JWST/MIRI Observations of Amorphous Silicates in Interstellar Comet 3I/ATLAS
Abstract
We present JWST/MIRI Low Resolution Spectroscopy (LRS) of 3I/ATLAS obtained post-perihelion at a heliocentric distance of 2.21 au. Our spectrum contains a prominent 10 micron silicate emission feature with a broad, smooth shape that is distinct from those commonly observed in Solar System comets. We model the spectrum using thermal emission from amorphous carbon, amorphous olivine, and amorphous pyroxene grains. Our models strongly favor a pyroxene-dominated, amorphous silicate composition and show a degeneracy between silicate grain size and the relative abundance of silicate and carbon dust. Across a set of representative models, the best-fitting crystalline abundance is zero, and we place an upper limit of 5.3% on the crystalline fraction of the silicates. We identify an emission feature near 6.83 microns that is consistent with aliphatic C-H-bearing material. The overwhelmingly amorphous silicate composition supports previous work that measured unusual volatile and isotopic composition in suggesting that the material incorporated into the object experienced little thermal processing and preserves information about the conditions in which this interstellar planetesimal formed.
Related
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- Very Large Telescope observations of interstellar comet 3I/ATLAS III: High-resolution monitoring of CN and forbidden oxygen emission across the perihelion passage with ESPRESSO
BibTeX
@misc{thomas2026jwstmiri,
title = {{JWST/MIRI Observations of Amorphous Silicates in Interstellar Comet 3I/ATLAS}},
author = {Cristina A. Thomas and Michael S. P. Kelley and David E. Harker and Martin A. Cordiner and Davide Farnocchia and Dennis Bodewits and Steven B. Charnley and Maria N. Drozdovskaya and Marco Micheli and Stefanie N. Milam and Cyrielle Opitom and Silvia Protopapa and Nathan X. Roth and Megan E. Schwamb},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.37540},
eprint = {2610.37540},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.37540},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - JWST/MIRI Observations of Amorphous Silicates in Interstellar Comet 3I/ATLAS
AU - Cristina A. Thomas
AU - Michael S. P. Kelley
AU - David E. Harker
AU - Martin A. Cordiner
AU - Davide Farnocchia
AU - Dennis Bodewits
AU - Steven B. Charnley
AU - Maria N. Drozdovskaya
AU - Marco Micheli
AU - Stefanie N. Milam
AU - Cyrielle Opitom
AU - Silvia Protopapa
AU - Nathan X. Roth
AU - Megan E. Schwamb
PY - 2026
DA - 2026/10/27
PB - arXiv
DO - 10.48550/arxiv.2610.37540
UR - https://arxiv.org/abs/2610.37540
AB - We present JWST/MIRI Low Resolution Spectroscopy (LRS) of 3I/ATLAS obtained post-perihelion at a heliocentric distance of 2.21 au. Our spectrum contains a prominent 10 micron silicate emission feature with a broad, smooth shape that is distinct from those commonly observed in Solar System comets. We model the spectrum using thermal emission from amorphous carbon, amorphous olivine, and amorphous pyroxene grains. Our models strongly favor a pyroxene-dominated, amorphous silicate composition and show a degeneracy between silicate grain size and the relative abundance of silicate and carbon dust. Across a set of representative models, the best-fitting crystalline abundance is zero, and we place an upper limit of 5.3% on the crystalline fraction of the silicates. We identify an emission feature near 6.83 microns that is consistent with aliphatic C-H-bearing material. The overwhelmingly amorphous silicate composition supports previous work that measured unusual volatile and isotopic composition in suggesting that the material incorporated into the object experienced little thermal processing and preserves information about the conditions in which this interstellar planetesimal formed.
ER -