Alkali Phenoxides In Comets

Potassium was first detected in spectra of the sungrazer comet Ikeya-Seki at the heliocentric distance rh = 0.15 au and, 48 years later, in comets PanSTARRS and ISON at r_h = 0.46 au.

The alkali tail photoionization model provides a Na/K ratio close to the solar value. 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.

Here we searched for the emissions of the alkali NaI, KI, and LiI in Comets C/2020 F3 and C/2024 G3. High-resolution spectra of the comets were taken with the 0.84 m telescope at the Schiaparelli Observatory at r_h = 0.36 and 0.15 au, the observations closest to the Sun since Ikeya-Seki.

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. 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, 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 10^4, 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 10^4 slower than sodium phenoxides.

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.

M. Fulle, P. Molaro, A. Rotundi, L. Tonietti, A. Aletti, L. Buzzi, P. Valisa

Comments: 8 pages, 10 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2506.02343 [astro-ph.EP] (or arXiv:2506.02343v1 [astro-ph.EP] for this version
https://doi.org/10.48550/arXiv.2506.02343
Focus to learn more
Submission history
From: Marco Fulle Dr.
[v1] Tue, 3 Jun 2025 00:46:55 UTC (5,987 KB)
https://arxiv.org/abs/2506.02343
Astrobiology, Astrochemistry,