The distribution of water in the solar nebula: Implications for solar system formation

Abstract

Water is important in the solar nebula both because it is extremely abundant and because it condenses out at 5 AU, allowing all three phases of H₂O to play a role in the composition and evolution of the solar system. In this work, a thorough examination of the inward radial drift of ice particles from 5 AU is undertaken. Drift model results are then linked to the outward diffusion of vapor, in one overall model which is numerically evolved over the lifetime of the nebula. Results of the model indicate that while the inner nebula is generally depleted in water vapor, there is a zone in which the vapor is enhanced by ∼40-100%, depending on the choice of ice grain growth mechanisms and rates. This enhancement peaks in the region from 0.1-2 AU and gradually drops off out to 5 AU. Conversely, ice abundance is enhanced over 3-5 AU. Representative hot (early) and cool (later) conditions during the quiescent phase of nebular evolution are examined. Additionally, the effect of the radial dependence of water depletion on nebular chemistry is quantified using a chemical equilibrium code that computes abundances of nebular elements and major molecular C, N, S, etc. species over a range of temperatures. In particular, changes in the local C/O ratio and organics abundance due to the radially dependent decrease in oxygen fugacity are tracked and plotted. Generally, the diffusion-drift model results in a more complex water distribution than previous models, with both radial and temporal variations in the C/O ratio which produce both relatively oxidizing and reducing nebular conditions across 1-5 AU. Depending on the value assumed for the solar C/O ratio, modest to significant enhancements of CH₄ and other organics abundances are produced in the inner nebula. These results coupled with the revised ice distribution may explain the radial signatures of hydration detections and darkening in asteroids, and perhaps the oxidation states of enstatite chondrites. The results also indicate that the inner nebula could have supplied organics and water to the terrestrial planets, as well as possibly to Europa and beyond, via outward mixing processes.