Characterizing the Native Ice Composition of Comet C/2022 E3 (ZTF) with NIRSpec at Keck 2

Overview. We present results of native ice composition for C/2022 E3 (hereafter E3), a long-period comet from the Oort cloud, based on near-IR (λ ~ 2.8 – 3.7 µm) observations using NIRSPEC, the long-slit (24"), cross-dispersed, high-resolution (λ/Δλ ~ 25,000) spectrograph at Keck 2. NIRSPEC was upgraded in 2018¹, resulting in substantially improved sensitivity. We present production rates and abundance ratios for H₂O and seven trace molecules (C₂H₆, CH₃OH, HCN, CH₄, C₂H₂, NH₃, and H₂CO). Compared with their mean abundances among Oort cloud comets,² our study reveals an overall depleted volatile composition for E3. We also compare spatial profiles of emissions in the coma, as these provide information regarding associations among ices in the nucleus. Spectral Studies. Post-perihelion spectra of E3 were obtained on UT 2023 January 20 & 21. To characterize the eight targeted parent volatiles, two L-band echelle/cross-disperser settings were used (KL1 & KL2), each one including many lines of H₂O plus multiple trace species. High signal-to-noise spectra were obtained in both settings on each date. KL1 includes strong emissions from H₂O, C₂H₆ and CH₃OH, thus relatively little time was spent on this setting (~20 minutes on-source/day). KL2 includes difficult-to-detect species having weaker emissions (C₂H₂, H₂CO, NH₃); therefore, ~80 minutes on-source/day were accrued. Both KL1 and KL2 also cover lines of CH₄ that require sufficient geocentric Doppler shift (Δ_dot) to displace cometary lines from their opaque telluric counterpart absorptions (for E3, Δ_dot ~ -45 km s^-1). Post-upgrade, the combined (KL1 + KL2) spectral coverage is more nearly complete from ~2.8 – 3.7 µm. This complements our concurrent study of E3 with iSHELL³ that features both L-band and M-band (~4.4 – 5.2 µm) spectra, the latter regime co-measuring CO and OCS with H₂O. Spatial Studies. The long slit permits studies of outgassing in the coma. One interesting question is whether differences exist for polar-dominated versus apolar-dominated ices, as has been observed in some comets (e.g., C/2007 W1 Boattini⁴, 103P/Hartley 2⁵). We will assess this for E3, comparing outgassing patterns and production rates/abundances from observations separated by approximately 24 hours. Acknowledgments: Support is recognized from NSF AARG and NASA SSO and EW programs, and from Keck Observatory staff to ensure the success of this study. References: ¹Martin E. C. et al. (2018) Proc. SPIE 10702, 107020A-1; ²Dello Russo, N. et al. (2016) Icar 278, 301; ³Rai et. al., this conference; ⁴Villanueva G. L. et al. (2011) Icar, 216, 227; ⁵Mumma, M. J. et al. (2011) ApJ, 734, L7