Spectroscopic Characterization Of Interstellar Object 3I/ATLAS: Water Ice In The Coma

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.

Bin Yang, Karen J. Meech, Michael Connelley, Jacqueline V. Keane

Comments: 8 pages, 4 figures, submitted to ApJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2507.14916 [astro-ph.EP] (or arXiv:2507.14916v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2507.14916
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Submission history
From: Bin Yang
[v1] Sun, 20 Jul 2025 11:10:21 UTC (1,951 KB)
https://arxiv.org/abs/2507.14916
Astrobiology,