Planetary Satellites Pluto EKOs

The large planetary satellites, Pluto, and the large EKOs fall into two broad groups. The Moon, Io, and Europa are terrestrial bodies in that they are predominantly rocky in composition. Ganymede, Callisto, Titan, Triton, Pluto, and the large EKOs (and presumably the small ones too) differ from the terrestrial bodies in that a substantial proportion of the mass of each of them consists of icy materials, so they are classified as icy-rocky bodies.

5.2.1 The Moon

Next to the Earth, the Moon is the most extensively studied planetary body. This is because it is by far the closest to us. It has been visited by numerous spacecraft, and is the only other body on which humans have walked, during the Apollo 11, 12, 14-17 missions, between 1969 and 1972.

The Moon is sufficiently small that its uncompressed mean density will not be much less than its actual mean density of 3340kgm-3. Even so, this is considerably less than the uncompressed mean density of the Earth and even of Mars, strongly indicating that iron is comparatively scarce in the Moon. The value of C/MR2 has been accurately obtained from a combination of J2 and the Moon's response (as determined by laser ranging) to tidal torques exerted by the Sun and the Earth. The value, 0.394 (Table 4.2), is slightly but significantly smaller than that of a homogeneous sphere. Because of the slight central compression, this indicates that the Moon is not homogeneous, but has a small dense core. If the core is nearly pure iron then it will be of 300-400 km radius, but if it contains a high proportion of iron-rich compounds, such as FeS, then because these are less dense than iron, the core radius will be somewhat larger. Figure 5.5 shows a model of the lunar interior in which the core, of 470 km radius, is assumed to include a fairly high proportion of iron-rich compounds. A pure FeS core would have a radius around 530 km.

Further evidence for an iron-rich core includes the effect of the Moon on the Earth's magnetic field lines, which indicates the presence of a considerable mass of an electric conductor. There is also depletion of siderophile elements in the mare basalts - these would have been carried downwards during core formation. Radiometric dating of lunar minerals places this differentiation at about 25-30 Ma after the impact that formed the Moon.

A rough indication of present-day internal temperatures has been obtained from measurements of the electric currents induced in the Moon by the magnetic field in the solar wind. These electric currents can be measured through the magnetic fields that they themselves generate. For a given solar wind magnetic field, the higher the internal temperatures, the larger the currents. The temperatures seem to increase with depth, reaching 1000 K or so at 300 km, and 2000 K or

Silicate-rich crust

Partial melting

Silicate-rich crust

Partial melting

Silicate-

rich mantle

Iron-rich core

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