Turbulent Radial Mixing in the Solar Nebula as the Source of Crystalline Silicates in Comets

Abstract

Abstract. There is much debate about the origin of crystalline silicates in comets. Silicates in the protosolar cloud were likely amorphous, however the temperature of the outer solar nebula was too cold to allow their formation in this region by thermal annealing or direct condensation. This paper investigates the formation of crystalline silicates in the inner hot regions of the solar nebula, and their diffusive transport out to the comet formation zone, using a turbulent evolutionary model of the solar nebula. The model uses time-dependent temperature and surface density profiles generated from the 2-D α-disk model of Hersant et al. (2001). It is shown that turbulent diffusion is an efficient process to carry crystalline silicates from inner to outer disk regions within timescales of a few 10 4 yr. The warmest solar nebula models which reproduce the D/H ratios measured in meteorites, comets, Uranus and Neptune (Hersant et al. 2001) provide a mass fraction of crystalline silicates in the Jupiter-Neptune region in agreement with that measured in comet Hale-Bopp.

Cometary Isotopic Measurements

“…The analogy between cometary and interstellar ice-composition indeed suggests that those materials formed by similar processes, i.e., ion-molecule and grain-surface reactions at low temperatures . However, the presence of crystalline silicates and other high temperature condensates in comets indicates that comets incorporated also materials formed in the hot innermost regions of the disk (Brownlee et al 2006;Bockelée-Morvan et al 2002;Wooden 2008), or which underwent processing through, e.g., nebular shocks (Harker and Desch 2002), or in the surface layers during protosolar luminosity outbursts (Ábrahám et al 2009). In the latter two cases, where a purely nebular origin may be indicated, mixing of material radially outwards or vertically downwards to the midplane, would also lead to cometary ices having a composition derived from a chemistry radically different from that of the interstellar medium.…”

Oxygen isotopic composition of chondritic interplanetary dust particles: A genetic link between carbonaceous chondrites and comets

“…Malfait et al, 1998;Van Boekel et al, 2004;Kessler-Silacci et al, 2006) are indeed formed in the inner part of the accretion disk rather than being annealed interstellar grains (e.g. Bockelee-Morvan et al, 2002).…”

Nonthermal crystallization of amorphous silicates in comets

“…In contrast, the interstellar composition of gas in the comae indicates the preservation of interstellar volatiles in the cold outer nebula (Biermann et al, 1982;Mumma, 1996). To reconcile hot and cold materials, crystalline silicates are considered as being transported from the inner solar nebula to the outer comet-formation region (Bockelée-Morvan et al, 2002). Nuth et al (2000) claim that the fractional abundance of crystalline silicate in the solar nebula increases with time and thus can be used to date a comet's formation age.…”