Strongly Depleted Methanol and Hypervolatiles in Comet C/2021 A1 (Leonard): Signatures of Interstellar Chemistry?

We measured the chemical composition of comet C/2021 A1 (Leonard) using the long-slit echelle grating spectrograph iSHELL/IRTF on 2021 December 20 and on 2022 January 8 and 9. We sampled 11 primary volatiles (H _2 O, HCN, NH _3 , CO, C _2 H _2 , C _2 H _6 , CH _4 , CH _3 OH, H _2 CO, OCS, and HCl) and three product species (CN, NH _2 , and OH) and retrieved their molecular abundances, which can serve as important cosmogonic indicators. The abundance ratios, relative to water, of almost all trace volatiles appear to be depleted relative to reference values, with methanol abundance among the lowest observed in a comet. The observed stronger depletion of CH _3 OH, relative to CO, CH _4 , and C _2 H _6 , could be evidence of an interstellar medium (ISM) chemistry signature in comet/Leonard ices. Both the detection of HCl and the detection of OCS support the idea of interstellar origin for comet/Leonard ices, since they are preferentially formed via solid-phase interstellar chemistry and are then found depleted in dense molecular clouds and protoplanetary disks, suggesting that their abundances in comets might retain a signature from the ISM era. The comet also revealed a complex outgassing pattern, with volatiles largely shifted toward the sunward direction, relative to the dust profiles that appeared centered on the nucleus-centric position. Here we present emission profiles measured along the Sun–comet line for brightest lines of H _2 O, HCN, C _2 H _6 , and CO, and we show that they follow the release of water in similar fashion, interpreting this as indication of a not strict relationship between polar and apolar ices.