Near-perihelion activity and composition of 3I/ATLAS from JUICE/MAJIS observations

A. Migliorini; B. Seignovert; C. Leyrat; C. Pilorget; C. Royer; D. Bockelee-Morvan; E. D'Aversa; E. Lellouch; E. Quirico; F. Poulet

REVIEW 1 major objections 8 minor 2 references

CO2 drives near-perihelion activity of interstellar comet 3I/ATLAS

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2026-07-10 04:35 UTC pith:UC35ONNG

load-bearing objection First spacecraft near-IR spectra of an interstellar comet near perihelion: solid production rates, circumstantial CO2-driven activity interpretation, and a tentative organic detection. the 1 major comments →

arxiv 2607.08603 v1 pith:UC35ONNG submitted 2026-07-09 astro-ph.EP astro-ph.GA

Near-perihelion activity and composition of 3I/ATLAS from JUICE/MAJIS observations

D. Bockelee-Morvan , F. Poulet , Y. Langevin , B. Seignovert , C. Leyrat , G. Piccioni , C. Royer , S. Rodriguez

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E. d'Aversa R. Brunetto J. Carter T. Cavalie M. De Sanctis E. Lellouch A. Migliorini C. Pilorget E. Quirico S. Robert F. Tosi

This is my paper

classification astro-ph.EP astro-ph.GA

keywords micronsactivityatlasconsistentemissionfluorescenceheliocentricidentified

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 reports the first near-perihelion infrared observations of the third known interstellar object, comet 3I/ATLAS, obtained by the MAJIS instrument on the JUICE spacecraft between November 2 and 25, 2025, at heliocentric distances of 1.36 to 1.68 au. The authors detect fluorescence emission from water (H2O) and carbon dioxide (CO2) in the coma and track their production rates over four observing dates. Water production declined from about 8 × 10^28 to 4 × 10^28 molecules per second, while the CO2/H2O ratio held steady near 10%. The radial distributions of both gases are consistent with release near the nucleus rather than from extended sources in the coma. The heliocentric evolution of CO2 production follows solar heating trends, and combined with the unusually low gas expansion velocities measured for this comet, the authors argue that CO2 sublimation, not water sublimation, was the primary driver of the comet's activity near perihelion. This would make 3I/ATLAS unusual among well-studied comets, where water typically dominates near-Sun activity. The authors also identify broad emission features in the 3.2 to 3.6 micron range that do not match the fluorescence signatures of common cometary carbon-bearing volatiles like methane or methanol. The spectral positions of these features align with aliphatic C-H stretching vibrations, suggesting that complex organic material is being released from dust grains in the coma. This detection was made only near perihelion, implying a temperature-dependent release process.

Core claim

The central claim is that CO2 sublimation was the dominant driver of near-perihelion activity for interstellar comet 3I/ATLAS, supported by three converging lines of evidence: a stable ~10% CO2/H2O production ratio, CO2 production rates tracking solar heating, and unusually low gas expansion velocities consistent with heavy-molecule-driven outflow. A secondary discovery is the tentative detection of aliphatic organic material released from coma dust grains, evidenced by broad 3.2 to 3.6 micron emission features that do not match any common cometary volatile fluorescence pattern.

What carries the argument

The argument rests on comparing observed H2O and CO2 fluorescence band intensities and radial profiles against synthetic spectra and coma density models (Haser model and power-law variants). Production rates are derived by fitting observed band intensities to Planetary Spectrum Generator simulations, assuming a constant gas expansion velocity of 0.6 km/s and heliocentric-distance-scaled photodissociation rates. The CO2-driven activity conclusion depends on combining these production rates with independently measured low gas expansion velocities and published CO abundances, then comparing the pattern to known cometary activity regimes.

Load-bearing premise

The production rates depend on a single assumed gas expansion velocity of 0.6 km/s applied to all four observing dates, even though the comet receded from 1.36 to 1.68 au over that period. The authors acknowledge this assumption is untestable with available data; if the velocity decreased with heliocentric distance as expected, the derived production rate trends would shift.

What would settle it

If future measurements of 3I/ATLAS or analogous comets show that the gas expansion velocity decreased significantly between 1.36 and 1.68 au, the production rate trends would change and the evidence for CO2-dominated activity would weaken. Additionally, if the broad 3.2 to 3.6 micron features are shown to arise from an instrumental artifact or from fluorescence of an as-yet-unmodeled simple molecule, the organic material claim would not hold.

