Sulfur Enrichment In Close-in Exoplanet Atmospheres Induced By Pebble Drift Across The Salt Line

Observations of JWST have revealed that several close-in exoplanets have sulfur-rich atmospheres through SO₂ detections. Atmospheric sulfur is often thought to originate from solid accretion during planet formation, whereas recent simultaneous detections of SO₂ and NH₃ challenge this conventional scenario.

In this study, we propose that ammonium salts, such as NH₄SH tentatively detected in comets and molecular clouds, play a significant role in producing sulfur-rich disk gases, which serve as the ingredient of giant planet atmospheres.

We simulated the radial transport of dust containing volatile ices and ammonium salts, along with the dissociation, sublimation, and recondensation of these materials, thereby predicting the atmospheric chemical structures and transmission spectra of planets inheriting these compositions. Assuming that ammonium salts sequester 20% of the elemental nitrogen and sulfur budgets, our results reveal that they enhance sulfur and nitrogen abundances in disk gases to 2-10 times the solar values near the salt dissociation line.

Photochemical simulations demonstrate that SO₂, NS, H₂S, NO, and NH₃ become the dominant N and S chemical species in the atmospheres on planets that inherited the gas compositions inside H₂O snowline. SO₂ features clearly appear in the infrared transmission spectra when the salt-bearing grains enhance the sulfur abundance of disk gas by pebble drift.

Our model provides a novel scenario that explains the SO₂ detected in some exoplanet atmospheres solely from disk gas accretion. Volatile-element ratios, particularly N/S and C/O, would provide a key to disentangle our scenario from the conventional solid-accretion scenario.

Transmission spectra of a planet with an atmospheric structure inheriting the elemental abundances of the disk at 1.5 au and 0.5 Myr. The black line represents the combined transmission spectrum of all molecules (H₂, He, H₂O, CO₂, CO, CH₄, NH3, H₂S, SO₂, CS, COS, NO, HCN, NS), while the shaded areas show the contributions of individual molecules. Upper panel: Spectrum for a planet formed in a salt-bearing disk. The corresponding atmospheric structure is shown in the upper left panel of Figure 7. The inset in the lower right corner provides an enlarged view of the 7–9 µm bands in the range of 0.99–1.015 Rsat transit radius, highlighting the small contributions of HCN and NS. Lower panel: Spectrum for a planet formed in a disk without salt. The corresponding atmospheric structure is shown in the lower left panel of Figure 7. — astro-ph.EP

Kanon Nakazawa, Ohno Kazumasa

Comments: Accepted for publication in ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2602.05300 [astro-ph.EP] (or arXiv:2602.05300v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2602.05300
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Submission history
From: Kanon Nakazawa
[v1] Thu, 5 Feb 2026 04:49:41 UTC (6,346 KB)
https://arxiv.org/abs/2602.05300

Astrobiology, Astrochemistry,