Distribution and evolution of water ice in the solar nebula: Implications for Solar System body formation

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 paper, we undertake a thorough examination of and model the inward radial drift of ice particles from 5 AU. We then link the drift results to the outward diffusion of vapor, in one overall model based on the two-dimensional diffusion equation, and numerically evolve the global model over the lifetime of the nebula. We find that while the inner nebula is generally depleted in water vapor, there is a zone in which the vapor is enhanced by 20-100%, depending on the choice of ice grain growth mechanisms and rates. This enhancement peaks in the region from 0.1 to 2 AU and gradually drops off out to 5 AU. Since this result is somewhat sensitive to the choice of nebular temperature profile, we examine representative hot (early) and cool (later) conditions during the quiescent phase of nebular evolution. Variations in the pattern of vapor depletion and enhancement due to the differing temperature profiles vary only slightly from that given above. Such a pattern of vapor enhancement and depletion in the nebula is consistent with the observed radial dependence of water of hydration bands in asteroid spectra and the general trend of asteroid surface darkening. This pattern of water vapor abundance will also cause variations in the C:O ratio, shifting the ratio more in favor of C in zones of relative depletion, affecting local and perhaps even global nebular chemistry, the latter through quenching and radial mixing processes. (C) 1998 Academic Press.