EARLY ACCRETIONAL HISTORY OF THE EARTH AND THE MOON-FORMING EVENT

The accretion of the Earth is considered in the broader perspective of the formation of the solar system. Formation of planets from dust, or from a giant gaseous protoplanet predict uniform planetary compositio ns with solar-type abundances. These are not observed. Evidence for th e accretion of the Earth from a hierarchical swarm of planetesimals, i ncludes the heavily cratered ancient surfaces of the Moon, Mars and Me rcury, the obliquities of the planets and compositional variations amo ng the planets, while the high density of Mercury and the low density of the Moon are both attributable to large collisional events. Followi ng separation of the solar nebula as a fragment from a molecular cloud , early violent T Tauri and FU Orionis stages of stellar evolution swe pt water and other uncondensed volatile elements out to a ''snow line' ' at 5 A.U. Condensation in this region increased the particle density , enabling a 15-20 Earth-mass core to form, which collected a hydrogen and helium envelope by gravitational attraction before the gaseous ne bula had dispersed. However, Jupiter has about 10 times the solar rock +ice/gas ratio, implying that the gaseous nebula was already partially dispersed by the time Jupiter had formed. This early formation of Jup iter depleted the region of the asteroid belt, and of Mars (which is 3 000 times less massive than Jupiter). Thus the formation of Mars, and by inference the other terrestrial planets, occurred after the gaseous nebula had been dispersed. The meteoritic evidence indicates that cho ndrules formed in the nebula very close to T-0 (4560 m.y.) from pre-ex isting silicate dust. Separate silicate, metal and sulfide phases were present and volatile element depletion had already occurred before th e chondrule-forming event, probably as a consequence of early violent solar activity. Very little mixing appears to have taken place, with t he chondrites accreting quickly from local regions of the nebula, perh aps only 0.1 A.U. wide. The wide diversity in chondrite compositions, oxygen isotopes and the lack of mixing among different classes implies heterogeneity in the nebula, which appears to be unrelated to helioce ntric distance. The K/U ratios (indexes of volatile/ refractory elemen t separation) for Earth, Venus and Mars indicate that volatile element depletion was widespread in the inner nebula. Judging from the K/U ra tios, Mars at 1.5 A.U. appears to contain about 50% more volatile elem ents than the Earth or Venus. The proportion of ''igneous'' asteroids in the main belt, nearly 100% sunwards of 2 A.U., decreases rapidly wi th increasing heliocentric distance. The source of heating is not esta blished, but it seems td be related to heliocentric distance. This lea ds to the inference that all bodies in the inner solar system (sunward s of the asteroid belt) from which the terrestrial planets were assemb led, were melted and differentiated. Metallic core formation in the te rrestrial planets was thus essentially coeval with the accretion of su ch bodies. Accretion of the terrestrial planets from those planetesima ls which survived the early violent solar activity in the inner nebula occurred on timescales of 10-50 m.y. The similarity in K/U ratios and uncompressed densities of Venus and the Earth (separated by 0.3 A.U.) probably indicates a similar bulk composition for the major elements for these two planets, in which case the inner planets accreted from z ones at least 0;3 A.U. wide. Mars, 0.5 A.U. distant, accreted from a d ifferent population of planetesimals. The composition of meteorites di ffers enough from that of the terrestrial planets that during their fo rmation, addition of material from the asteroid belt beyond 2 A.U. was probably minimal. At a late sta