# Evidence for Surprising Heavy Nitrogen Isotopic Enrichment in Comet 46P/Wirtanen's Hydrogen Cyanide

Martin Cordiner, Kemper Darnell, Dominique Bockelée-Morvan, Nathan Roth, Nicolas Biver, Stefanie Milam, Steven Charnley, Jérémie Boissier, Boncho Bonev, Chunhua Qi, et al.

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## To cite this version:

Martin Cordiner, Kemper Darnell, Dominique Bockelée-Morvan, Nathan Roth, Nicolas Biver, et al. Evidence for Surprising Heavy Nitrogen Isotopic Enrichment in Comet 46P/Wirtanen's Hydrogen Cyanide. The Planetary Science Journal, 2024, 5 (10), pp.221. 10.3847/PSJ/ad7829. hal-04791256

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Submitted on 19 Nov 2024

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# Evidence for Surprising Heavy Nitrogen Isotopic Enrichment in Comet 46P/Wirtanen's Hydrogen Cyanide

M. A. Cordiner¹,² ID, K. Darnell¹,², D. Bockelée-Morvan³ ID, N. X. Roth¹,² ID, N. Biver³ ID, S. N. Milam¹ ID, S. B. Charnley¹ ID, J. Boissier⁴ ID, B. P. Bonev⁵ ID, C. Qi⁶ ID, J. Crovisier³, and A. J. Remijan⁷ ID

¹ Astrochemistry Laboratory, NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA; martin.cordiner@nasa.gov

² Department of Physics, Catholic University of America, Washington, DC 20064, USA

³ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, F-92195 Meudon, France

⁴ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, F-38406, Saint Martin d'Hères, France

⁵ Department of Physics, American University, Washington, DC, USA

⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 42, Cambridge, MA 02138, USA

⁷ National Radio Astronomy Observatory, Charlottesville, VA 22903, USA

*Received 2024 June 17; revised 2024 August 26; accepted 2024 September 5; published 2024 October 8*

## Abstract

46P/Wirtanen is a Jupiter-family comet, probably originating from the solar system's Kuiper Belt, that now resides on a 5.4 yr elliptical orbit. During its 2018 apparition, comet 46P passed unusually close to the Earth (within 0.08 au), presenting an outstanding opportunity for close-up observations of its inner coma. Here we present observations of HCN, H¹³CN, and HC¹⁵N emission from 46P using the Atacama Compact Array. The data were analyzed using the SUBLIME non-LTE radiative transfer code to derive ¹²C/¹³C and ¹⁴N/¹⁵N ratios. The HCN/H¹³CN ratio is found to be consistent with a lack of significant ¹³C fractionation, whereas the HCN/HC¹⁵N ratio of 68 ± 27 (using our most conservative 1σ uncertainties), indicates a strong enhancement in ¹⁵N compared with the solar and terrestrial values. The observed ¹⁴N/¹⁵N ratio is also significantly lower than the values of ~140 found in previous comets, implying a strong ¹⁵N enrichment in 46P's HCN. This indicates that the nitrogen in Jupiter-family comets could reach larger isotopic enrichments than previously thought, with implications for the diversity of ¹⁴N/¹⁵N ratios imprinted into icy bodies at the birth of the solar system.

*Unified Astronomy Thesaurus concepts: Comets (280); Neutral coma gases (2158); Comae (271); Small Solar System bodies (1469); Astrochemistry (75); Cosmochemistry (331); Isotopic abundances (867); Radio interferometry (1346); Submillimeter astronomy (1647)*

## 1. Introduction

Comets consist of a mixture of ice, dust, and pebbles, which are thought to have accreted in the vicinity of the giant planets around 4.5 Gyr ago, and have remained relatively unaltered ever since. Measurements of their compositions therefore provide a unique tool for investigating chemical and physical processes that occurred in the protosolar accretion disk during (and prior to) the epoch of planet formation. Due to the difficulty of protoplanetary-disk midplane observations using even the most powerful ground- and space-based telescopes, important details regarding the chemistry of star and planet formation remain unknown (K. I. Öberg et al. 2023). Cometary observations are uniquely useful in their ability to provide fundamental, quantitative constraints on astrochemical models for star- and planet-forming regions, in particular, regarding the chemistry that occurred during the earliest history of our solar system.

