3D simulations of photochemical hazes in the atmosphere of hot Jupiter HD 189733b

TL;DR

This study uses 3D atmospheric simulations to explore how photochemical hazes of different sizes are transported and distributed in the atmosphere of hot Jupiter HD 189733b, affecting its observed spectra.

Contribution

First 3D simulations of haze transport in a hot Jupiter, analyzing size-dependent distribution and spectral effects, with insights into observed spectral discrepancies.

Findings

Small hazes accumulate in mid-latitude vortices and are more concentrated at the morning terminator.

Large hazes settle quickly on the nightside, leading to more hazes at the evening terminator.

Model spectra do not fully match observed steep spectral slopes, suggesting additional factors.

Abstract

Photochemical hazes have been suggested as candidate for the high-altitude aerosols observed in the transmission spectra of many hot Jupiters. We present 3D simulations of the hot Jupiter HD 189733b to study how photochemical hazes are transported by atmospheric circulation. The model includes spherical, constant-size hazes particles that gravitationally settle and are transported by the winds as passive tracers, with particle radii ranging from 1 nm to 300 $\mu$m. We identify two general types of haze distribution based on particle size: In the small-particle regime (<30 nm), gravitational settling is unimportant, and hazes accumulate in two large mid-latitude vortices centered on the night side that extend across the morning terminator. Therefore, small hazes are more concentrated at the morning terminator than at the evening terminator. In the large-particle regime (>30 nm), hazes settle out quickly on the nightside, resulting in more hazes at the evening terminator. For small particles, terminator differences in haze mass mixing ratio and temperature considered individually can result in significant differences in the transit spectra of the terminators. When combining both effects for HD189733b, however, they largely cancel out each other, resulting in very small terminator differences in the spectra. Transit spectra based on the GCM-derived haze distribution fail to reproduce the steep spectral slope at short wavelengths in the current transit observations of HD 189733b. Differing optical properties of hazes, hotter temperatures at low pressures because of heating by hazes, enhanced sub-grid-scale mixing, or star spots might explain the mismatch between the model and observations.