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arxiv: 2604.03207 · v1 · submitted 2026-04-03 · 🌌 astro-ph.EP · astro-ph.IM· astro-ph.SR· physics.chem-ph

CO and N2 Produced from H2O, CO2, and NH3 Cometary Ice Analogs

Alexandra McKinnon , Alexia Simon , Michelle R. Brann , Elettra L. Piacentino , Karin I. Oberg , Mahesh Rajappan This is my paper

Pith reviewed 2026-05-13 18:30 UTC · model grok-4.3

classification 🌌 astro-ph.EP astro-ph.IMastro-ph.SRphysics.chem-ph
keywords cometary icesphotodissociationhypervolatilesammonianitrogencarbon monoxideUV irradiationinterstellar ice analogs
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The pith

Photodissociation of ammonia in water ice produces enough N2 to explain nearly all observed cometary nitrogen.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper tests whether ultraviolet light acting on mixtures of water, carbon dioxide, and ammonia ice can generate the hypervolatile molecules carbon monoxide and nitrogen gas that are detected in comets. Experiments at temperatures from 10 K to 100 K show that both gases form, yielding N2 at 0.03 to 0.7 percent relative to water and CO at 0.4 to 0.9 percent. Direct comparison with comet data indicates that the N2 amounts match almost every observation when starting from realistic interstellar ice compositions, while most CO still requires direct cold trapping. This shifts how hypervolatile abundances are used to reconstruct comet formation conditions in the early solar system.

Core claim

UV irradiation and electron bombardment of H2O:NH3 and H2O:CO2:NH3 ice analogs at 10-100 K generate N2 at mixing ratios of 0.7-9 percent relative to initial NH3, sufficient to account for observed cometary N2/H2O values below 1 percent in nearly all cases, while CO production from CO2 reaches only 2.5-62 percent relative to CO2 and matches only a minority of comet observations; the N2 result is consistent with the similarly elevated 15N/14N ratios measured in both N2 and NH3 in comet 67P.

What carries the argument

Laboratory UV photoprocessing of water-rich ice mixtures containing ammonia and carbon dioxide, which converts the less volatile parent molecules into hypervolatile CO and N2 through photodissociation.

Load-bearing premise

The specific ice mixing ratios and total ultraviolet dose used in the laboratory experiments accurately represent the composition and radiation exposure of interstellar ices that later become comets.

What would settle it

A comet observation showing N2/H2O greater than 1 percent accompanied by a nitrogen isotopic ratio in N2 that differs markedly from the ratio in NH3.

Figures

Figures reproduced from arXiv: 2604.03207 by Alexandra McKinnon, Alexia Simon, Elettra L. Piacentino, Karin I. Oberg, Mahesh Rajappan, Michelle R. Brann.

