Phoebe's carbon isotope composition as evidence for self-shielding in the solar nebula

Clark et al. [Clark, R.N., Brown, R.H., Cruikshank, D.P., Swayze, G.A., 2019. Icarus, 321, 791–802] reported an extremely low value of the ¹²C/¹³C ratio in CO₂ ice on Phoebe, a likely captured moon of Saturn. Unless Phoebe did not form in the solar system, we interpret this value as indication that Phoebe accreted surface carbon from a region of the protosolar nebula where ¹³C was enriched due to self-shielding of ¹²CO from photodissociation. This could imply that Phoebe is also enriched in ^17,18O relative to most solar system objects sampled to date. Phoebe and other objects that may have sampled these ¹³C-rich regions, such as Pallas or Triton, may provide the opportunity to directly measure isotopic fractionations in endmembers of the self-shielded solar nebula.