The interior structures of the Earth and Moon are determined from seismic data. The existence and sizes of cores in other planets are inferred from observations of planetary sizes, masses, and shapes, which constrain their uncompressed mean densities and moment of inertia factors. Mantle and crust thicknesses can also be estimated from gravity data obtained by orbiting spacecraft. Successful models of planetary interiors constructed from compositional data must be consistent with observed densities and moments of inertia. High-pressure laboratory experiments can constrain the mineralogy of mantles and cores and the partitioning of elements between silicate and metal in the terrestrial planets. The interiors of the giant planets are not well understood, because of uncertainties in their compositions and internal temperatures and pressures. The states of hydrogen and helium in the interiors of Jupiter and Saturn, and the crystalline forms of ices in Uranus, Neptune, and icy satellites, are inferred from experimentally determined or calculated phase diagrams. The giant planets may have small rocky cores, with successive layers of either metallic hydrogen (Jupiter and Saturn) or ices (Uranus and Neptune), and molecular hydrogen. Planetary mantles and cores evolve over geologic time, through cooling and extraction (or reintroduction, in the case of Earth) of crustal components.

We present a brief overview of the planets, moons, dwarf planets, asteroids, and comets – intended as a primer for those with limited or no familiarity with planetary science. The terrestrial planets (Earth, Mars, Venus, and Mercury) are rocky bodies having mean densities that indicate metal cores; the giant planets are composed mostly of hydrogen and helium and can be divided into gas giants (Jupiter and Saturn) and ice giants (Uranus and Neptune), based on their physical states. Small bodies, composed of rock and ices, are either differentiated or not, depending on their thermal histories. Each section of this chapter is generally organized in the historical order in which the objects have been explored by spacecraft. We will return to these bodies repeatedly in the book, focusing on understanding their geologic characteristics and materials, and the processes that produced them.

Physical weathering of rocks on bodies other than the Earth occurs mostly through impact fragmentation, producing regoliths. The lunar regolith is finer-grained and contains more agglutinates than asteroidal regoliths, indicating its greater maturity. Mars exhibits both physical and chemical weathering, and its sedimentary deposits superficially resemble those on Earth. However, its basalt-derived sediments differ from those formed from felsic protoliths on Earth, and evaporation of its aqueous fluids is dominated by sulfates, distinct from terrestrial evaporites that are mostly carbonates and halides. On the surfaces of airless bodies, recondensation of vapor produced by micrometeorite impacts accounts for spectral changes, known as space weathering. In the interiors of carbonaceous chondrite asteroids, isochemical reactions of rocks with cold aqueous fluids produced by melting of ice have altered their mineralogy. Thermal metamorphism of dry chondritic asteroids has modified all but near-surface rocks. Hydrothermal metamorphism on Mars, likely associated with large impacts, has produced low-grade mineral assemblages in metabasalts and serpentinites. Conditions at Venus’ surface are severe enough to cause thermal metamorphism, and reactions with rocks may control the composition of the atmosphere. Because all bodies have gravity, some sloping topography, and some unconsolidated materials, mass wasting is among the most common processes modifying planetary surfaces.

In the previous chapter, some background was provided about types of reinforcement and their distribution within different matrices. Attention is now turned to predicting the behaviour of the resulting composites. The prime concern is with mechanical properties. The reinforcement is usually designed to enhance the stiffness and strength of the matrix. The details of this enhancement can be rather complex. The simplest starting point is the elastic behaviour of a composite with aligned long (continuous) fibres. This arrangement creates high stiffness (and strength) in the fibre direction. However, it is also important to understand the behaviour when loaded in other directions, so the treatment also covers transverse loading. In this chapter, and in the following one, perfect bonding is assumed at the fibre/matrix interface. Details concerning this region, and consequences of imperfect bonding, are considered in Chapter 6.

Composites are essentially materials comprising two or more distinct constituents, integrated into a single entity. An important aspect of composite theory concerns the properties that the material exhibits, expressed in terms of those of the constituents and the architecture of the integration. A case of interest is that of a two-constituent system in which one of them is just a void – possibly a vacuum, although more commonly a gas phase. Of course, voids have properties that are substantially different from those of constituents in conventional composites. For example, the stiffness will be effectively zero and the conductivity will tend to be very low. In practice, many materials contain at least some porosity, with the potential to affect certain properties, but in most cases it would not be considered appropriate to classify them as composites. However, very high porosity levels (say, >~30–40%) can justify treatment as a separate type of (composite) material. Sometimes the term ‘foam’ is used in such cases, although the word does carry connotations that would not necessarily apply to all highly porous materials. In this chapter, some composite theory approaches are applied to such materials and information is provided about their ‘microstructure’ (pore architecture), production and potential benefits.