A new pathway to SO₂

Revealing the NUV driven sulfur chemistry in hot gas giants

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, 1090GE Amsterdam, The Netherlands
² Department of Earth and Planetary Sciences, University of California, Riverside, CA, USA
³ SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁴ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁵ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

^★ Corresponding author; wiebedg@gmail.com

Received: 3 May 2024
Accepted: 4 November 2024

Abstract

Context. Photochemistry is a key process that drives planetary atmospheres away from local thermodynamic equilibrium. Recent observations of the H₂ dominated atmospheres of hot gas giants have detected SO₂ as one of the major products of this process.

Aims. We investigated which chemical pathways lead to the formation of SO₂ in an atmosphere, and we investigated which part of the flux from the host star is necessary to initiate SO₂ production.

Methods. We used the publicly available S–N–C–H–O photochemical network in the VULCAN chemical kinetics code to compute the disequilibrium chemistry of an exoplanetary atmosphere.

Results. We find that there are two distinct chemical pathways that lead to the formation of SO₂. The formation of SO₂ at higher pressures is initiated by stellar flux >200 nm, whereas the formation of SO₂ at lower pressures is initiated by stellar flux <200 nm. In deeper layers of the atmosphere, OH is provided by the hydrogen abstraction of H₂O, and sulfur is provided by the photodissociation of SH and S₂, which leads to a positive feedback cycle that liberates sulfur from the stable H₂S molecule. In the upper layers of the atmosphere, OH is provided by the photodissociation of H₂O, and sulfur can be liberated from H₂S either by the photodissociation of SH and S₂, or by the hydrogen abstraction of SH.

Conclusions. We conclude that the stellar flux in the 200–350 nm wavelength range as well as the ratio of near-UV to UV radiation are important parameters determining the observability of SO₂. In addition, we find that there is a diversity of chemical pathways to the formation of SO₂. This is crucial for the interpretation of SO₂ detections and derived elemental abundance ratios, and for overall metallicities.