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

If this is right

If CO2-driven activity is confirmed for 3I/ATLAS, it would distinguish this interstellar comet from solar system hyperactive comets like 103P/Hartley 2, where water sublimation appears to dominate, suggesting that interstellar planetesimals may exhibit different near-surface volatile regimes.
The tentative detection of aliphatic organics released near perihelion, if confirmed by higher signal-to-noise observations, would provide the first infrared spectroscopic evidence for complex organic material in an interstellar comet's dust, complementing the CO enrichment seen in 2I/Borisov.
The discrepancy between MAJIS water production rates and the larger-aperture SOHO/SWAN measurements, if robust, implies that MAJIS samples a near-nucleus water component while extended icy-grain sublimation produces additional water in the outer coma, a pattern testable with simultaneous small- and large-aperture observations.
The high inferred C/O ratio at large heliocentric distances, combined with the CO2-driven activity, supports formation beyond the CO2 snow line in the natal protoplanetary disk, constraining models of interstellar object origins.

Where Pith is reading between the lines

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

If CO2-driven activity is a general property of small interstellar comets formed beyond the CO2 snow line, future ISO detections may show systematically lower gas expansion velocities and higher CO2/H2O ratios than typical solar system comets at similar heliocentric distances, providing a remote diagnostic of extrasolar origin.
The temperature-dependent release of aliphatic organics only near perihelion, if a general phenomenon, suggests that organic inventories in interstellar dust may be underestimated by observations taken at large heliocentric distances where thermal desorption is inactive.
The discrepancy between JWST/MIRI and JWST/NIRSpec CO2 measurements noted by the authors hints that CO2 excitation in cometary comae is not fully understood, and resolving this discrepancy could change derived CO2 production rates and thus the strength of the CO2-driven activity argument.

Editorial analysis

A structured set of objections, weighed in public.

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

Desk Editor's Note

First spacecraft near-IR spectra of an interstellar comet near perihelion: solid production rates, circumstantial CO2-driven activity interpretation, and a tentative organic detection.

read the letter

The headline result: JUICE/MAJIS got the first spacecraft-based near-IR spectroscopy of an interstellar comet near perihelion. The H2O and CO2 detections, production rates, and the ~10% CO2/H2O ratio are solid and well-executed. The radial profile analysis showing near-nucleus release for both species is clean work. The 3.4-micron organic feature detection is genuinely novel for an ISO, though explicitly tentative and single-date with low S/N — the authors flag this honestly throughout. The dust spectral slope measurements fill in the near-perihelion gap in the photometric record nicely. This is a competent observational paper from a team that clearly knows how to reduce and interpret cometary IR spectra under difficult spacecraft constraints. The data quality is what it is — marginal in places — but the analysis is proportionate to the signal available. The constant-velocity assumption (0.6 km/s across all dates) is the weakness the reader flagged, but the stress-test note correctly identifies that this is secondary: the CO2/H2O ratio is velocity-insensitive since both species scale identically, and the ratio's constancy across dates is the paper's most robust result. If velocity actually decreased at larger rh, the CO2 production drop would be steeper than derived, which would strengthen the solar-heating argument rather than undermine it. The more substantive soft spot is the interpretive chain for CO2-driven activity. The three pillars are each individually circumstantial: the 10% CO2/H2O ratio is explicitly noted as typical for solar system comets at these distances, so it is not anomalous; the low expansion velocities rely on external SWI/IRAM data with model-dependent interpretations that could also reflect dust loading or geometry; and the 200% active fraction is somewhat circular — computed from a water production rate that includes icy-grain sublimation, then used to argue for CO2-driven icy grain release. The 103P/Hartley 2 analogy is explicitly undercut by Shou et al. 2025, which the paper itself cites. No single pillar uniquely points to CO2 as the driver. The paper hedges appropriately throughout — 'support a scenario,' 'consistent with' — so this is not an overclaim. But the gap between the solid observational results and the interpretive conclusion is where the argument is least secure. This deserves a serious referee. The observational results are publishable on their own merits; the referee should push the authors to either strengthen or soften the CO2-driven activity framing, and to address whether the 3.4-micron feature can be further constrained with the existing data. The paper is for cometary scientists, ISO specialists, and anyone tracking the growing 3I/ATLAS literature. It will be cited for the production rates and the organic detection claim regardless of how the activity interpretation holds up.

