SPHEREx Pre-Perihelion Mapping of ${\mathrm{H}}_2\mathrm{O}$, $\mathrm{C}{\mathrm{O}}_2$, and $\mathrm{CO}$ in Interstellar Object 3I/ATLAS

Abstract

From 01- to 15-Aug-2025UT, the SPHEREx spacecraft observed interstellar object 3I/ATLAS. Using $R=40-130$ spectrophotometry at $\hat{I}»=0.7-5$ $\hat{I}¼$m, light curves, spectra, and imaging of ATLAS were obtained. From these, robust detections of water gas emission at $2.7-2.8$ $\hat{I}¼$m and $C{O}_2$ gas at $4.23-4.27$ $\hat{I}¼$m plus tentative detections of ${}^{13}C{O}_2$ and $CO$ gas were found. A slightly extended $H_{2}O$ coma was detected, and a huge $C{O}_2$ atmosphere of extending out to at least $4.2\times 10^5$ km was discovered. Gas production rates and 1$\ddot{I}ƒ$ errors for $H_{2}O$, ${}^{12}C{O}_2$, ${}^{13}C{O}_2$, and $CO$ were $Q_{gas}=3.2\times 10^{26}\pm 20\%$, $1.6\times 10^{27}\pm 10\%$, $1.3\times 10^{25}\pm 25\%$, and $1.0\times 10^{26}\pm 25\%$, respectively. Co-addition of all $\hat{I}»=1.0-1.5$ $\hat{I}¼$m scattered light continuum images from produced a high SNR image consistent with an unresolved source. The scattered light lightcurve showed $\lesssim 15\%$ variability over the observation period. The absolute brightness of ATLAS at $1.0-1.5$ $\hat{I}¼$m is consistent with a $<2.5$ km radius nucleus surrounded by a 100 times brighter coma. The $1.5-4.0$ $\hat{I}¼$m continuum structure shows a strong feature commensurate with water ice absorption seen in KBOs and distant comets. The observed cometary behavior of ATLAS, including its preponderance of $C{O}_2$ emission, lack of $CO$ output, small size, and predominance of large icy chunks of material in a flux-dominant coma is similar to the behavior of short period comet 103P/Hartley 2, the ''hyperactive comet'' flyby target of the NASA Deep Impact extended mission in 2010. This correspondence suggests that ISOs can be significantly thermally processed before ejection into the ISM, and by comparison to 1I and 2I, can be widely variable in their physical outcome.

19 pages, 4 figures, 1 table, submitted to ApJL