Observational constraints on rocky-planet formation

The terrestrial planets and the Moon are differentiated, with dense iron-rich cores and rocky mantles. The uncompressed densities of Earth and Venus are similar. Mercury has a high density which suggests it has relatively large core. Conversely, the Moon has a low density, indicating a very small core. There is little observational evidence that asteroids are differentiated except for Vesta and Ceres (Thomas et al. 2005). However, iron meteorites from the cores of differentiated asteroids are quite common, and the irons found to date come from several dozen different parent bodies (Meibom Clark 1999). Most meteorites come from asteroids that never differentiated. These chondritic meteorites consist of intimate mixtures of heterogeneous material millimeter-sized rounded particles that were once molten, called chondrules, similarly sized calcium-aluminum-rich inclusions (CAIs), and micrometer-sized matrix grains. 

2 Chemical and isotopic compositions of the inner planets and asteroids 

The Cl class of chondritic meteorites are thought to have a similar elemental composition to that of the solar nebula (see Chapter 6 of this volume). All elements 

With the exception of oxygen, the Solar System has a remarkably uniform isotopic composition, with variations ofless than one partin 1000 for all objects 1 cm in size. Larger isotopic anomalies are seen only in a few submillimeter-sized CAIs, at the level of a few percent for some elements (e.g. MacPherson 2007), and in micrometer-sized presolar, circumstellar grains, which show large isotopic variations in all elements (Zinner 2007). Oxygen is a notable exception to this rule, with non-mass-dependent variations of up to 20% among Solar System materials 

Element 50% Condensation temperature (K) Earth mantle Mars mantle cv chondrite Solar photosphere 

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