Presentation Information
[PPS12-18]Evolution of oxygen isotope compositions via dust-gas exchange in the protosolar disk
*Lily Ishizaki1, Shogo Tachibana1, Shigeru Ida2,3 (1.University of Tokyo , 2.Department of Astronomy, School of Science, Westlake University, 3.Earth-Life Science Institute, Institute of Science Tokyo)
Keywords:
Protoplanetary disk,Protosolar disk,Gas–dust interaction,Monte Carlo simulation,Oxygen isotope
Introduction: Solar System bodies, including Earth, are depleted in 16O relative to the Sun, whereas calcium–aluminum-rich inclusions (CAIs) are thought to have formed from dust with solar-like oxygen isotope compositions. Because CAIs represent the oldest solids in the Solar System, formed within the first several ×105 years, this difference is likely to record the temporal evolution of oxygen isotope compositions in Solar System materials. Such evolution is attributed to oxygen isotope exchange of original silicate dust with CO gas enriched in 16O and water vaper depleted in 16O, compared with Sun. To explain the current isotopic compositions, previous studies have suggested that oxygen isotope exchange occurred in regions where 16O-depleted water vapor was concentrated in the inner disk (Yurimoto & Kuramoto 2004). The most plausible mechanism for such H2O enrichment is the inward drift and evaporation of icy pebbles at the H2O snowline.
Methods: To quantitatively investigate oxygen isotope evolution in the early Solar System and to identify plausible conditions for the protosolar disk, we developed a new Monte Carlo model that simulates bidirectional oxygen isotope exchange between amorphous silicate dust and gas species (H2O and CO) in a viscously evolving protoplanetary disk (e.g., Ida et al. 2016). The model explicitly tracks the independent motions of dust and gas super-particles and incorporates the effects of ice grain size. The isotope exchange reaction between amorphous silicate dust and H2O or CO is simulated based on experimentally determined reaction rates (Yamamoto et al. 2018, 2020).
Results and Discussion: Oxygen isotope exchange proceeds efficiently within narrow temperature ranges, referred to as oxygen isotope exchange lines (Ishizaki et al. 2023). Notably, indirect exchange mediated by silicate dust allows H2O and CO gases to attain mutual isotopic equilibrium at temperatures of ~700–800 K, substantially lower than those required for direct gas–gas exchange (>~1000 K).
The evolution of δ17O inside the exchange line is controlled by the degree of water vapor enrichment, which depends on the Stokes number (St) of icy grains outside the H2O snowline. Small grains (St<~10–4 for the viscous parameter α=10–3) fail to enrich the inner disk, whereas large grains (St>~10–3 for α=10–3) deliver water vapor too efficiently, producing excessively high δ17O values and rapidly depleting the outer icy reservoir. Intermediate grain sizes yield gradual enrichment toward a steady-state δ17O value that can be maintained over Myr timescales.
The timescale of enrichment is determined by disk gas dynamics: H2O enrichment begins when the gas stagnation line (boundary that separates inward and outward advection) crosses the snowline and reaches steady state once enriched vapor is advected to the inner disk edge. This timescale is independent of St and is determined by the disk model. In contrast, the steady-state δ17O value depends on the flux ratio of H2O vapor to silicate dust crossing the snowline and is governed by the dimensionless ratio St/α. The shift of δ17O in the inner region and the timing of enrichment were given by semi-analytical formula that well explain the results of numerical simulations.
The combined numerical and analytical results indicate that the disk viscous parameter of α=10–3 and the Stokes number of ~5 × 10–4 for H2O ice provide favorable conditions for reproducing the inferred oxygen isotope evolution of the early Solar System. In this regime, CAIs form from 16O-rich dust during the first several ×105 years, followed by sustained Earth-like dust compositions from ~1 Myr onward, consistent with oxygen isotope signatures of chondrules, terrestrial planets, and other Solar System materials.
Methods: To quantitatively investigate oxygen isotope evolution in the early Solar System and to identify plausible conditions for the protosolar disk, we developed a new Monte Carlo model that simulates bidirectional oxygen isotope exchange between amorphous silicate dust and gas species (H2O and CO) in a viscously evolving protoplanetary disk (e.g., Ida et al. 2016). The model explicitly tracks the independent motions of dust and gas super-particles and incorporates the effects of ice grain size. The isotope exchange reaction between amorphous silicate dust and H2O or CO is simulated based on experimentally determined reaction rates (Yamamoto et al. 2018, 2020).
Results and Discussion: Oxygen isotope exchange proceeds efficiently within narrow temperature ranges, referred to as oxygen isotope exchange lines (Ishizaki et al. 2023). Notably, indirect exchange mediated by silicate dust allows H2O and CO gases to attain mutual isotopic equilibrium at temperatures of ~700–800 K, substantially lower than those required for direct gas–gas exchange (>~1000 K).
The evolution of δ17O inside the exchange line is controlled by the degree of water vapor enrichment, which depends on the Stokes number (St) of icy grains outside the H2O snowline. Small grains (St<~10–4 for the viscous parameter α=10–3) fail to enrich the inner disk, whereas large grains (St>~10–3 for α=10–3) deliver water vapor too efficiently, producing excessively high δ17O values and rapidly depleting the outer icy reservoir. Intermediate grain sizes yield gradual enrichment toward a steady-state δ17O value that can be maintained over Myr timescales.
The timescale of enrichment is determined by disk gas dynamics: H2O enrichment begins when the gas stagnation line (boundary that separates inward and outward advection) crosses the snowline and reaches steady state once enriched vapor is advected to the inner disk edge. This timescale is independent of St and is determined by the disk model. In contrast, the steady-state δ17O value depends on the flux ratio of H2O vapor to silicate dust crossing the snowline and is governed by the dimensionless ratio St/α. The shift of δ17O in the inner region and the timing of enrichment were given by semi-analytical formula that well explain the results of numerical simulations.
The combined numerical and analytical results indicate that the disk viscous parameter of α=10–3 and the Stokes number of ~5 × 10–4 for H2O ice provide favorable conditions for reproducing the inferred oxygen isotope evolution of the early Solar System. In this regime, CAIs form from 16O-rich dust during the first several ×105 years, followed by sustained Earth-like dust compositions from ~1 Myr onward, consistent with oxygen isotope signatures of chondrules, terrestrial planets, and other Solar System materials.