Dominance of non-H2O volatiles in 3I/ATLAS’s coma

Determine whether the coma of interstellar object 3I/ATLAS is dominated by volatile species other than water—specifically carbon monoxide (CO), carbon dioxide (CO2), cyanogen (CN), and hydrocarbon species—rather than by H2O, as implied by the observed early OH detection and lack of CN detection.

Background

The paper reports a clear detection of OH emission near 3085 Å in 3I/ATLAS at 3.51 au, implying substantial H2O production at a distance where water sublimation is typically inefficient. This behavior, alongside strong coma reddening and indications of large icy grains, suggests potentially grain-driven water release.

The authors note the unusual sequence of detections—OH before any CN—and outline two contrasting hypotheses for the comet’s volatile composition: an H2O-rich scenario consistent with formation in low-metallicity systems, and a hypervolatile-rich (CO/CO2) scenario analogous to the post-perihelion behavior of 2I/Borisov. The explicit unresolved question is whether the coma will be dominated by volatiles other than water, motivating continued multiwavelength monitoring to assess CO, CO2, CN, and hydrocarbons.