Multi-Cycle monitoring of the volatile evolution of a returning planetesimal as it approaches perihelion

The 140 km diameter Oort Cloud comet C/2014 UN271 is likely one of the original planetesimals that formed in the protoplanetary disk and will reach a perihelion distance of ~10 au in 2031. Recent NIRSpec observations of UN271 when it was 18.2 au from the Sun show that it produces considerable quantities of CO2 and CO with icy grains. While CO can be active at ~18 au, the detection of CO2, normally active inside 13 au, implies that the comet is hyperactive and may posses other cometary volatiles such as NH3 and amorphous water ice which will become active or crystallize as the comet nears the Sun within the next couple of years. We propose a small muli-Cycle program of 24.66 h with JWST to take near-simultaneous observations of UN271 with NIRSpec and NIRCam to observe the evolution of its volatiles and ices over the next few years. The instruments will be complimentary with NIRSpec providing detailed spectral information the comets near-nucleus coma and NIRCam providing wide-field mapping of volatiles and ices. We will observe UN271 once per cycle with the first observation starting in June 2024 at the end of Cycle 2 when the comet is ~16.3 au from the Sun and ending in June 2026 during Cycle 4 when the comet is 14 au from the Sun. Combined with public NIRSpec data, our observations will provide coverage of the comet over a ~4 au range during which different cometary volatiles may become active such as NH3 and and water ice can crystalize. Observing the evolution of UN271 will provide groundbreaking and exciting constraints on the the volatile contents and evolution of the original planetesimals and protoplanetary disk.