Photochemically-produced SO$_2$ in the atmosphere of WASP-39b (2211.10490v2)

Abstract: Photochemistry is a fundamental process of planetary atmospheres that regulates the atmospheric composition and stability. However, no unambiguous photochemical products have been detected in exoplanet atmospheres to date. Recent observations from the JWST Transiting Exoplanet Early Release Science Program found a spectral absorption feature at 4.05 $\mu$m arising from SO$_2$ in the atmosphere of WASP-39b. WASP-39b is a 1.27-Jupiter-radii, Saturn-mass (0.28 M$_J$) gas giant exoplanet orbiting a Sun-like star with an equilibrium temperature of $\sim$1100 K. The most plausible way of generating SO$_2$ in such an atmosphere is through photochemical processes. Here we show that the SO$_2$ distribution computed by a suite of photochemical models robustly explains the 4.05 $\mu$m spectral feature identified by JWST transmission observations with NIRSpec PRISM (2.7$\sigma$) and G395H (4.5$\sigma$). SO$_2$ is produced by successive oxidation of sulphur radicals freed when hydrogen sulphide (H$_2$S) is destroyed. The sensitivity of the SO$_2$ feature to the enrichment of the atmosphere by heavy elements (metallicity) suggests that it can be used as a tracer of atmospheric properties, with WASP-39b exhibiting an inferred metallicity of $\sim$10$\times$ solar. We further point out that SO$_2$ also shows observable features at ultraviolet and thermal infrared wavelengths not available from the existing observations.

• The paper reveals a photochemical pathway where H₂S breakdown produces SO₂, challenging existing thermochemical models.
• It employs JWST NIRSpec PRISM and G395H data alongside four independent 1D models to confirm the spectral absorption at 4.05 µm.
• The study highlights SO₂ as a sensitive tracer for atmospheric metallicity, urging refined models to better understand exoplanet atmospheres.

Photochemical Production of Sulfur Dioxide in the Atmosphere of WASP-39b

The paper investigates the presence of photochemically-produced sulfur dioxide (SO₂) in the atmosphere of the exoplanet WASP-39b, leveraging data from the JWST Transiting Exoplanet Early Release Science Program. WASP-39b, a gas giant exoplanet, orbits a Sun-like star and presents an optimal subject for studying atmospheric composition due to its size and temperature. Prior to this paper, unequivocal evidence of photochemical products had not been identified in exoplanetary atmospheres.

Key Findings and Methodology

The researchers detected a spectral absorption feature at a wavelength of 4.05 µm attributed to SO₂, using JWST NIRSpec PRISM and G395H instrument modes, registering significances of 2.7σ and 4.5σ respectively. This observation challenges traditional atmospheric models that consider thermochemical equilibrium, as these models predict negligible SO₂ quantities given typical gas giant planet characteristics and metallicity. Instead, a photochemical pathway involving sulfur radicals resultant from hydrogen sulfide (H₂S) breakdown is proposed. These radicals are oxidized to SO₂ in the presence of UV irradiation, indicating that photochemical processes are crucial in the atmospheric composition of WASP-39b.

Several independent 1D photochemical models, specifically ATMO, ARGO, KINETICS, and VULCAN, were employed to simulate the sulfur species distribution. These models consistently demonstrated significant photochemical production of SO₂, corroborating the observed absorption feature. Notably, the SO₂ mixing ratio exhibits altitude dependence, with a pronounced peak between 0.01–1 mbar, varying from 10 to 100 ppm depending on the location (morning vs. evening terminator).

Implications and Sensitivity Analysis

The sensitivity of SO₂ as an atmospheric tracer is particularly noteworthy, offering insights into the metallicity of the exoplanet, estimated to be approximately 10 times that of solar mien. SO₂'s spectral features at ultraviolet and thermal infrared wavelengths further enhance its utility as a diagnostic tool for understanding atmospheric processes and conditions.

Compared to other proxies like H₂O and CO₂, SO₂ displays robust sensitivity to metallicity variations, affirming its applicability in discerning the presence of heavy elements in gas giant atmospheres. The paper's findings underscore the importance of including photochemical processes in atmospheric models to accurately interpret observational data from JWST and similar missions.

Future Research Prospects

The detection of SO₂ in WASP-39b opens several avenues for future exploration. A deeper understanding of the chemical kinetics underpinning photochemical SO₂ production necessitates further refinement of reaction rate constants and UV cross-section data at relevant temperatures. Moving beyond 1D models to incorporate horizontal transport dynamics would provide a more comprehensive view of atmospheric processes.

Moreover, the role of sulfur photochemistry at various planetary temperatures heralds potential advancements in characterizing exoplanet atmospheres, particularly those with elevated temperatures where sulfur compounds like SH and SO might become more prevalent. Consequently, ongoing and future observations, coupled with refined photochemical models, are anticipated to significantly expand the knowledge of atmospheric composition and processes on exoplanets.