JWST Reveals Refractory-Rich Water Ice in Interstellar Comet 3I/ATLAS: Evidence for a Continuum of Grain Properties across Protoplanetary Disks
Abstract
We present JWST/NIRSpec PRISM observations of the interstellar comet 3I/ATLAS obtained on 2025 August 6 (Epoch 1), 2025 December 22 (Epoch 2), and 2026 April 1 (Epoch 3), spanning eight months around perihelion at heliocentric distances of 3.3, 2.4, and 5.7 au, respectively. The spectra reveal a broad 3 μm absorption band together with H2O, CO2, and CO gas emission. Unlike previously reported water-ice-rich Solar System comae, the strong 3 μm absorption is accompanied by weak or absent 1.5 and 2.0 μm water-ice bands. Spectral modeling indicates that the observations are best reproduced by submicron- to micron-sized water-ice-bearing aggregates containing refractory material. Compared with Epoch 1, the Epoch 3 spectrum favors the presence of a second population of larger, micron-sized, ice-rich aggregates and exhibits a subtle Fresnel-like structure near 3.1 μm, consistent with crystalline water ice. The observations can be explained by either crystalline water ice at both epochs, with the spectral evolution arising primarily from changes in grain size and refractory mixing, or an evolution from an amorphous-like to crystalline state. The spectral properties of 3I bridge those of water-ice-bearing Solar System comae and several spectral classes of mid-sized trans-Neptunian objects, suggesting that the icy building blocks of planetesimals formed in different protoplanetary disks may span a continuum in the physical state of water ice, ranging from pure ice grains to water-ice-bearing aggregates with varying refractory content at submicron-to-micron scales, with 3I extending toward the refractory-rich end of this continuum.
Versions
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BibTeX
@misc{protopapa2026jwst,
title = {{JWST Reveals Refractory-Rich Water Ice in Interstellar Comet 3I/ATLAS: Evidence for a Continuum of Grain Properties across Protoplanetary Disks}},
author = {Silvia Protopapa and Michael S. P. Kelley and Martin A. Cordiner and Stefanie N. Milam and Dennis Bodewits and Steven B. Charnley and Maria N. Drozdovskaya and Sara Faggi and Davide Farnocchia and Aurélie Guilbert-Lepoutre and Marco Micheli and Nathan X. Roth and Megan E. Schwamb and Darryl Z. Seligman and Geronimo L. Villanueva},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.19286},
eprint = {2609.19286},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.19286},
}
RIS
TY - UNPB
TI - JWST Reveals Refractory-Rich Water Ice in Interstellar Comet 3I/ATLAS: Evidence for a Continuum of Grain Properties across Protoplanetary Disks
AU - Silvia Protopapa
AU - Michael S. P. Kelley
AU - Martin A. Cordiner
AU - Stefanie N. Milam
AU - Dennis Bodewits
AU - Steven B. Charnley
AU - Maria N. Drozdovskaya
AU - Sara Faggi
AU - Davide Farnocchia
AU - Aurélie Guilbert-Lepoutre
AU - Marco Micheli
AU - Nathan X. Roth
AU - Megan E. Schwamb
AU - Darryl Z. Seligman
AU - Geronimo L. Villanueva
PY - 2026
DA - 2026/09/16
PB - arXiv
DO - 10.48550/arxiv.2609.19286
UR - https://arxiv.org/abs/2609.19286
AB - We present JWST/NIRSpec PRISM observations of the interstellar comet 3I/ATLAS obtained on 2025 August 6 (Epoch 1), 2025 December 22 (Epoch 2), and 2026 April 1 (Epoch 3), spanning eight months around perihelion at heliocentric distances of 3.3, 2.4, and 5.7 au, respectively. The spectra reveal a broad 3 μm absorption band together with H_2O, CO_2, and CO gas emission. Unlike previously reported water-ice-rich Solar System comae, the strong 3 μm absorption is accompanied by weak or absent 1.5 and 2.0 μm water-ice bands. Spectral modeling indicates that the observations are best reproduced by submicron- to micron-sized water-ice-bearing aggregates containing refractory material. Compared with Epoch 1, the Epoch 3 spectrum favors the presence of a second population of larger, micron-sized, ice-rich aggregates and exhibits a subtle Fresnel-like structure near 3.1 μm, consistent with crystalline water ice. The observations can be explained by either crystalline water ice at both epochs, with the spectral evolution arising primarily from changes in grain size and refractory mixing, or an evolution from an amorphous-like to crystalline state. The spectral properties of 3I bridge those of water-ice-bearing Solar System comae and several spectral classes of mid-sized trans-Neptunian objects, suggesting that the icy building blocks of planetesimals formed in different protoplanetary disks may span a continuum in the physical state of water ice, ranging from pure ice grains to water-ice-bearing aggregates with varying refractory content at submicron-to-micron scales, with 3I extending toward the refractory-rich end of this continuum.
ER -