Wednesday, 23 April 2014

From Disks to Planets: Early Mixing and Transport in Young Disks

Background: Any theory of the formation of the solar system planets, and hence for the formation of planetary systems in general, must be consistent with basic observations of primitive meteorites and comets. Chondritic meteorites require that flash heating occurred frequently in regions of the disk that were otherwise cool enough to allow iron sulfide to be stable. Short-lived radionuclides were synthesized and injected heterogeneously into the disk, yet were rapidly homogenized by disk mixing. At the same time, fractionation of oxygen isotopes occurred and was not erased by mixing. The discovery of refractory grains in Comet Wild 2 by the Stardust Mission (Brownlee et al. 2006) showed that large-scale outward transport of materials from the hot inner regions to the cool outer disk must occur (Ciesla 2007).

Research To Date: While accretion disk models driven by a generic turbulent viscosity have been invoked to explain large-scale transport in the solar nebula, the detailed physics behind such an “alpha” viscosity remains unclear. Boss (2007) presented an alternative physical mechanism driving transport: gravitational torques associated with the transient spiral arms in a marginally gravitationally unstable disk. Time-evolving three-dimensional models of selfgravitating disks show that small dust grains are transported upstream and downstream (with respect to the mean inward flow of gas and dust) inside the disk on time scales of less than 1000 yr inside 10 AU (Boss 2008). These models further show that any initial spatial heterogeneities (e.g., in 26Al) will be homogenized by disk mixing down to a level of ~10%, preserving the use of short-lived isotopes as accurate nebular chronometers, while simultaneously allowing for the spread of stable oxygen isotope ratios.

Proposed Work: Co-I Alan Boss will continue this work on mixing and transport to extend the models to include an analysis of the time history of a population of individual dust grains as they traverse high and low temperature regions of the disk. Through a collaborative effort with Prof. Morris Podolak of Tel Aviv University, who has developed a model for silicate dust grains with water ice mantles, this will allow a determination of the extent to which water is transported as a solid by the dust grains. By following how water ice condenses or sublimates on a population of grains being transported around the disk, a better understanding of the distribution of water in the solar nebula will be achieved, with important implications for the delivery of water to the terrestrial planets.