The interiors of the planets arc totally inaccessible to direct observations. The deepest bole drilled into the Earth goes to a depth of about l0 km, just a small fraction of the 6378km distance to the centre. Information about the interior must therefore come from indirect methods and in practice the most important method by far is the study of SEISMIC WAVES. These waves are generated by earthquakes or artificial explosions, travel through the body of the Earth, and can be picked up by sensitive recording instruments called seismometers. There are four types of waves, of which two, LOVE WAVES and RAYLEIGH WAVES , only travel near the surface of the Earth and so tell us nothing about the interior. P-WAVES and S-WAVES travel through the- body of the Earth and are distinguished by the direction of the small displacements which make up the waves. A P-wave is like a sound wave in air and the vibrations are along the direction of travel of the wave, whereas they are at right angles for S-waves. An important consequence of this difference is that S-waves can travel only in solids whereas P-waves can travel in any medium. S- and P-waves typically have velocities around 10kms-1 but this varies with depth.By observing the times at which the waves from a particular event arrive at various places around the Earth, geophysicists have been able to deduce much about the variation of density with depth in the Earth . There is an outer superficial layer called the CRUST whose thickness varies between 30 to 40km beneath the continents and about 10km under the oceans. The composition of the continental crust is below the crust there is an abrupt increase in density in the region called the MANTLE.

The boundary between the crust and the mantle is called the MOHOROVICIC DISCONTINUITY (or MOHO for short). The mantle transmits both S- and P-waves and so it must be a solid layer. Its density varies from 3300 kg m-3 at the Mono to 5500 kg m-3 at its base, which is at a depth of 2900km. Here we meet the WIECHERT DISCONTINUITY where the density increases suddenly to 10 000 kg m-3. Below this discontinuity we are in the CORE. S-waves do not propagate into the core so its outer layers at least must be liquid. P-waves can enter the core and in 1936 it was shown that they suffer an abrupt change in velocity at a depth now measured to be 4980km (i.e. 1390km from the centre) so that the core is divided into distinct inner and outer parts. The inner core has been shown to be solid with a probable density of 13000kgni-3. On average, the core is about twice as dense as the mantle, so that although it occupies only 16 per cent of the Earth’s volume it has about 32 per cent of the mass. Overall the mean density of the Earth is 5518 kg m-3, calculated directly from the mass and radius.

Further subdivisions of the Earth’s interior can be made on the basis of sudden changes in the velocities of seismic waves at various depths. The main such division is at a depth of about 700km to give an upper and lower mantle.

At four places on the Earth’s surface there are outcrops of rock which are thought to have come from the mantle during mountain building. Even if this is mantle material, it is quite possible that it has been changed in some unknown way by the very process that put it into its present place. Apart from these outcrops, none of the material of the Earth’s interior is accessible for chemical study. Geophysicists can make estimates of the composition at various depths by using the seismic data discussed above. Although the increase of density towards the centre can be accounted for as being due to the enormous pressure inside the Earth, there must be an abrupt change of composition at the core-mantle boundary. It is generally assumed that the core is mainly iron with some nickel and that the mantle is mainly composed of silicates such as olivine (Mg,Fe)2Si04. The core may be similar to iron meteorites and the mantle to stony meteorites, but because meteorites vary widely in composition and probably come from parent bodies much smaller than the Earth, it is difficult to make reliable deductions about the Earth’s interior from them. By analogy with iron meteorites, the proportion of nickel in the core is often assumed to be about six per cent. Whatever the composition of the core it must have a high density and be an electrical conductor, to account for the magnetic field. Alternatives to a mixture of iron and nickel have been suggested but none has been found satisfactory. The solid inner core has a higher density than the liquid outer core, but the difference appears to be too great to be just that between a solid and a liquid of the same chemical composition and there must be some variation hi composition.

