Chemical heterogeneity and imperfect mixing in the solar nebula

Abstract

It is shown that for the standard accretion disk model of the solar nebula, the efficiency of mixing the products of thermochemical processing from to large disk radii depends only on the ratio of Djv ≡ k (D = eddy diffusivity, v = eddy viscosity). In the steady state limit, where mixing is most efficient, the fractional contamination at radius R which is due to thermochemical processing at radius R_p R_p, which is true for all but very low angular momentum models. Since most of the mass resides at large radii R » R_p, it is concluded that if k ≾ 1, then most of the solar nebula was not contaminated by the consequences of thermochemical equilibria that were established at small radii (e.g., of order 1 AU). This condition is almost certainly satisfied if the physical process responsible for v is thermal convection or waves, but has uncertain validity during the early phases of disk evolution where accretion-induced shear instabilities may dominate, as Prinn discusses. In most cases, and especially during the later most relevant stage of disk evolution, interstellar dominance is implied for most solar nebula speciation and is predicted for cometary speciation except possibly for a contamination which is due to catalyzed hydrogenation of CO to CH_4 and other hydrocarbons. If primordial giant planets possessed accretion disks, then the chemical speciation of the disk may have been partly that of the solar nebula. However, greater mixing and gas processing (including conversion of CO to CH_4 and N_2 to NH_3) might have occurred in these circumstances. The formalism developed here may have applicability to the interpretation of compositional gradients in the nebulae of Young Stellar Objects, and may be relevant to the survivability of interstellar dust grains.