Referee Report

1 major / 8 minor

Summary. This manuscript presents JUICE/MAJIS visible-to-infrared (0.5–5.56 μm) observations of the interstellar comet 3I/ATLAS obtained between 2025 November 2 and 25, shortly after perihelion. The authors report detections of H2O (2.7 μm) and CO2 (4.3 μm) fluorescence emission, derive production rates and the CO2/H2O ratio (~10%) at four epochs spanning 1.36–1.68 au, measure dust spectral slopes, and identify tentative broad emission features in the 3.2–3.6 μm region attributed to aliphatic C–H stretching modes. The central interpretive claim is that CO2 plays a major role in driving the near-perihelion activity of 3I/ATLAS, supported by the CO2/H2O ratio, the heliocentric evolution of CO2 production, low gas expansion velocities, and a computed active fraction of ~200%. The data are unique — no other instrument provides CO2 measurements at these heliocentric distances for this comet — and the analysis is generally careful with stated uncertainties.

Significance. The paper provides the only near-perihelion CO2 production rate measurements for 3I/ATLAS, filling a critical gap between pre- and post-perihelion JWST observations at larger heliocentric distances. The spatially resolved radial profiles of H2O and CO2, the time-resolved production rates, and the tentative organic feature detection are all scientifically valuable. The PSG forward-modeling approach is standard and appropriate. The identification of the 3.4 μm organic feature, while tentative, is a falsifiable claim that can be tested against higher-S/N or higher-resolution data. The combination of MAJIS, SWI, IRAM, and JWST data to build a coherent picture of the activity driver is a strength of the analysis.

major comments (1)

Section 4.2: The argument that 'CO2 plays a major role in driving the activity of 3I/ATLAS near perihelion' rests on three pillars, each of which has an alternative explanation that is not fully excluded. (1) The CO2/H2O ratio of ~10% is acknowledged to be 'consistent with values measured in solar system comets at similar heliocentric distances' — i.e., not anomalous. (2) The low expansion velocities (0.4–0.6 km/s) attributed to heavy-molecule-driven flow rely on external SWI/IRAM data and a model-dependent interpretation that could also reflect high dust loading or nucleus geometry. (3) The 200% active fraction is computed from the H2O production rate that the authors themselves state includes water from short-lived icy grains (Lp << 1000 km), making the hyperactivity argument partially circular: the icy-grain contribution inflates Q(H2O), which inflates the active fraction, which is然后用

minor comments (8)

Section 2: The spatial resolution is stated as ~10,000 km per pixel at closest approach on November 2, but the JUICE–comet distance Δ varies from 0.444 to 1.274 au across the four epochs (Table A1). The corresponding spatial resolutions for the other dates should be stated.
Section 3.2: The FWHM factor of 1.47 times the IFOV is stated without derivation. A brief explanation or reference would help reproducibility.
Section 3.2: The CO2 band shape is noted to be poorly reproduced by PSG, with the suggestion of a hot CO2 component. The rotational temperatures derived from PSG fits (80–148 K) are then used for production rate derivation. The sensitivity of Q(CO2) to this band-shape mismatch should be briefly discussed.
Section 4.1: The comparison of spectral slopes with literature values is complicated by different wavelength intervals. Figure C1 helps, but a small summary table of slopes and their wavelength ranges would make the comparison clearer.
Section 4.3: The 3.4 μm feature is described as having 'low S/N' and the detection is supported by the spatial coincidence shown in Figure B2. A quantitative estimate of the significance (e.g., in sigma) of the detection would strengthen the reader's ability to evaluate this tentative result.
Table A1: The H2O extraction for November 25 uses only the second of eight cubes, described as showing a 'hint of H2O emission.' The basis for selecting this single cube and rejecting the others should be more transparent, given the risk of selection bias.
Section 4.2: The discrepancy between MAJIS Q(H2O) and SOHO/SWAN values (factor of 4–6) is discussed but not fully resolved. The authors propose icy-grain sublimation in the outer coma as the explanation, but note that no evidence for ice particles is found in the MAJIS dust continuum. This tension should be acknowledged more explicitly.
Figure 2C: The synthetic spectra for CH4, C2H6, CH3OH, and H2CO assume specific abundance ratios (some arbitrary, e.g., H2CO/H2O = 2%). The sensitivity of the conclusion (that common volatiles cannot explain the observed features) to these assumed abundances should be briefly noted.

