Introduction

Mars, Venus and Mercury form with the Earth and the Moon a group of rather similar bodies. By comparison with the giant planets on the one hand and the small satellites on the other, the sizes lie in a relatively restricted range, while the mean densities are higher than those of most other bodies in the solar system. It is natural to think that their compositions are similar and that the structures of Mars, Venus and Mercury might be inferred from what is known of the Earth and the Moon.

Seismological data are, of course, not available for any of the planets other than the Earth, so that the structures of the terrestrial planets must be derived from the dynamical data, together with such inferences as may be drawn from the magnetic and electrical properties, together with analogies with the Earth and the Moon.

Unfortunately, the dynamical data themselves are less informative for Mars, Venus and Mercury than they are for the Earth and the Moon or for the major planets. The solar precession of Mars has not so far been observed and, in consequence, the moment of inertia cannot be derived from the value of J2 without making the assumption of hydrostatic equilibrium. Yet it is clear that Mars is not in hydrostatic equilibrium. The theory of the errors likely to be committed by making the assumption of hydrostatic equilibrium was given in Chapter 3 and subsequently in this chapter (section 6.6) it will be applied to Mars.