Accretion of the Earth—Missing Components?

Abstract

AbstractPrimitive meteorites preserve the chemical and isotopic composition of the first aggregates that formed from dust and gas in the solar nebula during the earliest stages of solar system evolution. Gradual increase in the size of solid bodies from dust to aggregates and then to planetesimals finally led to the formation of planets within a few to tens of million years after the start of condensation. Thus the rocky planets of the inner solar system are likely the result of the accumulation of numerous smaller primitive as well as differentiated bodies. The chemically most primitive known meteorites are chondrites and they consist mostly of metal and silicates. Chondritic meteorites are derived from distinct primitive planetary bodies that experienced only limited element fractionation during formation and subsequent differentiation. Different chondrite classes show distinct chemical and isotopic characteristics, which may reflect heterogeneities in the solar nebula and the slightly different pathways of their formation. To a first approximation the chemical composition of the bulk Earth bears great similarities to primitive meteorites. However, for some elements there are striking and significant differences. The Earth shows a much stronger depletion of the moderate to highly volatile elements compared to chondrites. In addition, mixing trends of specific isotopes reveal that the Earth is most enriched in $s$s-process isotopes compared to all other analysed bulk solar system materials. It is currently not possible to fully define and quantify the different chemical and isotopic materials that formed the Earth, because a major component seems missing in the extant collections of extraterrestrial samples. Variations in nucleosynthetic isotope compositions as well as the strong depletion of moderately and strongly volatile elements points towards a source in the inner solar system for this missing material. It is conceivable that Venus and Mercury contain a much larger fraction of this missing component. Thus, for a complete reconstruction of the conditions that led to the formation of the inner solar system planets (Mercury to Mars) samples from the inner planets Venus and Mercury are of great interest and importance. High precision chemical and isotopic analyses in the laboratory of rocky material from inner solar system bodies could complete the knowledge on the chemical, isotopic and mineralogical make-up of the solar nebula just prior to planet formation and enhance our understanding of the evolution of the solar nebula in general and the formation of the rocky planets in particular.