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 mu m. 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 mu m 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.