Microphysics of KCl and ZnS Clouds on GJ 1214b

TL;DR

This study models microphysical processes of KCl and ZnS clouds on GJ 1214b to better understand their impact on spectral features, emphasizing the importance of detailed cloud physics for exoplanet characterization.

Contribution

It introduces a microphysical cloud model for GJ 1214b, providing detailed predictions of cloud properties based on atmospheric conditions, improving upon simplified models.

Findings

KCl cloud mass and opacity increase with mixing and metallicity.

ZnS clouds require condensation nuclei and are affected by heterogeneous nucleation.

High metallicity and mixing are needed to explain GJ 1214b's flat spectrum.

Abstract

Clouds in the atmospheres of exoplanets confound characterization efforts by reducing, eliminating, and distorting spectral signatures of molecular abundances. As such, interpretations of exoplanet spectra strongly depend on the choice of cloud model, many of which are highly simplified and lack predictive power. In this work, we use a cloud model that treat microphysical processes to simulate potassium chloride (KCl) and zinc sulfide (ZnS) clouds in the atmosphere of the super Earth GJ 1214b and how they vary as a function of the strength of vertical mixing and the atmospheric metallicity. Microphysical processes control the size and spatial distribution of cloud particles, allowing for the computation of more physical cloud distributions than simpler models. We find that the mass and opacity of KCl clouds increase with mixing strength and metallicity, with the particle size and spatial distribution defined by nucleation, condensation, evaporation, and transport timescales. ZnS clouds cannot form without the presence of condensation nuclei, while heterogeneous nucleation of ZnS on KCl reduces particle sizes compared to pure KCl cases. In order to explain the flat transmission spectrum of GJ 1214b with homogeneously nucleated KCl clouds, the atmospheric metallicity must be at least 1000 $\times$ solar, and the eddy diffusivity must be at least 10$^{10}$ cm$^2$ s$^{-1}$. We predict that JWST observations of GJ 1214b may reveal the presence of methane, carbon monoxide, and water, allowing for constraints to be placed on atmospheric metallicity and C/O ratio.