CH3OH and HCN in Interstellar Comet 3I/ATLAS Mapped with the ALMA Atacama Compact Array: Distinct Outgassing Behaviors and a Remarkably High CH3OH/HCN Production Rate Ratio

We report the detection of methanol (CH₃OH) toward interstellar comet 3I/ATLAS using the Atacama Compact Array of the Atacama Large Millimeter/Submillimeter Array (ALMA) on UT 2025 August 28, September 18 and 22, and October 1, and of hydrogen cyanide (HCN) on September 12 and 15.

These observations spanned pre-perihelion heliocentric distances (rH) of 2.6 — 1.7 au. The molecules showed outgassing patterns distinct from one another, with HCN production being depleted in the sunward hemisphere of the coma, whereas CH₃OH was enhanced in that direction.

Statistical analysis of molecular scale lengths in 3I/ATLAS indicated that CH₃OH included production from coma sources at Lp>258 km at 99% confidence, although low signal-to-noise on long baselines prevented definitively ruling out CH₃OH as purely a parent species.

In contrast, HCN production was indistinguishable from direct nucleus sublimation. The CH₃OH production rate increased sharply from August through October, including an uptick near the inner edge of the H₂O sublimation zone at rH = 2 au.

Compared to comets measured to date at radio wavelengths, the derived CH₃OH/HCN ratios in 3I/ATLAS of 124+30−34 and 79+11−14 on September 12 and 15, respectively, are among the most enriched values measured in any comet, surpassed only by anomalous solar system comet C/2016 R2 (PanSTARRS).

Nathan X. Roth, Martin A. Cordiner, Dominique Bockelée-Morvan, Nicolas Biver, Jacques Crovisier, Stefanie N. Milam, Emmanuel Lellouch, Pablo Santos-Sanz, Dariusz C. Lis, Chunhua Qi, K. D. Foster, Jérémie Boissier, Kenji Furuya, Raphael Moreno, Steven B. Charnley, Anthony J. Remijan, Yi-Jehng Kuan, Lillian X. Hart

Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2511.20845 [astro-ph.EP] (or arXiv:2511.20845v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2511.20845
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From: Nathan Roth
[v1] Tue, 25 Nov 2025 20:48:30 UTC (3,283 KB)
https://arxiv.org/abs/2511.20845
Astrobiology, Astrochemistry,