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Protoplanetary disk models show ammonium salts dominate nitrogen and sulfur in the inner midplane.

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2026-08-04 12:54 UTC pith:7TBMIPCV

load-bearing objection A credible disk-chemistry model that predicts ammonium salts dominate N and S in the inner midplane, but the headlining cyanate result hinges on one unmeasured ice reaction; worth refereeing with a request for a sensitivity test. the 2 major comments →

arxiv 2608.02173 v1 pith:7TBMIPCV submitted 2026-08-03 astro-ph.EP astro-ph.GAastro-ph.SR

Ammonium salt formation and abundance in protoplanetary disks

Maxime Ruaud , Jean-Christophe Loison , Uma Gorti This is my paper
classification astro-ph.EP astro-ph.GAastro-ph.SR
keywords astrochemistryprotoplanetary disksammonium saltsnitrogen reservoirsulfur depletioncosmic-ray chemistrysnowlinescometary ices
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

This paper uses thermo-chemical disk models to argue that ammonium salts are not trace byproducts but a major reservoir in the comet-forming zone of protoplanetary disks. In the inner ~50 au midplane, grain-surface chemistry converts volatile nitrogen and sulfur carriers into salts: after 10 Myr of modeled evolution, nearly all available nitrogen sits in ammonium cyanate (NH4+OCN-) and nearly all available sulfur in ammonium hydrosulfide (NH4+SH-) on icy grains. The key mechanism is a cosmic-ray-driven sink inside ~30 au that progressively turns gas-phase CO and N2 into CO2 ice and ammonia ice over ~1 Myr, feeding salt formation and dragging the CO and N2 snowlines inward. The result matters because it offers a concrete, quantitative route to the long-missing nitrogen and sulfur reservoirs in comets, asteroids, and the inner solar system. The authors caution that the salt abundances are upper limits, since they include no ice destruction other than desorption.

Core claim

The paper argues that ammonium salts form readily in the inner midplane of protoplanetary disks (r ~ 50 au), and after ~10 Myr essentially all available nitrogen there sits in salts, chiefly ammonium cyanate (NH4+OCN-), while essentially all available sulfur sits in ammonium hydrosulfide (NH4+SH-) on grain surfaces. Formation proceeds by acid-base proton transfer in the ice: ammonia reacts with HCN, HNC, H2S, HNCO, HCOOH, and related acids; the dominant nitrogen salt forms via sNH3 + sHCN -> sNH4+CN- followed by sO + sNH4+CN- -> sNH4+OCN-. A cosmic-ray-driven sink inside ~30 au removes gas-phase CO and N2, converting them to CO2 ice and ammonia ice on ~1 Myr timescales, then to salts; this p

What carries the argument

The central mechanism is a coupled gas-grain chemical model with a two-phase ice treatment (surface and mantle), diffusion-driven surface reactions, and depth-dependent cosmic-ray ionization. Salt formation is modeled as proton transfer from acids (HCN, HNC, H2S, HNCO, HCOOH, CH3COOH, NH2COOH) to ammonia base in the ice, producing ammonium salts that are stable only in the ice and dissociate back to NH3 + HX on desorption. The specific chain that builds the dominant salt is sNH3 + sHCN -> sNH4+CN-, then sO + sNH4+CN- -> sNH4+OCN-, with the second step assumed rapid on ices. The cosmic-ray-driven sink that removes gas-phase CO and N2 via H3+ chemistry and water-ice photoprocessing is what amp

Load-bearing premise

Everything hinges on the unmeasured ice-surface reaction O + NH4+CN- -> NH4+OCN- being fast; the paper explicitly notes there is no theoretical or experimental study of this step.

What would settle it

Measure the rate of O-atom additions to ammonium cyanide ice at 10-50 K in the lab; if the reaction is slow or has an activation barrier, the predicted >80% nitrogen share of ammonium cyanate at 10-25 au collapses.

Watch this falsifier. Get emailed when new claim-graph text bears on it.

