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

Abstract

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 (H2O, HCN, NH3, CO, C2H2, C2H6, CH4, CH3OH, H2CO, OCS, and HCl) and three product species (CN, NH2, 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 CH3OH, relative to CO, CH4, and C2H6, 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 H2O, HCN, C2H6, 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.

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

“…Thus, our study of S3 provides a vital addition to the abundance measurements of OCS within the comet population. The OCS abundance we measured in S3 is consistent with measurements in other comets (both OCCs and Jupiter-family comets (JFCs)) obtained over R h ranging from 0.47 to 1.04 au: C/2012 S1 (Dello Russo et al 2016b), C/2015 ER61 (Saki et al 2021), 2P/Encke (Roth et al 2018), and C/2021 A1 (Leonard) (Faggi et al 2023).…”

“…Since observations of individual comets over a wide range of R h are generally unavailable, primarily due to observational constraints, we compare the S3 abundances to those obtained for the sample of OCCs measured within ∼0.8 au (see Table 3) and those observed at R h > 0.8 au. Table 4 compares the average abundances of S3 to those of all OCCs observed at R h greater than 0.8 au from the Sun, all et al 1996;Dello Russo et al 2002;Magee-Sauer et al 2002;DiSanti et al 2003), A1 ≡ C/2021 A1 (Faggi et al 2023;Lippi et al 2023). The abundances indicated for each species are weighted average values when there are multiple measurements for the comet.…”

Parent Volatile Outgassing Associations in Cometary Nuclei: Synthesizing Rosetta Measurements and Ground-based Observations

“…Among the molecules studied in the near-IR, there is a paucity in measurements of certain harder-to-detect species (e.g., H 2 CO, C 2 H 2 , and OCS; see Dello Russo et al 2016b) in both JFCs and OCCs, making developing a taxonomy and investigating the correlation between these species a challenging task. Among these, the sulfur-bearing molecule OCS has been sampled in only three JFCs and seven OCCs so far (Saki et al 2020;Faggi et al 2023). Atomic sulfur emissions require observations in the UV/near-UV wavelengths (which specifically requires spacebased observatories, as the UV is inaccessible from groundbased facilities) and the radiative transfer modeling of the atomic sulfur transitions.…”

Single-atom catalysis in space: Computational exploration of Fischer–Tropsch reactions in astrophysical environments

“…Methanol has been detected in comets (Hoban et al 1991;Reuter 1992;Davies et al 1993;Eberhardt' et al 1994;Le Roy et al 2015), which can arise from earlier stages of planetary evolution, including ices of primordial dust grains (Rubin et al 2019). Thus, the methanol abundance is expected to vary according to the protoplanetary region in which it accreted (and therefore its origin), but also to the conditions under which it evolved, such as the protostellar gas and the protoplanetary disk temperatures, the CO depletion level in the primordial volatile gases (Faggi et al 2023), or the irradiation conditions (Hudson & Moore 1999). While there is no direct evidence suggesting that FTT reactions take place during the cometary life, some of the organic components present in comets could have been inherited from FTT reactions occurring on grain surfaces in certain regions of the solar nebula (Llorca 2002), for example the presence of ethylene in cometary comas, whose origin is still elusive (Dartois 2021).…”