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REVIEW 4 major objections 8 minor 1 cited by

Water Production Rates from SOHO/SWAN Observations of Comets C/2017 K2 (PanSTARRS) and C/2022 E3 (ZTF)

T0 review · 4 major / 8 minor · reviewed 2026-08-15 · deepseek-v4-flash

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Pith's one-line read SOHO/SWAN hydrogen images yield nearly daily water production rates for two long-period comets, revealing K2's steady decline and E3's factor-of-two pre/post-perihelion asymmetry.

desk verdict Useful new SWAN water-production light curves for two long-period comets; the E3 'step' interpretation needs a closer look because the model may not accommodate the icy-grain source the paper invokes. read the letter →

arxiv 2505.11699 v1 pith:G2ZVNRPQ submitted 2025-05-16 astro-ph.EP

Michael Combi , Terhi Mäkinen , Jean-Loup Bertaux , Eric Quemerais , Stéphane Ferron This is my paper · ORCID
classification astro-ph.EP
keywords cometscometarycomaewaterproductionratesLyman-alphaSOHO/SWANlong-periodC/2017K2(PanSTARRS)C/2022E3(ZTF)
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

Using nearly daily all-sky hydrogen Lyman-alpha images from the SWAN camera on the SOHO spacecraft, the authors measured the water production rates of two long-period comets across months of their orbits. For C/2017 K2 (PanSTARRS) they obtained 83 rates, mostly after perihelion, showing a steady power-law decline with heliocentric distance, index -3.2, plus a factor-of-two outburst roughly two months after perihelion. For C/2022 E3 (ZTF) they obtained 80 rates around perihelion, revealing a factor-of-two higher water production before perihelion than after, with an abrupt drop about 15 days before closest approach. These results matter because water production is the most direct measure of cometary volatile activity, and the long time coverage lets the authors tie activity changes to possible seasonal effects or reservoirs of icy grains.

What carries the argument

The mechanism carrying the argument is the hybrid hydrogen-coma model of Mäkinen and Combi (2005) with the gas-dynamics treatment of Combi et al. (2004). It converts a SWAN Lyman-alpha image of the comet's hydrogen coma into a water production rate by simulating the outflow of water, its photodissociation into hydrogen atoms with ejection speeds of 8 and 20 km/s, and the partial thermalization of those atoms in the inner coma. For each image, the model's spatial brightness distribution is scaled against the observed coma plus the fitted interplanetary hydrogen background, yielding Q(H2O) and a formal 1-sigma fitting error.

What would settle it

Compare the SWAN water production rate for C/2022 E3 on dates when independent measurements exist: Biver et al. (2024) reported a 183.3 GHz water line value near perihelion and OH rates from Nançay, and Schleicher et al. (2023) give ground-based OH rates; if the ratio SWAN/independent varies systematically with the sunward-nightside asymmetry of the coma seen in HCN, the symmetric hybrid model is falsified. A direct test is to fit the two-dimensional SWAN Lyman-alpha image of E3 near perihelion with an asymmetric model that puts 3/4 of the production on the dayside and see whether the derived Q changes by tens of percent.

Watch

Extended reading notes

Core claim

The paper's central discovery is a long, nearly continuous record of water production for two comets observed by SOHO/SWAN: 83 values for C/2017 K2 and 80 for C/2022 E3. K2's post-perihelion water production fell as a power law with exponent -3.2 in heliocentric distance, typical of dynamically new comets, and the few pre-perihelion points are consistent with that trend. E3 behaved very differently: its water production was roughly twice as large before perihelion as after, with an abrupt change near 15 days before perihelion, and showed six activity peaks spaced about 15-25 days apart that cannot be explained by its ~8-hour rotation period. The authors argue the E3 asymmetry resembles that of comet C/2009 P1 (Garradd), where an extended source of icy grains sublimating on the pre-perihelion leg produced extra water, and they suggest a seasonal effect or grain reservoir as the explanation.

Load-bearing premise

The load-bearing assumption is that the hydrogen atoms producing the Lyman-alpha emission come from water photodissociation with the model's specific ejection speeds and partial thermalization; if a significant fraction of the hydrogen coma comes from other parents, such as icy grains releasing extra gas, or if the coma is strongly asymmetric as seen in E3's HCN emission, the inferred water production rates will be systematically biased.