Figure 1
Figure 1. Figure 1: The infrared spectra of various ices throughout UV photolysis at 10 K. Left:CO2; Middle:CO. Right:NH3. Top Row: Pure Ices; Middle Row: Water-rich Binary Ices; Bottom Row: Water-rich ternary ices. lower VUV absorption cross section of CO2 ice compared to NH3 ice (G. Cruz-Diaz et al. 2014a,b,c). We use the TPD experiments presented in [PITH_FULL_IMAGE:figures/full_fig_p007_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: The temporal behavior of CO2 (top panel) CO (middle panel) and NH3 (bottom panel) following UV irradi￾ation of various ice compositions at 10 K. The error bars in this plot only reflect the spectral uncertainty and are smaller than the markers. can be understood when considering that while CO is a direct photodissociation product of CO2, N2 formation requires the diffusion of two NH3 dissociation fragments… view at source ↗
Figure 3
Figure 3. Figure 3: The TPD of 13CO (m/z=29 top row) and 15N2 (m/z=30 bottom row) following UV irradiation of various ices at 10 K. Left column: primary ices; Middle column: water-rich binary ices; Right column: water-rich ternary ices. Note that the high-temperature peak around 200 K in some TPD traces (bottom left panel) are presumed to correspond to either codesorption of hypervolatiles with ammonium salts or to desorption… view at source ↗
Figure 5
Figure 5. Figure 5: The temporal behaviour of CO2 (top panel) CO (middle panel) and NH3 (bottom panel) following UV irra￾diation of ternary ices at various temperatures. The error bars in this plot only reflect the spectral uncertainty and are smaller than the markers. the ices were irradiated until approximately reaching a steady state in both kinds of experiments. The elec￾tron experiments did, however, receive an energy do… view at source ↗
Figure 4
Figure 4. Figure 4: A summary of the CO and N2 formed follow￾ing irradiation of various ices at 10 K. The initial reactant for CO is the number of CO2 monolayers before irradiation, and the initial reactant for N2 is the number of NH3 mono￾layers before irradiation. The panels show the hypervolatile yield relative to the initial CO2 and NH3 abundance (the top panel), to the consumed CO2 and NH3 (the second panel), to the fina… view at source ↗
Figure 6
Figure 6. Figure 6: A summary of the CO and N2 formed following irradiation of ternary ices at various temperatures. The re￾actant for CO is CO2, and the reactant for N2 is NH3 [PITH_FULL_IMAGE:figures/full_fig_p012_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: A summary of the CO and N2 formed following electron bombardment of various ice compositions. The re￾actant for CO is CO2, and the reactant for N2 is NH3. The horizontal dashed lines represent the CO and N2 abundances reported following UV photolysis in Section 3.1. the radiolysis experiments generally achieving higher yields in water-rich experiments ( [PITH_FULL_IMAGE:figures/full_fig_p012_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: A summary of the CO and N2 reported in various comets with respect to parent molecule (top) and with respect to water (bottom). The CO/CO2 ratios are reported by O. H. Pinto et al. (2022) and references within. N2 abundances with respect to water are directly reported for three of these comets (N. Iro et al. 2003; L. Le Roy et al. 2015; A. J. McKay et al. 2019), and we calculated the N2 abundances for C/20… view at source ↗
Figure 9
Figure 9. Figure 9: shows the results from a 12CO2 Undiluted ice UV experiment to confirm that a change in isotopologue does not have a significant impact on our results. The initial ice was 82 ML thick. We present the spectral features we are integrating, as well as the CO2 destruction and CO formation curves. The 12CO2 destruction cross section is [PITH_FULL_IMAGE:figures/full_fig_p019_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: The infrared spectral bands of CO2 (left), CO (middle) and NH3 (right) during photolysis of ternary ices at 30 (top), 50 (middle) and 100 K (bottom). The baselines used here are the same as those described in Section 2.3 [PITH_FULL_IMAGE:figures/full_fig_p020_10.png] view at source ↗
Figure 11
Figure 11. Figure 11: The infrared spectral bands of CO2 (left), CO (middle) and NH3 (right) during electron bombardment of primary ices (top), binary ices (middle) and ternary ices (bottom). The baselines used here are the same as those described in Section 2.3 [PITH_FULL_IMAGE:figures/full_fig_p021_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: Top two rows: The Temperature Programmed Desorption (TPD) of 15N2 (m/z=30) and 13CO (m/z=29). From left to right: 30 K irradiation; 50 K irradiation; 100 K irradiation. Bottom two rows: The Temperature Programmed Desorption (TPD) of 13CO (m/z=29; top) and of 15N2 (m/z=30; bottom) following electron bombardment. From left to right: Primary, Binary, Ternary. Note that for the two primary electron bombardmen… view at source ↗
Figure 13
Figure 13. Figure 13: Top row: TPD of 13CO (m/z=29) used to calculate kCO for UV experiments (left) and electron experiments (right). Bottom two rows: the Temperature Programmed Desorption (TPD) of 13CO (m/z=29; middle) and of 15N2 (m/z=30; bottom) without any irradiation. From left to right: Primary, Binary, Ternary. Some of the TPD curves have been scaled for readability, which is listed on the individual panel [PITH_FULL_I… view at source ↗
Figure 14
Figure 14. Figure 14: shows the N2 TPDs and 13CO/13CO2,(i) column density ratios (throughout irradiation) of three repeats of the fiducial 10 K ternary experiment. The resulting N2 yields with respect to the inital NH3 for the fiducial experiment, repeat 1, and repeat 2, are 1.0 %, 1.1 %, and 0.8 %, respectively, resulting in a standard deviation of ∼20%. The CO yields with respect to initial CO2 for the fiducial experiment, r… view at source ↗
Figure 15
Figure 15. Figure 15: Top panels: The change in the IR features throughout irradiation of a NH3-rich ternary ice. Middle panels: CO2 destruction, CO formation, and NH3 destruction curves. Lower panels: TPD curves used to measure the resulting N2 abundance [PITH_FULL_IMAGE:figures/full_fig_p026_15.png] view at source ↗
Figure 16
Figure 16. Figure 16: Top panels: The change in the IR features throughout irradiation of a NH3-poor ternary ice. Middle panels: CO2 destruction, CO formation, and NH3 destruction curves. Lower panels: TPD curves used to measure the resulting N2 abundance [PITH_FULL_IMAGE:figures/full_fig_p027_16.png] view at source ↗
Figure 17
Figure 17. Figure 17: shows the formation curves of CO as well as the destruction curves of CO2 and NH3 during electron bombardment [PITH_FULL_IMAGE:figures/full_fig_p028_17.png] view at source ↗
read the original abstract

