Coma Volatile Composition and Thermal Physics in Comet C/2022 E3 (ZTF) Measured Near Closest Approach to Earth with NASA-IRTF

The 2023 perihelion passage of comet C/2022 E3 (ZTF) afforded an opportunity to measure the abundances and spatial distributions of coma volatiles in an Oort cloud comet at high spatial resolution near its close approach to Earth ($Δ_\mathrm{min}\sim 0.28$ au on UT February 1). We conducted near-infrared spectroscopic observations of C/2022 E3 (ZTF) using iSHELL at the NASA Infrared Telescope Facility on UT 2023 February 9. Our measurements securely detected fluorescent emission from H$_2$O, CO, OCS, CH$_3$OH, CH$_4$, C$_2$H$_6$, C$_2$H$_2$, and HCN. For each instrumental setting we took exposures with the slit oriented parallel and also perpendicular to the projected Sun-comet vector, thereby enabling a test of the spatial distributions of these molecules. We report rotational temperatures, production rates, and abundance ratios (i.e., mixing ratios) for all sampled species. Our measurements found that molecular abundances in C/2022 E3 were depleted compared to their average values in Oort cloud comets with the exception of OCS, which was consistent. The H$_2$O production rate varied significantly and was likely tied to nucleus rotation effects. Measurements at the two slit orientations showed distinct column density and rotational temperature profiles for H$_2$O. Peak temperatures occurred off-nucleus and slower cooling was present in the anti-sunward hemisphere, consistent with the presence of icy grain sublimation in the coma.

💡 Research Summary

In this paper the authors present a detailed near‑infrared spectroscopic study of comet C/2022 E3 (ZTF) conducted with the iSHELL echelle spectrograph on the NASA Infrared Telescope Facility (IRTF) on 2023 February 9. The comet’s exceptionally close approach to Earth (minimum geocentric distance Δ ≈ 0.28 au on 2023 February 1) allowed the inner coma (≈ 750 km from the nucleus) to be sampled at a spatial resolution of ~45 km per pixel (0.167″). Observations were carried out in three iSHELL settings (a custom L‑band covering 2.81–3.09 µm, Lp1, and M2) with a 0.75″ slit (R ≈ 4.5 × 10⁴) and a 15″ long slit. For each setting the slit was alternated between being parallel and perpendicular to the projected Sun‑comet line, providing two independent spatial axes for each molecule.

The data reduction followed the standard iSHELL pipeline: ABBA nodding removed sky and thermal background, flux calibration was performed with bright infrared standards (BS‑1040 and BS‑2560), and atmospheric transmission was modeled with the NASA Planetary Spectrum Generator (PSG). Fluorescence g‑factors were also generated with PSG, allowing the authors to correct for the comet’s high geocentric velocity (≈ 38 km s⁻¹) that Doppler‑shifts the CO and CH₄ lines away from telluric absorption cores. Line intensities were fitted simultaneously across each echelle order using a Levenberg‑Marquardt algorithm, yielding high‑precision column densities even in crowded spectral regions.

Rotational temperatures (T_rot) were derived from excitation analyses of multiple rovibrational transitions. H₂O showed temperatures of 78 K (both slit orientations) in the L‑custom setting, rising to 87–90 K in the M2 setting. CO and OCS were colder (≈ 70–77 K), while HCN was warmer (≈ 108–116 K). CH₄, C₂H₆, and CH₃OH displayed temperatures consistent with H₂O within uncertainties. The authors report that T_rot values from the two slit orientations agree within 1σ for most species, but H₂O exhibits a modest systematic increase in the M2 setting, suggesting slight variations in the sampled coma region.

Production rates were calculated from nucleus‑centered column densities (Q_NC) and then scaled to global rates (Q) using growth factors derived from the Q‑curve method, which accounts for seeing‑induced loss of flux near the nucleus and for comet motion during exposures. The resulting global production rates are: H₂O = (6.0–8.3) × 10²⁵ mol s⁻¹, CO = (3.3–3.6) × 10²⁴ mol s⁻¹, OCS = 3.1 × 10²³ mol s⁻¹, CH₄ = (1.9–2.6) × 10²⁴ mol s⁻¹, C₂H₆ = (2.1–2.7) × 10²⁴ mol s⁻¹, CH₃OH = (5.3–5.9) × 10²³ mol s⁻¹, HCN = (8.0–9.7) × 10²² mol s⁻¹, and C₂H₂ = 4.5 × 10²² mol s⁻¹. Relative abundances with respect to water range from 0.05 % (C₂H₂) to 0.82 % (CH₃OH). Compared to the average Oort‑cloud comet composition, most species are depleted, with the notable exception of OCS, whose abundance (≈ 0.14 % of water) matches the canonical Oort‑cloud value.

Spatial profiles extracted along the slit for H₂O (directly and via its prompt OH* emission), HCN, CO, C₂H₆, and CH₄ reveal distinct morphologies. Water shows a relatively flat distribution out to ~750 km, with a peak column density offset from the nucleus and a slower decline on the anti‑sunward side. This behavior, together with the observed off‑nucleus temperature maximum, is interpreted as evidence for sublimation of icy grains released from the nucleus, providing an extended source of water that cools gradually as it moves away from the Sun. The CO and hydrocarbon profiles are more centrally peaked, consistent with direct nucleus sublimation, while HCN shows a modest asymmetry that may reflect localized active regions.

The authors discuss the implications of these findings. The depletion of most volatiles suggests that C/2022 E3 (ZTF) may be chemically evolved or that it originated from a region of the Oort cloud with intrinsically lower volatile content. The detection of an extended water source highlights the importance of grain‑driven activity even in relatively warm (r_H ≈ 1.12 au) comets. Moreover, the variation of water production rates between the two slit orientations points to nucleus rotation modulating outgassing, implying heterogeneous active areas on the surface.

In summary, this work demonstrates the power of high‑resolution near‑infrared spectroscopy combined with strategic slit orientation to dissect both the composition and thermal physics of a comet’s inner coma. By leveraging the rare close Earth approach of C/2022 E3 (ZTF), the authors provide one of the most detailed spatially resolved volatile inventories for an Oort‑cloud comet to date, offering valuable constraints for models of cometary nucleus structure, volatile processing, and the broader chemical diversity of the Oort cloud population.