Day–Night Transport-induced Chemistry and Clouds on WASP-39b: Gas-phase Composition

Abstract

JWST has recently detected the first robust photochemical product on an exoplanet: sulfur dioxide (SO2) on WASP-39b. The data from the NIRISS instrument also reveal signs of partial coverage of clouds. Most of the previous studies have focused on interpreting spectral data with 1D models. To explore how the chemical species and cloud particles are altered by global circulation, we applied a 2D photochemical model and a 2D microphysical cloud model separately to postprocess the thermal and dynamical structures simulated by a 3D general circulation model of WASP-39b. We found that SO2 produced by photochemistry on the dayside can be transported to the nightside owing to the efficient replenishment of horizontal transport. The morning–evening limb differences in methane (CH4) abundances predicted by the 1D models disappeared after horizontal transport is included. Similarly, the inclusion of horizontal transport also reduced the limb differences in SO2. Our modeling results suggest that the fast zonal wind results in minimal or negligible limb asymmetry in composition. Based on the synthetic spectra generated by our 2D atmosphere simulations, we propose that observing SO2 absorption in the emission spectra of WASP-39b at different phases may offer opportunities to probe the horizontal quenching process of photochemical products. We will focus on the gas-phase chemistry in this paper and leave the results regarding clouds in the subsequent paper as part of the series.

Biogenic Sulfur Gases as Biosignatures on Temperate Sub-Neptune Waterworlds

“…In the absence of photochemical destruction, biological sulfur compounds can in principle build up on the nightside of a tidally locked Hycean planet. Global circulation plays a central role in transporting species between the dayside and nightside, where distinct chemical sources and sinks are at play (Chen et al 2018;Tsai et al 2021bTsai et al , 2023c. To account for the day-night transport, we run VULCAN 2D (Tsai et al 2021b(Tsai et al , 2024 to model the equatorial region, using temperature and wind structures from the 3D general circulation model (GCM) Exo-FMS (Lee et al 2021;Innes & Pierrehumbert 2022) as input.…”