Silicate Sundogs: Probing the Effects of Grain Directionality in Exoplanet Observations

Abstract

Crystalline ice in Earth’s atmosphere can produce spectacular phenomena due to orientation-dependent attenuation, such as sundogs and halos, providing diagnostics of the external processes acting on the aerosol grains. Crystalline mineral aerosols, such as quartz (SiO2) and enstatite/forsterite (MgSiO3/Mg2SiO4), have long been predicted to form in hot Jupiter atmospheres, with JWST’s Mid-Infrared Instrument Low Resolution Spectrometer (MIRI LRS) verifying the existence of crystalline quartz observationally. Due to the strong horizontal winds (∼1–5 km s−1) and small aerosol grains (<1 μm) found in hot Jupiter atmospheres, we show that aerosols could be mechanically aligned with the winds. We then derive direction-dependent optical properties of quartz, enstatite, and forsterite and model transmission and emission spectra assuming random and mechanically aligned orientations, finding that the orientation of all three crystalline aerosols can impart ≥100 ppm differences in observed spectra (8–12 μm). We run retrievals on JWST/MIRI LRS transmission and emission data of WASP-17b, and find that directionality alone cannot physically explain the transmission data, pointing towards polymorphs or insufficient laboratory data, and find weak hints of directionality (1.0–1.3σ) in the emission data. This work demonstrates the power of JWST/MIRI LRS in detecting aerosol directionality with future observations, and a technique by which to probe how aerosols interact with atmospheric dynamical processes. To foster the exploration of aerosols in exoplanet data, the open-source code POSEIDON has been updated (v1.3.1) to include 144 new direction- and temperature-dependent aerosols with precomputed optical properties, alongside new aerosol models.