H₂S ice sublimation dynamics

Experimentally constrained binding energies, entrapment efficiencies, and snowlines

¹ Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² Center for Astrophysics, Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
³ UC Berkeley Department of Chemistry, Berkeley, CA 94720, USA

^★ Corresponding author: santos@strw.leidenuniv.nl

Received: 7 February 2025
Accepted: 18 April 2025

Abstract

Context. Hydrogen sulfide (H₂S) is thought to be an important sulfur reservoir in interstellar ices. It serves as a key precursor to complex sulfur-bearing organics, and has been proposed to play a significant role in the origin of life. Although models and observations both suggest H₂S is present in ices in non-negligible amounts, its sublimation dynamics remain poorly constrained.

Aims. In this work, we present a comprehensive experimental characterization of the sublimation behavior of H₂S ice under astro-physically relevant conditions.

Methods. We used an ultrahigh vacuum chamber to deposit pure multilayer H₂S ice, submonolayer H₂S ice on top of compact amorphous solid water (cASW), and ice mixtures of H₂S and H₂O. The sublimation behavior of H₂S was monitored with a quadrupole mass spectrometer during temperature-programmed desorption experiments. These experiments were used to determine binding energies and entrapment efficiencies of H₂S, which were then employed to estimate its snowline positions in a protoplanetary disk midplane. Results. We derive mean binding energies of 3159 ± 46 K for pure H₂S ice and 3392 ± 56 K for submonolayer H₂S desorbing from a cASW surface. These values correspond to sublimation temperatures of around 64 K and 69 K in the disk midplane, placing its sublimation fronts at radii just interior to the CO₂ snowline. We also investigated the entrapment of H₂S in water ice and find it to be highly efficient, with ~75 − 85% of H₂S remaining trapped past its sublimation temperature for H₂O:H₂S mixing ratios of ~5−17:1. We discuss potential mechanisms behind this efficient entrapment.

Conclusions. Our findings imply that, in protoplanetary disks, H₂S will mostly be retained in the ice phase until water crystallizes, at radii near the water snowline, if it forms mixed into water ice. This has significant implications for the possibility of H₂S being incorporated into icy planetesimals and its potential delivery to terrestrial planets, which we discuss in detail.