If this is right

  • If the model is right, the nitrogen missing from cometary volatiles is hidden as ammonium cyanate in grain ice, with salt/H2O ratios of 10-30% at t ~ 1 Myr.
  • The long-standing sulfur depletion puzzle would be solved by ammonium hydrosulfide, which can carry ~100% of available sulfur between ~10 and ~50 au when sulfur starts at solar abundance.
  • CO and N2 snowlines shift inward by several au over 1-10 Myr, so gas-phase tracers such as N2H+ and CO emission evolve with time even at fixed disk structure.
  • Upon inward drift and sublimation at ~1-3 au (T ~ 120-200 K), salts release NH3 and HX, predicting enhanced inner-disk abundances of NH3, H2S, HNCO, and HCOOH from salt dissociation.
  • JWST-accessible spectral signatures, notably the broad 5.3 micrometer NH4+ + SH- band, could provide a direct observational test.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The assumption that the second salt-formation step (sO + sNH4+CN- -> sNH4+OCN-) is fast is untested; if laboratory work shows a barrier or slower rate, the predicted dominance of ammonium cyanate over 10-30 au would weaken, and formate or acetate salts could become the main nitrogen carriers inside ~15 au.
  • Because the model treats salt formation as diffusion-limited, salt abundances in earlier prestellar or collapsing stages may be underestimated; JWST detections of NH4+ and OCN- in dense clouds hint that a sizeable salt reservoir may already exist before the disk forms.
  • If vertical mixing is stronger than assumed (alpha ~ 10^-2), the cosmic-ray-driven sink on N2 is reduced, which would damp the salt yield; disks with weak mixing would be the best places to look for salt-dominated nitrogen reservoirs.
  • The paper's salt reservoir predicts that planetesimals formed at ~10-50 au inherit a different N/C ratio than their gas, offering a potential compositional tracer of the disk region from which comets and asteroids accreted.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

2 major / 5 minor

Summary. The paper presents thermo-chemical disk models with an extended gas-grain network that includes ammonium salt formation via acid-base reactions on ices. Using the Ruaud & Gorti (2019) framework, it predicts that in the inner disk midplane (r ≲ 50 au) salts form efficiently, with ammonium cyanate (sNH4+OCN-) becoming the dominant nitrogen carrier (>80% at 10–25 au after 10 Myr) and ammonium hydrosulfide (sNH4+SH-) capturing most of the sulfur. The authors identify a cosmic-ray-driven sink that converts gas-phase CO and N2 into CO2 and salts over timescales ≳1 Myr, shifting the CO and N2 snowlines inward. They explore variations in the elemental sulfur abundance and cosmic-ray ionization rate, and connect the results to comet 67P and JWST observations of OCN-. The results are explicitly framed as upper limits because salt destruction processes other than thermal desorption are not included.

Significance. If the predicted salt production is robust, the paper supplies a plausible mechanism for the refractory nitrogen and sulfur reservoirs inferred in comets, and it connects disk chemistry to observed OCN- and NH4+ ice features. The modeling is detailed and largely transparent: desorption parameters are taken from experimental and quantum-chemical studies, the authors clearly identify the unmeasured sO + sNH4+CN- reaction as a key assumption, and they include a cosmic-ray-rate sensitivity test. The central nitrogen-budget claim, however, hangs on this single unmeasured surface reaction; a zero-rate or varied-rate sensitivity test is essential before the quantitative predictions can be accepted. With such a test, the paper would be a strong contribution to disk astrochemistry.

major comments (2)
  1. [§3.1, Table 1] The dominant pathway for sNH4+OCN- is assumed to be sNH3 + sHCN → sNH4+CN- followed by sO + sNH4+CN- → sNH4+OCN-. The text states 'There is no theoretical or experimental study of the second step' but argues it is rapid. This assumption is load-bearing for the central claim in §3.4 that sNH4+OCN- carries ≳80% of the available nitrogen at 10≲r≲25 au at 10 Myr. If this reaction has a barrier or is slow, sNH4+CN- would remain abundant (contradicting the stated <10^-10 abundance) and the cyanate abundance would collapse, undermining the comparison to observed OCN-. The upper-limit caveat in §4.3 covers destruction, not branching. Please run a sensitivity test with this rate set to zero (or varied over a plausible range) and report the resulting nitrogen partitioning. Without this, the quantitative nitrogen-budget claim is not supported.
  2. [§3.2] The cosmic-ray-driven sink effect that converts gas-phase N2 into sNH3 and then salts is a central mechanism for the 10-Myr enhancement of salt abundances, and it is highlighted in the abstract. However, vertical mixing is not included in the model. The authors justify this by citing Furuya & Aikawa (2014), who find that strong turbulence (α=10^-2) reduces the sink, and by noting that typical disk α values are lower. This is a reasonable argument, but no quantitative test is performed for the present model. Since the predicted abundance evolution from t=1 Myr to t=10 Myr depends on this sink, please either implement a simple vertical mixing prescription or use published results to estimate the maximum effect on the salt abundances. This would make the time-dependent claims more robust.
minor comments (5)
  1. [§3.1, Appendix A] Typographical and notation issues: in §3.1 'ammonium salts efficiently from' should be 'form'; in Eq. (A4) the product should be sNH4+NH2COO- (ammonium carbamate) rather than sNH4+NH2COOH-.
  2. [Fig. 5 caption] The caption reads 'CR = 10 18 s 1' and should be 'ζ_CR = 10^-18 s^-1'.
  3. [§4.1] HD163296 should be written with a space: 'HD 163296'.
  4. [§2.2] The desorption treatment of salts is described only briefly: 'when salts desorb, they dissociate into the stable products NH3 + HX'. Please clarify how the desorption of the salt is implemented in the model—specifically, whether both NH3 and HX are released at the salt's binding temperature, and how this relates to the apparent desorption temperature of, e.g., NH3 in cometary comae.
  5. [§3.1] The text says 'A small fraction of sNH4+OCN- also forms directly from sNH3 + sHNCO.' Since this direct route is chemically more secure than the two-step route, please quantify this fraction at a representative radius (e.g., 10–25 au) to support the claim that the two-step pathway dominates.

