Isotopic evidence for the formation of the Moon in a canonical giant impact

Nd isotope variation between the Earth-Moon system and enstatite chondrites

Reconstructing the building blocks that made Earth and the Moon is critical to constrain their formation and compositional evolution to the present. Neodymium (Nd) isotopes identify these building blocks by fingerprinting nucleosynthetic components. In addition, the ¹⁴⁶Sm-¹⁴²Nd and ¹⁴⁷Sm-¹⁴³Nd decay systems, with half-lives of 103 million years and 108 billion years, respectively, track potential differences in their samarium (Sm)/Nd ratios. The difference in Earth's present-day ¹⁴²Nd/¹⁴⁴Nd ratio compared with chondrites^1,2, and in particular enstatite chondrites, is interpreted as nucleosynthetic isotope variation in the protoplanetary disk. This necessitates that chondrite parent bodies have the same Sm/Nd ratio as Earth's precursor materials². Here we show that Earth and the Moon instead had a Sm/Nd ratio approximately 2.4 ± 0.5 per cent higher than the average for chondrites and that the initial ¹⁴²Nd/¹⁴⁴Nd ratio of Earth's precursor materials is more similar to that of enstatite chondrites than previously proposed^1,2. The difference in the Sm/Nd ratio between Earth and chondrites probably reflects the mineralogical distribution owing to mixing processes within the inner protoplanetary disk. This observation simplifies lunar differentiation to a single stage from formation to solidification of a lunar magma ocean³. This also indicates that no Sm/Nd fractionation occurred between the materials that made Earth and the Moon in the Moon-forming giant impact.

A Review of the Lunar 182Hf-182W Isotope System Research

In recent years, the extinct nuclide 182Hf-182W system has been developed as an essential tool to date and trace the lunar origin and evolution. Despite a series of achievements, controversies and problems exist. As a review, this paper details the application principles of the 182Hf-182W isotope system and summarizes the research development on W isotopes of the Moon. A significant radiogenic ε182W excess of 0.24 ± 0.01 was found in the lunar mantle, leading to heated debates. There are three main explanations for the origin of the excess, including (1) radioactive origin; (2) the mantle of the Moon-forming impactor; and (3) disproportional late accretion to the Earth and the Moon. Debates on these explanations have revealed different views on lunar age. The reported ages of the Moon are mainly divided into two views: an early Moon (30–70 Ma after the solar system formation); and a late Moon (>70 Ma after the solar system formation). This paper discusses the possible effects on lunar 182W composition, including the Moon-forming impactor, late veneer, and Oceanus Procellarum-forming projectile. Finally, the unexpected isotopic similarities between the Earth and Moon are discussed.

Fossil records of early solar irradiation and cosmolocation of the CAI factory: A reappraisal

Calcium-aluminum–rich inclusions (CAIs) in meteorites carry crucial information about the environmental conditions of the nascent Solar System prior to planet formation. Based on models of ⁵⁰V–¹⁰Be co-production by in-situ irradiation, CAIs are considered to have formed within ~0.1 AU from the proto-Sun. Here, we present vanadium (V) and strontium (Sr) isotopic co-variations in fine- and coarse-grained CAIs and demonstrate that kinetic isotope effects during partial condensation and evaporation best explain V isotope anomalies previously attributed to solar particle irradiation. We also report initial excesses of ¹⁰Be and argue that CV CAIs possess essentially a homogeneous level of ¹⁰Be, inherited during their formation. Based on numerical modeling of ⁵⁰V–¹⁰Be co-production by irradiation, we show that CAI formation during protoplanetary disk build-up likely occurred at greater heliocentric distances than previously considered, up to planet-forming regions (~1AU), where solar particle fluxes were sufficiently low to avoid substantial in-situ irradiation of CAIs.