A JWST Study of the Remarkable Oort Cloud Comet C/2017 K2 (PanSTARRS)

Abstract: Comets, during their journeys into the inner solar system, deliver volatile gases, organics, and particulates into their comae that provide crucial information for assessing the physico-chemical conditions in the outer disk from which they formed. Here we present observational and modeling results of a JWST NIRSpec and MIRI MRS integral-field-unit (IFU) spatial-spectral study of the inner coma of the Oort Cloud comet C/2017 K2 (PanSTARRS) at a heliocentric distance of 2.35 au. We find the comet is hyperactive (water ice active fraction greater than or equal to 86%), with a nucleus radius of $<$4.2 km, exhibiting strong emission from H${2}$O, $^{12}$CO, $^{13}$CO, and CO${2}$ as well as CN, H$2$CO, CH$_3$OH, CH$_4$, C$_2$H$_6$, HCN, NH$_2$, and OH prompt emission. The water ortho-to-para ratio is greater than or equal to 2.75. The modeled dust composition (relative mass fraction of the sub-micron grains) in the coma is dominated by amorphous carbon ($\simeq 25$%), amorphous Mg:Fe olivine ($\simeq 19$%), amorphous Mg:Fe pyroxene ($\simeq 16$%), and Mg-rich crystalline olivine ($\simeq 39$%) and the crystalline mass fraction of the sub-micron grains in the coma is, $f{cryst} \simeq 0.384 \pm 0.065$. Analysis of residuals in 3 to 8 $\mu$m region of the spectral energy distribution strongly suggests the presence of polycyclic aromatic hydrocarbon (PAHs) species in the coma.