Comets and Asteroids

The Evolution in Coma Molecular Composition of Comet C/2017 K2 (PanSTARRS) Across the H2O Sublimation Zone: ALMA Imaging of an H2O-Dominated Coma

By Keith Cowing

Status Report

astro-ph.EP

December 29, 2025

Filed under ALMA, astro-ph.EP, astrochemistry, astrogeology, biochemistry, Biosignatures, C/2017 K2 (PanSTARRS), organic chemistry, Spectroscopy

The Evolution in Coma Molecular Composition of Comet C/2017 K2 (PanSTARRS) Across the H2O Sublimation Zone: ALMA Imaging of an H2O-Dominated Coma — A)–(D). Spectrally integrated flux maps for HCN, CO, CH3OH, and CS in K2. Contour intervals in each map are given in multiples of the rms noise. The rms noise (σ, mJy beam−1 km s−1 ) is indicated in the upper left corner of each panel. Sizes and orientations of the synthesized beam (Table 1) are indicated in the lower left corner of each panel. The comet’s observer-centered illumination (ϕ ∼ 25◦ ), as well as the direction of the Sun and dust trail, are indicated in the lower right. The black cross indicates the position of the peak continuum flux. A spectrum extracted from a 10′′ diameter aperture centered on the nucleus is shown in the upper right. Contours are in 10σ increments for HCN and 5σ increments for CO, CH3OH, and CS. — astro-ph.EP

We report a survey of molecular emission from cometary volatiles using the Atacama Large Millimeter/Submillimeter Array (ALMA) toward comet C/2017 K2 (PanSTARRS) carried out on UT 2022 September 21, 22, and 23 at a heliocentric distance (rh) of 2.1 au.

These measurements of HCN, CS, CO, CH₃OH, and H₂CO (along with continuum emission from dust) sampled molecular chemistry in C/2017 K2 at the inner edge of the H₂O sublimation zone, discerning parent from daughter or extended source species.

This work presents spectrally integrated flux maps, production rates, and parent scale lengths for each molecule. CH₃OH, CO, and HCN were produced within ∼250 km of the nucleus, potentially including contributions from sublimation of icy grains.

CS was consistent with production from CS₂ photolysis, and H₂CO required production from extended sources in the coma. An ortho-to-para ratio OPR=2.9±0.4 for H₂CO was derived from simultaneously measured transitions of each spin species. The continuum was extended and spatially resolved, consistent with thermal emission from dust in the coma.

Analysis of the continuum visibilities provided an upper limit on the nucleus diameter d<6.6 km and coma dust masses of 1.2−2.4×10¹¹ kg.

Nathan X. Roth, Stefanie N. Milam, Martin A. Cordiner, Anthony J. Remijan, Dominique Bockelee-Morvan, Nicolas Biver, Jeremie Boissier, Steven B. Charnley, Charles E. Woodward, Lillian X. Hart, Timothy N. Proudkii

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2511.05662 [astro-ph.EP] (or arXiv:2511.05662v1 [astro-ph.EP] for this version
https://doi.org/10.48550/arXiv.2511.05662
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Submission history
From: Nathan Roth
[v1] Fri, 7 Nov 2025 19:02:00 UTC (2,775 KB)
https://arxiv.org/abs/2511.05662
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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