Early accretional history of the Earth and the Moon-forming event

Abstract

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 compositions with solar-type abundances. These are not observed. Evidence for the accretion of the Earth from a hierarchical swarm of planetesimals, includes the heavily cratered ancient surfaces of the Moon, Mars and Mercury, the obliquities of the planets and compositional variations among the planets, while the high density of Mercury and the low density of the Moon are both attributable to large collisional events. Following separation of the solar nebula as a fragment from a molecular cloud, early violent T Tauri and FU Orionis stages of stellar evolution swept 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 Earthmass core to form, which collected a hydrogen and helium envelope by gravitational attraction before the gaseous nebula 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 Jupiter depleted the region of the asteroid belt, and of Mars (which is 3000 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 chondrules formed in the nebula very close to T 0 (4560 m.y.) from pre-existing silicate dust. Separate silicate, metal and sulfide phases were present and volatile element depletion had already occurred before the chondrule-forming event, probably as a consequence of early violent solar activity. Very little mixing appears to have taken place, with the chondrites accreting quickly from local regions of the nebula, perhaps 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 heliocentric distance. The K U ratios (indexes of volatile/refractory element separation) for Earth, Venus and Mars indicate that volatile element depletion was widespread in the inner nebula. Judging from the K U ratios, Mars at 1.5 A.U. appears to contain about 50% more volatile elements than the Earth or Venus. The proportion of “igneous” asteroids in the main belt, nearly 100% sunwards of 2 A.U., decreases rapidly with increasing heliocentric distance. The source of heating is not established, but it seems to be related to heliocentric distance. This leads to the inference that all bodies in the inner solar system (sunwards of the asteroid belt) from which the terrestrial planets were assembled, were melted and differentiated. Metallic core formation in the terrestrial planets was thus essentially coeval with the accretion of such bodies. Accretion of the terrestrial planets from those planetesimals 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 zones at least 0.3 A.U. wide. Mars, 0.5 A.U. distant, accreted from a different population of planetesimals. The composition of meteorites differs enough from that of the terrestrial planets that during their formation, addition of material from the asteroid belt beyond 2 A.U. was probably minimal. At a late stage in the accretion of the terrestrial planets, a large (0.15 earthmass) differentiated body impacted the Earth. This object was disintegrated and its silicate mantle was spread out into Earth orbit. The metallic core was accreted to the Earth and a Moon-sized object formed, mostly from the silicate mantle of the impactor. The Moon contains 50% more FeO than the terrestrial mantle, consistent with derivation from a different body. The energy involved in this event is sufficient to melt the Earth, but melting anyway is a probable consequence of the accretion of the Earth from a hierarchical suite of planetesimals. Late veneers are a possible source for the Earth of mantle siderophile elements, water and noble gases, but are not apparent on the Moon.