Ice-Coated Pebble Drift as a Possible Explanation for Peculiar Cometary CO/H2O Ratios

TL;DR

This study models how ice-coated pebble drift in protoplanetary disks can lead to regions with high CO/H2O ratios, potentially explaining the peculiar compositions observed in some comets.

Contribution

It introduces a novel model linking pebble drift to enhanced CO ice regions, providing a new explanation for high CO/H2O ratios in comets.

Findings

Regions of enhanced CO/H2O form and expand over time in disks.

Most models achieve CO/H2O ratios of at least 1, some exceed 10.

Approximately 40% of disk ice mass becomes CO-rich after 1 Myr.

Abstract

To date, at least three comets -- 2I/Borisov, C/2016 R2 (PanSTARRS), and C/2009 P1 (Garradd) -- have been observed to have unusually high CO concentrations compared to water. We attempt to explain these observations by modeling the effect of drifting solid (ice and dust) material on the ice compositions in protoplanetary disks. We find that, independent of the exact disk model parameters, we always obtain a region of enhanced ice-phase CO/H2O that spreads out in radius over time. The inner edge of this feature coincides with the CO snowline. Almost every model achieves at least CO/H2O of unity, and one model reaches a CO/H2O ratio > 10. After running our simulations for 1 Myr, an average of 40% of the disk ice mass contains more CO than H2O ice. In light of this, a population of CO ice enhanced planetesimals are likely to generally form in the outer regions of disks, and we speculate that the aforementioned CO-rich comets may be more common, both in our own Solar System and in extrasolar systems, than previously expected.