Isotopic ratios such as D/H and ¹⁵N/¹⁴N within cometary molecules are especially sensitive to the physical conditions prevalent during the formation and accretion of cometary matter. Isotopic fractionation is the process by which different isotopes of a given atom can become concentrated in a (gas or solid-phase) molecular reservoir, leading to isotopic abundance ratios that differ from the elemental ratios of the bulk reservoir.

As described in the review by H. Nomura et al. (2023), isotopic fractionation occurs in interstellar, protostellar, protoplanetary-disk, and planetary environments through a broad range of gas- and solid-phase processes.

In dense interstellar clouds, strong depletion of ¹⁵N in N₂H⁺ gas is commonly found relative to the local interstellar medium (L. Bizzocchi et al. 2013; E. Redaelli et al. 2018). On the other hand, P. Hily-Blant et al. (2018) found evidence for ¹⁵N enrichment in HC₃N toward the L1544 prestellar core. Atacama Large Millimeter/submillimeter Array (ALMA) observations of protoplanetary disks have recently revealed significant ¹⁵N enrichment in gas-phase HCN (V. V. Guzmán et al. 2017; P. Hily-Blant et al. 2019), and this can be explained as a result of isotope-selective photodissociation of N₂ (H. Nomura et al. 2023). As shown by R. Visser et al. (2018), self shielding of the dominant N₂ isotopologue leads to a region of the disk enriched in gas-phase ¹⁵N, which becomes incorporated into other gas-phase molecules, resulting in enhanced ¹⁵N/¹⁴N ratios. When the density is high enough and the temperature is low enough, such isotopically enriched gas-phase molecules freeze out onto dust grain surfaces to form ice mantles, which are later incorporated into comets and other icy bodies.

Consistent with this picture, the observed protoplanetary disk HC¹⁴N/HC¹⁵N ratios of ~100–200 are similar to those found in comets (H. Nomura et al. 2023), which corroborates our basic understanding of the genetic relationship between protoplanetary disk and cometary compositions. Unlike protoplanetary disks, however, comets show a surprising degree of

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<table><caption>**Table 1**
Observed Spectral Line Details</caption><thead><tr><th>Species</th><th>Transition<br>(J''–J')</th><th>Freq.<sup>a</sup><br>(GHz)</th><th>Obs. Date<br>(UT)</th><th>θ<sub>B</sub><br>(arcsec)</th><th>Δν<br>(kHz)</th><th>Δ<br>(au)</th><th>∫S<sub>ν</sub>dv<br>(mJy km s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>HCN</td><td>4–3</td><td>354.505477</td><td>2018-12-02 02:59</td><td>4.5 × 2.8</td><td>122</td><td>0.12</td><td>2806 ± 37</td></tr><tr><td>HC<sup>15</sup>N</td><td>3–2</td><td>258.156996</td><td>2018-12-07 23:58</td><td>7.6 × 4.1</td><td>977</td><td>0.09</td><td>41 ± 9</td></tr><tr><td>H<sup>13</sup>CN</td><td>3–2</td><td>259.011864</td><td>2018-12-07 23:58</td><td>7.6 × 4.1</td><td>977</td><td>0.09</td><td>32 ± 10</td></tr><tr><td>CH<sub>3</sub>OH</td><td>5<sub>K</sub>–4<sub>K</sub></td><td>241.7–241.9</td><td>2018-12-07 23:58</td><td>7.0 × 4.2</td><td>244</td><td>0.09</td><td>3472 ± 68</td></tr></tbody></table>

**Note.**

uniformity in their $^{14}$N/$^{15}$N ratios (among different molecules and across different comets), with a weighted average value of 144 ± 3 from HCN, CN, and NH$_2$ in 31 comets (P. Hily-Blant et al. 2017). A similar nitrogen isotopic fingerprint was also found in the molecular nitrogen gas emitted by comet 67P ($^{14}$N/$^{15}$N ~ 130 ± 30; K. Altwegg et al. 2019).

