Ice Sublimation in Planetesimals Formed at the Outward Migrating Snowline

Abstract

Abstract Isotopic studies of meteorites suggest that planetesimals were formed as two distinct populations: noncarbonaceous (NC) and carbonaceous (CC) reservoirs. A recent model explains this dichotomy by considering planetesimal formation at the snowline during its migration in the protoplanetary disk, suggesting that NC planetesimals were formed during the outward migration and CC planetesimals were formed during the inward migration. This model has been suggested to contradict meteorite observations because planetesimals formed at the snowline are expected to be rich in H2O and, therefore, develop more oxidizing environments than those inferred from NC iron meteorites. However, if the accreted ice sublimates into vapor without transitioning into a liquid state, the planetesimals may lose most water without being oxidized because reactions between vapor and solids are negligibly slow at temperatures relevant to direct ice sublimation. Here, we investigate the transport of vapor inside a planetesimal and suggest that the pore pressure would have been sufficiently low for direct ice sublimation if (1) the planetesimals were formed during the outward snowline migration (such that they lay inside the snowline after formation and had surfaces permeable to water vapor), (2) these planetesimals were formed by dust-aggregate boulders through “streaming instabilities” instead of being formed directly by submicrometer dust grains, and (3) the boulders were between a few centimeters to ~10 m in size. With these results, the snowline model for NC/CC planetesimal formation may be reconciled with the observations of iron meteorite oxidation states.