The consequences of a potential rainout for a planetary atmosphere can be manyfold. First of all the rainout removes metals from the atmosphere, relocating them to deeper layers. Hence the corresponding grain or droplet opacity will be missing from higher atmospheric layers. Because we do not include cloud opacities in our calculations we make the implicit assumption of a rainout of the condensed particles, although we do not model it, the net effect being the removal of metals from the higher layers. It has to be kept in mind, however, that the chemical equilibrium solution of the gas abundances in chemical contact with the condensed species is not necessarily the same as it would be when assuming a rainout. Our implicit assumption of a rainout is also applicable when considering the gaseous Na and K alkali abundances. In our models MgSiO3 condenses at temperatures below ∼1600 K. In principle this silicate material can further react with the alkali atoms to form alkali feldspars (such as albite and orthoclase), removing the gaseous alkalis from the gas for T ≲  1600 K (see, e.g., Lodders 2010). We do not consider these feldspars in our condensation model, such that the alkali atoms stay in the gas, as they would in a silicate rainout scenario. It has been found that alkali atoms are present in cool brown dwarf atmospheres, indicating that silicate rainout may occur in these objects (Marley et al. 2002; Morley et al. 2012). Another consequence of condensed material can be the formation of a cloud deck, close to and above the layers of the atmosphere hot enough the evaporate the in-falling cloud particles again. Such cloud decks can heat the atmosphere locally and in the layers below, by making the atmosphere more opaque to the planet’s intrinsic flux, effectively acting like a blanket covering the lower layers of the atmosphere (see, e.g., Helling & Casewell 2014; Morley et al. 2014). If the cloud layer is optically thick close to the planet’s photosphere it will leave an imprint on the planet’s spectral appearance and and may reduce the contrast of absorption features. The height of the cloud deck depends critically on the planets effective temperature and also on its surface gravity since the condensation temperature is pressure-dependent. The cooler an object is, the deeper in its interior the clouds will reside. Therefore the spectral imprint of clouds will vary with temperature, similar to the behavior in brown dwarf atmospheres. Silicate clouds with a high optical depth are thought to reside in the photospheres L4–L6 type brown dwarfs (





 ∼ 1500–1700 K) where they affect the spectra. For cooler objects the cloud deck lies below the photosphere and the clouds are no longer seen (see, e.g., Lodders & Fegley 2006). In our atmospheres we checked the possible locations of the cloud decks (i.e., the layers below which the condensates evaporate). We found that the silicate evaporation layer of planets with 





 = 1000 K  and 





 K is always located at pressures far higher than that of the photosphere, such that we do not expect any spectral impact of a cloud layer. For effective temperatures between 1500 and 1750 K the evaporation layer lies close to and above the photosphere (in altitude), such that a cloud deck could potentially affect the spectrum. For increasing log(g) the photosphere shifts to layers of deeper pressure, but so does the evaporation layer, as condensation is pressure dependent. Note that this temperature range is close to the effective temperature where L4–L6 dwarfs are thought to be most strongly affected by silicate clouds. For higher temperatures the evaporation layer is far above the photosphere such that we do not expect clouds to be of importance.
Citation: Morley et al. 2012
Citation function: Motivation