials, and by the relative abundances of the elements in the Solar System.

The temperatures in the interior of a body at any one time depend on the rates of energy gains and the rates of energy losses at all earlier times. The interior temperatures are a factor in determining whether differentiation has occurred, and they also determine the dynamics of the interior and of the surface.

Energy sources no longer active include accretional energy and heat from short-lived radioactive isotopes. Differentiation can be a primordial source of energy, or it can still be an active source. Other active sources include heat from long-lived radioactive isotopes, tidal energy, and solar radiation.

Energy is ultimately lost in the form of infrared radiation emitted to space by the surface or atmosphere of the body. To reach these outer regions energy is conveyed by radiation, conduction, convection (in liquids or solids), and advection. Radiative transfer is negligible (except perhaps in certain zones in Jupiter and Saturn), and the proportions of the overall flow carried by the other three processes vary considerably from body to body.

Some insight into the thermal state of the interior of a body is provided by energy flow measurements, by gravitational and seismic data, by its history of geological activity, and by the magnetic field now and in the past.

There are three broad features of the thermal history of a planetary body:

(1) Except for very early on in Solar System history, interior temperatures have either been roughly constant, or been declining.

(2) The temperatures today increase with depth.

(3) If all other things are equal, the smaller the planetary body the faster it loses internal energy per unit mass.

Table 4.2 summarises some of the observational constraints relevant to interior models of various planetary bodies. Other relevant data are given in Tables 1.5, 4.3-4.5, and in Figures 4.6 and 4.9.

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