Toward Tracing Redox State Evolution in the Protoplanetary Disk with High Resolution

Introduction: We apply high-resolution 26 Al-26 Mg chronometry to primitive meteorites of various oxidation states. The objective is to test the hypothesis that the fluctuating redox states of the protoplanetary disk were governed by radial transport of water ice and organic materials, evaporating, respectively, after crossing the snowline and tarline in the solar nebula. The chronology of redox state changes in the solar nebula may provide constraints on the formation timescale of giant planets like Jupiter, as their formation will prevent the rapid migration of materials from the outer disk. Observational and Theoretical Context: Proto-planetary disks evolve over their lifetimes. Mass is transferred through the disk and accreted onto the central star at rates up to 10-6 solar masses per year. The rate progressively decreases to 10-10 solar masses per year over timescales of ~10 Ma [1], as the disk cools and becomes less massive. The transport of solids and gaseous molecules by advection, diffusion, and gas drag changes the physical and chemical structure of the disk as it evolves [2,3]. Recent theoretical studies suggest that the inner solar nebula became enriched in H 2 O, and thus more oxidized, by inward transport and sublimation of water ice crossing the snowline at around 3 AU [2]. Dust particles in circumstellar orbit migrate inward faster than gas. Meter-sized bodies achieve maximum inward radial drift velocities. Transport of H 2 O in the early stages of disk evolution was dominated by the coagulation of icy dust particles in the outer disk into meter-sized rubble that then moved rapidly inward. The disk immediately inside the snowline became enriched in water vapor. Over time, the influx of icy bodies from the outer regions of the disk diminished, due to the depletion of dust particles and meter-sized " fast drifters " , as well as to the rapid formation of larger planetesimals and planets in the outer disk regions. Diffusion became a dominant mechanism at this stage, with H 2 O vapor carried outward, where it condensed. The concentration of water vapor in the inner disk thus decreased over time [3]. Primitive chondrites must have witnessed these processes and may hold key clues to the details of protoplanetary chemistry. Redox conditions in the inner solar nebula are preserved in the mineralogy and O-isotopic compositions of primitive chondrite components such as calcium-aluminum-rich inclusions (CAIs) and chondrules. CAIs appear to have formed in an 16 O-rich nebular …