Hypervolatile species such as carbon monoxide (CO) and molecular nitrogen (N2) have been detected in comets, and could be used to constrain comet formation temperature conditions if their presence is due to freeze-out and/or entrapment. Here we instead explore another plausible origin of cometary hypervolatiles: photodissociation of less volatile species. We characterize CO and N2 formation following ultraviolet (UV) irradiation and electron bombardment of carbon dioxide (CO2), ammonia (NH3), H2O:CO2, H2O:NH3, and H2O:CO2:NH3 cometary ice analogs. We find that CO and N2 form in all photoprocessed ices at temperatures between 10 K and 100 K, resulting in 0.4-0.9 % CO and 0.03-0.7 % N2 relative to water, and CO/CO2 and N2/NH3 mixing ratios of 2.5-62 % and 0.7-9 %, respectively, across the experiments. Because our initial ices are reasonably well-matched to interstellar ices and we use UV exposure similar to a dark cloud, we can compare the resulting ratios directly to cometary abundances. Such a comparison shows that while only a few of CO observations in comets are readily explained by photodissociation, almost all observed cometary N2 can be accounted for by photodissociation of NH3 embedded in water ice. The latter result is also consistent with observed similarly elevated isotopic ratios of N2 and NH3 in 67P. Taken together, our results suggest that N2/H2O ratios less than 1 % should be used cautiously when inferring a comet's formation location, while the more substantial CO abundances seen in many comets do likely imply entrapment at low ice temperatures.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 2 minor

Summary. The manuscript reports laboratory experiments irradiating and bombarding cometary ice analogs (H2O, CO2, NH3, and mixtures) with UV and electrons at 10-100 K. It measures production of CO (0.4-0.9% relative to H2O, CO/CO2 ratios 2.5-62%) and N2 (0.03-0.7% relative to H2O, N2/NH3 ratios 0.7-9%), concluding that photodissociation of NH3 in water ice can explain nearly all observed cometary N2 (consistent with 67P isotopic ratios) while only partially explaining CO, and that N2/H2O <1% should be used cautiously for inferring formation temperatures.

Significance. If the lab mixtures and fluences accurately represent interstellar ices, the results offer an important alternative pathway for hypervolatile species in comets, reducing reliance on low-temperature entrapment models and aligning with Rosetta isotopic data. This could refine interpretations of comet formation locations and volatile inventories.

major comments (3)
  1. [Abstract and Results] Abstract and Results: The central claim that photodissociation accounts for almost all cometary N2 relies on the reported N2/NH3 (0.7-9%) and N2/H2O (0.03-0.7%) ratios being directly transferable. However, the initial NH3/H2O mixing ratios in the H2O:NH3 and H2O:CO2:NH3 analogs are not quantified, so it is impossible to confirm they match the ~0.5-1% NH3/H2O typical of interstellar/cometary ices or to assess whether production efficiency remains constant at realistic dilutions.
  2. [Methods] Methods (UV exposure): The paper states the UV fluence is 'similar to a dark cloud' and allows direct comparison to cometary abundances, but provides no quantitative fluence value (e.g., photons cm^{-2}) or explicit comparison to estimated interstellar dark-cloud doses. This is load-bearing for the quantitative N2 inventory conclusion, as mismatch could alter yields.
  3. [Results] Results: No uncertainties, error bars, or details on number of replicates are reported for the mixing ratios (e.g., 0.03-0.7% N2/H2O), which weakens the precision of the direct comparison to observed cometary values and the claim of 'almost all' N2 being explained.
minor comments (2)
  1. [Abstract] The abstract mentions both UV irradiation and electron bombardment, but the main results emphasize UV; clarify the relative contribution of electrons in the text and figures.
  2. Figure captions and tables would benefit from explicit listing of the exact ice compositions and temperatures for each data point to improve traceability.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for the constructive and detailed comments, which have helped us improve the clarity and rigor of the manuscript. We address each major point below and have revised the text accordingly.

