Insights into the origins of the Earth-Moon system are provided by analysis of calcium isotope compositions of various inner-Solar-System objects, as reported in this week’s Nature. Calcium is a key constituent of rock-forming minerals and thus can yield clues about the material that went on to form the Solar System’s rocky planets (Mercury, Venus, Earth and Mars).
Variations in the isotopic composition of objects in the inner Solar System can be used to study the relationship between meteorites and the rocky planets. This approach generally assumes that isotopic variability reflects the object’s distance from the centre of the protoplanetary disk. Thus, given the isotopic similarity of Earth and the Moon, this assumption is difficult to reconcile with the standard model for the formation of the Earth-Moon system, which assumes the collision of a Mars-sized body into the proto-Earth.
Martin Schiller and colleagues analysed the calcium isotope composition of samples from meteorite parent bodies, the asteroid Vesta, Mars and Earth. The authors found that the calcium isotope ratios correlate with the masses of the parent asteroids and planets, providing a proxy for their accretion timescales. They infer that this correlation arises from a slow, steady evolution of the bulk calcium isotopic composition in the rocky-planet-forming region of the protoplanetary disk, reflecting the introduction of pristine outer-Solar-System material to the thermally processed inner protoplanetary disk, which is associated with the accretion of mass to the proto-Sun. They also conclude that the very similar calcium isotope composition of Earth and the Moon implies that the Moon-forming impact involved protoplanets that formed near the end of the life of the protoplanetary disk.