Structure and transport in the solar nebula from constraints on deuterium enrichment and giant planets formation

A simplified analytical model of an evolutionary nebula is used to generate temperature-density radial profiles following the procedure elaborated by Dubrulle (Icarus 106, 59, 1993), Each nebula disk is characterized by its initial mass M(D), its initial radius R(D), and the coefficient of turbulent viscosity alpha. We show that these parameters may be constrained by comparing temperature-density profiles to properly chosen physical and chemical Solar System data. Relatively weak constraints come from theories of formation of giant planets. The deuterium over hydrogen ratios observed in fossil water in primitive objects of the Solar System are much more constraining. For each model of the nebula, the temporal and radial evolution of the deuterium enrichment factor in water with respect to the protosolar abundance (in H(2)) is calculated for the first time by integrating the equation of diffusion and is compared to observed deuterium enrichments in LL3 meteorites, giant planets, and comets. Observations cannot be fitted when we assume that H(2)O is uniquely produced in the hot inner part of the nebula. The agreement with observed data requires highly enriched deuterium ices initially infalling from the presolar cloud onto the whole nebula discoid, In order to fit measurements of D/H in LL3 meteorites, M(D), R(D), and alpha must be between 0.03 and 0.3 M(.), 8 and 28 AU, and 0.003 and 1.0, respectively. The source of the turbulence consistent with these a values is discussed. High viscosity disks are characterized by MHD turbulence, while the low viscosity disks (alpha between 0.003 and 0.01) are characterized by hydrodynamical turbulence. Magnetic fields in the selected nebulae are also calculated Their lifetime is found to be equal to 10(4) years for alpha = 0.1 and 10(5) years for alpha = 0.003, Scenarios providing us with an interpretation of the high D/H ratio observed in comets are discussed. The first scenario in which comets coming from the Oort cloud were formed very rapidly in the Uranus-Neptune region of the turbulent nebula and expelled toward the Oort cloud prior to the complete formation of these planets implies some reprocessing in the nebula of the cometary matter coming from the presolar cloud. It may not be easy however to expel comets toward the Oort cloud quite early in the history of the Solar System. It might be, according to the second scenario, that both comets coming from the Oort cloud and comets of the Jupiter family were formed farther than Neptune in a nonturbulent region of the nebula. In such a case, comets might have conserved to a large extent the chemical signature of the interstellar medium. Possible tests of the scenarios are considered. (C) 1999 Academic Press.