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Exoplanet atmosphere characterisation has become an important tool inunderstanding exoplanet formation, evolution. However, clouds remain a keychallenge for characterisation: upcoming space telescopes (e.g. JWST, ARIEL)and ground-based high-resolution spectrographs (e.g. CRIRES+) will produce datarequiring detailed understanding of cloud formation and cloud effects. We aimto understand how the micro-porosity of cloud particles affects the cloudstructure, particle size, and material composition. We examine thespectroscopic effects of micro-porous particles, the particle sizedistribution, and non-spherical cloud particles. We expanded our kineticnon-equilibrium cloud formation model and use a grid of prescribed 1D(T_gas-p_gas) DRIFT-PHOENIX profiles. We applied the effective medium theoryand the Mie theory to model the spectroscopic properties of cloud particleswith micro-porosity and a derived particle size distribution. We used astatistical distribution of hollow spheres to represent the effects ofnon-spherical cloud particles. Highly micro-porous cloud particles (90% vacuum)have a larger surface area, enabling efficient bulk growth higher in theatmosphere than for compact particles. Increases in single-scattering albedoand cross-sectional area for these mineral snowflakes cause the cloud deck tobecome optically thin only at a wavelength of ~100 ${\rm \mu m}$ instead of atthe ~20 ${\rm \mu m}$ for compact cloud particles. A significant enhancement inalbedo is also seen when cloud particles occur with a locally changing Gaussiansize distribution. Non-spherical particles increase the opacity of silicatespectral features, which further increases the wavelength at which the cloudsbecome optically thin. JWST MIRI will be sensitive to signatures ofmicro-porous and non-spherical cloud particles based on the wavelength at whichclouds are optically thin.

Mineral snowflakes on exoplanets and brown dwarfs: Effects of micro-porosity, size distributions, and particle shape