METEORITES AND THE ORIGIN OF THE SOLAR SYSTEM

Abstract

During the past two centuries, theories of the origin of the solar system have been strongly influenced by observations and theories about meteorites. I review this history up to about 1985. During the 19th century the hypothesis that planets formed by accretion of small solid particles ('the meteoritic hypothesis') competed with the alternative 'nebular hypothesis' of Laplace, based on condensation from a hot gas. At the beginning of the 20th century Chamberlin and Moulton revived the meteoritic hypothesis as the 'planetesimal hypothesis' and joined it to the assumption that the solar system evolved from the encounter of the Sun with a passing star. Later, the encounter hypothesis was rejected and the planetesimal hypothesis was incorporated into new versions of the nebular hypothesis. In the 1950s, meteorites provided essential data for the establishment by Patterson and others of the presently accepted 4500 Ma age of die Earth and the solar system. Analysis of the Allende meteorite, which fell in 1969, inspired the 'supernova trigger' theory of the origin of the solar system, and furnished useful constraints on theories of planetary formation developed by Urey, Ringwood, Anders and others. Many of these theories assumed condensation from a homogeneous hot gas, an assumption that was challenged by astrophysical calculations. The meteoritic-planetesimal theory of planet formation was developed in Russia by Schmidt and later by Safronov. Wetherill, in the United States, established it as the preferred theory for formation of terrestrial planets. © The Geological Society of London 2006.