A JWST Spectroscopic Study of Comet C/2017 K2 (PanSTARRS)

Dynamically new Oort cloud comets, by their journeys into the inner solar system, deliver particulates and volatile gases into their comae that reveal records of the most primitive materials in the solar system. The abundances and spatial distribution of major gas species (H₂O, CO₂, CO), and organics yield insight into comet nucleus composition as well as sublimation or radiative dissociation of species in the coma. JWST spectroscopic observations of such comets provide a unique look into the physico-chemical properties of materials sublimed from the nucleus into the coma. Spectroscopy with MIRI MRS and NIRSpec integral-field-units (IFUs) deliver spectral-spatial cubes with wavelength access to key diagnostics of coma materials produced by comet nucleus sublimation activity, including molecular emission bands, organic signatures, and dust features in the spectral energy distribution (SEDs). Here, we highlight observations and modeling of a JWST study of the inner coma of the Oort cloud comet C/2017 K2 (PanSTARRS), a hyperactive (water ice active fraction ≥86%) comet at a heliocentric distance of 2.35 au. Our observations and analysis [1] reveals: (a) emission lines from H2O are detected in both NIRSpec and MRS spectra including faint H2O ro-vibrational lines from the ν₃-ν₂ and ν₁-ν₂ hot bands in the 4.5 to 5.2 μm range, and strong lines from the ν₂ fundamental vibrational band; (b) a wealth of trace volatile species (molecular emission) is superimposed on the continuum including CN (band emission centered at 4.9 μm), H₂CO, CH₃OH, CH₄, C₂H₆, HCN, NH₂, and OH prompt emission (after UV photolysis of water); (c) a particle differential size distribution (n(a)da) and relative abundances of amorphous carbon, amorphous silicates, and crystalline silicates commensurate with those found in other comets[2]. In C/2017 K2 (PanSTARRS) the average (spatially averaged across the coma) amorphous carbon mass fraction has a value of the order 0.247 ± 0.010, the silicate-to-carbon ratio is  of the order 3.049 ± 0.163, and the crystalline mass fraction of the sub-micron grains, f_cryst, is of the order 0.384 ± 0.065; (d) residual spectral emission features at 3.42 um and between 6.1 and 8.6 μm, with a prominent 6.9 um peak, in the JWST spectrum strongly support the contention that polycyclic aromatic hydrocarbons (PAHs) are present in the inner coma of this comet. We briefly discuss comparisons of the spectra of other comets observed with JWST early in the mission and explore the in-field and out-of-field impact of JWST comet surveys in the context of advancing our understanding of protoplanetary disk evolution and the formation of small bodies in the outer solar system. 

This work is based in part on observations made with the NASA/ESA/CSA James Webb Space Telescope Cycle 1 GO program 1556 whose analysis was supported via StSci grant JWST-GO-01566.001. 

References:[1] Woodward, C.E.  et al. 2025, “A JWST Study of the Remarkable Oort Cloud Comet C/2017 K2 (PanSTARRS)”, Planetary Sci. J. (in press) doi: 10.3847/PSJ/add1d5. [2] Harker, D.E. et al. 2024, “Dust Properties of Comets Observed by Spitzer”, Planetary Sci. J. 4, 242 doi: 10.3847/PSJ/ad0382.