Do Photosphere and Initial bulk Sun Compositions Differ?
[See Web Document F for a less abreviated discussion of the same material]. It is widely assumed that the average solar system elemental composition is preserved by the material on the surface of the Sun. This is because the present surface mixing zone (SMZ) and radiative interior of the Sun are calculated to have formed very early, before thermonuclear burning of H could produce abundance changes. Recent solar models [e.g. Proffitt, 1994] allow for gravitational settling, thermal gradient diffusion, and differential radiation pressure which collectively produce compositional gradients beneath a well-mixed SMZ. “Settling out” of heavier elements at the base of the SMZ can occur in principle, but the turbulent nature of the mixing, (e.g. convective overshoots) counters the settling. Theoretical differences [e.g. Proffitt, 1994] between SMZ (photospheric) and initial abundances are predicted to be small (order percents for elements other than He), but it is important to make observationa
