Effect of H 2 S on the Near-infrared Spectrum of Irradiation Residue and Applications to the Kuiper Belt Object (486958) Arrokoth

Abstract

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 μ 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 H 2 S” CH 3 OH:NH 3 :H 2 S:H 2 O (3:3:3:1) and “without H 2 S” CH 3 OH:NH 3 :H 2 O (3:3:1) were measured to study the effect of H 2 S. The “with H 2 S” sulfur-rich laboratory-synthesized organic residue displays an absorption band at 1.8 μ m that is absent in the spectrum of “without H 2 S” sample. This feature matches the Arrokoth spectrum better than any other expected material. This suggests the past presence of H 2 S ice on the surface of Arrokoth and a role for Kuiper Belt objects as a key S reservoir in the early solar system.