The internal evolution of planetary-sized objects

The importance of ‘creep’ in controlling the internal thermal state of large objects with physical properties corresponding to a roughly homogeneous meteoritic composition is reviewed. Some results of this study are used to justify a picture of evolution as a quasistatic process. An attempt is made to show that the viscous dissipation of the motions that occur in the lifetime of such bodies formed about 4.5 × 10⁹ yr ago gives them an innate capacity to chemically differentiate if their external radius exceeds a few hundred kilometres. The capacity to differentiate increases rapidly with external radius and for objects of lunar size and greater, the process is not yet complete. During the ‘active’ stage of evolution the convective cores of these objects tend to grow smaller and hotter with time, giving a secular change in the composition of the differentiating phases. It is suggested that by a curious coincidence of dehydration curves and the horizontally averaged temperature distribution, water of dehydration can still be present at depth in the planets and is the cause of the observed seismic attenuation at the Moon's centre and in the Earth's upper mantle. It is also noted that if water is present at tenths of a percent level the temperature within objects with radii < 3000 km is kept below the Curie point of pure iron for long periods - a situation that could have significant bearing on the present magnetisation of the planetary objects in this size range.