- © The Mineralogical Society Of America
Central to the goals of mineral physics is an elucidation of how material behavior governs planetary processes. Planetary accretion, differentiation into crust, mantle, and core, ongoing processing by magmatism, dynamics and thermal evolution, and the generation of magnetic fields, are all processes controlled by the physical properties and phase equilibria of planetary materials. Knowledge of these processes has advanced in large measure by progress in the study of material behavior at extreme conditions of pressure and temperature.
Thinking of a planet as an experimental sample emphasizes the relationship between planetary processes and material behavior via properties that control its response to natural perturbations. We may explore the response of the planet to a sudden increase in energy, provided, for example, by a giant impact of the type that is thought to have formed Earth’s moon (Canup 2004). Responses include an increase in temperature, controlled by the heat capacity, a decrease in density controlled by the thermal expansivity, and phase transformations controlled by the free energy. Phase transformations also contribute to changes in temperature and density via the heats and volumes of transformation. A giant impact also perturbs the stress state, to which the planet responds via compression (controlled by the bulk modulus), adiabatic heating (Grüneisen parameter) and phase transformations (free energy).
A planet is an unusual experimental sample because its pressure is self-generated via gravitational self-compression, and its temperature via adiabatic compression and radioactive decay. Indeed one of the major challenges in understanding planetary-scale processes is that the characteristic pressure (1 Mbar) and temperature (several thousand K) are so large. Consider an Earth-like planet in which the mantle makes up 2/3 of the mass and the core makes up the remainder. The pressure PM at the base of the mantle depends approximately linearly on planetary mass M: PM …