Abstract
Primitive 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-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.
Highlights
The bulk composition of the Earth and that of the distinct terrestrial reservoirs is essential to understand and quantify the major differentiation processes that led to the present-day highly differentiated planet (e.g., McDonough and Sun 1995; Palme and O’Neill 2014)
The bulk composition of the Earth is required to reconstruct the formation of the Earth as a planet and to deduce the different processes that led to its specific chemical composition
It is possible to quantify the relative abundances of the different material that contributed to the growing Earth or other rocky planets whose bulk composition can be estimated (e.g., Sanloup et al 1999; Lodders 2000; Fitoussi et al 2016; Liebske and Khan 2019)
Summary
The bulk composition of the Earth and that of the distinct terrestrial reservoirs is essential to understand and quantify the major differentiation processes that led to the present-day highly differentiated planet (e.g., McDonough and Sun 1995; Palme and O’Neill 2014). The disparities in the chemical and isotopic composition of the rocky planets could reflect the different contributions of different known meteorite classes to their formation Based on this assumption, it is possible to quantify the relative abundances of the different material that contributed to the growing Earth or other rocky planets whose bulk composition can be estimated (e.g., Sanloup et al 1999; Lodders 2000; Fitoussi et al 2016; Liebske and Khan 2019). The Earth seems to be an apparent end member for many geochemical parameters among the rocky materials in the solar system studied so far (e.g., Wänke and Dreibus 1988; Dauphas et al 2004; Burkhardt et al 2011; Akram et al 2013, 2015; Akram and Schönbächler 2016; FischerGödde et al 2015; Liebske and Khan 2019) This contribution explores the chemical and isotope characteristics of different meteorites classes and their temporal evolution in order to evaluate their potential contributions to the formation of the Earth. Particular questions addressed are: What are the characteristics of this potentially missing component and where might it be found within the solar system? Is the Earth possibly an end member in its chemical composition or does the Earth sample a common component of the inner solar system that is dominantly present in the inner solar system planets like Venus and Mercury? What is the timing of formation for planetesimals and planets, and is there a relationship with their nucleosynthetic compositions?
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