Circularity Check

0 steps flagged

No circularity found; derivation chain is standard forward modeling with independently measured inputs

full rationale

The paper's derivation chain is self-contained and non-circular. Production rates are derived from observed MAJIS band intensities via PSG forward modeling (Haser model + fluorescence synthesis) with explicitly stated assumptions (v_exp = 0.6 km/s, photodissociation rates). The CO2/H2O ratio is a ratio of two independently measured quantities and is insensitive to the shared velocity assumption. The CO2-driven activity conclusion is an interpretive synthesis of multiple independent evidence streams (production rate trends, external SWI expansion velocities, IRAM CO data, active fraction calculation), none of which reduce to each other by construction. The 200% active fraction is computed from Q(H2O) and nucleus size, and the paper transparently acknowledges this includes icy-grain contributions rather than feeding it back as a premise. Rotational temperatures from PSG fits are cross-validated against an independent non-LTE model. Self-citations (Bockelée-Morvan et al. 2019, Biver et al. 2026) provide external data or methodology, not load-bearing logical steps. No 'prediction' or 'first-principles result' is equivalent to its inputs by definition.

Axiom & Free-Parameter Ledger

4 free parameters · 3 axioms · 0 invented entities

No new physical entities or forces are postulated. The analysis relies on standard cometary physics (fluorescence, Haser model, photodissociation) and known molecular species. The organic material interpretation invokes known aliphatic C-H functional groups, not a new substance.

free parameters (4)

Gas expansion velocity (vexp) = 0.6 km/s
Assumed constant across all four observing dates for production rate derivation (Section 3.3). Derived from SWI measurements on Nov 2-6 and held fixed.
H2O photodissociation rate scaling = 1.49e-5 s^-1 at 1 au, scaled as rh^-2
Used for Haser model and production rate calculations (Section 3.2). Corrected for solar activity using F10.7cm index.
CO2 photodissociation rate scaling = 3.2e-6 s^-1 at 1 au, scaled as rh^-2
Used for CO2 production rate calculations (Section 3.2).
Power-law index for CO2 radial density = -2.2
Fitted to match observed CO2 radial profiles on Nov 2 and 12 instead of standard Haser r^-2 (Section 4.2). 3-sigma uncertainty of 0.1.

axioms (3)

domain assumption Fluorescence emission is the dominant excitation mechanism for H2O and CO2 vibrational bands in the coma at these heliocentric distances.
Standard cometary assumption used to justify PSG modeling (Section 3.2). Well-established for IR cometary emission.
domain assumption The non-LTE excitation model constrained by CH3OH millimeter lines (Biver et al. 2026) accurately predicts H2O rotational temperature behavior in the MAJIS extraction apertures.
Used to validate derived Trot values for H2O (Section 3.2, Table A2). Relies on external data and model assumptions.
ad hoc to paper The 3.2-3.6 micron emission features arise from C-H stretching modes of aliphatic organic material rather than thermal emission or unmodeled volatile fluorescence.
The authors state this is tentative evidence (Section 4.3). The assignment is based on spectral position but the emission mechanism (thermal vs. fluorescence vs. scattering) is not definitively established.

pith-pipeline@v1.1.0-glm · 20365 in / 2624 out tokens · 314548 ms · 2026-07-10T04:35:34.978038+00:00 · methodology

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read the original abstract

We present visible-to-infrared (0.5-5.56 microns) observations of the interstellar comet 3I/ATLAS obtained with the Moons and Jupiter Imaging Spectrometer (MAJIS) aboard the Jupiter Icy Moons Explorer (JUICE) spacecraft between 2025 November 2 and 25, shortly after perihelion. The fluorescence emission from H2O at 2.7 microns and CO2 at 4.3 microns is detected at heliocentric distances of 1.36-1.68 au. A weak dust-scattered continuum is identified from which we determine the spectral slope, with values ranging from ~15% per 100 nm in the 0.65-0.9 microns region to 1-3% per 100 nm from 0.9 to 2.6 microns. Spatially resolved measurements show that the radial distributions of H2O and CO2 species are consistent with release in the near-nucleus environment. We derive H2O production rates that decreased from 8 x 1028 s-1 to 4 x 1028 s-1 over the period November 2-25, while the CO2/H2O ratio remained nearly constant at ~10%. The heliocentric evolution of the CO2 production rate indicates activity controlled by solar heating. Combined with published CO measurements and the low gas expansion velocities, our analyses support a scenario in which CO2 plays a major role in driving the activity of 3I/ATLAS near perihelion. In addition, broad emission features are identified in the 3.2-3.6 microns region that cannot be explained by the fluorescence of common cometary CH-bearing volatiles. Their spectral characteristics are consistent with aliphatic C-H functional groups and provide tentative evidence for the release of complex organic material from dust grains in the coma.

Figures

Figures reproduced from arXiv: 2607.08603 by A. Migliorini, B. Seignovert, C. Leyrat, C. Pilorget, C. Royer, D. Bockelee-Morvan, E. D'Aversa, E. Lellouch, E. Quirico, F. Poulet, F. Tosi, G. Piccioni, J. Carter, M. De Sanctis, R. Brunetto, S. Robert, S. Rodriguez, T. Cavalie, Y. Langevin.

