Mixing in the solar nebula: Implications for isotopic heterogeneity and large-scale transport of refractory grains

Abstract

The discovery of refractory grains amongst the particles collected from Comet 81P/Wild 2 by the Stardust spacecraft (Brownlee et al. 2006) provides the ground truth for large-scale transport of materials formed in high temperature regions close to the protosun outward to the comet-forming regions of the solar nebula. While accretion disk models driven by a generic turbulent viscosity have been invoked as a means to explain such large-scale transport, the detailed physics behind such an "alpha" viscosity remains unclear. We present here an alternative physical mechanism for large-scale transport in the solar nebula: gravitational torques associated with the transient spiral arms in a marginally gravitationally unstable disk, of the type that appears to be necessary to form gas giant planets. Three dimensional models are presented of the time evolution of self-gravitating disks, including radiative transfer and detailed equations of state, showing that small dust grains will be transported upstream and downstream (with respect to the mean inward flow of gas and dust being accreted by the central protostar) inside the disk on time scales of less than 1000 yr inside 10 AU. These models furthermore show that any initial spatial heterogeneities present (e.g., in short-lived isotopes such as 26 Al) 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. This finite level of nebular spatial heterogeneity appears to be related to the coarse mixing achieved by spiral arms, with radial widths of order 1 AU, over time scales of ∼ 1000 yrs.

Cosmochemical evidence for astrophysical processes during the formation of our solar system

“…And third, the isotopic composition of condensed (i.e., rocky) matter in the solar system is defined entirely by a mixture of supernova plus nova plus red giant grains. This scenario may be consistent with intermittent phases of gravitational instability in the gaseous disk (38).…”

“…37) suggested that particle diffusion within the disk itself led to grains being transported significant distances outward from the Sun over time scales of 0.1 to 1 Myr. Boss (38) showed that a phase of marginal gravitational instability is able to rapidly transport small particles from the inner disk to the outer disk while simultaneously homogenizing most initial isotopic heterogeneity, over time scales less than 0.001 Myr. It is possible that all three mechanisms operated in transporting grains outward from the Sun, but results from the Stardust sample return mission from Comet Wild 2 comet dust may pose a problem for the disk diffusion model.…”

Photometric observations of comet 81P/Wild 2 during the 2010 perihelion passage

“…Heating to temperatures greater than 1100 K can anneal silicates to crystallinity, but no protoplanetary heating sources have been identified that were sufficiently strong to heat materials in the outer nebula, where comets are believed to form, to such high temperatures. Possible explanation scenarios reinforced the conclusion that large-scale mixing indeed played a key role in the small bodies' formation in the protoplanetary disk (Bockelée-Morvan et al 2002;Boss 2008).…”

Oxygen Isotopic Compositions of Solar Corundum Grains

“…The only reported magnesium isotopic compositions of the 16 O-rich micron-sized corundum grains from Murchison (Virag et al 1991) are consistent with their early formation. Although the exact mechanism of radial mixing remains controversial (Shu et al 1996(Shu et al , 1997Bockelée-Morvan et al 2002;Gail 2001Gail , 2004Dullemond et al 2006;Ciesla 2007;Boss 2008), it was probably most effective at transporting materials outward at the very earliest stages of disk evolution, when the rates of mass and angular momentum transport are at their highest, allowing for the large-scale redistribution of disk materials.…”