Formation of Silicate and Titanium Clouds on Hot Jupiters

TL;DR

This study models the size distribution and properties of silicate and titanium clouds on hot Jupiters, revealing their dependence on atmospheric conditions and implications for observed spectra.

Contribution

First application of a microphysical and vertical transport model to predict detailed cloud particle size distributions on hot Jupiters.

Findings

Cloud size distributions are often bimodal and irregular.

Total cloud mass correlates negatively with temperature and positively with vertical mixing.

Cloud opacities are relatively constant across broad wavelengths, with significant forward scattering.

Abstract

We present the first application of a bin-scheme microphysical and vertical transport model to determine the size distribution of titanium and silicate cloud particles in the atmospheres of hot Jupiters. We predict particle size distributions from first principles for a grid of planets at four representative equatorial longitudes, and investigate how observed cloud properties depend on the atmospheric thermal structure and vertical mixing. The predicted size distributions are frequently bimodal and irregular in shape. There is a negative correlation between total cloud mass and equilibrium temperature as well as a positive correlation between total cloud mass and atmospheric mixing. The cloud properties on the east and west limbs show distinct differences that increase with increasing equilibrium temperature. Cloud opacities are roughly constant across a broad wavelength range with the exception of features in the mid-infrared. Forward scattering is found to be important across the same wavelength range. Using the fully resolved size distribution of cloud particles as opposed to a mean particle size has a distinct impact on the resultant cloud opacities. The particle size that contributes the most to the cloud opacity depends strongly on the cloud particle size distribution. We predict that it is unlikely that silicate or titanium clouds are responsible for the optical Rayleigh scattering slope seen in many hot Jupiters. We suggest that cloud opacities in emission may serve as sensitive tracers of the thermal state of a planet's deep interior through the existence or lack of a cold trap in the deep atmosphere.