Chemical Tracers for 3D Atmospheric Asymmetries on WASP-69 b

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Abstract
Warm giant exoplanets exhibit strong three-dimensional temperature contrasts that can significantly alter atmospheric chemistry through quenching and photochemistry, yet transmission spectra are commonly interpreted using one-dimensional, limb-averaged models. Such simplifications may bias inferred atmospheric properties, particularly metallicity and C/O ratio. In this work, we investigate the relative influence of atmospheric composition and three-dimensional thermal structure on atmospheric chemistry and transmission spectra using WASP-69 b as a test case. WASP-69 b is a ∼900 K warm Saturn, residing in a thermal regime especially sensitive to disequilibrium chemistry. We use three-dimensional general-circulation-model-derived pressure–temperature profiles as inputs for a one-dimensional photochemical–kinetics model to resolve longitudinal and latitudinal chemical asymmetries across the atmosphere. We find that CH 4 exhibits strong latitudinal variations linked to deep quench temperatures, while SO 2 shows longitudinal asymmetries driven by upper-atmospheric photochemistry and irradiation geometry. In contrast, CO 2 remains comparatively insensitive to spatial thermal variations and emerges as a robust tracer of atmospheric metallicity. Synthetic transmission spectra reveal that three-dimensional chemical asymmetries can produce spectral variations comparable to those induced by metallicity itself, demonstrating that limb-averaged interpretations can mask substantial spatial structure in warm giant exoplanet atmospheres.

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