The temperature at the centre of the Earth is about 4000 K and decreases steadily towards the surface. Since heat flows from high to low temperatures there is an outward flow; at the surface it is about 0.06 watt m-2 on average, and although this is much less than the heat received from the Sun. it is the heat from the interior that supplies most of the energy for volcanoes, earthquakes and mountain building. It is now generally believed that the Earth formed from solid particles with a temperature of f 500 K or less, so we must ask where the Earth’s heat has come from. The answer is from the radioactive decay of elements such as uranium and thorium. There is also an appreciable contribution from elements such as potassium, which are only weakly radioactive but are sufficiently common to be almost as important as uranium. Although there is some uncertainty about the exact amounts of radioactive elements inside the Earth, there is no question that they can provide the necessary energy to explain the present temperatures.

Radioactive decay is not the only source of heat. As the Earth grew, its gravitational attraction increased and with it the energy released by an infalling body; the gravity is sufficient in certain circumstances, such as meteors, to cause complete vaporization. The total energy released during the formation of the Earth was sufficient to heat it to a temperature of at least 20000K. This energy was released at or near the surface and it is likely that most of it was quickly radiated away, leaving radioactive decay as the main internal energy source.

Roughly speaking, the radioactive elements important to the Earth’s heating can be divided into two types: short-lived with half-lives of one to ten million years and long-lived with half-lives of a billion years or so. If there were sufficient quantities of the short-lived elements, then the Earth would have heated up quickly after it was formed, whereas the long-lived elements would have taken much of the Earth’s lifetime to heat it up. Since ail the short-lived elements have long since decayed, it is not known how important their contribution was. At some stage the central regions of the Earth must have melted and it was probably at this time that the separation into a core and a mantle took place. A similar process occurs when iron is melted in a steelworks and the non-metallic parts separate out from the iron to form a lower density slag on the surface.

Although the mantle is solid, it is not completely rigid. The outermost layer, the LITHOSPHERE. is between 70 and 100km thick and has considerable strength, but below this is a weaker layer called the ASTHENOSPHERE, which extends to a depth of a few hundred kilometers. Geophysicists believe that the material of this layer can flow and that there are convection currents which carry some of the outward flow of the Earth’s heat. Since the material is solid, its motion can be only very slow, but over a long period of time there can be significant movement. It is thought that these convection currents are responsible for movements of the Earth’s crust giving rise to continental drift (see plate tectonics below). The mantle below the asthenosphere, the MESOSPHERE, is thought to have appreciable strength, like the lithosphere; neither of these layers exhibits convection.

Among the many instruments carried to the Moon have been several seismometers which have returned much important information. Their observations of lunar seismic waves, including some induced artificially by deliberately crashing spacecraft onto the surface, have shown that the Moon has a crust, mantle and dense core. The core has a radius of about 500km and the mantle is 1200km thick. The core therefore occupies only two per cent of the Moon’s volume compared to 16 per cent for the Earth and there¬fore the mantle is relatively more important on the Moon than on the Earth and the mean density is lower. Similarly the crust is relatively much thicker on the Moon. The seismic observations allow a rigid lithosphere to be distinguished from an asthenosphere and show that it extends to a depth of 1000km. The thin lithosphere on the Earth is fractured into plates which can move about and is thin enough to allow lava to be transported from the asthenosphere to the surface. The enormous armoured layer of the Moon’s lithosphere just cannot be fractured in this way and so, apart from the small core, the Moon is a dead body. The Moon could have been heated up by radioactive decay as was the Earth-and acquired a dense core in the process, but because of its smaller size; it would have cooled down more quickly to give the thicker lithosphere and mantle that we observe today.

Although the results are by no means certain, both theory and observation indicate that Mercury, Venus and Mars are also differentiated into a mantle and a dense core. Because of their different distances from the Sun, the inner planets are likely to have formed with different proportions of the elements. According to theoretical studies. Mercury and Venus should have massive iron-nickel cores whereas the core of Mars should also contain sulphur so that all, or nearly all the iron is present as ferrous sulphide (FeS) and there is little free iron. Ferrous sulphide has a lower density than iron, and a core of this material would explain why the mean density of Mars is lower than that of Mercury, Venus or the Earth. Mercury’s iron core appears to contain about 80 per cent of the planet’s mass and to have a radius of 1800 km. The core of Venus has a radius of 3100 km, so that as far as we can tell the interior of Venus is very much like the Earth

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