# Water Production of Interstellar Comet 3I/ATLAS from SOHO/SWAN Observations after Perihelion

Michael R Combi, Terhi Mäkinen, Jean-Loup Bertaux, Eric Quémerais,
Stéphane Ferron, Rosine Lallement, Walter Schmidt

► To cite this version:

Michael R Combi, Terhi Mäkinen, Jean-Loup Bertaux, Eric Quémerais, Stéphane Ferron, et al.. Water Production of Interstellar Comet 3I/ATLAS from SOHO/SWAN Observations after Perihelion. The Astrophysical Journal Letters, 2026, 998 (17), pp.4. 10.3847/2041-8213/ae3bd8. hal-05505504

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# Water Production of Interstellar Comet 3I/ATLAS from SOHO/SWAN Observations after Perihelion

M. R. Combi¹, T. Mäkinen², J. L. Bertaux³, E. Quémerais³, S. Ferron⁴, R. Lallement⁵, and W. Schmidt²

¹Dept. of Climate and Space Sciences and Engineering, University of Michigan, 2455 Hayward Street, Ann Arbor, MI 48109-2143, USA; mcombi@umich.edu

²Finnish Meteorological Institute, Box 503, SF-00101 Helsinki, Finland

³LATMOS/IPSL, UVSQ Université Paris-Saclay, Sorbonne Université, CNRS, Guyancourt, France

⁴ACRI-st, Sophia-Antipolis, France

⁵LIRA, Observatoire de Paris, Meudon, France

Received 2025 December 24; revised 2026 January 15; accepted 2026 January 19; published 2026 February 5

## Abstract

The Solar Wind Anisotropies all-sky hydrogen Lyman-alpha camera on the Solar and Heliosphere Observatory observed the hydrogen coma of interstellar comet 3I/ATLAS, also called C/2025 N1 (ATLAS), beginning on 2025 November 6, 9 days after perihelion. Water production rates were calculated from each image of 3I/ATLAS using the methodology of J. T. T. Mäkinen and M. R. Combi, and fluorescence rates and g-factors were calculated using the daily solar Lyman-alpha fluxes from the LASP database (https://lasp.colorado.edu/lisird/data) corrected for solar rotation and for the comet's heliocentric velocity. The method has been used for over 90 comet apparitions. A water production rate of $3.17 \times 10^{29} \text{ s}^{-1}$ was found on November 6 when the comet was at a heliocentric distance of 1.40 au and at a sufficient solar elongation angle. It decreased over time after that, down to $1-2 \times 10^{28} \text{ s}^{-1}$ around 40 days postperihelion (December 9).

*Unified Astronomy Thesaurus concepts: Interstellar objects (52); Free floating planets (549); Comet volatiles (2162); Ultraviolet astronomy (1736)*

## 1. Introduction

The third known object to originate outside of our solar system and the second comet, 3I/ATLAS = C/2025 N1 (ATLAS), was discovered with the Asteroid Terrestrial-Impact Last Alert System (ATLAS) 0.5 m telescope in Rio Hurtado, Chile, on 2025 July 1 (L. Denneau et al. 2025). It was almost immediately known to have a surrounding coma (M. Minev et al. 2025), and prediscovery observations (e.g., J. Martinez-Palomera et al. 2025) confirmed its interstellar origin with a hyperbolic orbit with eccentricity of 6 (Minor Planets Center). Its orbital inclination is ~175°, making its orbit retrograde and only ~5° out of the plane of the solar system (ecliptic). Because this comet comes from essentially infinity, its orbital inclination could have any value with equal probability.

The first confirmed interstellar object, 11/(2017 U1) 'Oumuamua, did not have a detected gaseous or solar-continuum scattered dust coma surrounding it. Comet 2I/Borisov, the first interstellar comet to be discovered, on the other hand, had a well-developed coma with CN (A. Fitzsimmons et al. 2019), a common cometary coma gas species, and dust and water (A. J. McKay et al. 2020; Z. Xing et al. 2020). It was found to be carbon-chain-depleted, similar to the class of carbon-chain-depleted comets of solar system origin (T. Kareta et al. 2020).

