The volatile content of giant Oort cloud comet C/2014 UN271 during its return to the planetary region.

Comets are relatively-pristine remnants of the original planetesimals that were ejected from the planetary region in the late stages of planetary formation. The Oort cloud comet C/2014 UN271, hereafter UN271, is ~140 km in diameter, large enough that it could be an intact example of a planetesimal that formed in the protoplanetary disk before being ejected into the Oort cloud. UN271 is currently beyond 17 au from the Sun as it moves to a perihelion distance of 11 au in 2031. Given its large heliocentric distance, the temperatures on and below its surface are too low for efficient sublimation of water ice. It is likely that its activity is driven by hypervolatiles such as CO or CO2 and not H2O. It is dynamically new on its first inbound trip to the Solar System's planetary region providing the rare opportunity to study the volatile content of one of the original planetesimals in a pristine state. We present JWST/NIRSpec prism IFU observations of UN271 with R~100 covering the spectral range where we can detect emission features from the vibrational bands of the gas-phase all the main volatiles found in comets (such as CO2, CO and C2H6). Moreover, this range also includes absorption bands of solid-phase water-ice grains. Our observations, taken on December 22, 2022, when the comet was at 18.6 au from the Sun, provide constraints on the comet's CO and CO2 production rates, the activity driving mechanism, the presence and nature of water-ice grains in its coma and can be compared with similar spectra of Kuiper Belt bodies. We will discuss the implications for the timing of the formation of the original planetesimals as well as UN271's formation environment within the primordial Kuiper Belt.