Continued studies of cometary isotopic ratios are therefore of interest to explore the distribution of $^{14}$N/$^{15}$N values for comparison with observations of protoplanetary disks and models for the formation of our own solar system, with the aim of constraining the physics and chemistry of these crucial planet-forming environments. In this article, we present new results on the HC$^{14}$N/HC$^{15}$N abundance ratio in the Jupiter-family comet 46P/Wirtanen, which was observed by M. A. Cordiner et al. (2023) using the ALMA during its exceptional 2018 apparition. The unusually close Earth–comet distance allowed the detection of weak spectral lines not typically detectable in Jupiter-family comets from the ground, resulting in the first map of HC$^{15}$N in a Jupiter-family comet and new insights into the possible diversity of $^{14}$N/$^{15}$N ratios among the comet population.

## 2. Observations

Observations of comet 46P/Wirtanen were conducted using ALMA during 2018 December 2–7, when the comet was around 0.1 au from Earth and 1.06–1.07 au from the Sun (the comet’s perihelion date was 2018 December 12). This study focuses on the Atacama Compact Array (ACA) data, which incorporated 12 × 7 m antennas covering baselines in the range 9–50 m. The shorter baselines of the ACA compared with the main (12 m) ALMA array make it more sensitive to extended coma emission; the resulting synthesized beam size (angular resolution) was $\theta_B = 4''5 \times 2''8$ at 354 GHz. Observations were conducted of the HCN ($J=4-3$), H$^{13}$CN ($J=3-2$), and HC$^{15}$N ($J=3-2$) transitions, using the Band 6 and 7 receivers. Multiple lines of the CH$_3$OH $J_K = 5_K-4_K$ band were observed in the range 241–242 GHz in order to derive the coma kinetic temperature. Additional observational parameters are given in Table 1, including the spectral resolution ($\Delta\nu$), geocentric distance ($\Delta$), and spectrally integrated line intensity ($\int S_{\nu}d\nu$, with $\pm 1\sigma$ statistical errors derived from the actual noise level inside a $\pm 90$ km s$^{-1}$ region adjacent to each spectral line). For CH$_3$OH, the integrated line intensity was summed over the 14 detected transitions of the $J_K = 5_K-4_K$ band.

Data flagging, calibration and continuum subtraction were performed as described by M. A. Cordiner et al. (2023). Imaging was performed using the CASA tclean (Högbom) algorithm with natural weighting and a pixel size of 0.5". Deconvolution was carried out within a 30"-diameter circular

mask centered on the comet, with a flux threshold of twice the rms noise level ($\sigma$).

## 3. Results

Spectrally integrated flux maps for the three HCN isotopologues are shown in Figure 1, integrated over velocity ranges $\pm 1.2$ km s$^{-1}$ with respect to the line rest velocities. Angular distances on the sky have been converted to spatial coordinates at the distance of the comet, with the origin at the HCN peak (for HCN), and at the CH$_3$OH peak for HC$^{15}$N and H$^{13}$CN (observed simultaneously with CH$_3$OH). For the weaker (HC$^{15}$N and H$^{13}$CN) lines, the spectral integration ranges were determined based on the velocity width of the stronger (HCN) line.

Spectra were extracted from the (0,0) position in each map (shown in Figure 2). Based on the spectrally integrated line fluxes, HCN is detected at a high significance (76$\sigma$), while HC$^{15}$N and H$^{13}$CN are detected at $4.6\sigma$ and $3.2\sigma$, respectively. The observed CH$_3$OH spectrum is shown in Figure 3. The spectral line profiles are well resolved for HCN and CH$_3$OH, showing a characteristic double-peaked substructure due to Doppler motion of the quasi-isotropically expanding coma along the line of sight, whereas no substructure is expected for the lower-resolution H$^{13}$CN and HC$^{15}$N observations.