Early observations of 3I/ATLAS indicated that the comet was also carbon-chain-depleted but showed a strong CN emission (L. E. Salazar Manzano et al. 2025). Observations of 3I/ATLAS after perihelion on November 29 (E. Jehin et al. 2025a) show strong $C_2$ and $C_3$ emissions with a more typical $C_2$/CN ratio with $\log(Q-C_2/Q-CN) = +0.04$.

Measurements of the gases and byproducts that gave clues to the main volatile constituents then followed. Z. Xing et al. (2025) observed OH in 3I with the Swift satellite on 2025 July 31–August 1, when the comet was at a heliocentric distance of 2.9 au. These indicated a total water production rate of $(0.74 \pm 0.50) \times 10^{27} \text{ s}^{-1}$. JWST observations on August 6 (M. Cordiner et al. 2025) measured a lower water production rate of $(2.23 \pm 0.08) \times 10^{26} \text{ s}^{-1}$, whereas later observations with Swift on August 18–20 found again a much larger water production rate of $(1.26 \pm 0.35) \times 10^{27} \text{ s}^{-1}$. Z. Xing et al. 2025 suggested that the difference might be due to an extended source of icy particles released and moving away from the nucleus that would contribute to larger apparent water production rates in the large field of view of Swift compared with the smaller aperture of JWST. The JWST observations of M. Cordiner et al. (2025) also provided a $CO_2$ production rate of $(1.70 \pm 0.01) \times 10^{27} \text{ s}^{-1}$ and of CO of $3.7 \pm 0.02) \times 10^{26} \text{ s}^{-1}$, both larger than that of H$_2$O. At a heliocentric distance of 3.32 au, this is not too surprising, as the postperihelion measurements at >3 au of the relative production rates of $CO_2$ and CO were also larger or comparable to that of H$_2$O in the Rosetta target comet 67 P/Churyumov–Gerasimenko (M. Combi et al. 2020).

D. Hutsemékers et al. (2025) reported log OH production rates from ground-based echelle spectroscopic observations of 3I/ATLAS with the Very Large Telescope on August 8, September 3–4, September 10, and September 12 of 26.37 ± 0.04, 26.59 ± 0.02, 27.05 ± 0.13, and 27.15 ± 0.10. These give water production rates of approximately 2.8, 4.6, 13.4, and 16.8 in $10^{26} \text{ s}^{-1}$.

J. Crovisier et al. (2025) reported the results of 1667 MHz observations of the OH 18 cm line with the Nançay radio telescope during the period of 2025 October 13–19. They found an OH production rate of $(5.7 \pm 0.6) \times 10^{28} \text{ s}^{-1}$, which scales to an approximate water production rate of $6.7 \times 10^{28} \text{ s}^{-1}$. This amounts to an increase in water

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production by a factor of 60 over 30 days from the earlier observations.

E. Jehin et al. (2025a) found an OH production rate on November 29 of $1.67 \pm 0.27 \times 10^{28} \text{ s}^{-1}$ corresponding to a water production rate of about $1.98 \times 10^{28} \text{ s}^{-1}$ assuming an OH/H$_2$O yield of 0.84. E. Jehin et al. (2025b) reported a smaller OH production rate of $(6.93 \pm 1.13) \times 10^{27} \text{ s}^{-1}$ corresponding to a water production rate of $8.4 \times 10^{27} \text{ s}^{-1}$ on December 7. These are notably smaller than those from J. Crovisier et al. (2025).

## 2. H$_2$O Rates from SWAN Observations of H Ly$\alpha$

The Solar Wind Anisotropies (SWAN) all-sky Ly$\alpha$ camera on the Solar and Heliosphere Observatory (SOHO) satellite located in a halo orbit around the L1 Lagrange point between the Earth and Sun (J. L. Bertaux et al. 1995) observed the hydrogen Ly$\alpha$ coma of 3I/ATLAS beginning 9 days after perihelion, once it emerged from the solar obstruction area of SWAN. It was not detectable when it was bright enough around and before perihelion, as it was located in the solar obstruction area. Because solar UV photodissociation of water produces a net of 2 H atoms per water molecule with OH as an intermediary for half, the water production rate of the comet can be calculated from an analysis of the H coma with a well-established procedure (J. T. T. Mäkinen & M. R. Combi 2005). To date, SWAN has observed over 90 comet apparitions, including multiple apparitions each of several short-period comets over the past 30 yr and many long-period Oort Cloud comets (M. R. Combi et al. 2019; M. Combi 2022). A special operation procedure was implemented in the SWAN data operation procedure to move a regularly occurring artifact that shows itself on one side next to the spacecraft obstruction in the full sky images to the opposite side. This enabled obtaining an extra week's worth of results to be obtained for 3I.

