Determine the H2O/CO ratio in the coma of 3I/ATLAS

Determine the water-to-carbon monoxide (H2O/CO) abundance ratio in the coma of the interstellar comet 3I/ATLAS by detecting and quantifying CO and CO2 emissions or production rates, in order to assess whether the current activity is driven primarily by water-ice sublimation or by super-volatile outgassing from the subsurface.

Background

The authors detect water ice signatures in the near-infrared spectrum of 3I/ATLAS and infer that activity may be driven by sublimation at heliocentric distances around 4 au. However, no gas emissions have yet been observed from 3I/ATLAS, and super-volatiles such as CO or CO2 could also be responsible for activity.

Establishing the H2O/CO ratio is critical for constraining the dominant volatiles driving the activity and for understanding the formation region and thermal processing history of 3I/ATLAS. The authors emphasize the need for targeted searches for CO and CO2 with ground- and space-based facilities to resolve this uncertainty.

References

The H$_2$O/CO ratio remains unknown, making future searches for CO and CO$_2$ with ground- and space-based facilities essential.

— Spectroscopic Characterization of Interstellar Object 3I/ATLAS: Water Ice in the Coma  (2507.14916 - Yang et al., 20 Jul 2025) in Section 5 (Discussion)

However, major questions remain about the mechanisms driving cometary activity and compositional evolution.

— WALTzER: Wide-band Atmospheric Laboratory for Transiting Exoplanet Research  (2610.21705 - Sreejith et al., 15 Oct 2026) in Section 2.2.2, “Active bodies in the Solar System”