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 July 14. The optical spectrum shows a red slope of approximately 10% per 1000 angstroms between 0.5 and 0.8 microns, closely resembling that of typical D-type asteroids. At longer wavelengths, the near-infrared spectrum flattens significantly to approximately 3% per 1000 angstroms from 0.9 to 1.5 microns, consistent with the spectral behavior of large water ice grains in the coma. Spectral modeling with an areal mixture of 70% Tagish Lake meteorite and 30% 10-micron-sized water ice successfully reproduces both the overall continuum and the broad absorption feature near 2.0 microns. The 1.5-micron water ice band, however, is not detected, likely due to the limited signal-to-noise of the IRTF data and dilution by refractory materials. The ~30% ice fraction should be interpreted as an approximate, order-of-magnitude estimate of the coma composition. The agreement between the GMOS and SpeX spectra, taken nine days apart, suggests short-term stability in the coma's optical properties. Our observations reveal that 3I/ATLAS is an active interstellar comet containing abundant water ice, with a dust composition more similar to D-type asteroids than to ultrared trans-Neptunian objects.