  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.
