Effect of H₂S on the Near-infrared Spectrum of Irradiation Residue and Applications to the Kuiper Belt Object (486958) Arrokoth

On 2019 January 1, the New Horizons spacecraft flew by (486958) Arrokoth, a small body in the Kuiper Belt that is the most distant object ever visited by a spacecraft. A strong unidentified absorption band was observed in the spectrum of Arrokoth at 1.8 μm. We report here experimental evidence suggesting that the near-infrared spectrum of Arrokoth is indicative of sulfur-rich, tholin-like organic residue. The spectra of organic residues produced by irradiating ice mixtures "with H₂S" CH₃OH:NH₃:H₂S:H₂O (3:3:3:1) and "without H₂S" CH₃OH:NH₃:H₂O (3:3:1) were measured to study the effect of H₂S. The "with H₂S" sulfur-rich laboratory-synthesized organic residue displays an absorption band at 1.8 μm that is absent in the spectrum of "without H₂S" sample. This feature matches the Arrokoth spectrum better than any other expected material. This suggests the past presence of H₂S ice on the surface of Arrokoth and a role for Kuiper Belt objects as a key S reservoir in the early solar system.