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

In mid-August 2025, 0.75-5.0 μm SPHEREx imaging spectrophotometric and ancillary NASA-IRTF/SpeX 0.7-2.5 μm low-resolution spectral observations of 3I/ATLAS were obtained. The combined spectrophotometry is dominated by features due to water ice absorption and CO2 gas emission. A bright, ~3’ radius CO2 gas coma was clearly resolved, corresponding to Qgas,CO2~9.4x1026 molec/sec. From the SPHEREx photometry, we put conservative, preliminary 3s upper limits on the gas production rates for H2O and CO of 1.5x1026 and 2.8x1026 molec/sec, respectively. 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 >99% of the measured SPHEREx continuum flux is from coma dust.