Table 1 gives the observational circumstances and the water production rates as a function of time in days from perihelion on 2025 October 29.48. The uncertainty given is the formal uncertainty associated with the fit of the coma model and interplanetary background to the image of the comet, which is rather small. It does not account for uncertainties in the instrument calibration, the solar Ly$\alpha$ flux calibration, or the photochemical lifetimes and other parameters in the coma model. There are also contributions to uncertainty from unaccounted for field stars, which are numerous with SWAN's 1° pixels. We have estimated that all those could amount to up to 30%. Figure 1 shows the water production rates plotted also as a function of time from perihelion. Figure 2 shows a representative sample of a SWAN image of 3I/ATLAS from 2025 November 12. For very bright comets observed with SWAN that have higher signal-to-noise and cover much more of the sky, the shape of the coma influenced by radiation pressure is more evident (M. R. Combi et al. 2000). This is clearly not the case for 3I/ATLAS or many fainter comets.

The first few values of the production rate agree extremely well with those determined from OH spectra measurements made over a very wide field of view encompassing essentially all of the OH in the coma out to several hundred thousand kilometers with the MAVEN spacecraft UVIS instrument (M. Stevens and J. Deighan, private communication) over the same time period. A description of the MAVEN UVIS instrument is given by W. E. McClintock et al. (2014).

<table><caption>**Table 1**
SOHO/SWAN Observations of 3I/ATLAS and Water Production Rates</caption><thead><tr><th>ΔT<br>(Days)</th><th>R<br>(au)</th><th>Δ<br>(au)</th><th>g<br>(s<sup>-1</sup>)</th><th>Q<br>(10<sup>28</sup> s<sup>-1</sup>)</th><th>δQ<br>(10<sup>28</sup> s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>9.344</td><td>1.401</td><td>2.187</td><td>0.002153</td><td>31.72</td><td>0.04</td></tr><tr><td>11.345</td><td>1.420</td><td>2.161</td><td>0.002348</td><td>23.10</td><td>0.03</td></tr><tr><td>12.345</td><td>1.431</td><td>2.149</td><td>0.002266</td><td>23.46</td><td>0.04</td></tr><tr><td>14.345</td><td>1.456</td><td>2.123</td><td>0.002202</td><td>23.62</td><td>0.04</td></tr><tr><td>15.345</td><td>1.470</td><td>2.110</td><td>0.002181</td><td>24.54</td><td>0.04</td></tr><tr><td>16.345</td><td>1.484</td><td>2.097</td><td>0.002197</td><td>20.96</td><td>0.04</td></tr><tr><td>17.346</td><td>1.499</td><td>2.084</td><td>0.002179</td><td>21.86</td><td>0.04</td></tr><tr><td>18.346</td><td>1.514</td><td>2.071</td><td>0.002194</td><td>19.09</td><td>0.04</td></tr><tr><td>20.346</td><td>1.548</td><td>2.045</td><td>0.002270</td><td>25.08</td><td>0.04</td></tr><tr><td>21.346</td><td>1.565</td><td>2.032</td><td>0.002314</td><td>32.89</td><td>0.49</td></tr><tr><td>22.346</td><td>1.584</td><td>2.019</td><td>0.002317</td><td>21.90</td><td>0.04</td></tr><tr><td>28.162</td><td>1.699</td><td>1.946</td><td>0.002407</td><td>14.30</td><td>0.04</td></tr><tr><td>34.503</td><td>1.841</td><td>1.874</td><td>0.002750</td><td>4.08</td><td>0.07</td></tr><tr><td>35.504</td><td>1.865</td><td>1.864</td><td>0.002785</td><td>0.76</td><td>0.27</td></tr><tr><td>36.504</td><td>1.888</td><td>1.854</td><td>0.002799</td><td>2.43</td><td>0.10</td></tr><tr><td>37.504</td><td>1.912</td><td>1.845</td><td>0.002703</td><td>1.14</td><td>0.22</td></tr><tr><td>38.504</td><td>1.937</td><td>1.836</td><td>0.002649</td><td>3.68</td><td>0.08</td></tr><tr><td>39.504</td><td>1.961</td><td>1.827</td><td>0.002616</td><td>2.04</td><td>0.14</td></tr><tr><td>40.505</td><td>1.986</td><td>1.819</td><td>0.002626</td><td>1.00</td><td>0.25</td></tr><tr><td>41.505</td><td>2.011</td><td>1.811</td><td>0.002636</td><td>1.60</td><td>0.13</td></tr></tbody></table>

