2026ApJ...999L..32R

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 (r_H) 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 L_p > 258 km at 99% confidence. However, low signal-to-noise ratio (S/N) on long baselines, which sample emission on small spatial scales closest to the nucleus, 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 r_H = 2 au. Compared to comets measured to date at radio wavelengths, the derived CH₃OH/HCN ratios in 3I/ATLAS of $12{4}_{-34}^{+30}$ and $7{9}_{-14}^{+11}$ 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 (Pan-STARRS).