Constancy of optical gas mixing ratios in 3I/ATLAS near perihelion

Determine whether the relative abundances (mixing ratios) of optical gas species such as NH2, C2, and CN in interstellar comet 3I/ATLAS remained approximately constant during 2025 September–October as heliocentric distance decreased from about 2.2 au to 1.36 au, thereby validating the assumption that optical gas production rates scale with H2O or overall gas production.

Background

To compare optical brightness with OH (proxy for H2O) production rates, the authors enumerate four conditions that must hold, including that mixing ratios of the optical gases remain constant over the observation period.

Conditions related to gas dominance, aperture coverage, and the Swings effect appear plausible/satisfied for NH2, but the constancy of mixing ratios is not established, making the comparability of brightness and production rates uncertain.