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

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.