Impact of dust evolution during protoplanetary disk buildup on the CO-to-water ratio of pebbles and planetesimals

The volatile composition of icy bodies provides important clues to the chemical inheritance linking molecular clouds, protoplanetary disks, and forming planetary systems. While water ice is generally expected to dominate the volatile inventory of bodies formed in the outer Solar System, observations of comets and trans-Neptunian objects reveal substantial diversity in ice abundances, including unexpectedly high CO-to-water ratios in some objects. Understanding how such CO-rich reservoirs form and evolve is therefore essential for connecting disk ice chemistry with the volatile composition of planetesimals and planets.

In this contribution, I present results of a recent study (Drazkowska 2026) using a one-dimensional disk model including dust coagulation, fragmentation, radial drift, volatile evaporation and recondensation, and planetesimal formation via the streaming instability. In particular, I explore how the disk buildup stage influences the chemical evolution of icy solids and the preservation of volatile-rich reservoirs.

CO-rich pebbles naturally form near the CO snow line due to the cold-finger effect, where outward-diffusing CO vapor recondenses onto drifting grains. This process efficiently enhances the CO ice abundance in solids and occurs regardless of whether disk buildup is included. However, models incorporating the buildup phase produce significantly stronger CO enrichment relative to water in the outer disk, demonstrating that early disk evolution can substantially modify the volatile composition inherited by icy solids. Despite the formation of CO-rich pebbles, smooth disk models do not produce CO-rich planetesimals. This suggests that additional mechanisms are required to retain or trap CO-rich solids.

These results highlight the importance of volatile transport and ice reprocessing in regulating the chemical inheritance of forming planetary systems. In particular, models aiming to connect disk chemistry with cometary compositions and exoplanet atmospheric C/O ratios should account for the disk buildup stage, which can significantly alter the spatial distribution and incorporation of volatile species into solids.