SPHEREx Pre-Perihelion Mapping of H2O, CO2, and CO in Interstellar Object 3I/ATLAS

From 01- to 15-Aug-2025 UT, the SPHEREx spacecraft observed interstellar object 3I/ATLAS. Using R=40-130 spectrophotometry at λ=0.7-5μm, light curves, spectra, and imaging of 3I were obtained.

From these, robust detections of water gas emission at 2.7-2.8μm and CO₂ gas at 4.23-4.27μm plus tentative detections of ¹³CO₂ and CO gas were found. A slightly extended H₂O coma was detected, and a huge CO₂ atmosphere extending out to at least 4.2×10⁵km was discovered. Gas production rates for H₂O, ¹²CO₂, ¹³CO₂, and CO were Qgas=3.2×10²⁶±20%, 1.6×10²⁷±10%, 1.3×10²⁵±25%, and 1.0×10²⁶±25%, respectively.

Co-addition of all λ=1.0-1.5μm scattered light continuum images produced a high SNR image consistent with an unresolved source. The scattered light lightcurve showed ≲15% variability over the observation period. The absolute brightness of 3I at 1.0-1.5μm is consistent with a <2.5km radius nucleus surrounded by a 100 times brighter coma.

The 1.5-4.0μm continuum structure shows a strong feature commensurate with water ice absorption seen in KBOs and distant comets. The observed cometary behavior of 3I, including its preponderance of CO₂ emission, lack of CO output, small size, and predominance of large icy chunks of material in a flux-dominant coma is reminiscent of the behavior of short period comet 103P/Hartley 2, target of the NASA Deep Impact extended mission in 2010 and a “hyperactive comet” near the end of its outgassing lifetime.

This correspondence places 3I closer to barely- or non-active 1I/Oumuamua than primitive, ice rich 2I/Borisov, suggesting that ISOs are often highly thermally processed before ejection into the ISM.

Carey M. Lisse, Yoonsoo P. Bach, Sean A. Bryan, Brendan P. Crill, Phil M. Korngut, Ari J. Cukierman, Michael W. Werner, Asantha Cooray, Michael Zemcov, Volker Tolls, Gary J. Melnick, Andreas L. Faisst, C. Darren Dowell, Seungwon Choi, Jooyeon Geem, Masateru Ishiguro, Hangbin Jo, Bumhoo Lim, Max Mahlke, Joseph L. Hora, Yun-Ting Cheng, Spencer Everett, Jeong-Eun Lee, Zafar Rustamkulov, Sunho Jin, Howard Hui, Daniel C. Masters, Chi H. Nguyen, Roberta Paladini, Yujin Yang, James J. Bock, O. Doré, M. L. Sitko, C. Champagne, M. Connelley, J. P. Emery, Y. R. Fernandez, W. T. Reach

Comments: 19 pages, 4 figures, 1 table, submitted to ApJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2512.07318 [astro-ph.EP] (or arXiv:2512.07318v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2512.07318
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Submission history
From: Yoonsoo Bach
[v1] Mon, 8 Dec 2025 09:02:41 UTC (1,880 KB)
https://arxiv.org/abs/2512.07318
Astrobiology, Astrochemistry, Astrogeology,