Editorial extensions

If this is right

  • K2's post-perihelion power-law index of -3.2 provides a reference activity curve for a nearly dynamically new comet in the 1.8-2.4 au range, against which other pre- and post-perihelion observations of K2 can be compared.
  • E3's factor-of-two pre/post-perihelion asymmetry and abrupt drop ~15 days before perihelion imply that its activity is controlled by a seasonally varying source, likely sublimation of an icy-grain reservoir, rather than by simple water-ice sublimation from the nucleus surface.
  • The six activity peaks in E3, spaced 15-25 days apart, indicate discrete outbursts; because they are much longer than the ~8-hour rotation period they must arise from dynamical processes such as grain release events rather than rotational modulation.
  • The published tables of 83 and 80 water production rates give the community a dense dataset for modeling the outgassing of these two comets and for checking consistency of other measurements.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • If E3's asymmetry is indeed caused by a pre-perihelion icy-grain halo, then other SWAN-observed long-period comets should show the same pattern: elevated water production at larger heliocentric distances before perihelion, followed by a drop once the grains sublimate; this is a testable prediction for future apparitions.
  • The abrupt activity change 15 days before perihelion could be a seasonal equinox on E3's nucleus; combined with the reported ~8.5-hour rotation period and the dust/water asymmetry, it could be used to constrain the pole orientation, an analysis the authors do not carry out.
  • The scatter in K2's rates is attributed to unidentified faint background stars; a re-reduction using a modern star catalog to subtract field stars pixel by pixel could lower the scatter and verify whether the reported ~15-day outburst is real or an artifact of background contamination.
  • The authors' suggestion that visual magnitude (dust) and water production have differently shaped asymmetries about perihelion, as in Garradd, implies that dust and gas can be decoupled in long-period comets; comparing SWAN gas rates with dust photometry for other comets would show how common this decoupling is.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

4 major / 8 minor

Summary. The paper presents SOHO/SWAN hydrogen Lyman-alpha observations of two long-period comets, C/2017 K2 (PanSTARRS) and C/2022 E3 (ZTF), and derives water production rates using the hybrid coma model of Mäkinen and Combi (2005). The authors tabulate daily water production rates for 83 dates for K2 (mostly post-perihelion) and 80 dates for E3 (around perihelion), and interpret the time series: K2 shows a post-perihelion power-law decline with exponent -3.2, while E3 shows a pre- to post-perihelion asymmetry of roughly a factor of two, an abrupt change near 15 days before perihelion, and several activity peaks. The paper is primarily a data paper, with the tables and figures as the central product, and the physical interpretations are presented as plausible but not definitive.

Significance. The strength of the paper is its homogeneous, multi-month water production time series for two comets from a mature and well-calibrated instrument. The tables provide a useful reference for future multi-wavelength comparisons, and the systematic uncertainty of roughly 30% is honestly stated. The paper does not claim to test or modify the underlying model, and the central numbers are produced by a published, peer-reviewed forward-modeling approach. The physical claims about seasonal effects and icy-grain halos are interesting but are not uniquely constrained by the data; they would gain credibility from explicit modeling of the proposed extended source and from statistical testing of the reported transitions.

