C
Chemical Fractionation During Solar Accretion

 

It is generally assumed that the Sun formed from its parent cloud without significant fractionation from the average cloud composition. The approximate agreement between photospheric and CI chondrite composition provides some justification for this assumption, although, in principle, fractionation could occur before the separation of solar and planetary material, e.g. by fragmentation from an initially larger cloud or during a stage characterized by strong bipolar outflows. Evidence for the representativeness of the original nebular cloud by CI chondrite composition comes from the CI relative abundance pattern, which defines a smooth curve for heavy (A > 60) odd-mass nuclei (Document B, Anders and Grevesse, 1989; Burnett et al., 1989), a regularity that must be nucleosynthetic in origin (Suess, 1947).

The photospheric abundances do not have sufficient precision to test for smoothness. Factors of 2 fractionation among heavy elements between the parent cloud and the solar system would have produced deviations from smoothness in the CI abundance curves which are not present. General fractionations of 30% or less cannot be ruled out, and, at some lighter masses, fractionations in the 30%-factor of 2 range are possible. Moreover, there is one class of elements whose abundances are not well constrained either in the photosphere or in CI chondrites: the volatile elements. Volatiles are depleted in the CI chondrites and, in part, are not observable in the solar photosphere due to the high first excited atomic state of gases, resulting in the absence of characteristic absorption lines. Therefore, a relatively large solid/gas fractionation in the formation of the sun and/or the planets would not be obvious by any means studied so far.

Volatile/nonvolatile fractionation could have occurred by a number of different mechanisms during solar system formation and solar nebula dispersal. These have been reviewed by Wiens et al., (1991, 1992) and will be briefly reiterated here:

  1. Fractionation due to intense stellar winds early in solar history. Efficient sinks for angular momentum and magnetic fields are needed during solar formation, and substantial mass loss appears to occur during and immediately preceding a T-Tauri phase (e.g., Lada and Shu, 1990). By analogy with the present-day solar wind, fractionation by first ionization potential (FIP) might result because of the preferential ionization of elements with low FIP. Since volatile elements all have high FIP this mechanism effectively leads to low nonvolatile/volatile elemental ratios in the sun even though volatility per se is not the important parameter. The intensity of fractionation could have been far greater than the present-day solar wind factor of ~4. Assuming that the sun lost a substantial fraction of its original mass during an early T-Tauri stage, the overall fractionation of the remaining material in the sun could be significant.
  2. A variation of (1) at lower temperature with Si in grains but Xe and Kr in the gas phase. A low level of ionization and separation of the bulk material from originally embedded magnetic fields would produce a fractionation of gas ions from solids and ions from neutral atoms and molecules. This mechanism may have been active in the interstellar cloud long before ignition of the sun, since magnetohydrodynamic processes are thought to play a crucial role in causing local inhomogeneities leading to stellar formation. The most easily ionized species (molecules, Xe, Kr) would be preferentially extracted compared to elements bound in grains.
  3. Solid/gas fractionation due to the Poynting-Robertson effect, which causes solid particles to slowly spiral toward the sun. Today only solids spiral toward the sun. The mass flux is insignificant relative to the mass of the solar convection zone, and it is not clear whether the atoms in the incoming particles are accreted to the sun or vaporized and swept out with the solar wind. However, this mechanism could have been significant in the early solar system when unaccreted materials were abundant and the solar surface temperature much lower. This would lead to enrichment of solids relative to gases in the photosphere. In addition to electromagnetic drag, a strong early solar wind may have caused an enhanced corpuscular radiation analogue of the Poynting-Robertson effect on grains smaller than ~1 µm (Burns et al., 1979).
  4. Enhancement of volatiles in the sun as the result of early large-scale planetesimal formation in the outer solar system.
  5. Similar to (4) but with fractionation in the galaxy as a whole due to removal of all elements condensible at temperatures of ~20-50 K by large-scale comet and ice formation (Tinsley and Cameron, 1974). This would only affect H and He.
  6. Fractionation due to comets or planetesimals in sun-grazing comet-like orbits, which would enrich the sun in solids (Joss, 1974).
Solar wind composition can provide relatively precise constraints on the amount of fractionation between the sun and the original solar nebula. While volatile/nonvolatile solar wind ratios such as (He,Ne)/(Mg,Si,Fe) are becoming available from in-situ spacecraft instruments (Document G.2), they are not helpful, because there are essentially no nucleosynthetic constraints on the relative abundances of these elements, but for elements heavier than Ni, abundance smoothness and s-process theory offers significant constraints (Wiens et al., 1991, 1992). S-process theory has been able to model elemental abundances quantitatively only in the high mass (> 100) range, but this complements constraints set by smoothness. Attempts have been made to use solar wind Kr (Wiens et al., 1991) and Xe (Wiens et al., 1992) implanted in lunar mineral grains, in conjunction with Si abundances from spacecraft solar wind data, to place constraints on solid/gas fractionation. Their estimate of solar Kr is shown in Fig. E1, along with CI chondrite data for the surrounding odd-mass isotopes (Burnett et al., 1989). The uncertainties only constrain solid/gas fractionation to within a factor of ~2. The large errors are mostly due to the convoluted way in which the (Kr,Xe)/Si ratios were obtained, since solar wind Si cannot be measured in lunar soils. Additional uncertainty comes from correcting for the FIP fractionation the solar wind experiences during acceleration, to obtain solar abundances from the solar wind.



Figure C1. Solar-system abundances of odd-mass isotopes in the range of A = 75-101, after Wiens et al.(1991).
All data except Kr are average CI concentrations from Burnett et al.(1989), normalized to Si = 106. The Kr data point is averaged from lunar ilmenite results and corrected for photosphere/solar-wind fractionation using a value of 4.2±1.5 relative to Si. The solar Kr abundance thus derived is within uncertainty of interpolation and s-process Kr estimates, which assume no solid/gas fractionation.

Because the redistribution of solar wind ions by diffusion and contamination by micrometeorite impacts and regolith gardening is so severe, lunar soils will not be able to provide any closer constraints. The heavy mass (A > 70) elements in the solar wind will not be analyzed by any other proposed spacecraft detector. However, the heavy noble gases will be some of the most fail-safe measurements for Genesis (proposal Section 2.E). Even in a worst-case scenario for the instrument, accurate constraints for the solid/gas fractionation problem will be possible.
 

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