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

We report production rates of H _2 O and nine trace molecules (C _2 H _6 , CH _4 , H _2 CO, CH _3 OH, HCN, NH _3 , C _2 H _2 , 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 H _2 O indicate that S3 is depleted in CH _3 OH compared to its mean abundance relative to H _2 O 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 H _2 O. 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 H _2 CO, NH _3 , and C _2 H _2 , 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 C _2 H _6 and CH _4 are nearly symmetric and closely track each other, while those of CO and HCN co-measured with H _2 O (using different instrument settings) are similar to each other and are asymmetric in the antisunward direction.