The Volatile Composition of Dynamically New Comet C/2023 A3 (Tsuchinshan-ATLAS) Based on High-Resolution Near-Infrared Spectroscopy

Comets are rich in organics and volatiles, and are considered among the least altered bodies in the solar system. The materials preserved in comets are thought to have experienced a wide range of thermal and density environments, from the interstellar molecular cloud core to the protoplanetary disk. Isotopic and mineralogical analyses of cometary dust and (independently) comparisons between relative volatile abundances in comets with predictions from astrochemical models have suggested the existence of large-scale radial transport processes within the disk (e.g., Aikawa et al., 2024; Willacy et al., 2022; Altwegg et al., 2019 and references therein). Thus, cometary materials were likely incorporated through complex evolutionary pathways. After their formation in the inner region of the early solar system (~5 to 30 au from the Sun), comets are thought to have been dynamically scattered into distant reservoirs such as the Kuiper Belt and Oort Cloud. Jupiter-family comets (JFCs), which are trapped by Jupiter’s gravity, have made multiple passages through the inner solar system, during which highly volatile species may have been selectively depleted due to solar heating. In contrast, dynamically new comets (DNCs), which are on their first passage into the inner solar system since being ejected to the Oort Cloud, have remained in cold and distant regions for most of their orbital histories. Thus, their chemical compositions are less likely to have been altered and can provide key constraints on the physical and chemical processes that occurred from the molecular cloud to the solar nebula.

High-resolution near-infrared spectroscopy enables direct detection of volatile species sublimated from the ices in the cometary nucleus. To date, the number of DNCs studied with this technique remains very limited (~10 comets, not all studied in detail, e.g., Dello Russo et al., 2016). For this reason, each new opportunity for an in-depth compositional analysis of a DNC has the potential to bring significant insights.

C/2023 A3 (Tsuchinshan–ATLAS) is a dynamically new comet that brightened to V-magnitude ~-4 near perihelion in late September 2024 and became a prominent visible object in the evening sky around mid-October during its closest approach to Earth. We conducted high-resolution near-infrared spectroscopic observations using NIRSPEC on the Keck II telescope on UT 2024 October 31 (heliocentric distance 0.91 au, geocentric distance 0.95 au, and was then receding from Earth at +60.6 km/s).  Despite its dust-rich nature, emission lines of volatiles were detected, including H₂O, OH, HCN, C₂H₂, CH₄, C₂H₆, and CH₃OH. We derived the water production rate and molecular mixing ratios relative to H2O. In this presentation, we compare the composition of C/2023 A3 with other DNCs and with comets from other dynamical classes, to assess its chemical characteristics in the context of differing compositional abundances among the comet population. We also compare the mixing ratios with the results derived from the optical spectra taken by the Subaru Telescope with the high-dispersion spectrograph (HDS) on the same night. The results from these optical observations will be presented separately (Kawakita et al., 2025; Shinnaka et al., 2025; Tsujimoto et al., 2025).

This study is financially supported by NASA SSO grants 80NSSC22K1401 (RJV, NDR, BPB) and 22-SSO22_0013 (MD), and NSF AST-AST-2009398 (NDR, BPB).

Rererences
Aikawa et al. 2024, in Comets III, edited by Karen. J. Meech et al., University of Arizona Press, pp. 33–62.
Willacy et al. 2022, ApJ, 931, 164
Altwegg et al. 2019, Ann. Rev. Astron. Astrophys., 57, 113
Dello Russo et al. 2016, Icarus, 278, 301
Kawakita et al. 2025, EPSC-DPS Joint meeting 2025
Shinnaka et al. 2025, EPSC-DPS Joint meeting 2025
Tsujimoto et al. 2025, EPSC-DPS Joint meeting 2025