3I/ATLAS chemistry points to formation in unusually cold conditions

The importance of the nitrogen result comes from the extreme volatility of molecular nitrogen. N₂ is difficult to retain as comet-forming material warms, so an elevated N₂/CO ratio can preserve information about the low temperatures at which volatile ices were trapped.

Ferellec et al. compare the 3I/ATLAS measurement with Solar System comets and laboratory trapping experiments and interpret it as consistent with cold formation conditions.

That information matters because 3I/ATLAS formed outside the Solar System. Its chemistry offers a rare direct comparison between material assembled around another star and the comet populations used to reconstruct conditions in the early Solar System.

The new nitrogen measurement does not identify the comet’s parent star, but it narrows the physical conditions under which some of its most volatile material could have been incorporated and preserved.

Ferellec et al. (2026) observed 3I/ATLAS after perihelion on November 30 and December 2, 2025 with the WHT Enhanced Area Velocity Explorer (WEAVE) on the William Herschel Telescope. They simultaneously detected N₂⁺, CO⁺, CO₂⁺, H₂O⁺ and CH⁺ in the antisolar plasma tail. The measured N₂⁺/CO⁺ ratio constrains the abundance of the neutral parent molecules to N₂/CO >0.023 ± 0.001, placing 3I/ATLAS in an N₂-rich regime relative to Solar System comets.

The ion ratios remained broadly stable along the observed tail. Only CH⁺ showed a marginal decline with increasing cometocentric distance, while the abundance ratios of the other detected ions showed no corresponding change. Ferellec and colleagues cautioned that most ion ratios cannot be converted directly into neutral-molecule abundances because chemical processing in the coma complicates that relationship; N₂⁺/CO⁺ provides the cleaner formation-sensitive constraint used in their analysis.

The accepted MNRAS manuscript appeared on July 23, 2026, followed by the corrected and typeset Version of Record on September 7. The publication adds an independent chemical line of evidence to isotope measurements reported earlier in 2026.

Cordiner et al. (2026) measured a water D/H ratio of 0.98 ± 0.06% in 3I/ATLAS using the James Webb Space Telescope. The deuterium-to-hydrogen ratio provides a tracer of formation chemistry because low-temperature reactions can strongly alter the relative abundance of deuterium incorporated into water. The Nature study constrains formation of most of the comet’s water ice to ≲30 K, about −243°C (−406°F), with little contribution from water processed at higher temperatures.

The same study measured ^12C/^13C ratios of 141–191 in CO₂ and 123–172 in CO, above typical values reported for Solar System material, nearby interstellar clouds and protoplanetary disks. Cordiner and colleagues interpret the carbon-isotope composition as evidence for formation in a relatively metal-poor environment.

The isotope measurements are independent of the nitrogen constraint: they probe different molecules with a different instrument and rely on different chemical processes.

Millimeter observations add another kind of difference without providing a separate formation-temperature measurement. Biver et al. (2026) observed 3I/ATLAS with the Institut de Radioastronomie Millimétrique (IRAM) 30-m telescope on November 1–3, 2025, shortly after its October 29 perihelion. HCN, CH₃OH, CO and H₂CO were securely detected, detections of CS and CH₃CN reached about 4σ, and H₂S was not detected.

Several molecular abundance ratios fell toward the upper end of values measured in Solar System comets, while the sulfur-to-carbon abundance ratio was at or below the minimum of the comparison sample.

The result does not establish where 3I/ATLAS formed, but it adds to the evidence that its volatile inventory differs from the ranges commonly measured in Solar System comets.

The IRAM spectra also showed unusually narrow molecular lines. Spectral-line width records the range of gas velocities along the line of sight, so narrower lines correspond to slower coma expansion. An isotropic model gave an expansion velocity of 0.37 km/s (0.23 miles/s), about half the 0.7–0.8 km/s (0.43–0.50 miles/s) measured in comets of similar activity at comparable heliocentric distances.

An asymmetric model reproduced the profiles with 0.42 km/s (0.26 miles/s) in the sunward hemisphere and 0.32 km/s (0.20 miles/s) in the antisunward hemisphere. Biver et al. found that a coma dominated by heavier molecules such as CO₂ can account for part of the low velocity; substantial gas production from sublimating icy grains provides another explanation. IRAM issued an institutional summary of the peer-reviewed results on September 9.

The three observing approaches do not measure the same quantity. WEAVE maps ions in the plasma tail and uses N₂⁺/CO⁺ to constrain the neutral nitrogen-to-carbon-monoxide abundance. JWST’s Near Infrared Spectrograph measures isotopologues of neutral volatile molecules, providing deuterium and carbon isotope ratios. IRAM measures rotational molecular emission and line profiles, providing molecular abundances and coma-gas velocities. Agreement across formation-sensitive measurements is more informative than repeated measurements of a single chemical proxy.

The chemistry still does not identify the exact stellar system or time in which 3I/ATLAS formed. Cordiner et al. report that reconstructing the object’s Galactic trajectory becomes increasingly difficult beyond roughly 10 million years because gravitational encounters make backward orbital solutions chaotic. Chemical and isotopic measurements can preserve evidence of temperature and elemental environment even after the orbital path to the source system has been lost.

That distinction sets the present limit of the result: observations can constrain the physical environment in which 3I/ATLAS acquired its volatile inventory, but they cannot yet reconstruct a unique birthplace. Further interstellar objects will be needed to determine whether the nitrogen-rich and isotopically unusual chemistry of 3I/ATLAS represents a wider population of cold-formed extrasolar comets or an unusual individual case.

References:

¹ Ferellec, L., Opitom, C., & Snodgrass, C. (2026). Ion abundances in the plasma tail of 3I/ATLAS show that it is N₂-rich. Monthly Notices of the Royal Astronomical Society, 551(3), stag1402. https://doi.org/10.1093/mnras/stag1402

² Biver, N., Bockelée-Morvan, D., Moreno, R., Crovisier, J., Paubert, G., Zakharov, V., Boissier, J., Cordiner, M. A., & Roth, N. X. (2026). Perihelion observations of interstellar comet 3I/ATLAS with the IRAM 30-m telescope. Astronomy & Astrophysics, 708, L16. https://doi.org/10.1051/0004-6361/202659316

³ Secrets of interstellar comet 3I/ATLAS revealed – Royal Astronomical Society – September 8, 2026