【惑星科学】地球の構成単位の特性を解明する

Insights into the composition of the meteoritic material added to the Earth during its formation are provided in two papers published in this week’s Nature. The Earth formed by the accretion of Moon-sized to Mars-sized bodies from various sources, but determining the isotopic composition of these meteoritic building blocks has remained challenging.

Nicolas Dauphas shows that the isotopic nature of the Earth’s building blocks over time can be deciphered using signatures in the mantle of elements with differing affinities for metal. Lithophile (‘stone-loving’) elements, such as oxygen and calcium, tend to remain in the rocky mantle, whereas siderophile (‘iron-loving’) elements tend to move into the underlying core. The analyses suggest that the meteorites accreted by the Earth have not changed much over time and always contained a large proportion of material of isotopic composition similar to that of enstatite meteorites. This finding could also help to explain the similar isotopic compositions of the Earth and Theia, the hypothetical giant Moon-forming impactor.

In a second paper, Mario Fischer-Godde and Thorsten Kleine report that the Earth seems to have obtained its volatiles, including water, throughout its growth, rather than late in the Earth’s history through the accretion of chondrites (a type of stony meteorite) or comets. Siderophile elements, such as gold, iron and ruthenium, that are now present in the Earth’s mantle are thought to result from the ‘late veneer’ of material that accumulated after the Moon-forming impact. This study finds that all chondrites have ruthenium isotope compositions that are more different from that of the Earth’s mantle the further from the Sun they formed. This suggests that the late veneer was not, in fact, the primary source of volatiles and water on the Earth, and instead had a composition similar to that of enstatite meteorites.