Protoplanetary Core Formation by Rain-Out of Minerals

Models of giant gaseous protoplanets calculated by DeCampli and Cameron (1979) indicate that iron and probably other minerals in the interior of a planet would be in the liquid state during part of the protoplanet evolution. Liquid drops in a protoplanet would grow by coalescence much as cloud drops in the Earth's atmosphere grow to rain drops. We have modeled this process by using the ‘stochastic collection’ equation (Slattery, 1978) for various initial conditions. In all of the cases considered, the growth time (to centimeter-sized droplets) is much shorter than the time, as estimated by detailed evolutionary calculations, that the drops are in the liquid state. Brownian collection is effective in quickly coalescing tiny liquid droplets to an average radius of about 0.005 cm with very few drops remaining with radii less than 0.001 cm. For radii larger than 0.005 cm gravitational collection is dominant. Since the particles are rapidly swept from interstellar grain sizes to much larger sizes, the opacity in the cloud layer is expected to drop sharply following melting of the grains.