Circularity Check

0 steps flagged

No significant circularity: salt abundances are genuine network outputs; the main cyanate route is an explicitly acknowledged assumption, not a fitted or definitional input.

full rationale

The derivation chain is not circular. Salt abundances are outputs of a time-dependent gas-grain chemical network with rate coefficients taken from laboratory experiments and quantum-chemical calculations; no parameter is fitted to the comet or JWST data used for comparison. The dominant salt, sNH4+OCN-, is produced through sNH3 + sHCN -> sNH4+CN- followed by the assumed reaction sO + sNH4+CN- -> sNH4+OCN-. The paper explicitly states in Sec. 3.1 that 'There is no theoretical or experimental study of the second step, but we argue that it is reasonable to assume that this reaction will be rapid on ices' and repeats in Sec. 4.3 that this reactivity 'has not been thoroughly investigated, either theoretically or experimentally.' This is an acknowledged modeling assumption and a genuine sensitivity risk, not a circularity: the predicted cyanate abundance is not defined as the assumed reaction rate, and a slow rate would change the result, showing the prediction is not tautological. The cosmic-ray-driven sink that converts gas-phase CO and N2 into sCO2 and sNH3 is not merely imported by self-citation. It is demonstrated in the paper itself: Figure 2 shows the midplane evolution of CO, N2, sCO2, and sNH3 in a model with salt formation neglected, and the same sink was found in K. Furuya & Y. Aikawa (2014) and Y. Aikawa et al. (2015), so the Ruaud & Gorti (2019) self-citation is not the sole load-bearing support. The disk model and photodesorption yields cite the authors' prior work, but those are methodological references, not results reconstructed from the salt claim. The high-sulfur model result that sNH4+SH- carries nearly 100% of available sulfur between ~10 and 50 au is a network consequence of the assumed sNH3 + sH2S reaction and the chosen elemental abundance; it is a model output rather than a rename of the input. The upper-limit caveat in Sec. 4.3 affects robustness, not circular structure. Overall, no step reduces by construction, and no prediction is equivalent to its fitted inputs.

Axiom & Free-Parameter Ledger

5 free parameters · 6 axioms · 0 invented entities

The model relies on a network of assumed rate coefficients and physical parameters, most taken from prior literature or quantum-chemistry calculations. No new physical entities are introduced. The key ad hoc assumption is the unmeasured O + CN- surface reaction that produces the dominant salt, and the deliberate neglect of salt destruction, which the authors explicitly flag as making their predictions upper limits.

free parameters (5)
  • E_diff/E_bind ratio for surface species = 0.4 (surface), 0.8 (mantle)
    Diffusion barriers poorly constrained; assumed fixed fraction of binding energy (Sec. 2.1). Controls mobility of acidic reactants and thus salt formation efficiency.
  • Photodesorption yield = 10^-3 molecules/photon for all species
    Assumed similar for all species (Sec. 2.1); affects ice composition and the sink effect.
  • Initial elemental sulfur abundance (S/H) = 1e-6 (fiducial), 1.5e-5 (solar)
    Varied in Sec. 3.3; controls NH4SH abundance and N/S partitioning.
  • Cosmic-ray ionization rate = depth-dependent ζ_CR ~1e-17 to 1e-16.5 s^-1 (fiducial); 1e-18 s^-1 (low-CR model)
    Central to the sink effect; Sec. 3.5 explores sensitivity.
  • Viscous turbulence parameter α = 5e-3
    Sets dust settling and justifies neglect of vertical mixing in the fiducial model (Sec. 2.1, 3.2).
axioms (6)
  • domain assumption Acid-base reactions in ice form ammonium salts at cryogenic temperatures with efficiency only weakly dependent on H2O dilution.
    Taken from lab experiments (van Broekhuizen et al. 2004); basis for all salt formation routes.
  • domain assumption Salts desorb by dissociating into NH3 + HX.
    From lab/calculations (Kruczkiewicz et al. 2021; Loison et al. 2025); used for desorption treatment.
  • ad hoc to paper The O + CN- reaction is rapid in ice by analogy with gas-phase anion chemistry.
    No experimental/theoretical study; authors argue by analogy (Sec. 3.1). Load-bearing for the dominant salt NH4OCN-.
  • ad hoc to paper No destruction processes for salts in ice other than thermal desorption.
    Acknowledged by authors as making results upper limits (Sec. 4.3).
  • domain assumption Cosmic-ray-driven photodissociation of water ice produces sOH, converting sCO to sCO2 on ~1 Myr timescale.
    From the authors' prior model (Ruaud & Gorti 2019), used to drive the sink effect.
  • domain assumption Vertical mixing is negligible (α ~ 5e-3, less than 1e-2).
    Assumed to neglect vertical mixing that could reduce the N2 sink (Sec. 3.2).

