The Inbound Gas and Dust Evolution of Interstellar Comet 3I/ATLAS from Optical Spectroscopy and Multiband Photometry*

Year: 2026

Type: article

Abstract: Interstellar objects (ISOs) provide direct constraints on the composition and evolution of extrasolar planetary systems. As the third confirmed ISO, comet 3I/ATLAS offers a rare opportunity to study pristine material from another star system. We present a systematic multiband monitoring campaign conducted from 2025 July to September. To minimize systematic uncertainties, we utilized a homogeneous dataset of Sloan Digital Sky Survey (SDSS) g ′ , r ′ , i ′ , z s and Johnson–Cousins BVR imaging obtained with MuSCAT3/4 on the 2.0 m Faulkes Telescopes and the 1.5 m Maidanak telescope. We employed a novel multiaperture technique that provides a robust, distance-independent characterization of the coma, alongside optical spectroscopy from SALT and the Nordic Optical Telescope. 3I/ATLAS exhibited a stable reddish color with no significant secular variation during its inbound journey. Spectroscopic analysis reveals carbon-chain depletion, with CN as the only prominent molecular emission. Coma morphology, analyzed via azimuthal averaging and rotational gradient filters, shows a persistent sunward jet at a position angle of ∼280 ∘ , indicating sustained and localized outgassing from a high-latitude active region. Modeling of light scattering demonstrates that the observed colors and polarization are consistent with dense aggregates of organic matter and silicate minerals, while water-ice sublimation remained insufficient to alter the optical properties. The stability of the coma structures and the homogeneous nature of our dataset indicate that 3I/ATLAS underwent a steady activation phase, preserving a volatile-rich but carbon-chain-poor composition. This characterization points to an object formed in a distinct protoplanetary environment and provides a definitive record of its pre-perihelion evolution.

Source: The Astrophysical Journal

DOI: 10.3847/1538-4357/ae8dbb

Authors: T. Santana-Ros, A. Sergeyev, O. Ivanova, S. Mykhailova, J. Markkanen, I. Luk’yanyk, T. Kwiatkowski, D. Oszkiewicz, A. Penttilä, N. Erasmus, K. Ergashev, Y. Krugly

Institutions: University of Alicante, Universitat de Barcelona, Université Côte d'Azur, Communauté d'universités et d'établissements Université Côte d'Azur, V. N. Karazin Kharkiv National University, National Academy of Sciences of Ukraine, Main Astronomical Observatory, Astronomical Institute of the Slovak Academy of Sciences, Adam Mickiewicz University in Poznań, Taras Shevchenko National University of Kyiv, University of Helsinki, Stellenbosch University, South African Astronomical Observatory, Ulugh Beg Astronomical Institute, Université Paris Sciences et Lettres

Language: English

Topic: Astro and Planetary Science

Subfield: Space and Planetary Science

Field: Earth and Planetary Sciences

Domain: Physical Sciences

Open Access status: gold

Estimated APC paid: $4,499

Funders: Slovenská Akadémia Vied, European Commission, Cardiff University, Deutscher Akademischer Austauschdienst, Deutsche Forschungsgemeinschaft, Generalitat Valenciana, Narodowym Centrum Nauki, Ministerio de Ciencia e Innovación, Agentúra na Podporu Výskumu a Vývoja, Universitat de Barcelona, Science and Technology Facilities Council, Japan Society for the Promotion of Science, European Regional Development Fund, Agencia Estatal de Investigación, Institut de Ciències del Cosmos, Precursory Research for Embryonic Science and Technology

OpenAlex ID: W7203713991