The raw statistical significance of our HC$^{15}$N and H$^{13}$CN detections is less than optimal, but the evidence for both molecules is strengthened by the properties of the spectral line profiles, which match (within the noise) the rest velocity and FWHM of the (high-significance) HCN line. In the case of HC$^{15}$N, the four spectral channels that make up the line peak are all at a significance of greater than $3\sigma$ (where $\sigma = 4.7$ mJy). Furthermore, the HC$^{15}$N emission peak coincides with the known location of the comet’s nucleus, as derived from the peak position of the spectrally integrated CH$_3$OH data. As a general rule of thumb in radio interferometry, a detection can be considered real if it appears above the $3\sigma$ level for a source with a known location (as is the case here) or above $5\sigma$ if the location is unknown; both HC$^{15}$N and H$^{13}$CN therefore fulfill the detection criteria.

Spectral modeling was performed using the spherically symmetric (1D) version of SUBLIME: a time-dependent, non-LTE radiative transfer code for cometary comae (M. A. Cordiner et al. 2022), and the 1D version of the model was found to provide a sufficiently good fit to these ACA data. In the molecular excitation calculation, collision rates between CH$_3$OH and H$_2$O were assumed to be the same as CH$_3$OH with H$_2$ (D. Rabli & D. R. Flower 2010). HCN–H$_2$O collision rates are from M. L. Dubernet & E. Quintas-Sanchez (2019) and were assumed to apply equally for all three HCN

<footnote><sup>a</sup> Spectral line frequencies were obtained from the CDMS database (C. P. Endres et al. 2016). For the HCN isotopologues, the frequency of the strongest hyperfine component is given.</footnote>
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<figure><img src="image_7.png" /><figcaption>**Figure 1.** ALMA ACA maps of spectrally integrated HCN, H$^{13}$CN, and HC$^{15}$N emission from comet 46P/Wirtanen. Contour intervals are in units of 5σ for HCN and 1σ for H$^{13}$CN and HC$^{15}$N. Beam size (angular resolution) is shown in the lower left; sky-projected comet–Sun vectors are shown in the lower right. The HCN map is centered on the emission peak, whereas the H$^{13}$CN and HC$^{15}$N maps are centered on the stronger (simultaneously observed) CH$_3$OH emission peak (not shown).</figcaption></figure>

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<figure><img src="image_10.png" /><figcaption>**Figure 2.** ALMA ACA spectra of HCN, H$^{13}$CN, and HC$^{15}$N in comet 46P/Wirtanen, extracted from the central position(s) shown in Figure 1. Best-fitting SUBLIME radiative transfer models (including hyperfine structure) are overlaid in orange. The 3σ feature in the HC$^{15}$N spectrum around -5 km s$^{-1}$ is interpreted as an unusually strong (>3σ) noise spike.</figcaption></figure>

isotopologues. Rovibrational pumping due to the solar radiation field was calculated using molecular data for HCN and CH$_3$OH from the HITRAN and Planetary Spectrum
46P/Wirtanen CH₃OH UT 2018-12-07 02:33

<figure><img src="image_11.png" /><figcaption>**Figure 3.** ALMA ACA spectrum of CH₃OH observed in comet 46P/Wirtanen (extracted from the CH₃OH spectrally integrated brightness peak). Best-fitting SUBLIME radiative transfer model is overlaid in orange, demonstrating a gas kinetic temperature of T<sub>kin</sub> = 93 ± 3 K.</figcaption></figure>

<table><caption>**Table 2**
Production Rates and Abundances for H₂O and the Observed 46P HCN Isotopologues</caption><thead><tr><th>Species</th><th>Obs. Date</th><th>Q(H<sub>2</sub>O)<br>(s<sup>-1</sup>)</th><th>Q(Species)<br>(s<sup>-1</sup>)</th><th>Abundance</th></tr></thead><tbody><tr><td>HCN</td><td>2018-12-02</td><td>5.67 × 10<sup>27</sup></td><td>(5.20 ± 0.09) × 10<sup>24</sup></td><td>(9.17 ± 0.16) × 10<sup>-4</sup></td></tr><tr><td>HC<sup>15</sup>N</td><td>2018-12-07</td><td>7.56 × 10<sup>27</sup></td><td>(1.03 ± 0.25) × 10<sup>23</sup></td><td>(1.36 ± 0.33) × 10<sup>-5</sup></td></tr><tr><td>H<sup>13</sup>CN</td><td>2018-12-07</td><td>7.56 × 10<sup>27</sup></td><td>(7.70 ± 2.49) × 10<sup>22</sup></td><td>(1.03 ± 0.32) × 10<sup>-5</sup></td></tr></tbody></table>

Generator databases (see G. L. Villanueva et al. 2018 and M. A. Cordiner et al. 2023 for details). Hyperfine structure was included in the model spectra for all three HCN isotopologues, assuming equilibrium line-strength ratios among the hyperfine components in a given *J* state.