**Figure 1.** Figure 1: (A) The trajectory of 3I/ATLAS overlaid on the sky with the comet position at the dates of the MAJIS observations. The black squares show the NavCam field-of-view and the blue lines show the MAJIS slit. The heliocentric distance rh of the comet and the JUICE distance to the comet Δ are indicated. (B) Orbital motion of 3I/ATLAS (black square) projected onto the ecliptic plane. The positions of JUICE, Earth,… view at source ↗

**Figure 2.** Figure 2: Sample spectra of 3I/ATLAS acquired on 2025 November 2 with MAJIS. Panel A: Normalized dust reflectance derived from the MAJIS measurements and scaled to unity at 1 µm. The black dots are the reflectance values evaluated for each spectral element of the VISNIR channel, while the red curve corresponds to the reflectance spectrum smoothed over 30 spectral elements. The two thick blue lines indicate linear fi… view at source ↗

**Figure 3.** Figure 3: displays the radial profiles of the H2O and CO2 band intensities with respect to the estimated photocenter on November 2 and 12. The dashed grey curves show the expected radial variation assuming isotropic release of these species from the nucleus at constant velocity (i.e., using the Haser model, L. Haser 1957). For these calculations, we adopted the expansion velocity vexp of 0.6 km/s estimated from Dopp… view at source ↗

**Figure 4.** Figure 4: Heliocentric evolution of the H2O, CO2 and CO production rates (A) and their relative abundances (B). Panel A: H2O (CO2) production rates are indicated by blue (red) symbols, with open and filled symbols for pre-perihelion and post-perihelion dates, respectively; green symbols are used for the CO production rates. Panel B: CO2/H2O, CO/H2O and CO/CO2 ratios are indicated by red, green, and blue symbols, res… view at source ↗

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Works this paper leans on

2 extracted references · 2 canonical work pages · 1 internal anchor

[1]

F., Belton, M

A'Hearn, M. F., Belton, M. J. S., Delamere, W. A., et al. 2011, Science, 332, 6036, 1396 Belyakov, M., Wong, I., Bolin, B. T., et al. 2026, ApJL, 1001, L11 Bockelée-Morvan, D., Leyrat, C., Erard, S., et al. 2019, A&A, 630, A22 Boissier, J., Bockelée-Morvan, D., Biver, N., et al. 2014, Icarus, 228, 197 Biver, N., Bockelée-Morvan, D., Crovisier, J., et al. ...

work page arXiv 2011
[2]

F., & Mukherjee, J

Huebner, W. F., & Mukherjee, J. 2015, Planetary and Space Science, 106, 11 Harrington Pinto, O., Womack, M., Fernandez, Y., et al. 2022, PSJ, 3, 11, 247 Hoogendam, W. B., Kuesters, D., Shappee, B. J., et al. 2025, arXiv:2512.09020 Hui, M.-T., Ye, Q.-Z., Fôhring, D., et al. 2020, AJ, 160, 2, 92 Hui, M.-T., Jewitt, D., Mutchler, M. J., et al. 2026, ApJL, 99...

work page internal anchor Pith review arXiv 2015

[1] [1]

F., Belton, M

A'Hearn, M. F., Belton, M. J. S., Delamere, W. A., et al. 2011, Science, 332, 6036, 1396 Belyakov, M., Wong, I., Bolin, B. T., et al. 2026, ApJL, 1001, L11 Bockelée-Morvan, D., Leyrat, C., Erard, S., et al. 2019, A&A, 630, A22 Boissier, J., Bockelée-Morvan, D., Biver, N., et al. 2014, Icarus, 228, 197 Biver, N., Bockelée-Morvan, D., Crovisier, J., et al. ...

work page arXiv 2011

[2] [2]

F., & Mukherjee, J

Huebner, W. F., & Mukherjee, J. 2015, Planetary and Space Science, 106, 11 Harrington Pinto, O., Womack, M., Fernandez, Y., et al. 2022, PSJ, 3, 11, 247 Hoogendam, W. B., Kuesters, D., Shappee, B. J., et al. 2025, arXiv:2512.09020 Hui, M.-T., Ye, Q.-Z., Fôhring, D., et al. 2020, AJ, 160, 2, 92 Hui, M.-T., Jewitt, D., Mutchler, M. J., et al. 2026, ApJL, 99...

work page internal anchor Pith review arXiv 2015