REVIEW 2 major objections 1 minor 1 cited by
Methane depletion within the first million years keeps inner disc C/O ratios at or below 1 even when pebbles drift inward.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · grok-4.3
2026-06-28 20:07 UTC pith:JFN3GHNN
load-bearing objection The paper shows cosmic rays and ice chemistry can suppress inner-disc C/O below 1 via fast methane depletion and traps, but the 10% surface-reaction threshold is an untested control knob. the 2 major comments →
Metamorphoses of carbon and oxygen in protoplanetary discs: how chemistry and radial drift transform inner disc C/O ratios
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Methane depletion within less than 1 Myr prevents the delivery of carbon-rich gas to the inner disc and yields C/O less than or equal to 1 for cosmic-ray ionization rates at or above 10 to the minus 17 per second. Dust traps collect water and carbon-rich ices formed via methane destruction, further lowering the inner disc metallicity and C/O ratio. Cosmic-ray driven photodissociation of ices can convert water to O2 and carbon-bearing molecules to CO, allowing ices to escape the trap if at least 10 percent of the dissociated products can participate in surface reactions.
What carries the argument
The coupled action of radial pebble drift carrying ices, dust traps that sequester those ices, and cosmic-ray photodissociation plus surface chemistry that can release a fraction of the ices back into the gas.
Load-bearing premise
At least 10 percent of the fragments produced by cosmic-ray dissociation of ices must participate in surface reactions that let the ices escape dust traps.
What would settle it
An inner-disc C/O ratio clearly above 1 measured in a disc with a cosmic-ray ionization rate of 10 to the minus 17 per second or higher would falsify the claim that methane depletion and trap sequestration dominate.
If this is right
- Observed inner-disc C/O ratios should remain at or below 1 across a wide range of disc sizes and accretion rates once cosmic-ray ionization exceeds 10 to the minus 17 per second.
- The location and efficiency of dust traps directly control how much water and carbon-rich ice is removed before it reaches the inner disc.
- Changing the cosmic-ray ionization rate alters both the speed of methane depletion and the fraction of ices that can leave traps.
- Destruction of carbon grains supplies an additional source of gas-phase carbon that can raise C/O only if the cosmic-ray rate is low enough to preserve methane.
Where Pith is reading between the lines
- The same mechanism could explain why JWST spectra show inner-disc compositions that correlate with disc size and stellar mass through the efficiency of drift and trapping.
- Laboratory measurements of surface reaction yields on ices under cosmic-ray bombardment would directly test the 10-percent escape threshold.
- Discs around stars with different local cosmic-ray environments should exhibit systematically different inner C/O ratios if the chemistry is the controlling process.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript models how chemical processing of ices on drifting pebbles, driven by cosmic-ray ionization rate ζ and dissociation efficiency, alters inner-disc C/O ratios in a solar-mass protoplanetary disc. It incorporates variable dust radial-drift efficiencies (set by fragmentation velocity), dust traps at different radii, and carbon-grain destruction. The central results are that methane depletion within <1 Myr yields C/O ≲1 for ζ ≥ 10^{-17} s^{-1}, that traps further reduce inner-disc metallicity and C/O by sequestering water and carbon-rich ices, and that cosmic-ray photodissociation (converting H2O to O2 and CH4-derived species to CO) permits ices to escape traps provided ≳10 % of the products participate in surface reactions; the authors conclude that cosmic rays are the dominant factor controlling chemistry’s effect on inner-disc composition.
Significance. If the quantitative outcomes survive scrutiny of the chemical network and parameter choices, the work supplies a concrete mechanism linking cosmic-ray-driven ice chemistry to the observed JWST correlations between inner-disc C/O, disc size, accretion rate and stellar mass, while highlighting an under-appreciated role for dust traps and cosmic rays in setting the volatile budget delivered to forming planets.
major comments (2)
- [Abstract] Abstract (final paragraph): the requirement that ≳10 % of cosmic-ray-dissociated ice products must participate in surface reactions to escape dust traps is introduced without laboratory calibration, without a sensitivity study across 1–50 %, and without demonstration that the adopted network produces sufficient mobile species (O2, CO) at that efficiency. This threshold, together with the chosen fragmentation velocities that control drift, is load-bearing for the claim that cosmic rays dominate the C/O outcome; altering either parameter can remove the reported C/O ≲1 result for ζ ≥ 10^{-17} s^{-1}.
- [Abstract] Abstract: the chemical network that converts H2O to O2 and carbon-bearing molecules to CO under cosmic-ray processing is not specified or validated, preventing assessment of whether the stated conversions are robust or whether they actually yield enough mobile products to satisfy the 10 % escape criterion.
minor comments (1)
- The abstract states that ζ and fragmentation velocity are varied but does not report the numerical ranges or the specific values that produce the quoted C/O ≲1 threshold.
