Infrared Compositional Measurements in Comet C/2017 K2 (Pan-STARRS) at Heliocentric Distances Beyond 2.3 au

Comet C/2017 K2 (Pan-STARRS) provided a rare opportunity to investigate the evolution of coma composition and outgassing patterns over a transitional heliocentric distance (R h) range where activity drivers in comets are thought to change from “hypervolatile” (CO, CH4, C2H6, and/or CO2)-dominated to H2O-dominated. We performed high-resolution ( λ / Δ λ ≈ 25,000–42,000), cross-dispersed, near-infrared spectroscopy of C/2017 K2 with iSHELL at the NASA Infrared Telescope Facility and NIRSPEC at the W. M. Keck II Observatory. We report gas rotational temperatures (T rot) and molecular production rates (Q; molecules per second) or upper limits for the “hypervolatile” species CH4, CO, and C2H6, together with less volatile ices (CH3OH, H2O, HCN, C2H2, NH3, and OCS) over a range of pre-perihelion distances, R h = 3.15–2.35 au. We also report (or stringently constrain) abundance ratios (mixing ratios) of the targeted species with respect to CO, C2H6, and (when detected) H2O. All volatiles were enriched relative to water in C/2017 K2 when compared to their mean values among Oort Cloud comets, whereas abundances relative to C2H6 were consistent with their average values from other long-period comets.