**Notes.** ΔT (days from perihelion 2025 October 29.48),

r: heliocentric distance (au),

Δ: comet-SOHO distance (au),

g: solar Ly$\alpha$ g-factor (photons s$^{-1}$) at 1 au,

Q: water production rates for each image (s$^{-1}$),

δQ: internal $1\sigma$ uncertainties.

Observations by MAVEN UVIS of comet C/2013 A1 (Siding Spring) were described by M. M. J. Crismani et al. (2015) and M. Mayyasi et al. (2020). Those measurements were fairly constant between 2.2 and $3.2 \times 10^{29} \text{ s}^{-1}$, going back to 6 days before perihelion. Because they include the whole coma, they are only dependent on the g-factor and the lifetime of OH radicals. Both MAVEN and SWAN therefore include all water production from the nucleus and any extended sources if present.

Ground-based observations of OH with the Trappist telescope (E. Jehin et al. 2025a, 2025b, and private communication) from late November to early December are consistent with water production rates that are 7 times smaller than SWAN on November 29 to 2 times smaller on December 7. Similar discrepancies that seem to depend on the size of the field of view (FOV) of the observation were noted by Z. Xing et al. (2025), who had observations of OH from the Swift satellite compared with those very small FOV observations of H$_2$O from M. Cordiner et al. (2025) using JWST. About this apparent discrepancy, Z. Xing et al. 2025 raised a possible role of an extended source of water from released solid ice particles that subsequently sublimate at larger distances from the nucleus. This would also be consistent with the possible detection of water ice features by C. M. Lisse et al. (2025).

The long-period Oort Cloud comet C/2006 P1 (Garradd) showed strong evidence of an important extended water source from icy grains (M. R. Combi et al. 2013; D. Bodewits et al. 2014). Similarly, the Jupiter-family comet 103 P/Hartley 2 has been shown directly and indirectly to have a substantial extended source of icy grains (M. F. A'Hearn et al. 2011;
<figure><img src="image_4.png" /><figcaption>**Figure 1.** Water production rate of comet 3I/ATLAS as a function of time from perihelion (2025 October 29.48). The open squares give the water production rate in $s^{-1}$ from single images. The error bars give the $1\sigma$ formal random fitting errors for each value. There is a potential ~30% uncertainty from the model parameters and calibration not included in the plotted error bars.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Ly$\alpha$ intensity map in the region of the comet 3I/ATLAS (Rayleigh, *R*). The hydrogen cloud is clearly visible around 195° ecliptic longitude, near 0° latitude. The resolution is 1° pixel$^{-1}$. It is a small portion of the full-sky map recorded on 2025 November 12. There is a blind zone that cannot be observed with SWAN, too near the Sun (outside the map, on the left). The very bright UV star SPICA is just below the blind zone. The intensity is color-coded, and some particular contour isophotes are drawn: a thick dashed black line is at 560 *R*, a thick gray line is at 600, and the other thin lines are, respectively, at 545 *R* (one around the comet), and 400, 450, and 480. The 545 *R* isophote around the comet (at a distance to Earth of 2.13 au) has an approximate size of 16 × 10$^6$ km in longitude by 13 × 10$^6$ km in latitude.</figcaption></figure>

