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Spectroscopic Characterization of Interstellar Object 3I/ATLAS: Water Ice in the Coma
Bin Yang (Planetary Science Institute, Universidad Diego Portales), Karen J. Meech (University of Hawaiʻi at Mānoa, University of Hawaii System), Michael S. Connelley (University of Hawaiʻi at Mānoa, University of Hawaii System), Ruining Zhao (Chinese Academy of Sciences, National Astronomical Observatories), J. V. Keane (U.S. National Science Foundation)
Résumé : Abstract We present optical and near-infrared spectroscopy of the interstellar object 3I/ATLAS, obtained with Gemini-S/GMOS and NASA IRTF/SpeX on 2025 July 5 and 14. The optical spectrum shows a red slope of ∼11% per 1000 Å between 0.5 and 0.8 μm, resembling typical D-type asteroids and distinct from ultrared trans-Neptunian objects. At longer wavelengths, the near-infrared continuum flattens to ∼3% per 1000 Å between 0.9 and 1.5 μm, with a broad absorption feature near 2.0 μm indicative of water-ice grains in the coma. Spectral modeling with a mixture of 63% amorphous carbon and 37% 1 μm-sized water ice reproduces both the continuum and the 2.0 μm band, while the 1.5 μm water-ice band is not detected, likely due to limited signal-to-noise in the IRTF data and dilution by refractory material. The close agreement between the GMOS and SpeX spectra, taken 9 days apart, indicates short-term stability in the coma’s optical properties. These observations demonstrate that 3I/ATLAS is an active interstellar comet containing abundant water ice, consistent with the theoretical expectation that its home planetary system had a high bulk fraction of water ice by mass.