Spectroscopic Characterization of Interstellar Object 3I/ATLAS: Water Ice in the Coma

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.