2025A&A...699A.110F

Context. Potassium was first detected in spectra of the sungrazer comet C/1965 S1 Ikeya-Seki at the heliocentric distance r_h = 0.15 au and, 48 years later, in comets C/2011 L4 PanSTARRS and C/2012 S1 ISON at r_h = 0.46 au. The alkali tail photoionization model provides a Na/K ratio close to the solar value in comets C/1965 S1 and C/2011 L4. No lithium was detected in any comet: the lower limit of the Na/Li ratio was almost one order of magnitude greater than the solar ratio.
Aims. Here we searched for the emissions of the alkali NaI, KI, and LiI in Comets C/2020 F3 NEOWISE and C/2024 G3 ATLAS.
Methods. High-resolution spectra of the comets were taken with the 0.84 m telescope at the Schiaparelli Observatory at r_h = 0.36 and r_h = 0.15 au, respectively, the observations closest to the Sun since C/1965 S1. To model the data, we assumed that alkali phenoxides are present in the aromatic fraction of organic dust at the nucleus surface where they react with carbon dioxide ejecting alkali atoms. Results. NaI and KI were detected in emission lines of exceptional intensity in both comets, with no evidence of LiI emission. The NaI/KI ratios were determined: 31 ± 5 and 26 ± 8 in comets C/2020 F3 and C/2024 G3, respectively, whereas solar Na/K ≈ 15. This excess and its observed trend with the heliocentric distance are consistent with chemistry between CO₂ and alkali phenoxides at the nucleus surface. The Li upper limit for comet C/2020 F3 is very stringent at Na/Li > 3.4 × 10⁴, a factor of 34 greater than the solar value. This Li depletion is consistent with the reaction rate of lithium phenoxides, which is a factor of 10⁴ slower than sodium phenoxides.
Conclusions. The widespread chemistry of carbon dioxide with organic dust may provide a significant energy and mass sink of carbon dioxide in all comets also at r_h > 1 au, reconciling recent models of cometary activity with Rosetta CO₂ measurements. At r_h < 0.5 au potassium was observed in all comets, so that we predict the formation of a KI tail spatially resolved from the NaI tail.