major comments (4)
  1. [Section 3, Table 3] The abrupt change in E3's water production near -15 days is a central physical claim, but it rests on two adjacent observations: at DT = -14.061 days, Q = 5.00e28 ± 0.20e28 s⁻¹, and at DT = -12.559 days, Q = 3.03e28 ± 0.32e28 s⁻¹. Given the large day-to-day scatter in the E3 series, the authors should report a formal significance test (e.g., a change-point test or a comparison of this step with the local variance). More importantly, the paper invokes an icy-grain extended source to explain the pre/post asymmetry, yet the Mäkinen and Combi (2005) model used to convert Lyman-alpha brightness to Q(H2O) assumes a spherically symmetric, nucleus-centered water source with canonical photodissociation ejection speeds. If a substantial fraction of pre-perihelion water is released by sublimating grains far from the nucleus, the inferred Q values are systematically biased exactly in the epoch that defines the break. The paper should either quantify this bias with a model that includes an extended source or explicitly list it as a limitation that prevents a firm interpretation of the abrupt change.
  2. [Section 4 and Highlights] The power-law exponents for K2 (-3.2) and for E3 (-1.3 pre-perihelion, -0.4 post-perihelion) are quoted without uncertainties. Since the K2 data show wide scatter attributed to unidentified background stars, and the E3 data are irregular with several outbursts, the exponents are not self-evidently robust. The authors should state the fitting method (e.g., weighted least squares on the logarithms), give formal parameter errors or confidence intervals, and report the goodness of fit (e.g., rms scatter or chi-squared). Without this, the reader cannot judge whether -3.2 is significantly different from typical values or whether the E3 pre/post exponent difference is statistically meaningful.
  3. [Section 3 and Summary] The paper claims that the SWAN results are 'fairly consistent' with other published water production rates, but it also reports an unexplained large discrepancy with a TRAPPIST value from Jehin et al. (2022b) for E3 and does not provide a quantitative comparison for K2. The consistency claim should be supported by explicit ratios and uncertainties for each comparison dataset, or it should be restricted to the datasets for which agreement within the stated errors is actually demonstrated. As written, the assertion of consistency is not falsifiable and conflicts with the later statement that 'there is no explanation at this time for these larger than normal differences.'
  4. [Section 2] The pre-perihelion behavior of K2 is described as 'qualitatively similar' to the post-perihelion trend on the basis of only six usable images. Given the large scatter in the SWAN values, this statement is not statistically supported. The authors should either show the pre-perihelion points alongside the post-perihelion fit and discuss the comparison explicitly, or soften the claim to note that the pre-perihelion data are too sparse to constrain the slope.
minor comments (8)
  1. [Table 2 note] The note says 'Perihelion December 12.68, 2020' but the perihelion date given in Table 1 is 2022 December 19.69; the year is wrong and should be corrected.
  2. [Table 3 note] The note says 'Perihelion January 3.30, 2022' but the perihelion date given in Table 1 is 2023 January 12.78; the year and day are inconsistent.
  3. [Figure 2 caption] The caption reads 'Comet C/2023 E3(ZTF)' but the comet is C/2022 E3 (ZTF); the year is a typo.
  4. [Section 3] The sentence 'Finally, Schleicher et al. also reported a rotation period of K2 of 8.7 +/- 0.1 hours' appears in the E3 section and should refer to E3, not K2; this is likely a typo in the comet name.
  5. [Section 2] The phrase 'one value from a et al. (2022)' is an incomplete citation; the reference should be identified (probably Jehin et al. 2022a) and listed properly.
  6. [References] The reference for Mäkinen and Combi (2005) is formatted as 'Mäkinen, J., Teemu, T., Combi, Michael, R., 2005' but should be 'Mäkinen, J.T.T., Combi, M.R., 2005' to match the authors' names used in the text.
  7. [References] The reference for Ejeta et al. (2025) begins with 'Chemeda Ejeta' but should use the conventional surname-first format, e.g., 'Ejeta, C., Gibb, E., DiSanti, M.A., et al.'; the current format is inconsistent with the rest of the list.
  8. [Section 3] The visual-magnitude URL (aerith.net) should be cited properly or replaced with a published source; as written, it is a bare hyperlink that may not be stable.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: the water production rates are derived from a fixed forward model published earlier by the same group, but the model was not refit to these comets and the paper makes no claim to test it; the E3 icy-grain asymmetry is an interpretation, not a model input.

full rationale

The paper's core derivation is the conversion of SOHO/SWAN Lyman-alpha coma brightness into water production rates using the hybrid model described in Mäkinen and Combi (2005) and Combi et al. (2004). The text states: "water production rates can be calculated from each H Lya image of the comet observed by SWAN using the methods described in detail by Mäkinen and Combi (2005)." This is a self-citation, but it is not load-bearing in a circular sense: the model is a fixed physical forward model with parameters derived from prior physics and prior comet observations, not fitted to the K2 or E3 data in this paper. The Q(H2O) values are scale factors obtained by fitting the model spatial distribution to each image, so the input (Lyman-alpha brightness) and output (production rate) are distinct physical quantities connected by an independently established model. No equation in the paper reduces to its own input, and no fitted parameter is renamed as a prediction. The E3 pre-perihelion asymmetry is explained by invoking an icy-grain extended source, but that explanation is a post hoc interpretation, not an input to the model; if the model lacks that source, the inferred rates may be biased, but that is an accuracy limitation, not circularity. The paper also compares its results with external observations from Biver et al. (2024), Schleicher, and Jehin et al. (2022a,b), providing independent anchors. Therefore no circular step can be exhibited from the paper's own equations or self-citation chain.