B. Hermalyn et al. 2013; M. S. Kelley et al. 2013; S. Protopapa et al. 2014; L. M. Feaga & J. M. Sunshine 2025; Y. Shou et al. 2025), which might explain so-called hyperactive comets, generally speaking (Y. Khan et al. 2023). If an extended source

from icy grains is responsible for the larger water production rates obtained with larger FOVs, then the contribution from those icy grains decreases substantially between early November and early December.
### 3. Discussion

From the SWAN data (9–41 days postperihelion; Figure 1), it can be computed that a total of 13.5 million metric tons of H₂O has been released from the comet during this period. The total active area of 3I/ATLAS ranged from a high of 193.7 down to 9.9 km² using the model of J. J. Cowan & M. F. A'Hearn (1979). This decrease would be consistent with the relative decrease in the contribution of an extended source of icy grains over this period. If water production came exclusively from a simple rotating nucleus, then the minimum radius would correspond to only 0.88 km if the surface were 100% active, only slightly larger than Jupiter-family comets 46 P/Wirtanen and 103 P/Hartley 2, and similar to that of Oort Cloud comet 1996 B2 (Hyakutake); see, e.g., M. R. Combi et al. (2005). See also the extensive table in the Letter by M. R. Combi et al. (2019). Comets that seem to be nearly 100% (or more) active appear to have an extensive contribution to water production from an extended source of released icy particles (Y. Khan et al. 2023).

H₂O production rates are an essential reference for all other gas production rates for the chemical and isotopic composition of volatile compounds in comets. The D/H ratio in water is of particular interest, as it constrains the deuterium abundance in primitive ice grains. Comet formation models indicate that, in the solar system, the D/H ratio of primitive ice was about twice larger than the average D/H ratio in comets (e.g., J. J. Kavelaars et al. 2011). If we make the reasonable assumption that the formation process of comets is similar in other stellar systems, then the D/H ratio for interstellar comets such as 3I/ATLAS would also indicate an about twice larger D/H ratio in its prestellar nebula. It should then provide valuable information about the origin of the observed variability of the D/H ratio in the interstellar medium (ISM) gas. As a matter of fact, one debated interpretation is that all D atoms missing in the gas have accumulated into icy grains. In this case, up to 4 orders of magnitude of deuteration are predicted to occur in ices of very dense clouds, based on the D/H depletion measured in interstellar gas along several lines of sight (B. T. Draine 2004; J. L. Linsky et al. 2006, R. Lallement et al. 2008, S. D. Friedman et al. 2023). Alternatively, in view of the suggested release of icy particles from the surface of the nucleus, it cannot be totally excluded that what is seen in the coma comes from a layer of interstellar material that would have been accreted during one or several crossings of the nucleus through very dense and cold interstellar clouds, during its long Galactic journey.

Finally, we want to emphasize that the larger water production rates from observations over larger FOVs mean that calculations of D/H ratios in 3I using different instruments for HDO and H₂O may need to be interpreted more carefully.

### Acknowledgments

SOHO is an international mission between ESA and NASA. M.C. acknowledges support from NASA grant 80NSSC23K0030 from the Solar System Observations Program. T.T.M. and W.S. were supported by the Finnish Meteorological Institute (FMI). J.-L.B. and E.Q. acknowledge support from CNRS and CNES. We obtained orbital elements from the JPL Horizons website (http://ssd.jpl.nasa.gov/horizons.cgi). For classification of the dynamical ages of the comets, we used the Minor Planets Center website https://www.minorplanetcenter.net/db_search tool. The composite solar Lyα data were taken from the LASP website at the University of Colorado (https://lasp.colorado.edu/lisird/data). We acknowledge the personnel who have been keeping SOHO and SWAN operational for 30 yr. We thank the MAVEN UVIS team for sharing the preliminary results of their observations before publication. We especially thank the reviewer, Jacques Crovisier, whose review helped to greatly improve this Letter.

### ORCID iDs

M. R. Combi https://orcid.org/0000-0002-9805-0078

T. Mäkinen https://orcid.org/0000-0001-9489-8154

J. L. Bertaux https://orcid.org/0000-0003-0333-229X

S. Ferron https://orcid.org/0000-0002-0631-5674

R. Lallement https://orcid.org/0000-0002-1284-875X

W. Schmidt https://orcid.org/0000-0002-8210-3868

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