Mineralogy of the planets: a voyage in space and time

Abstract

The mineralogy of the planets must be considered in the context of chemical and physical differentiation of the solar nebula. Astronomical observations reveal warm dust clouds around young stars and evidence for violent processes in stellar nebulae including rapid expansion, strong rotation, high magnetic field, very powerful solar wind and big mass loss. Theoretical analyses are extremely difficult and the results depend greatly on the initial assumptions. An orthodox interpretation using a hot, massive nebula involves: local instability of several solar masses in galactic cloud; contraction to give Sun; temperature rising to over 2000 K at centre to a few degrees at 100 AU; progressive condensation to give dust; aggregation by several mechanisms including magnetic attraction; clearing and rapid cooling by radiation loss; acceleration to mid-plane of nebula and gravitational instability; multiple collisions producing streamline rotation; growth of planetesimals and ultimately planets; early melting; stunting of Mercury and Mars because of increased velocity of planetesimals near the Sun and Jupiter; inhibition of planet in asteroid region; formation and destruction of moons; intense early bombardment of proto-crust by decaying population of planetesimals; lucky preservation of residual debris as asteroids; ‘clean-up’ of inner region and removal of atmospheres of terrestrial planets by intense solar wind. Many features should be applicable to models based on a cooler, less massive nebula.