![Orbit of 3I/ATLAS showing the 17 UVES/VLT observing epochs. Open and filled circles represent pre- and post-perihelion observations, respectively. Planet positions with filled circles are shown for the date of perihelion. The open diamond marks Earth’s position at the discovery of 3I/ATLAS, while the open circles along Earth’s orbit indicate its position at each observing epoch. [astro-ph.EP]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/09/High-resolution.png?fit=1200%2C678&ssl=1)
[astro-ph.EP] Comets preserve volatile material from the early stages of planetary-system formation. The discovery of the interstellar comet 3I/ATLAS enables a direct comparison between a comet formed around another star and comets in the Solar System.
We characterise the coma composition of 3I/ATLAS across perihelion using homogeneous high-resolution near-UV and optical spectroscopy with UVES on the ESO Very Large Telescope, from August 2025 (rh=3.14 au) pre-perihelion to February 2026 (rh=4.34 au) post-perihelion.
From flux-calibrated spectra, we measured OH, NH, CN, C3, CH and C2 emissions, derived production rates, examined abundance ratios and heliocentric-distance trends, constrained NH2, and compared OH-based water production with estimates from forbidden oxygen lines. Gas production rates show broadly symmetric pre- and post-perihelion behaviour, with steeper heliocentric-distance dependences than typically observed in Solar System comets.
NH and NH2 are strongly depleted, indicating a severe deficiency of ammonia-related volatiles. The C2/CN ratio indicates carbon-chain depletion and evolves with heliocentric distance, consistent with coma-driven effects such as hyperactivity rather than intrinsic compositional changes.
Water production from forbidden oxygen lines exceeds OH-based estimates except near perihelion, suggesting varying contributions from H2O and CO2 to coma chemistry. Relative abundance correlations place 3I/ATLAS outside the canonical Solar System comet cluster and closest to strongly carbon-chain depleted comets, including G-Z-type comets, while showing strong depletion of NH- and C3-bearing species relative to CN.
3I/ATLAS therefore extends the known diversity of cometary compositions, demonstrating that volatile reservoirs in other planetary systems can differ substantially from those sampled by Solar System comets.
K. Aravind, E. Jehin, C. Opitom, J. Manfroid, D. Hutsemékers, D. Bodewits, M. Bannister, R. C. Dorsey, L. Ferellec, H. Kawakita, M. Lippi, C. Snodgrass
Comments: 16 pages (4 page appendix), 13 Figures, 4 Tables, Accepted for publication in A&A
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2609.26577 [astro-ph.EP] (or arXiv:2609.26577v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2609.26577
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Submission history
From: K Aravind
[v1] Tue, 22 Sep 2026 15:25:45 UTC (3,160 KB)
https://arxiv.org/abs/2609.26577
Astrobiology,
