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SPHEREx Pre-Perihelion Mapping of $\mathrm{H_2O}$, $\mathrm{CO_2}$, and $\mathrm{CO}$ in Interstellar Object 3I/ATLAS

Carey M. Lisse, Yoonsoo P. Bach, Brendan P. Crill, Phil M. Korngut, Ari J. Cukierman, Sean A. Bryan, Asantha Cooray, C. Darren Dowell, Michael W. Werner, Joseph L. Hora, Zafar Rustamkulov, Jeong-Eun Lee, Bumhoo Lim, Y. R. Fernandez, Volker Tolls, W. T. Reach, O. Doré, Michael Zemcov, James J. Bock, Yun-Ting Cheng, C. Champagne, Seungwon Choi, M. Connelley, J. P. Emery, Spencer Everett, Andreas L. Faisst, Jooyeon Geem, Howard Hui, Masateru Ishiguro, Sunho Jin, Hangbin Jo, Max Mahlke, Daniel C. Masters, Gary J. Melnick, Chi H. Nguyen, Roberta Paladini, M. L. Sitko, Yujin Yang

TL;DR

This study leverages SPHEREx's $R=40$–$130$ spectrophotometry over $0.7$–$5\,\mu$m to map H$_2$O, CO$_2$, and CO in interstellar object 3I/ATLAS during its pre-perihelion passage. The analysis detects robust water emission at 2.7–2.8 μm, a dominant CO$_2$-emission shell extending to at least several 10^5 km, and tentative CO/^{13}CO$_2$ signals, yielding gas production rates of $Q_{\mathrm{H_2O}}=3.2\times10^{26}$ s$^{-1}$, $Q_{^{12}\mathrm{CO_2}}=1.6\times10^{27}$ s$^{-1}$, $Q_{^{13}\mathrm{CO_2}}=1.3\times10^{25}$ s$^{-1}$, and $Q_{\mathrm{CO}}=1.0\times10^{26}$ s$^{-1}$. A 1.0–1.5 μm continuum image is unresolved, while the 1.5–4.0 μm continuum shows water-ice absorption features; radial profiles indicate CO$_2$ outflow declines faster than $1/\rho$, consistent with acceleration or destruction. The nucleus remains unresolved with an upper size limit of about $2.5$ km, and 3I displays comet-like activity reminiscent of 103P/Hartley 2, suggesting strong thermal processing prior to ejection and marking ISOs as often thermally evolved prior to interstellar transit. These findings support a picture in which ISOs are not uniformly primitive but instead exhibit a spectrum of thermal histories, with 3I lying toward the end of the active lifetime and CO$_2$-rich, CO-poor compositions. The results motivate population-wide SPHEREx ISO studies to test the prevalence of thermal processing in the ISO population and to refine models of ISO origin and evolution.

Abstract

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 $4.2\times10^{5}\,$km was discovered. Gas production rates for H$_2$O, $^{12}$CO$_2$, $^{13}$CO$_2$, and CO were $Q_{\mathrm{gas}} = 3.2\times10^{26} \pm 20\%$, $1.6\times10^{27} \pm 10\%$, $1.3\times10^{25} \pm 25\%$, and $1.0\times10^{26} \pm 25\%$, 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.

SPHEREx Pre-Perihelion Mapping of $\mathrm{H_2O}$, $\mathrm{CO_2}$, and $\mathrm{CO}$ in Interstellar Object 3I/ATLAS

TL;DR

This study leverages SPHEREx's – spectrophotometry over –m to map HO, CO, and CO in interstellar object 3I/ATLAS during its pre-perihelion passage. The analysis detects robust water emission at 2.7–2.8 μm, a dominant CO-emission shell extending to at least several 10^5 km, and tentative CO/^{13}CO signals, yielding gas production rates of s, s, s, and s. A 1.0–1.5 μm continuum image is unresolved, while the 1.5–4.0 μm continuum shows water-ice absorption features; radial profiles indicate CO outflow declines faster than , consistent with acceleration or destruction. The nucleus remains unresolved with an upper size limit of about km, and 3I displays comet-like activity reminiscent of 103P/Hartley 2, suggesting strong thermal processing prior to ejection and marking ISOs as often thermally evolved prior to interstellar transit. These findings support a picture in which ISOs are not uniformly primitive but instead exhibit a spectrum of thermal histories, with 3I lying toward the end of the active lifetime and CO-rich, CO-poor compositions. The results motivate population-wide SPHEREx ISO studies to test the prevalence of thermal processing in the ISO population and to refine models of ISO origin and evolution.