Simulated Author's Rebuttal
We thank the referee for their constructive and detailed report. We address each major comment below and will revise the manuscript to incorporate clarifications and additional analyses where needed.
read point-by-point responses
-
Referee: [Abstract] Abstract (final paragraph): the requirement that ≳10 % of cosmic-ray-dissociated ice products must participate in surface reactions to escape dust traps is introduced without laboratory calibration, without a sensitivity study across 1–50 %, and without demonstration that the adopted network produces sufficient mobile species (O2, CO) at that efficiency. This threshold, together with the chosen fragmentation velocities that control drift, is load-bearing for the claim that cosmic rays dominate the C/O outcome; altering either parameter can remove the reported C/O ≲1 result for ζ ≥ 10^{-17} s^{-1}.
Authors: We agree that the ≳10% efficiency threshold is a key assumption whose robustness should be demonstrated more explicitly. In the revised manuscript we will add a dedicated sensitivity study (new figure and text) varying the surface-reaction participation fraction from 1% to 50% while holding other parameters fixed, showing that the C/O ≲1 outcome for ζ ≥ 10^{-17} s^{-1} persists above ~5–10% for the fiducial fragmentation velocities. We will also expand the discussion of fragmentation-velocity choices, noting that they lie within the range used in prior radial-drift studies and illustrating how modest changes affect the timing of methane depletion. On laboratory calibration, direct experimental constraints on the precise fraction of dissociated products that remain mobile and react on grain surfaces are indeed limited; our adopted value is presented as an illustrative threshold motivated by typical surface-reaction branching ratios in the astrochemical literature. We will add a short paragraph citing relevant ice-irradiation experiments and models to make this motivation explicit. revision: yes
-
Referee: [Abstract] Abstract: the chemical network that converts H2O to O2 and carbon-bearing molecules to CO under cosmic-ray processing is not specified or validated, preventing assessment of whether the stated conversions are robust or whether they actually yield enough mobile products to satisfy the 10 % escape criterion.
Authors: The reactions converting H2O → O2 and CH4-derived species → CO are described in the Methods section (cosmic-ray dissociation rates and subsequent surface chemistry), drawing on standard astrochemical networks. To improve clarity we will (i) add an explicit cross-reference from the abstract to the relevant Methods subsection, (ii) include a short validation paragraph comparing the resulting O2 and CO abundances against published ice-irradiation experiments and other models, and (iii) show in the supplementary material that, at the 10% participation efficiency, the network indeed supplies sufficient mobile O2 and CO to allow escape from traps. These additions will allow readers to assess the robustness of the conversions directly. revision: yes
Circularity Check
No circularity detected; parameter exploration with independent forward modeling
full rationale
The provided abstract and context describe a parameter study that varies ζ (cosmic-ray ionization rate), dust fragmentation velocity, and cosmic-ray dissociation efficiency of ices, then reports resulting inner-disc C/O ratios and metallicity. No quoted equations, claims, or steps reduce any reported outcome to an input by construction (e.g., no fitted parameter renamed as prediction, no self-definition of C/O via the 10% threshold, and no load-bearing self-citation chain). The ≳10% surface-reaction efficiency is introduced as a modeling assumption controlling ice escape, but is not shown to be derived from or equivalent to the target C/O result. The derivation chain therefore remains self-contained against external benchmarks and does not match any enumerated circularity pattern.
Axiom & Free-Parameter Ledger
free parameters (3)
- cosmic ray ionisation rate ζ
- dust fragmentation velocity
- surface-reaction efficiency after dissociation
axioms (2)
- domain assumption Standard thermal structure and dust properties of a protoplanetary disc around a solar-mass star
- domain assumption Particular chemical network governing gas-ice reactions for C and O species
read the original abstract
The chemical composition of a protoplanetary disc is sensitive to its thermal structure and dust properties, and can provide insights into the disc evolution. Recent observations with the James Webb Space Telescope (JWST) reveal correlations of the inner disc compositions with disc size, accretion rate and stellar mass, explained by the key role of dust radial drift in redistributing primordial volatiles. We explore how chemical reactions change the composition of ices carried with pebbles and how they affect the inner disc C/O ratios in a disc around a solar mass star. We consider different drift efficiencies set by dust fragmentation velocity and include dust traps at different locations. We vary the incident cosmic ray ionisation rate $\zeta$ and the efficiency of cosmic ray dissociation of ices, and consider the effect of carbon grain destruction. We find that methane depletion within $<1$ Myr prevents the delivery of carbon-rich gas to the inner disc and yields $\mathrm{C/O} \lesssim1$ for $\zeta\geq10^{-17}$ s$^{-1}$. Dust traps collect water and carbon-rich ices formed via methane destruction, further lowering the inner disc metallicity and C/O ratio. Cosmic-ray driven photodissociation of ices can convert water to O$_2$ and carbon-bearing molecules to CO, allowing ices to escape the trap if $\gtrsim10 \%$ of the dissociated products can participate in surface reactions. We discuss the observational implications and conclude that cosmic rays and their effect on ices are the key factors that determine the impact of chemistry on the inner disc composition.
Forward citations
Cited by 1 Pith paper
-
The Influence of Dust Composition on Accretion Outbursts
Using 1D simulations with dust evaporation and condensation, the paper shows that dead-zone accretion outbursts vaporize dust out to about 0.5 au and that higher dust sublimation temperatures produce stronger but less...
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.