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The bulk chemical composition of the Earth is poorly constrained because samples are limited to only the outer 10% of its radius. Meteorite data are used to represent the huge zones that we cannot sample, but for this approach to provide a realistic model of the Earth, we need to know where various meteorites originated and what processing they underwent. Currently popular models of the Solar System presume that the Sun formed first, followed by assembly of the planets from mineral dust in a disk that condensed locally from hot gas (e.g., Boss, 1998). More recent findings that comets are mixtures of phases formed at high and low temperature (Zolensky et al., 2006) came as a great surprise to planetary scientists because this contraindicates the existence of a condensation gradient as proposed by Lewis (1974). The alternative to dust condensation around a pre-formed Sun is a nebula containing dust inherited from older generations of stars. Meteorites possess small amounts of isotopically distinctive pre-solar material, proving that the nebula included some dust that predates our Sun (e.g., Bernatowicz & Zinner, 1997). Recent detection of silicate dust in molecular clouds (van Breeman et al., 2011), which are loci of star formation, suggests that pre-solar dust was abundant. Disk models also have problems conserving angular momentum (Armitage, 2011) and fail to explain the first order characteristics of the Solar System (upright axial spin and nearly circular orbits). Thus, independent evidence points to formation of planets and Sun simultaneously from a 3-dimensional dusty nebula (Hofmeister & Criss, 2012), in which case, virtually all meteoritic material has been processed. What represents the bulk Earth?

Origin of HED Meteorites from the Spalling of Mercury - Implications for the Formation and Composition of the Inner Planets