H₂O production rates for comet 46P (Q(H₂O)) were obtained for our observation times from cubic spline interpolation of the SOHO Lyα-derived measurements by M. R. Combi et al. (2020) and are given in Table 2. These Q(H₂O) values are consistent with the average water production rate of 8 × 10²⁷ s⁻¹ measured by D. C. Lis et al. (2019) using the SOFIA telescope a few days closer to perihelion (between December 14 and 20) and with the average value of 7 × 10²⁷ s⁻¹ obtained using the Infrared Telescope Facility between December 6 and 21 (Y. Khan et al. 2023). The fact that the values from three different observatories (spanning ultraviolet to far-infrared wavelengths) are consistent despite the large differences in their beam sizes implies that the SOHO water production rates should be applicable at the spatial resolution of our ACA data.

Prior to spectral modeling, the measured ACA spectra were corrected for interferometric flux loss factors of 0.79 for HCN and 0.73 for HC¹⁵N and H¹³CN. These were derived based on initial best-fit SUBLIME model image cubes, which were spectrally integrated, then processed using CASA `simobserve` according to the particular sky position and observation time of each line, and then were subject to the same cleaning and deconvolution as the observations. The purpose of this loss factor is to account for the fact that the ACA images are missing a fraction of the flux from the extended coma because the interferometer is only sensitive to structures less than λ/D in spatial extent, where λ is the observed wavelength and D is the minimum antenna separation (9 m in this case).

A beam-averaged gas kinetic temperature of T<sub>kin</sub> = 93 ± 3 K was derived using a least-squares fit to the CH₃OH spectrum (Figure 3). By fitting the HCN *J* = 4–3 line profile, a coma outflow velocity of 0.53 ± 0.01 km s⁻¹ and Doppler shift of -0.097 ± 0.006 km s⁻¹ were derived; these values were used in subsequent fits to the spectrally less well-resolved (and lower signal-to-noise) HCN isotopologue lines. Best-fitting production rates and abundances (relative to H₂O) for the three HCN isotopologues are given in Table 2. Formal (1σ) uncertainties were derived from the diagonal elements of the covariance matrix of the least-squares fit. To account for the correlation between adjacent spectral channels introduced by the ACA correlator, the rms noise measurement on each spectrum was multiplied by a factor of 1.29, following C. A. Nixon et al. (2020).

The maximum optical depth of our radiative transfer model at the HCN central peak is 0.2, but as the coma opacity falls rapidly with nucleocentric distance, the mean optical depth inside the ACA beam is only 0.04. Hence, the HCN *J* = 4–3 line is largely optically thin, as demonstrated by the ratio of the main HCN line peak with respect to the hyperfine satellites, which are too weak to be clearly detected, consistent with the optically thin limit (see Figure 2). Considering the cometary HCN data are typically well reproduced by a spherically symmetric coma model (see also M. A. Cordiner et al. 2014, 2019, 2023 and N. X. Roth et al. 2021), it is therefore unlikely that the presence of spurious, high opacity HCN clumps or jets would significantly impact our results.

