In the previous two chapters, we discussed the atmospheres and surface geology of planets. Both of these regions of a planet can be observed directly from Earth and/or space. But what can we say about the deep interior of a planet? We are unable to observe the inside of a planet directly. For the Earth and the Moon we have seismic data, revealing the propagation of waves deep below the surface and thereby providing information on the interior structure (§6.2). The interior structure of all other bodies is deduced through a comparison of remote observations with observable characteristics predicted by interior models. The relevant observations are the body's mass, size (and thus density), its rotational period and geometric oblateness, gravity field, characteristics of its magnetic field (or absence thereof), the total energy output, and the composition of its atmosphere and/or surface. Cosmochemical arguments provide additional constraints on a body's composition, while laboratory data on the behavior of materials under high temperature and pressure are invaluable for interior models. Quantum mechanical calculations are used to deduce the behavior of elements (especially hydrogen) at pressures inaccessible in the laboratory.

In this chapter we discuss the basics of how one can infer the interior structure of a body from the observed quantities.