Abstract

From 01- to 15-Aug-2025 UT, the SPHEREx spacecraft observed interstellar object 3I/ATLAS. Using - spectrophotometry at -m, light curves, spectra, and imaging of 3I were obtained. From these, robust detections of water gas emission at -m and CO gas at -m plus tentative detections of CO and CO gas were found. A slightly extended HO coma was detected, and a huge CO atmosphere extending out to at least km was discovered. Gas production rates for HO, CO, CO, and CO were , , , and , respectively. Co-addition of all -m scattered light continuum images produced a high SNR image consistent with an unresolved source. The scattered light lightcurve showed variability over the observation period. The absolute brightness of 3I at -m is consistent with a km radius nucleus surrounded by a 100 times brighter coma. The -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 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.

Paper Structure

This paper contains 16 sections, 3 equations, 4 figures.

Table of Contents

  1. Introduction
  2. Observations
  3. Data Reduction
  4. Results
  5. Lightcurves
  6. Spectra
  7. Gas Production Rates
  8. Images
  9. Radial profiles
  10. Nucleus Size
  11. Discussion
  12. Other Hypotheses
  13. Home System Implications
  14. ISO Evolution
  15. Conclusions and Future Work
  16. ...and 1 more sections

Figures (4)

  • Figure 1: Motion of 3I. (a) Color composite from SPHEREx data at 1.185, 1.716, and 2.194 $\,\mathrm{\mu m}$ for red, green, and blue, respectively. The trajectory of 3I is overplotted, with observed positions marked by crosses. The starting and ending points of our dataset are indicated separately (see legend). (b) Orbital motion of 3I (black) projected onto the ecliptic plane. The orbits of Earth, Mars, and Jupiter are shown in blue, red, and purple, respectively. Numbers indicate the positions of the bodies at the first day of each month. 3I's retrograde motion and perihelion are marked with arrows. (c) Lightcurve of 3I produced by taking all $\lambda = 1.0 \mathrm{-} 1.8 \,\mathrm{\mu m}$ SPHEREx flux measurements and dividing them by the JWST flux (Fig. \ref{['fig:mjy+refl']}b; 2025ApJ...991L..43C) to leave only lightcurve variations. Also included are SOLO clear-filter brightness measurements scaled to match their median to that of the SPHEREx/JWST ratio. The resulting lightcurve amplitude is within $\pm 15\,\mathrm{\%}$, so the derived reflectance should be correct to within this range considering any lightcurve modulation effects.
  • Figure 2: (a) The flux density of the measurements, scaled to the geometry of $r_\mathrm{obs} = r_\mathrm{hel} = 1 \,\mathrm{au}$. The JWST measurements 2025ApJ...991L..43C are scaled by a constant factor of 1.5 to match the SPHEREx measurements. For flags, see Table \ref{['tab:obs']} and text for explanation. The skyblue points are the JWST measurements, convolved with the pixel spectral throughput at the SPHEREx pixel for that observation, using the SPHEREx Sky Simulator 2024SPIE13092E..3NH2025ApJS..281...10C. Blue shades indicate important spectral features. Colors of "good" measurements indicate $F_\mathrm{field}$ (Eq. \ref{['eq:F_field']}). (b) The distance-corrected ratio of the SPHEREx to JWST flux, showing that SPHEREx measurements are systematically higher than JWST. This indicates the spatial extendedness of the object, 3I. The error-bars are not shown for $S/N < 2$ data. (c) The reflectance, normalized at $1.2\,\mathrm{\mu m}$ for SPHEREx, JWST, and the IRTF measurements. The blue line shows the moving median of the IRTF data with a $0.2\,\mathrm{\mu m}$ window.
