5:15 PM - 6:45 PM
[PCG22-P05] Theoretical estimation of oxygen isotope exchange rates between amorphous silicate dust aggregates and ambient water vapor
Keywords:Oxygen isotope, Dust aggregate
Meteorites and their components exhibit a diverse range of oxygen isotope compositions due to exchanges between 16O-rich primitive amorphous silicate dust particles and 16O-poor water vapor in the early solar system. Yamamoto et al. (2018) determined the exchange rates between amorphous forsterite grains and water vapor. They found that for grains with a size of 1 micron, the timescale for oxygen isotope exchange is shorter than the lifetime of the solar nebula at temperatures above approximately 600 K. Additionally, at temperatures below 800 K, isotope exchange occurs faster than crystallization. However, in the solar nebula, solid particles exist as aggregates, necessitating consideration of aggregate oxygen isotope exchange rates in discussions on the evolution of oxygen isotope compositions in the early solar system.
In this study, we theoretically estimated the oxygen isotope exchange rates between amorphous silicate dust aggregates and ambient water vapor. The exchange process between aggregates and water vapor can be divided into four processes: (1) supply of water vapor to the aggregate surface, (2) diffusion of water vapor within the aggregate, (3) isotope exchange on the constituent grain surfaces, and (4) diffusion within grains. We evaluated the timescales of these processes and assessed which one becomes the rate-determining step based on aggregate size, temperature, and water vapor pressure. Our analytical calculations revealed that for dust aggregates smaller than mm size, the isotope exchange rate is the same as that of the constituent particles. In contrast, for aggregates larger than cm size, the isotope exchange rate would be controlled by the supply rate of water vapor to the aggregate surface.
In this study, we theoretically estimated the oxygen isotope exchange rates between amorphous silicate dust aggregates and ambient water vapor. The exchange process between aggregates and water vapor can be divided into four processes: (1) supply of water vapor to the aggregate surface, (2) diffusion of water vapor within the aggregate, (3) isotope exchange on the constituent grain surfaces, and (4) diffusion within grains. We evaluated the timescales of these processes and assessed which one becomes the rate-determining step based on aggregate size, temperature, and water vapor pressure. Our analytical calculations revealed that for dust aggregates smaller than mm size, the isotope exchange rate is the same as that of the constituent particles. In contrast, for aggregates larger than cm size, the isotope exchange rate would be controlled by the supply rate of water vapor to the aggregate surface.