pith-pipeline@v1.3.0-daily-deepseek · 16661 in / 12168 out tokens · 82087 ms · 2026-08-04T12:54:30.220115+00:00 · methodology

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read the original abstract

Ammonium salts may represent an important reservoir of volatile species in Solar system primitive bodies, but the question of how and when these salts can form during the star formation process remains unknown. In this paper, we use thermo-chemical models to study the formation of ammonium salts during the protoplanetary disk stage. We show that ammonium salts form efficiently in the inner disk midplane (i.e. $r \lesssim 50 $ au), inside the comet forming region. In this region, our model predicts that almost all the available nitrogen is in the form of salts (i.e. mainly in ammonium cyanate) at the surface of grains after evolving for 10 Myrs. For sulfur, we show that almost all the available S is in the form of ammonium hydrosulfide in the inner disk midplane. We show that inside $r\sim 30$ au, ammonium salt formation is enhanced by a cosmic-ray-driven sink effect that progressively converts gas-phase CO and N$_2$ into carbon dioxide and salts, respectively, at the surface of grains on a timescale $\gtrsim 1$ Myr. This impacts the location of the CO and N$_2$ radial snowlines which both shift closer to the star as a function of time.

Figures

Figures reproduced from arXiv: 2608.02173 by Jean-Christophe Loison, Maxime Ruaud, Uma Gorti.

Figure 1
Figure 1. Figure 1: Computed abundance maps of ammonium cyanate, ammonium formate, ammonium acetate, ammonium hydrosulfide and ammonium carbamate in the solid phase for r < 50 au and computed after 1 Myr and 10 Myrs. We also show the ratio of the vertically integrated abundances of these salts relative to water ice [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Computed midplane abundance of CO, N2, sCO2 and sNH3 as a function of time and at r = 10 au. For this figure, salts formation was neglected. that converts gas-phase N2 into sNH3 and subsequently to ammonium salts after t = 1 Myr. This sink effect on timescales ≳ 1 Myr, is driven by cosmic rays, for which we use a depth dependent expres￾sion from M. Padovani et al. (2018) (see also M. Ruaud & U. Gorti 2019)… view at source ↗
Figure 3
Figure 3. Figure 3: Sulfur partitioning in the disk midplane for the model with an elemental abundance of sulfur of 1.5 × 10−5 . The top panel is at t = 1 Myr and the bottom panel at t = 10 Myrs. We note that there a no major differences in the sulfur partitioning when using an elemental sulfur abundance of 10−6 . its surface (K. Altwegg et al. 2022). In fact, the simul￾taneous detection of large quantities of H2S and NH3 af￾… view at source ↗
Figure 4
Figure 4. Figure 4: Nitrogen partitioning in the disk midplane for the model with an elemental abundance of sulfur of 10−6 (left panels) and 1.5 × 10−5 (right panels). Top panels are at t = 1 Myr and bottom panels at t = 10 Myrs. salts is explained by the efficient diffusion of sHCN at the surface of the ice compared to other acidic reactants like sHCOOH which is nevertheless abundant in the ice (typically on the order of few… view at source ↗
Figure 5
Figure 5. Figure 5: Nitrogen partitioning in the disk midplane com￾puted at t = 10 Myrs for the model with an elemental abun￾dance of sulfur of 10−6 and a low cosmic-ray ionization rate of ζCR = 10−18s −1 . the results. We adopt a constant cosmic-ray ionization rate of ζCR = 10−18 s −1 throughout the disk, which is the median value inferred from observations of N2H+ in the exoALMA disk sample by L. Trapman et al. (2025) as we… view at source ↗

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