Glittery clouds in exoplanetary atmospheres?

Abstract

AbstractCloud formation modelling has entered astrophysics as a new field of research for planetary and brown dwarf atmospheres. Clouds are a chemically and physically very active component of an atmosphere since they determine the remaining gas phase and change the object's albedo depending on their material composition. The grains can also provide a surface where the molecular constituents for life can be physisorbed for possible pre-biotic evolution. This paper summarizes our model for the kinetic formation of dirty dust grains which make up the atmospheric clouds of extraterrestrial giant gas planets. We include seed formation, surface growth and evaporation, the gravitational settling that influences the dust formation, element depletion that determines the remaining gas phase abundances, and convective overshooting that is needed for a dust model to be applicable to hydrostatic atmosphere simulations. We demonstrate the evolution of the material composition of the cloud itself and the distribution of the grain sizes in the cloud layer, exemplary for a giant gas planet parameter combinations (Teff, log g). In general, substellar clouds are composed of small, dirty grains with a high silicate content at the cloud deck. They grow in size and gradually purify to iron/corundum grains when they move into denser and hotter atmospheric regions. Comparing these results with experimental data from condensation experiments leads to the conclusion that cloud grains that gravitationally settle in the atmosphere of a giant planet can easily change their lattice structure from the disordered amorphous state they are forming in, into the ordered lattice of a crystal.