Composition and Properties of Accessible Materials

4.4.1 Surface Materials

Even though the only planetary materials available for direct compositional analysis are from the surface or near surface, the composition of such materials can give us a useful indication of the composition, temperature, and density deeper down. For example, the Earth's surface consists of rocks with a density of about 2700 kg m-3 on the continents and about 2900 kg m-3 in the ocean basins. (In many regions these rocks are covered by a very thin veneer of soil or sediment, but that is of no relevance here.) By contrast, the Earth has a mean density of 5520 kg m-3, and it is not possible for the internal pressures to be high enough to squeeze the surface rocks to such a high mean value. Therefore, the interior must consist largely of intrinsically denser materials.

As well as densities, we can also constrain the range of minerals that could comprise the interior. This is done by choosing a mineral mix such that when it is subjected to various geological processes that produce surface rocks, it would produce those seen. We shall look at this more closely in Chapters 5-8.

For the giant planets there is no solid surface, and they are all covered in atmospheres that blend into deep outer envelopes. Therefore, determination of the composition of these atmospheres is of particular importance in building models of their interiors.

4.4.2 Elements, Compounds, Affinities

A powerful constraint on composition is provided by observations, not of the body in question, but of other bodies, notably the Sun and meteorites. The relative abundances of the elements in the Solar System are obtained mainly from the composition of the Sun outside its core (Section 1.3) and from the composition of the least altered meteorites, the C1 chondrites (Section 3.3.2). Table 1.5 shows the relative abundances of the 15 most abundant chemical elements. It is to these elements, and to compounds containing them, that we must look to account for most of the mass of a planetary body. In doing so, we must take account of the density of the substance. Table 4.3 lists the densities of some important substances, and distinguishes the more dense from the less dense.

Table 4.3 Densities of some important substances

Substance

Chemical formula

Density"/kgm 3

Iron-nickel

Fe plus ~ 6% Ni by mass

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