Earth formed from an unknown selection of meteoritic material. Writing in Nature, Fischer-Gödde et al.1 report that the composition of ruthenium isotopes in ancient rocks from southwest Greenland contains evidence of a previously unrecognized building block of Earth. Surprisingly, the inferred isotopic composition of ruthenium in the material does not match known meteorite compositions. The authors’ findings suggest that Earth’s volatile components, such as water and organic compounds, could have arrived during the final stages of the planet’s growth.

Read the paper: Ruthenium isotope vestige of Earth’s pre-late-veneer mantle preserved in Archaean rocks

Our planet is the product of a series of collisions of increasingly large celestial bodies2–4. These building blocks accreted from a protoplanetary disk of dust and gas that orbited the proto-Sun about 4.6 billion years ago. Identifying the compositions of Earth’s building blocks is difficult because of our limited access to the disk’s remnants, and because of the complex, long-term geological processing of the mantle that has mixed Earth’s ancient ingredients.

Potential answers to the question of what Earth is made of can come from studies in which the isotopic compositions of terrestrial rock samples are compared with those of meteorites that formed within the first few million years of the Solar System’s history. These meteorites are presumed to be representative of the smaller bodies that ultimately coalesced to form the rocky planets. Consequently, meteorites are our most promising candidates for Earth’s building blocks.

Fischer-Gödde and colleagues’ study builds on the finding that meteorites have characteristic isotope compositions that serve as fingerprints to distinguish different types of potential building block. For example, meteorites such as carbonaceous chondrites, which are often ‘wet’ (that is, they contain volatile components), have different isotopic fingerprints from meteorites that are generally ‘dry’5. The differences in isotopic composition originate from the heterogeneous distribution of stardust in the protoplanetary disk, and are known as nucleosynthetic isotope variations. If the fingerprints could be identified in terrestrial rock samples, this might provide evidence of the material from meteorites that Earth was built from.

The documentation of fingerprints in terrestrial rocks could help to constrain estimates of when volatile elements were delivered to Earth and where they came from. This is because the abundances of certain isotopes of some elements — ruthenium-100 (100Ru), for example — not only distinguish between wet and dry building blocks, but also trace different stages of Earth’s accretion history.

A chronometer for Earth’s age

Ruthenium is classified as a highly siderophile (iron-loving) element, because it collects in metal-rich phases of Earth’s interior. Consequently, most of our planet’s ruthenium is concentrated in its metallic core. There are, however, traces of ruthenium and other highly siderophile elements (HSEs) in the mantle, and their relative proportions approximate to those measured in primitive meteorites6. One interpretation of this is that the HSEs were added to the mantle after the core formed, during an event called the late veneer — when the final approximately 0.5% (of the total percentage weight) of Earth’s mass accreted7,8. If so, then ruthenium and other HSEs in the mantle record the composition of the last material that accreted to Earth9.

It has been proposed that Earth’s volatile elements were also added during the late veneer, possibly by the accretion of carbonaceous chondrites10,11. Studies in the past few years, however, have found a mismatch between the 100Ru-isotope composition (the abundances of 100Ru in terrestrial rocks) in Earth’s mantle and that in carbonaceous chondrites12,13. It could therefore be concluded that carbonaceous chondrites did not form part of the late veneer, thus casting doubt on the timing of the delivery of volatiles to Earth13.

This conclusion rests on the assumption that HSEs in the mantle do not contain significant quantities of material from before the late veneer — a reasonable assertion, given that there is limited direct evidence of this. If the pre-late-veneer mantle did contain a substantial amount of 100Ru that did not collect in the core, and that was identifiable by having a different 100Ru-isotope composition from that of the modern mantle, then carbonaceous chondrites could still have been accreted during the late veneer.

Nucleosynthetic ruthenium-isotope variations have not been reported for terrestrial rocks before now. This is, in part, because Earth has active plate tectonics and mantle convection, which mix and dilute the fingerprints of its building blocks. However, in the past few years, analytical methods14 have been further developed that enable isotope variations to be measured on the scale of parts per million, making it possible to search for these primitive isotopic signatures.

By comparing the 100Ru-isotope compositions of terrestrial rocks with those of meteorites, Fischer-Gödde and co-workers report that an ancient part of Earth, preserved in rocks from southwest Greenland, retains the fingerprints of an unusual building block (Fig. 1). The fact that the inferred isotope compositions do not match known meteorite compositions indicates that current meteorite collections are considerably limited in their sampling of the protoplanetary disk.

Figure 1 | A scenario for the preservation of ancient material in Earth’s mantle. a, Between 4.6 billion and about 4.5 billion years ago, Earth formed from the accretion of material from meteorites. Siderophile elements, which have a strong affinity for metals, segregated into the core. b, The final approximately 0.5% of the total percentage weight of Earth’s mass accreted from meteorites during an event called the late veneer, after the core had formed. c, Fischer-Gödde et al.1 report that ancient rocks from southwest Greenland have an unusual ruthenium-isotope composition. They attribute this to the presence of pre-late-veneer mantle material in the rocks. The distribution of pre-late-veneer material shown here is speculative; the actual amount and distribution cannot be derived from the available data.

The authors interpret their unusual 100Ru data as the isotopic signature of pre-late-veneer ruthenium in the source of these rocks. Considering their findings in the context of the compositions of other HSEs in the mantle, the authors suggest that the modern composition of the mantle can be reconciled with their new data only if the late veneer contained carbonaceous chondrites to counterbalance the composition of the pre-late-veneer component of the mantle. This would mean that volatiles could have been delivered to Earth during the final stages of the planet’s formation.

Fischer-Gödde and colleagues’ data answer the long-standing question of whether Earth’s diverse building blocks are preserved and accessible for study. But the data also raise key questions, the answers to which will undoubtedly determine the importance of the new findings. For example, how representative of the pre-late-veneer mantle is the suite of rock samples from southwest Greenland? Are nucleosynthetic fingerprints observed in the isotopic compositions of other elements in the mantle? What is the composition of the ‘missing’ meteorites that dominated the ruthenium composition of the pre-late-veneer mantle, and why has it not yet been identified? And how was the isotopic signature of these meteorites preserved in the convecting mantle? These questions can be addressed only by expanding the search for nucleosynthetic fingerprints in the mantle.