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Paper: The Chemical Evolution of Dust and Gas in Substellar Atmospheres
Volume: 414, Cosmic Dust—Near and Far
Page: 125
Authors: Helling, C.
Abstract: The formation of clouds, an ensemble of dust grains composed of a silicate/oxide mixture, is triggered by the formation of possibly homogeneous seeds followed by the heterogeneous growth of a grain mantle in substellar atmospheres. The mantle compounds can grow simultaneously by a number of chemical surface reactions. These compounds act as element sinks, hence, they determine the remaining gas phase composition in the atmosphere until complete evaporation. Also gravitational settling (drift) and convective mixing do alter the cloud evolution. Modelling these processes by momentum equations allows to study the evolution of the material composition of the cloud itself but also the chemical composition of the remaining gas phase. Generally, substellar clouds are composed of dirty grains which are small and have a high silicate content at the cloud deck, and which grow in size and gradually purify to iron/corundum grains when they move into denser and hotter atmospheric regions. Interestingly and counter-intuitively, the maximum gas-phase depletion occurs at higher atmospheric latitudes than the maximum dust-to-gas ratio, a region where the atmospheric gas remains supersaturated. A comparison study that included phase-equilibrium and phase-non-equilibrium approaches shows the impact of the cloud modelling on the remaining gas phase.
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