Sulfur Enrichment in Close-in Exoplanet Atmospheres Induced by Pebble Drift across the Salt Line

TL;DR

This paper proposes a new model where ammonium salts in protoplanetary disks enhance sulfur and nitrogen in planetary atmospheres, explaining observed SO$_2$ in exoplanets through disk gas accretion rather than solid accretion.

Contribution

It introduces a novel scenario involving ammonium salts affecting disk gas composition, leading to sulfur-rich atmospheres in close-in exoplanets, supported by simulations of dust transport and chemical processes.

Findings

Ammonium salts increase sulfur and nitrogen in disk gases by 2-10 times solar values.

SO$_2$ features are detectable in infrared spectra due to pebble drift effects.

The model explains SO$_2$ in exoplanet atmospheres without solid accretion.

Abstract

Observations of JWST have revealed that several close-in exoplanets have sulfur-rich atmospheres through SO$_2$ detections. Atmospheric sulfur is often thought to originate from solid accretion during planet formation, whereas recent simultaneous detections of SO$_2$ and NH$_3$ challenge this conventional scenario. In this study, we propose that ammonium salts, such as NH$_4$SH 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 SO$_2$, NS, H$_2$S, NO, and NH$_3$ become the dominant N and S chemical species in the atmospheres on planets that inherited the gas compositions inside H$_2$O snowline. SO$_2$ 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 SO$_2$ 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.