Search

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, CH _4 , C _2 H _6 , and/or CO _2 )-dominated to H _2 O-dominated. We performed high-resolution ( $\lambda $ / ${\rm{\Delta }}\lambda $ ≈ 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 CH _4 , CO, and C _2 H _6 , together with less volatile ices (CH _3 OH, H _2 O, HCN, C _2 H _2 , NH _3 , 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, C _2 H _6 , and (when detected) H _2 O. 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 C _2 H _6 were consistent with their average values from other long-period comets.