SPHEREx Discovery of Strong Water Ice Absorption and an Extended Carbon Dioxide Coma in 3I/ATLAS
C. M. Lisse, Yoonsoo P. Bach, Sean Bryan, B. P. Crill, A. Cukierman, Olivier Doré, Beth Fabinsky, Andreas L. Faisst, Phillip Korngut, Gary J. Melnick, Zafar Rustamkulov, Volker Tolls, M. W. Werner, Michael L. Sitko, Chansey Champagne, Michael S. Connelley, Joshua P. Emery, Y. R. Fernández, Bo Yang, the SPHEREx Science Team
Abstract
Abstract 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.