Kavli Affiliate: James Bock
| Summary:
From 01- to 15-Aug-2025 UT, the SPHEREx spacecraft observed interstellar object 3I/ATLAS. Using $R = 40$-$130$ spectrophotometry at $λ= 0.7$-$5μ$m, light curves, spectra, and imaging of 3I were obtained. From these, robust detections of water gas emission at $2.7$-$2.8,μ$m and CO$_2$ gas at $4.23$-$4.27,μ$m plus tentative detections of $^13$CO$_2$ and CO gas were found. A slightly extended H$_2$O coma was detected, and a huge CO$_2$ atmosphere extending out to at least $4.2times10^5,$km was discovered. Gas production rates for H$_2$O, $^12$CO$_2$, $^13$CO$_2$, and CO were $Q_mathrmgas = 3.2times10^26 pm 20%$, $1.6times10^27 pm 10%$, $1.3times10^25 pm 25%$, and $1.0times10^26 pm 25%$, respectively. Co-addition of all $λ= 1.0$-$1.5,μ$m scattered light continuum images 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 3I at $1.0$-$1.5,μ$m is consistent with a $< 2.5,$km radius nucleus surrounded by a 100 times brighter coma. The $1.5$-$4.0,μ$m continuum structure shows a strong feature commensurate with water ice absorption seen in KBOs and distant comets. The observed cometary behavior of 3I, including its preponderance of CO$_2$ emission, lack of CO output, small size, and predominance of large icy chunks of material in a flux-dominant coma is reminiscent of the behavior of short period comet 103P/Hartley 2, target of the NASA Deep Impact extended mission in 2010 and a “hyperactive comet” near the end of its outgassing lifetime. This correspondence places 3I closer to barely- or non-active 1I/Oumuamua than primitive, ice rich 2I/Borisov, suggesting that ISOs are often highly thermally processed before ejection into the ISM.
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