Eimission spectra of 3I/ATLAS taken with BFOSC and YFOSC. For clarity, the full spectral range is divided into three panels with different vertical offsets to emphasize specific features. The left panel highlights emission lines of Ni i (blue dashed lines) and Fe i (red dashed lines), and the CN violet band (blue shaded). The middle panel shows molecular bands of C3 (green shaded), CH (orange shaded), and C2 (red shaded), and lists the heliocentric distance for each spectrum. The right panel displays the [O i] λλ6300, 6364 doublet (green dashed lines). — astro-ph.EP
We present multi-epoch optical spectroscopy of the interstellar comet 3I/ATLAS obtained from December 2025 to January 2026 (heliocentric distances 1.8 to 3.3 au). From these spectra, we derive post-perihelion production rates and/or mixing ratios for daughter molecules (CN, C3, C2, and CH) and gaseous metals (Fe I and Ni I).
We also estimate a lower limit on the CO abundance based on the [O I] λ6300 line. The resulting outgassing profiles reveal a pronounced perihelion asymmetry, with volatile production declining more gradually outbound than during the inbound phase. In addition, the coma becomes less depleted in C2 following perihelion, and shows a substantial increase, relative to H2O, in the production rates of metals and potentially also CO.
These trends may indicate the activation of subsurface material or compositional heterogeneity revealed by seasonal effects. The inferred high CO abundance further suggests possible decoupling of CO from H2O and CO2, similar to the behavior observed in solar system comets.
The potential post-perihelion enhancement of both CO and metal production, if confirmed, would also be consistent with metal carbonyls contributing to the release of gaseous metals in cometary comae.
Ruining Zhao, Xiliang Zhang, Bin Yang, Xiangyu Fan, Shu Wang, Yang Huang, Jifeng Liu
Comments: 13 pages, 3 figures, 2 tables, submitted to ApJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2603.07718 [astro-ph.EP] (or arXiv:2603.07718v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2603.07718
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Submission history
From: Ruining Zhao
[v1] Sun, 8 Mar 2026 16:42:57 UTC (432 KB)
https://arxiv.org/abs/2603.07718
Astrobiology, Astrochemistry,