  • Figure 3: Median combined images ($\approx 5\arcmin \times 5\arcmin$) and radial profiles. Left column: The stacked images, after simulation and background subtractions, in MJy/sr unit (see scale bars). The orange bars at the bottom indicate aperture diameter ($24\arcsec$) and $1 \arcmin$. Middle column: The radial profile of 3I in the stacked image (green; blue if the signal is above the 1-$\sigma$ scatter), mean (gray) and median (black) of 30 brightest field stars within $\lesssim 10\arcmin$, respectively. Each profile is normalized to the central pixel. The red solid and dashed lines are for $\rho^{-1.5}$ and $\rho^{-1}$ profiles, respectively, for the radius $\rho$. The nominal aperture radius (orange vertical lines) and corresponding length at 3I (title) are shown. Right column: Same as the left column but convolved with a standard deviation of 1.5-pixel gaussian filter for visualization. In the left and right columns, celestial North (white thick arrow), East (white thin arrow), anti-solar (red solid) and anti-velocity (yellow dashed) vectors are shown in the top left corner. The white and red contours (also shown in the color bar to the right of each panel) are 2- and 5-$\sigma$ above the background, respectively.
  • Figure 4: (a)$\mathrm{CO}/\mathrm{CO}_2$ gas production rate ratios of solar system comets 2022PSJ.....3..247H. The $\mathrm{CO}$ or $\mathrm{CO}_2$ dominance criteria of $1 \pm 0.33$ is indicated as horizontal shadow 2022PSJ.....3..247H. The ratio of 3I makes it extremely $\mathrm{CO}$ poor and equivalent to the most thermally processed comet in the 2022 survey, 103P. (b) Pre-perihelion optical lightcurves observed for comet 103P during the 2010--2011 apparition and 3I in May - Oct 2025. Published data from 2025ApJ...994L...3J2025arXiv250905562T2025ApJ...993L..31Y have been shifted to match the absolute magnitude of Minor Planet Center (MPC) Database, the T-mag from JPL Horizons ephemerides service, and the data kindly shared by S. Yoshida (2025, priv. comm.). There is very good agreement between the two inbound lightcurve trends. An inherent spread of $\sim 0.5 \,\mathrm{mag}$ in the measurements between nights and between observers can be seen. (c) Close flyby imaging of the $\sim 1.5 \times 0.6 \times 0.6 \,\mathrm{km}$ comet 103P nucleus as obtained by the Deep Impact Extended mission 2011Sci...332.1396A. A marked bilobate nuclear morphology was seen, with unusually large (mm to dm) chunks of $\mathrm{CO}_2$ rich ice and gas emanating from the end of the smaller lobe and the middle of the larger lobe. The surrounding coma, filled with large long lived icy grains, was found to be at least 5 times brighter and more actively emitting gas than the nucleus. While the nucleus of 3I has never been resolved, given its similar behavior, it could possibly share similar morphological characteristics. (d) Schematic of the 3I object derived from SPHEREx $\mathrm{CO}_2$ photometric mapping. The $< 2.5 \,\mathrm{km}$ radius nucleus 2025ApJ...990L...2J was not resolved at the SPHEREx pixel scale of $6.15 \arcsec \approx 11,600 \,\mathrm{km}$ at $2.6 \,\mathrm{au}$ distance, but a notional nucleus image has been included at the origin to give the reader its location and a sense of scale. Two distances have been marked off: The $\sim 38,000 \,\mathrm{km}$ (white dashed circle) inside of which we think coma ice is actively sourcing $\mathrm{CO}_2$ by sublimation; and $\sim 340,000 \,\mathrm{km}$ (orange dashed oval), the minimum radial distance to which we detect $\mathrm{CO}_2$ gas at 2-$\sigma$ level.