Overview

Earth is part of the solar system and it cannot be completely understood in isolation. The chemistry of meteorites and the Sun provide constraints on the composition of the planets. The properties of the planets provide ideas for and tests of theories of planetary formation and evolution. The Earth is often assumed to have been formed by the slow accumulation of planetesimals – small cold bodies present in early solar system history. In particular, types of stony meteorite called chondrites have been adopted as the probable primary material accreted by the Earth. This material, however, has to be extensively processed before it is suitable.

Study of the Moon, Mars and meterorites demonstrates that melting and basaltic volcanism is ubiquitous, even on very small bodies. Planets form hot, or become hot, and begin to differentiate at a very early stage in their evolution, probably during accretion. Although primitive objects have survived in space for the age of the solar system, there is no evidence for the survival of primitive material once it has been in a planet. One would hardly expect large portions of the Earth to have escaped this planetary differentiation, and to be ‘primordial’ and undegassed. The present internal structure of the Earth was mainly established 4.57 billion years ago.