Complex cloud models in retrieval

Clouds are ubiquitous in solar system planets, and based on theory,
expected to be present in most temperature regimes in exoplanet
atmospheres. Regardless of their chemical composition; water, methane,
sulfur in solar system planets, or iron, magnesium, silicate clouds in
hot exoplanetary atmospheres; clouds are quite difficult to
model. Until recently, inverse-retrieval approaches were only
able to model gray clouds, i.e. an opaque cloud deck at a certain
altitude or exponentially increasing opacity from the cloud top. These
are good assumptions given limited wavelength coverage and low
resolution of current telescopes. However, with the advent of wider
wavelength range and higher resolution instruments like JWST, TESS,
PLATO etc, the exoplanetary field has an express need for more
advanced cloud models. Here, we present two complex cloud models, a
thermal stability cloud model and a kinetic cloud model, both capable
of determining the cloud composition, particle size distribution,
cloud particle number density and the cloud extent. The thermal
stability model uses a chemical equilibrium assumption, where the
cloud bottom forms at the point where the vapor pressure curve of the
condensate crosses the planetary temperature profile. Clouds nuclei
are assumed to already exist in the gas. On the other hand, the
kinetic cloud model follows the formation of the cloud nuclei from the
top of the atmosphere and produces a mixture of cloud species as it
settles, self-consistently changing the chemical structure of the
atmosphere.

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