Compositions of carbonaceous-type asteroidal cores in the early solar system

Compositions of iron-meteorite parent bodies constrain the structure of the protoplanetary disk

Magmatic iron-meteorite parent bodies are the earliest planetesimals in the Solar System, and they preserve information about conditions and planet-forming processes in the solar nebula. In this study, we include comprehensive elemental compositions and fractional-crystallization modeling for iron meteorites from the cores of five differentiated asteroids from the inner Solar System. Together with previous results of metallic cores from the outer Solar System, we conclude that asteroidal cores from the outer Solar System have smaller sizes, elevated siderophile-element abundances, and simpler crystallization processes than those from the inner Solar System. These differences are related to the formation locations of the parent asteroids because the solar protoplanetary disk varied in redox conditions, elemental distributions, and dynamics at different heliocentric distances. Using highly siderophile-element data from iron meteorites, we reconstruct the distribution of calcium-aluminum-rich inclusions (CAIs) across the protoplanetary disk within the first million years of Solar-System history. CAIs, the first solids to condense in the Solar System, formed close to the Sun. They were, however, concentrated within the outer disk and depleted within the inner disk. Future models of the structure and evolution of the protoplanetary disk should account for this distribution pattern of CAIs.

First classification of iron meteorite fragment preserved in Chang'e-5 lunar soils

Lunar soil preserves numerous fragments of meteorites impacting on the Moon, providing a unique opportunity to investigate the distribution of the types of projectiles over billions of years. Here we report the first discovery of an iron meteorite fragment from the Chang'e-5 lunar soil, which consists mainly of martensite (quenched from taenite), kamacite, and schreibersite, with a trace of pentlandite. The meteorite fragment is Ni- and P-rich, S-poor, and based on its mineral chemistry and bulk composition, can be classified into the IID-group, a rare and carbonaceous group of iron meteorite originating in the outer Solar System. This meteorite fragment experienced only limited partial melting followed by fast cooling, suggestive of efficient preservation of intact remnants of iron meteorites impacting on the porous lunar regolith. Alternatively, it is a relic of a low-velocity impact of submillimeter-sized metal grains originated from an IID-like iron meteorite. Our observations demonstrate that it is feasible to achieve the type distribution of meteorites impacting on the Moon via systematically analyzing a large number of metal grains separated from lunar soils, thus shedding light on the dynamic evolution of the Solar System.