## 4. Discussion

46P/Wirtanen is only the second Jupiter-family comet to date in which the minor (¹⁵N and ¹³C) isotopologues of HCN have been detected. The first was 17P/Holmes, which had
HCN/HC¹⁵N = 139 ± 26 (observed using the IRAM 30 m telescope during the comet's major outburst in 2007 October; D. Bockelée-Morvan et al. 2008). Accounting for purely statistical errors, the HCN/HC¹⁵N production rate ratio in comet 46P is 67 ± 16, and the HCN/H¹³CN ratio is 90 ± 28. Within the uncertainties, the ¹²C/¹³C ratio is consistent with previous cometary observations (D. Bockelée-Morvan et al. 2015; M. A. Cordiner et al. 2019), whereas the ¹⁴N/¹⁵N ratio is surprisingly enriched in the minor (¹⁵N) isotope—the (error-weighted) average of prior HCN/HC¹⁵N measurements in four previous comets is 146 ± 11 (D. Bockelée-Morvan et al. 2008; N. Biver et al. 2016). A tentative (3σ) detection of HC¹⁵N was also obtained in comet 46P by N. Biver et al. (2021) using the IRAM 30 m telescope between 2018 December 12 and 2018 December 18, leading to a HCN/HC¹⁵N ratio of 77 ± 26. The combination of these IRAM and ALMA results adds credibility to the conclusion that comet 46P's HCN was surprisingly enhanced in ¹⁵N.

Because HCN and its minor isotopologues were observed in comet 46P on different dates (almost 6 days apart), variations in the HCN/H₂O production rate ratio, as well as uncertainties in Q(H₂O), should be incorporated into our isotopic ratio uncertainties. Based on six infrared spectroscopic measurements of Q(HCN) and Q(H₂O) in comet 46P between December 6 and 21 (B. P. Bonev et al. 2021; Y. Khan et al. 2023), the HCN abundance relative to H₂O remained apparently constant, with a standard deviation on the Q(HCN)/Q(H₂O) ratio of only 0.00013 (corresponding to 6% of the HCN/H₂O value), and the error-weighted mean was Q(HCN)/Q(H₂O) = 0.0020 ± 0.0001. It is therefore reasonable to assume that variations in the comet's HCN/H₂O ratio contribute a negligible source of uncertainty to our result. Errors on Q(H₂O) may be more significant, however, considering the scatter in M. R. Combi et al. (2020)'s measurements as a function of time, which amount to an rms of 7.6 × 10²⁶ s⁻¹ (~10%) with respect to the best-fitting linear trend in Q(H₂O) between 2018 November 28 and 2018 December 11. Adding this fractional uncertainty in quadrature with the statistical uncertainty on Q(HC¹⁵N)/Q(H₂O), combined with a further 10% uncertainty on Q(HCN) and Q(HC¹⁵N) to account for possible inaccuracies in the absolute ACA flux scale, gives a value of HCN/HC¹⁵N = 67 ± 20.

The possibility of more extreme temporal variability in Q(H₂O) (or greater variability in HCN/H₂O) as a function of time cannot be ruled out, however. In that case, we combine the error on the observed H¹³CN abundance ((1.03 ± 0.32) × 10⁻⁵) with the expected HCN/H¹³CN ratio (≈90; based on measurements of ¹²C/¹³C ratios in a diverse range of comets and other solar system bodies; H. Nomura et al. 2023) to obtain a more conservative HCN abundance (and associated uncertainty) of (9.27 ± 2.88) × 10⁻³ at the time of our HC¹⁵N observation. This leads to a more conservative HCN/HC¹⁵N ratio (and uncertainty) of 68 ± 27. For comparison with meteoritic measurements, whereby the isotopic ratio is typically expressed as a fractional enhancement of the minor isotope, δ¹⁵N, with respect to the terrestrial standard ratio (¹⁴N/¹⁵N)Earth (A. O. Nier 1950), we calculate δ¹⁵N = (¹⁴N/¹⁵N)/(¹⁴N/¹⁵N)Earth - 1 = (3015 ± 1200)‰.

In the context of prior ¹⁴N/¹⁵N ratios measured throughout the solar system and beyond, our value in comet 46P is statistically unusual. Cometary ¹⁴N/¹⁵N ratios are known to be systematically lower than those found in the terrestrial and

giant planets, as well as the Sun, but the distribution of cometary values measured to date appears surprisingly uniform. The weighted average ¹⁴N/¹⁵N ratio for HCN, CN, and NH₂ in a sample of 31 comets is 144 ± 3 (P. Hily-Blant et al. 2017), which is significantly enriched in ¹⁵N compared with the bulk terrestrial and solar values of 273 and 459, respectively (A. O. Nier 1950; B. Marty et al. 2011). Indeed, within the error bars, all prior cometary ¹⁴N/¹⁵N measurements (within various molecules) may be consistent with a value ≈140, including the in situ mass spectrometry measurements of the 67P coma made by the Rosetta spacecraft (see Figure 4). Our new ¹⁴N/¹⁵N value in comet 46P is 2.8σ less than the weighted average of 144 and therefore represents an unexpected outlier with respect to the overall comet population, as well as to other (bulk) solar system bodies.

