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

In mid-August 2025, 0.75–5.0 μ m SPHEREx imaging spectrophotometric and ancillary NASA-IRTF/SpeX 0.7–2.5 μ m low-resolution spectral observations of 3I/ATLAS were obtained. The combined spectrophotometry is dominated by features due to water ice absorption and CO 2 gas emission. A bright, ∼3′ radius CO 2 gas coma was clearly resolved, corresponding to Q gas,CO2 ∼9.4 × 10 26 molecules s −1 . From the SPHEREx photometry, we put conservative, preliminary 3 σ upper limits on the gas production rates for H 2 O and CO of 1.5 × 10 26 and 2.8 × 10 26 molecules s −1 , respectively. No obvious jet, tail, or trail structures were found in SPHEREx images. If we assume all observed 1 μ m flux is scattered light from a p v = 0.04 albedo spherical nucleus, then the radius would be R nuc ∼ 23 km. Given the nucleus size limit R nuc < 2.8 km from D. Jewitt et al., we conclude >99% of the measured SPHEREx continuum flux is from coma dust.