PHYSICOCHEMICAL PROCESSES IN PLANETARY EVOLUTION

Data returned from exploration of the planets teach us that planets ar e highly individualistic despite the assumed underlying universality o f the processes. It is argued that the diversity of evolutionary outco mes arises mainly because of the interplay of thermal and compositiona l effects, both in the assembly of planets and in their subsequent con vective evolution. The role of compositional differences and phase tra nsitions invalidates any view of planets that relies heavily on models consisting of a small number of homogeneous layers. Four illustrative examples of real planetary behaviour are discussed: sold-solid phase transitions in terrestrial mantles, volcanic styles and recycling in V enus, heat flows of the giant planets and the role of the Lorentz forc e in the dynamics of the non-metallic portions of giant planets.