Meteoritic organics typically have ¹⁴N/¹⁵N ratios in the range 200–270 (somewhat enriched relative to the Earth; see Figure 4). Carbonaceous chondrite meteorites also contain small, micron-sized isotopic “hot spots,” which exhibit strong ¹⁵N enrichment (with ¹⁴N/¹⁵N values as low as 65 ± 14; H. Busemann et al. 2006), which is similar to our 46P value. Considering the anomalous nature of our value compared with the numerous previous cometary ¹⁴N/¹⁵N measurements, it is interesting to speculate that comets (or comet 46P in particular) could also contain isotopically heterogeneous material. Our HC¹⁵N observation occurred over a time period of 1 hr (between UT 2018 December 7 23:58 and 2018 December 8 00:59) with a beam size ≈3.′′5 (probing radial distances ~114 km from the nucleus). For an outflow velocity of 0.53 km s⁻¹ and volatile mass-loss rate of 320 kg s⁻¹ (based on Q(H₂O), Table 2, and a typical CO₂/H₂O ratio of 17%; T. Ootsubo et al. 2012), the mass of volatiles within the ACA beam was ~69,000 kg. This is indeed small compared with the total mass of the comet (~4 × 10¹⁴ kg, adopting a mean radius of 560 m and density of 0.6 g cm⁻³), so it is plausible that our ACA measurement is representative of a spatially isolated ¹⁵N enhancement confined to a relatively small part of the comet's nucleus. This idea is supported by the fact that Y. Moulane et al. (2023) measured a nominal CN/C¹⁵N ratio of 150 ± 30 using Very Large Telescope (VLT) ultraviolet spectroscopy only 1 day later (on 2018 December 9 at UT 00:41), which could be more indicative of the comet's bulk ¹⁴N/¹⁵N value. However, it should be noted that CN in cometary comae often has an additional source that cannot be explained by HCN photolysis alone (N. Fray et al. 2005; H. Cottin & N. Fray 2008), so its nitrogen isotopic ratio need not necessarily be the same as HCN. As shown by Figure 4, there is no prior evidence for differing CN/C¹⁵N and HCN/HC¹⁵N ratios in comets, but this possibility should be investigated further, and could help shed light on the extent of the chemical link between CN and HCN in comets.

Given the 9.1 hr rotation period of the 46P nucleus (T. L. Farnham et al. 2021), the time difference of ≈24 hr between our ACA HCN observations and the VLT CN observations of Y. Moulane et al. (2023) (corresponding to 2.6 nucleus rotations) also allows for the possibility that different regions of the nucleus have different nitrile ¹⁴N/¹⁵N ratios, in the case of outgassing dominated by a solar-facing jet (as was deduced for this comet by M. A. Cordiner et al. 2023).

¹⁵N enrichment in cometary nitriles and meteoritic organics probably occurred as a consequence of isotope-selective photodissociation of N₂ in the protosolar nebula
<figure><img src="image_12.png" /><figcaption>**Figure 4.** Nitrogen isotope measurements in primitive solar system materials, expressed as ¹⁴N/¹⁵N (lower *x*-axis, with smaller values indicating greater ¹⁵N enrichment toward the right); the fractional isotopic enrichment δ¹⁵N is shown on the upper *x*-axis. This figure was adapted from D. Bockelée-Morvan et al. (2015). The meteoritic bulk value is from CR chondrite insoluble organic matter (IOM) measurements (H. Nomura et al. 2023), while the ¹⁵N hot spots are regions that present strong isotopic enrichments relative to the surrounding meteoritic material. Gray ellipses represent the range of values from multiple laboratory sample analysis measurements. The CN, HCN, and NH₂ measurements in cometary gases are the averages from D. Bockelée-Morvan et al. (2008), J. Manfroid et al. (2009), Y. Shinnaka et al. (2016), and N. Biver et al. (2016). 67P values are from K. Altwegg et al. (2019), and the 46P CN value is from Y. Moulane et al. (2023).</figcaption></figure>

