The Evolution in Coma Molecular Composition of Comet C/2017 K2 (PanSTARRS) across the H₂O Sublimation Zone: ALMA Imaging of a H₂O-dominated Coma

We report a survey of molecular emission from cometary volatiles using the Atacama Large Millimeter/submillimeter Array toward comet C/2017 K2 (PanSTARRS) carried out on UT 2022 September 21, 22, and 23 at a heliocentric distance (r_H) 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, the region from r_H = 2 to 3 au where H₂O begins vigorously subliming and increasingly dominating comet activity, 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 of 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.