ALMA Imaging of Comet C/2017 K2 (PanSTARRS) En Route to the H2O Sublimation Zone

Comets afford a window into the chemistry and physics of planet formation. Through remote sensing of coma gases, the composition of nucleus ices can be inferred and placed into the context of the protoplanetary disk midplane and solar system formation. As all-sky survey capabilities continually improve, comets are being discovered at ever larger heliocentric distances (r_H), enabling investigations of their outgassing behavior far outside the r_H = 2-3 au window where H₂O, the dominant ice in most comets, first begins vigorously subliming. On the other hand, some of these comets will never cross the H₂O sublimation zone. Understanding what clues to solar system formation are preserved in the nuclei of these distantly active comets requires relating compositional measurements at much larger r_H to the majority of remote sensing work, which takes place when comets are at or inside 1 au from the Sun.

Here we report analysis of ALMA observations of distantly active comet C/2017 K2 (PanSTARRS) in three epochs when the comet was at r_H = 4 au, 3.5 au, and 3 au. We observed emission from HCN, CO, CH₃OH, H₂CO, CS, HNC, and thermal emission from the nucleus and dust coma at millimeter wavelengths. We will discuss the evolution of the spatial distributions, outgassing kinematics, and molecular abundances in the coma as the comet approached the H₂O sublimation zone and compare our results to measurements taken when H₂O sublimation had activated [1,2] thereby placing them into context with the larger comet population.

This work makes use of ALMA data ADS/JAO.ALMA #2021.1.00862.S.

References:

[1] Woodward et al. 2025, “A JWST Study of the Remarkable Oort Cloud Comet C/2017 K2 (PanSTARRS)”, PSJ, In Press, doi: https://doi.org/10.3847/PSJ/add1d5

[2] Ejeta et al. 2025, “Infrared Compositional Measurements of Comet C/2017 K2 (Pan-STARRS) at Heliocentric Distances Beyond 2.3 au”, AJ, 169, 102