Neglected Silicon Dioxide Polymorphs As Clouds in Substellar Atmospheres

Direct mid-infrared signatures of silicate clouds in substellar atmospheres were first detected in Spitzer observations of brown dwarfs, although their existence was previously inferred from near-infrared spectra. With the Mid-Infrared Instrument (MIRI) instrument on JWST, we can now more deeply probe silicate features from 8 to 10 microns, exploring specific particle composition, size, and structure. Recent characterization efforts have led to the identification in particular of silica (silicon dioxide, SiO_2) cloud features in brown dwarfs and giant exoplanets. Previous modeling, motivated by chemical equilibrium considerations, has primarily focused on magnesium silicates (forsterite, enstatite), crystalline quartz, and amorphous silica to match observations. Here, we explore the previously neglected possibility that other crystalline structures of silica, i.e. polymorphs, may be more likely to form at the pressure and temperature conditions of substellar upper atmospheres. We show how these polymorphs may be distinguished from each other with current JWST observations. We also explore how such particles could form and be dynamically lofted and sedimented throughout the atmosphere, and implications for the underlying chemical and dynamical processes governing these objects. We ultimately propose that accounting for the distinct opacities arising from the possible crystalline structure of cloud materials may act as a powerful, observable diagnostic tracer of atmospheric conditions, where particle crystallinity records the history of the atmospheric regions through which clouds formed and evolved. Finally, we highlight that high fidelity, accurate laboratory measurements of silica polymorphs are critically needed to draw meaningful conclusions about the identities and structures of clouds in substellar atmospheres.