Characterizing the Properties of Water Ice in Distant Oort Cloud Comets with JWST

Introduction  Water ice, the most abundant solid in protoplanetary disks, plays a critical role in planet formation by enhancing grain growth and  planetesimal formation. In the Solar System, it likely contributed to the cores of giant planets and the volatiles of the terrestrial planets. The physical structure of water ice, amorphous versus crystalline, is sensitive to the temperature and pressure at the time of formation, as well as to the subsequent thermal evolution. Comets, as some of the best-preserved icy relics of the protoplanetary disk, offer a unique window into the physical properties of primordial materials. Recent discoveries of distant, active Oort cloud comets, beyond the water-ice sublimation line, present rare opportunities to study intrinsically bright, minimally evolved nuclei under conditions of reduced coma activity relative to comets observed at smaller heliocentric distances. Targeted observations of kilometer-sized, non-outbursting distant comets are an important piece of the puzzle for probing the properties of water ice released from the surface and/or interior of minimally processed nuclei, shedding light on the characteristics of ices in the Oort Cloud reservoir.
Observations  We conducted near-infrared (IR) spectroscopic observations of three typical-sized, quiescent distant comets—C/2019 O3, C/2019 E3, and C/2019 F2, using the Near-Infrared Spectrograph (NIRSpec) Integral Field Unit (IFU) on board the James Webb Space Telescope (JWST). All targets have perihelia beyond 8.8 au, where the local blackbody temperature is ~93 K. At such low temperatures, if amorphous water ice is present in the nucleus, it may never undergo large-scale crystallization. Our primary goal was to detect and characterize diagnostic features of crystalline water ice, particularly the 1.65-μm absorption band and the 3.1-μm Fresnel reflection peak. We also searched for supervolatiles species (CO, CO₂) and organics, including methanol, HCN, and aromatic and aliphatic hydrocarbons.
Results and Discussion JWST spectra reveal diagnostic water ice absorption features in all three comets, along with fluorescence emissions from CO and CO₂. Despite similar heliocentric distances, the comets display striking diversity in both volatile production rates and the characteristics of the water ice features, including variations in band depth and spectral shape. While comets C/2019 O3 and C/2019 E3 exhibit a typical 1/ρ coma profile consistent with steady-state outflow, comet C/2019 F2 shows a markedly different morphology, suggesting a non-standard ejection mechanism or evolutionary history. We explore possible explanations for this unusual coma structure and discuss its implications for understanding the drivers of activity in distant comets.  These findings enhance our knowledge of the physical state of water ice and contribute to a broader understanding of volatile evolution in outer solar system bodies.