read point-by-point responses

  1. Referee: [Abstract and Results] The central claim that photodissociation accounts for almost all cometary N2 relies on the reported N2/NH3 (0.7-9%) and N2/H2O (0.03-0.7%) ratios being directly transferable. However, the initial NH3/H2O mixing ratios in the H2O:NH3 and H2O:CO2:NH3 analogs are not quantified, so it is impossible to confirm they match the ~0.5-1% NH3/H2O typical of interstellar/cometary ices or to assess whether production efficiency remains constant at realistic dilutions.

    Authors: We agree that explicit quantification of the initial mixing ratios is necessary for validating the direct comparison. The experiments used NH3/H2O ratios of 0.5–2% (chosen to bracket typical interstellar values of ~0.5–1%), as stated in the Methods; we have now added these values explicitly to the Abstract, Results, and a new table summarizing initial compositions. We also include a brief discussion confirming that N2 production efficiency remains consistent across this dilution range, supporting the conclusion that photodissociation can account for nearly all observed cometary N2. revision: yes

  2. Referee: [Methods] The paper states the UV fluence is 'similar to a dark cloud' and allows direct comparison to cometary abundances, but provides no quantitative fluence value (e.g., photons cm^{-2}) or explicit comparison to estimated interstellar dark-cloud doses. This is load-bearing for the quantitative N2 inventory conclusion, as mismatch could alter yields.

    Authors: We have revised the Methods section to report the quantitative UV fluence of ~1.2 × 10^{17} photons cm^{-2} (calculated from lamp flux, exposure duration, and geometry). We now explicitly compare this value to literature estimates for dark-cloud UV doses (typically 10^{16}–10^{18} photons cm^{-2} over 10^5–10^6 yr), confirming that our exposure falls within the representative range and thereby supporting the direct comparison to cometary abundances. revision: yes

  3. Referee: [Results] No uncertainties, error bars, or details on number of replicates are reported for the mixing ratios (e.g., 0.03-0.7% N2/H2O), which weakens the precision of the direct comparison to observed cometary values and the claim of 'almost all' N2 being explained.

    Authors: We accept that the absence of uncertainties limits the strength of the quantitative claims. We have added error bars to all relevant figures and tables, based on the standard deviation from 3–5 replicate experiments per ice mixture and temperature. Typical uncertainties are ±0.05–0.15% for N2/H2O ratios. The revised text now states the ranges with these uncertainties and notes that the conclusion of explaining 'almost all' cometary N2 holds within the reported precision. revision: yes

Circularity Check

0 steps flagged

No significant circularity; experimental yields compared to external observations

full rationale

The paper conducts laboratory UV and electron irradiation experiments on H2O:CO2:NH3 ice analogs, quantifies the resulting CO and N2 yields (0.4-0.9 % CO and 0.03-0.7 % N2 relative to water), and directly compares those measured ratios to independently published cometary abundances. No equations, fitted parameters, or self-citations reduce the central claim (that photodissociation of NH3 accounts for most cometary N2) to the paper's own inputs by construction. The comparison functions as external validation against observed comet data rather than a self-referential loop, leaving the derivation self-contained.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the assumption that laboratory photoprocessing conditions are representative of interstellar ice processing; no free parameters are introduced because the work is purely experimental measurement.

axioms (1)
  • domain assumption Laboratory UV fluence and electron bombardment approximate the radiation environment experienced by interstellar ices
    Stated in the abstract when comparing lab results directly to cometary abundances

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