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

TL;DR

This study used SPHEREx and NASA-IRTF observations to detect strong water ice absorption and a prominent CO2 gas coma in interstellar object 3I/ATLAS, revealing its composition and activity.

Contribution

First detection of water ice absorption and a large CO2 coma in 3I/ATLAS using combined spectrophotometry and spectroscopy.

Findings

Detected a bright CO2 gas coma with a production rate of 9.4 x 10^26 molecules/sec.

Placed upper limits on H2O and CO gas production rates.

Most of the observed flux is from coma dust, not the nucleus.

Abstract

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 Qgas,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.