Locked in ice: how pebble drift and volatile entrapment can significantly impact carbon and oxygen ratios in evolving protoplanetary discs

ABSTRACT The complex interplay between the growth, drift, and sublimation of ice-covered pebbles can strongly influence the volatile distribution and evolution of disc composition, and therefore impact the composition of forming planets. Classic pebble drift models treat volatile species individually as sublimating at their respective snowlines, although observations from the James Webb Space Telescope (JWST) suggest that ices are likely mixed; laboratory studies suggest ice mixtures can exhibit more complex sublimation behaviours, remaining trapped beyond their nominal sublimation temperatures. We present the first model that couples pebble growth and drift with CO entrapment inside water ice – preventing a fraction (up to $\sim$60 per cent) of the CO from sublimating at its snowline, instead desorbing via volcanic desorption at the water crystallization front, at 130 K. Our models show that CO entrapment will significantly impact the carbon and oxygen distributions, enhancing the gas-phase C/O and C/H inside the water snowline by up to a factor of 10 over 1 Myr and a factor of a few around the CO$_2$ snowline; O/H is also increased around the CO$_2$ snowline, but is water-dominated in the inner disc. Entrapment therefore provides a means of introducing more carbon to the inner disc whilst retaining a large amount of water. We discuss connections to planet formation, noting that CO entrapment can increase the gas-phase heavy element content around the water snowline by up to 150 per cent. We also consider links to JWST observations and highlight the importance of entrapment for pebble drift models to accurately model disc composition.