Are there chemical gradients in the inner solar system?

The solar system is apparently stratified with regard to the contents of vo latile constituents, as judged from the rocky, volatile-poor inner solar sy stem planets and meteorites and the huge volatile-rich outer planets. Howev er, beyond this gross structure there is no evidence for a systematic incre ase of the volatiles' abundances with distance from the Sun. Although meteo rites show comparatively large differences in volatile element contents the y also differ in many other respects, such as Mg/Si-ratios, bulk Fe and ref ractory element contents. These variations reflect variations in the nebula r environment from which meteorites formed. The various conditions of meteo rite formation cannot, however, be related in a simple way to heliocentric distances. There are also no systematic variations in the chemistry of the inner plane ts Mercury, Venus, Earth, Moon, Mars, and including the fourth largest aste roid Vesta, that could be interpreted as a relationship between volatility and composition. Although Mars (as judged from the composition of Martian m eteorites) is more oxidized and contains more volatile elements than Earth, this trend cannot be extrapolated to the dry volatile poor Vesta (sampled by HED meteorites) in the asteroid belt. If the Earth-Mars trend reflects g lobal inner solar system gradients then Vesta must have formed inside Earth 's orbit and moved out later to its present location. The quality of Mercur y and Venus composition data is not sufficient to allow reliable extrapolat ion to distances closer to the Sun. Recent nebula models predict small temperature gradients in the inner solar system supporting the view that no large variations in volatile element co ntents of inner solar system materials are expected.