Vertical mixing of mineral clouds in hot Jupiter atmospheres

Recent JWST transmission and emission spectroscopic observation of hot Jupiters have demonstrated that sub-micron sized silicate mineral clouds may be common hot Jupiter atmospheres. In addition, ground-based high-resolution spectroscopic observations of both transit and eclipse have implied horizontal asymmetries that may be amplified by clouds. Silicate mineral clouds have long been predicted to form and persist on the nightside and western dayside of hot Jupiters by cloud microphysical models and 3D General Circulation Models. Given the capability of current ground-based high resolution spectrographs, complementary JWST observations, as well as recent advancements in modelling techniques, the time is right to determine the prevalence and spatial and particle size distribution of mineral clouds across the parameter regime of hot Jupiters. This effort will provide a detailed test of our present theoretical understanding of cloud nucleation, transport and growth processes, and the radiative feedback of clouds on the atmospheric circulation and climate of hot Jupiters. In this work, we present a combination of analytical and numerical models of the vertical mixing of mineral clouds in hot gas giant atmospheres. Our scaling analyses naively predict that clouds of a broad range of particle sizes (up to 10 microns) should be well-mixed throughout gas giant atmospheres to low pressures (~0.1 mbars). However, the local nature of vertical mixing in more detailed three-dimensional simulations prevents such well-mixed micron-sized mineral clouds in many cases. In addition, we hypothesise that fragmentation may play a role in setting the characteristic maximum particle size of silicate mineral clouds in hot Jupiter atmospheres, much like it impacts the build-up of silicate grains in protoplanetary disks. We predict how mixing scales with cloud particle size and composition using analytic theory and three-dimensional numerical simulations including cloud tracers. We discuss implications for ground-based high resolution characterisation of hot Jupiters, especially with time-resolved spectroscopy.