Coma Abundances of Volatiles at Small Heliocentric Distances: Compositional Measurements of Long-period Comet C/2020 S3 (Erasmus)

We report production rates of H2O and nine trace molecules (C2H6, CH4, H2CO, CH3OH, HCN, NH3, C2H2, OCS, and CO) in long-period comet C/2020 S3 (Erasmus) using the high-resolution, cross-dispersed infrared spectrograph (iSHELL) at the NASA Infrared Telescope Facility, on two pre-perihelion dates at heliocentric distances R h = 0.49 and 0.52 au. Our molecular abundances with respect to simultaneously or contemporaneously measured H2O indicate that S3 is depleted in CH3OH compared to its mean abundance relative to H2O among the overall comet population (Oort Cloud comets and Jupiter-family comets combined), whereas the eight other measured species have near-average abundances relative to H2O. In addition, compared to comets observed at R h < 0.80 au at near-infrared wavelengths, S3 showed enhancement in the abundances of volatile species H2CO, NH3, and C2H2, indicating possible additional (distributed) sources in the coma for these volatile species. The spatial profiles of volatile species in S3 in different instrumental settings are dramatically different, which might suggest temporal variability in comet outgassing behavior between the nonsimultaneous measurements. The spatial distributions of simultaneously measured volatile species C2H6 and CH4 are nearly symmetric and closely track each other, while those of CO and HCN co-measured with H2O (using different instrument settings) are similar to each other and are asymmetric in the antisunward direction.