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SPHEREx Discovery of Strong Water Ice Absorption and an Extended Carbon Dioxide Coma in 3I/ATLAS

By Keith Cowing

Status Report

astro-ph.EP

August 22, 2025

Filed under 3I/ATLAS, astro-ph.EP, astrochemistry, astronomy, Comet, Interstellar, spectrophotometry, SPHEREx, water ice

SPHEREx Discovery of Strong Water Ice Absorption and an Extended Carbon Dioxide Coma in 3I/ATLAS — SPHEREx 3.0/4.26/4.7 μm H2O/CO2/CO gas coma from left to right respectively. The object is undetected in H2O and CO, with 2-pixel radius preliminary conservative upper limits of 0.45 and 0.99 mJy. By contrast, a bright CO2 coma with central PSF flux = 33mJy that extends out to at least 30-pixel (=348,000 km) was found. This observed minimum size for the CO2 coma corresponds to ~700,000-second mean lifetime at vgas = 0.48 km/sec. The reported photodissociation lifetime for CO2 at 1au is 135hrs = 486,000 seconds (Huebner+1992), which scales by 1/r2 to ~4,670,000 seconds at 3.2au. — astro-ph.EP

In mid-August 2025, 0.75-5.0 micron SPHEREx imaging spectrophotometric and ancillary NASA-IRTF SpeX 0.7-2.5 micron low-resolution spectral observations of Interstellar Object 3I ATLAS were obtained.

The combined spectrophotometry is dominated by features due to water ice absorption and CO2 gas emission. A bright, 3 arcmin radius CO₂ gas coma was clearly resolved, corresponding to Q_gas,CO₂ = 9.4 x 10{^26} molec/sec. From the SPHEREx photometry, we put conservative, preliminary 3sigma upper limits on the gas production rates for H₂O and CO of 1.5 x 10{^26} and 2.8 x 10{^26} molec/sec.

No obvious jet, tail, or trail structures were found in SPHEREx images. Assuming all observed 1-um flux is scattered light from an pv = 0.04 albedo spherical nucleus, its radius would be 23 km.

Compared to the nucleus size limit r = 2.8km of Jewitt+ 2025, this suggests that greater than 99 percent of the measured SPHEREx continuum flux is from coma dust.

C.M. Lisse, Y.P. Bach, S. Bryan, B.P. Crill, A. Cukierman, O. Doré, B. Fabinsky, A. Faisst, P. M. Korngut, G. Melnick, Z. Rustamkulov, V. Tolls, M. Werner, M.L. Sitko, C. Champagne, M. Connelley, J.P. Emery, B. Yang, the SPHEREx Science Team

Comments: 6 pages, 4 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR); General Relativity and Quantum Cosmology (gr-qc)
Cite as: arXiv:2508.15469 [astro-ph.EP] (or arXiv:2508.15469v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2508.15469
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Submission history
From: Carey Lisse
[v1] Thu, 21 Aug 2025 11:46:08 UTC (960 KB)
https://arxiv.org/abs/2508.15469
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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