Accretion in the inner nebula: The relationship between terrestrial planetary compositions and meteorites*

Abstract

Abstract-The bulk compositions ofthe terrestrial planets are assessed. Venus and Earth probably have similar bulk compositions, but Mars is enriched in volatile elements. The inner planets are all depleted in volatile elements, as shown by KJU ratios, relative to most meteorites and the CI primordial values. Terrestrial upper mantle Mg/Si ratios are high compared with CI data. If they are representative of the bulk Earth, then the Earth accreted from a segregated suite of planetesimals that had non-chondritic major element abundances. The CI meteorite abundances, despite aqueous alteration, match the solar data and provide the best estimate for the composition of the solar nebula, including the iron abundance. The widespread depletion of volatile elements in the inner solar nebula is most likely caused by heating related to early violent solar activity (e.g., T Tauri and FU Orionis stages) which, for example, drove water out to a "snow line" in the vicinity of Jupiter. The variation in composition among the meteorites and the apparent lack of mixing among the groups indicates accretion from narrow feeding zones. There appears to have been little mixing between meteorite and planetary formation zones, as shown by the oxygen isotope variations, lack of mixing of meteorite groups, and differences in KJU ratios. In summary, it appears that the final accretion of planets did not result in widespread homogenization, and that mixing zones were not more than about 0.3 A.U. wide. Although the composition of the Moon is unique, and its origin due to an essentially random event, its presence reinforces the planetesimal hypothesis and the importance of stochastic processes during planetary accretion in the inner solar system.

The composition and thickness of the crust of Mars estimated from rare earth elements and neodymium‐isotopic compositions of Martian meteorites

“…Remote sensing observations (Singer and McSween, 1993), and the bulk composition of Martian surface dust measured by the Viking and Pathfinder landers (Taylor, 1991;Rieder et al, 1997) are also consistent with a basaltic crust on Mars and allow only minor volumes of intermediate or granitic crust at the surface. Isotopic compositions of the Martian meteorites suggest that this crust formed very early in Martian history (Harper et al, 1995;Borg et al, 1997) and has remained largely isolated from the mantle since that time.…”

Not so rare Earth? New developments in understanding the origin of the Earth and Moon

“…Taylor, 1991). On the one hand, cosmochemists see many similarities between the Earth and Moon, such as Mn-V-Cr abundances, O isotopes and Nb/Ta ratios (Dreibus and Wanke, 2002;Wiechert et al, 2001;Münker et al, 2003, respectively).…”