(R. Visser et al. 2018; S. Lee et al. 2021). Although such fractionation appears to have operated to produce relatively uniform ¹⁴N/¹⁵N ratios (in the range 140~170) within the bulk of the icy materials found in the comets and moons of the outer solar system (E. Furi & B. Marty 2015), the full range of ¹⁴N/¹⁵N values present in the planet-forming regions of our protosolar disk remains unknown. The surprisingly low HCN/HC¹⁵N ratio in comet 46P therefore presents a new challenge for our understanding of the physical and chemical processes that occurred during solar system formation.

Although the ¹²C/¹³C values measured in molecules in interstellar clouds and star-forming regions exhibit some genuine diversity, the ¹²C/¹³C ratios measured throughout the solar system are relatively uniform (H. Nomura et al. 2023). In contrast to HC¹⁵N, our HCN/H¹³CN ratio in 46P is consistent with the values of ¹²C/¹³C ~ 90 found in meteorites, terrestrial and giant planets and their icy satellites, as well as other comets. Consequently, we confirm the findings of M. A. Cordiner et al. (2019) suggesting a lack of strong carbon isotopic fractionation in cometary HCN during the formation of the solar system.
## 5. Conclusion

We observed rotational emission lines from HCN and its two minor isotopologues $HC^{15}N$ and $H^{13}CN$ in the coma of comet 46P during its exceptionally close 2018 December apparition, using the ACA. The $HC^{15}N$ ($J=3-2$) line was surprisingly strong compared with the $H^{13}CN$ ($J=3-2$) line, allowing $HC^{15}N$ to be mapped for the first time in a comet. The spectral line data were subject to non-LTE radiative transfer modeling, from which we derived a HCN/$HC^{15}N$ production rate ratio of $67 \pm 17$ (or a more conservative value of $68 \pm 27$, based on the observed $H^{13}CN$ and $HC^{15}N$ abundances, with an assumed HCN/$H^{13}CN$ ratio of 90). These values are significantly lower than previously measured in any N-bearing molecules in comets or in any other large solar system bodies. Within the solar system, comet 46P's HCN was therefore surprisingly enriched in $^{15}N$, reaching a value similar to those found in the most $^{15}N$-enriched “hot spots” in spatially isolated meteoritic samples. This result implies that cometary $^{14}N/^{15}N$ ratios could be more diverse, and potentially $^{15}N$ rich, than previously thought. More studies of $^{14}N/^{15}N$ in comets are warranted to better understand the diversity of isotopic fractionation processes taking place in protoplanetary disks.

## Acknowledgments

This work was supported by the National Science Foundation under grant Nos. AST-2009253, AST-2009398, and by NASA's Planetary Science Division Internal Scientist Funding Program through the Fundamental Laboratory Research work package (FLaRe). This work makes use of ALMA data set ADS/JAO.ALMA#2018.1.01114.S. ALMA is a partnership of ESO, NSF, NINS, NRC, NSC and ASIAA, in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ.

## ORCID iDs

M. A. Cordiner https://orcid.org/0000-0001-8233-2436

D. Bockelée-Morvan https://orcid.org/0000-0002-8130-0974

N. X. Roth https://orcid.org/0000-0002-6006-9574

N. Biver https://orcid.org/0000-0003-2414-5370

S. N. Milam https://orcid.org/0000-0001-7694-4129

S. B. Charnley https://orcid.org/0000-0001-6752-5109

J. Boissier https://orcid.org/0000-0002-1545-2136

B. P. Bonev https://orcid.org/0000-0002-6391-4817

C. Qi https://orcid.org/0000-0001-8642-1786

A. J. Remijan https://orcid.org/0000-0001-9479-9287

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