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

TL;DR

This work tackles the origin of sulfur-bearing species, notably $SO_2$, in close-in exoplanet atmospheres by proposing that semi-volatile ammonium salts (e.g., $NH_4SH$) embedded in dust can librate nitrogen and sulfur into disk gas as dust drifts inward. It develops an end-to-end model that couples 1D viscous disk transport of rocks, ices, and salts with salt-line chemistry, followed by photochemical evolution of planetary atmospheres and their transmission spectra using VULCAN and petitRADTRANS. The key findings are that salt-dissociation can raise disk gas abundances of $N$ and $S$ by factors of 2–10 near the salt line within ~0.1–1 Myr, producing observable $SO_2$ at 7–8 μm in atmospheres inherited from such gas for planets with $T_{ m eq}$ roughly from 800–1200 K, while cooler planets show diminished sulfur features; the study also outlines how volatile-element ratios like $N/S$ and $C/O$ can break degeneracies with solid accretion scenarios. Overall, the paper offers a gas-driven pathway for atmospheric sulfur enrichment that complements solid-delivery scenarios and provides concrete observational diagnostics to constrain planet-formation histories in disks.

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.

Figures (14)

• Figure 1: The calculation flow in our model. We model the dynamics of gas and dust containing volatile ices and semi-volatile salts to calculate the C, O, N, and S abundances in the disk. Subsequently, we simulate the evolution of the planetary atmospheric structure that inherits the disk's composition and predict its transmission spectrum. The colored input parameters at the top of the figure correspond to the results obtained from the previous calculation step.
• Figure 2: Initial dust surface density (left) and initial vapor surface density (right). Different colored lines show the surface density of different chemical species. In the left panel, the gray and black lines show the dust surface density of rocky component and the sum of rock and icy components, respectively.
• Figure 3: Time evolution of the disk temperature $T$ from the fiducial model as a function of the distance from the central star $r$. The blue dotted, dashed, and solid lines are the snapshots at times $t = 0, 0.1$ and 1.0 Myr, respectively.
• Figure 4: Evolution of dust surface density (upper) and vapor surface density (lower). The left and right panels are the snapshots at times $t = 0.1$ and 1.0 Myr. Solid and dashed lines correspond to models with and without salt, respectively. Vertical dash-dotted lines in the Figure represent the salt dissociation lines.
• Figure 5: Time evolution of the grain size $a$ as a function of the distance from the central star $r$. The light blue, blue and navy lines are the snapshots at times $t = 0, 0.1$ and 1.0 Myr, respectively. The solid and dashed lines correspond to models with and without salts, respectively.