Assumptions & free parameters 4 free parameters · 3 assumptions · 0 invented entities

The paper introduces no new entities or physical parameters. Its central output is an observational data product derived from an established forward model. The only numbers fitted to data are descriptive power-law slopes and the location of E3's activity break; none of these are inputs to the model. The dominant assumptions are inherited from the cited hydrogen coma model and the water-only parent assumption.

free parameters (4)
  • K2 post-perihelion power-law exponent = -3.2
    Fit to 83 SWAN water production rates versus heliocentric distance; no uncertainty is given.
  • E3 pre-perihelion power-law exponent = -1.3
    Descriptive fit to SWAN values before perihelion; no uncertainty is given.
  • E3 post-perihelion power-law exponent = -0.4
    Descriptive fit to SWAN values after perihelion; no uncertainty is given.
  • E3 activity break time = -15 days relative to perihelion
    Identified from the light curve as the approximate time of the abrupt production-rate change; used to support the seasonal or icy-grain interpretation.
assumptions (3)
  • domain assumption The observed hydrogen coma is produced solely by H2O photodissociation, yielding two H atoms per water molecule.
    Load-bearing for converting H Lyman-alpha brightness to water production; invoked in Section 1 and in the cited Mäkinen and Combi (2005) model.
  • domain assumption The transport of hydrogen atoms is described by the hybrid model of Mäkinen and Combi (2005) with H ejection speeds of 8 and 20 km/s and partial thermalization.
    The model parameters come from prior literature; the paper does not revalidate them for these two comets.
  • domain assumption The SWAN absolute calibration and the time-varying solar Lyman-alpha g-factor from LASP are correct.
    These contribute the stated roughly 30 percent systematic uncertainty and are assumed throughout the derivation.

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Cite this review

Pith. "Pith review of Water Production Rates from SOHO/SWAN Observations of Comets C/2017 K2 (PanSTARRS) and C/2022 E3 (ZTF)." pith.science (2026). https://pith.science/paper/G2ZVNRPQ

@misc{pith2026250511699,
  author       = {Pith},
  title        = {Pith review of: Water Production Rates from SOHO/SWAN Observations of Comets C/2017 K2 (PanSTARRS) and C/2022 E3 (ZTF)},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/G2ZVNRPQ}},
  note         = {Machine review of arXiv:2505.11699}
}
read the original abstract

In 2022 and 2023 the hydrogen comae of two long period comets, C/2017 K2 (PanSTARRS) and C/2022 E3 (ZTF), were observed with the Solar Wind ANisotropies (SWAN) all-sky hydrogen Lyman-alpha camera on the SOlar and Heliosphere Observer (SOHO) satellite. SWAN obtains nearly daily full-sky images of the hydrogen Lyman-alpha distribution of the interstellar hydrogen as it passes through the solar system yielding information about the solar wind and solar ultraviolet fluxes that eat away at it by ionization and charge exchange. The hydrogen comae of comets, when of sufficient brightness, are also observed. Water production rates have been calculated over time for each of these comets, covering about 6 months mostly of the post-perihelion period of C/2017 K2 (PanSTARRS) and about 3 months around perihelion of C/2022 E3 (ZTF).

Figures

Figures reproduced from arXiv: 2505.11699 by the authors.

Figure 1
Figure 1. Water production rate of comet C/2017 K2 (PanSTARRS) as [PITH_FULL_IMAGE:figures/full_fig_p014_1.png] view at source ↗
Figure 2
Figure 2. Water production rate in comet C/2023 E3(ZTF)as a [PITH_FULL_IMAGE:figures/full_fig_p015_2.png] view at source ↗

Discussion (0). Continue with ORCID to comment.

Forward citations

Cited by 1 Pith paper

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. Full citation record

  1. Variations in Volatile-Driven Activity of Comet C/2017 K2 (PanSTARRS) Revealed by Long-Term Multi-Wavelength Observations

    astro-ph.EP 2025-07 conditional novelty 6.0 of 10

    Long-term TRAPPIST, UVES, and CRIRES+ observations show C/2017 K2's volatile-driven activity transitioning from CO/CO2 to water near 3 au, with typical-to-enriched composition and HCN as the main CN parent.

Reference graph

Works this paper leans on

23 extracted references · 23 canonical work pages · cited by 1 Pith paper

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    INTRODUCTION Two long period comets were observed by the Solar Wind Anisotropies (SWAN) all-sky hydrogen Lyman-a (H Lya) camera on the Solar and Heliosphere Observer (SOHO) satellite during 2022 and 2023: C/2017 K2 (PanSTARRS) and C/2022 E3 (ZTF). The principal investigation of SWAN is observing all-sky images at H Lya to measure the changing spatial stru...

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