Origins of the raw materials from which the Earth was formed

Two studies on volatile elements discovered in meteorites provide revealing new information about the origins of the raw materials from which the Earth was formed.

In two different studies, published in the academic journal Science, it has been possible to identify nucleosynthetic isotopic anomalies in the volatile elements potassium and zinc present in meteorites. This makes it possible to delimit the sources from which the material from which the Earth was formed came. According to both studies, about 90% of the Earth’s mass comes from non-carbonaceous material from the area of ​​the solar system near the Sun and about 10% from carbonaceous chondrite material from the area of ​​the solar system far from the Sun. The carbonaceous chondrite deposit provided Earth with about 20% of its potassium and half of its zinc. Both studies indicate that the volatile elements were not evenly distributed in the hot solar nebula that formed the solar system.

Nucleosynthetic abnormalities are small differences in the isotopic ratios of chemical elements that occur when the elements are formed. During the formation of the solar system, elements carrying these nucleosynthetic anomalies condensed from the gas phase to form solid dust, which was then incorporated into meteorites and into Earth and other terrestrial (rocky) planets.

Material from different parts of the early solar system inherited various nucleosynthetic anomalies. The origin of the material that formed the Earth can be determined by measuring the nucleosynthetic anomalies of meteorites. However, nucleosynthetic anomalies of volatile elements (those that condense at low temperatures) have been difficult to measure, so their origin has not been accurately determined.

Artist’s impression of the Earth at a time remote in the past, not long after its formation. (Illustration: NASA’s Goddard Space Flight Center / Francis Reddy)

Nicole Nie and Da Wang’s team, both from the Carnegie Institute of Science in the United States, measured three isotopes of potassium (potassium 39, potassium 40 and potassium 41) in 32 meteorites. The researchers found nucleosynthetic anomalies in the potassium 40 isotope, which were larger and more variable in carbonaceous chondrite meteorites than in noncarbonaceous material. According to the findings, the proportion of potassium-40 nucleosynthetic anomalies in terrestrial rocks closely matches that of noncarbonaceous material, suggesting that most of the Earth’s potassium was contributed by noncarbonaceous material and less than 20% by carbonaceous chondrite material.

This study is titled “Meteorites have inherited nucleosynthetic anomalies of potassium-40 produced in supernovae”.

In another study, the team of Rayssa Martins and Sven Kuthning, both from Imperial College London in the UK, focused on another volatile element, zinc. The study authors analyzed the 5 stable isotopes of zinc in 18 meteorites. They identified nucleosynthetic anomalies in zinc isotopes that differ between carbonaceous chondrite and noncarbonaceous material meteorites. When compared to the isotopic signature of terrestrial zinc, the findings by Martins, Kuthning and their colleagues suggest a mixed source of the element. The study authors estimate that approximately 10% of the Earth’s total mass comes from carbonaceous chondrite meteorites, including 50% of its zinc. These findings indicate that carbonaceous chondrite material from the periphery of the solar system could have contributed substantially to increasing the presence on Earth of the other volatile elements on Earth.

This study is entitled “Nucleosynthetic isotope anomalies of zinc in meteorites constrain the origin of Earth’s volatiles”. (Source: AAAS)