Recently, Professor Wang Wenzhong from the University of Science and Technology of China, in collaboration with international scholars, has revealed that two key stages, early protoplanetary disk volatile loss and late volatile-rich material addition, collectively determine the abundance of nitrogen in silicate Earth, providing new insights into the origin of Earth's volatiles. These findings were published in Nature Communications.
Early differentiation of volatile and late accretion of volatiles on the Earth. Image provided by the University of Science and Technology of China.
Nitrogen is one of the fundamental elements of life on Earth, widely present in numerous organic molecules. Despite the vital importance of nitrogen to life, the current nitrogen content in the silicate Earth (including the atmosphere, crust, and mantle) is relatively low compared to the primordial Earth biomass, at only about 2 ppm (parts per million). A thorough investigation into the evolutionary history of nitrogen in the Earth is crucial for understanding the origin of life-related elements on Earth and the evolution of habitability.
Currently, there are two main models in academia regarding the accretion of volatiles on Earth. The first model, known as the "late accretion model," suggests that the initial Earth biomass that formed contained almost no volatiles, including nitrogen, and the current abundance of volatiles in the silicate Earth was mainly acquired in the late accretion phase by incorporating small amounts of volatile-rich materials. The second model, the "early evolution model," proposes that the initial Earth biomass was originally rich in volatiles, and a series of evolutionary processes led to the relative depletion of volatiles in the silicate Earth compared to the initial composition.
Nitrogen has two stable isotopes, 14N and 15N. Nitrogen isotopes can be used to trace the evolutionary history of volatiles on Earth during planetary accretion processes, providing a crucial research tool for studying the origin and evolution of volatile components on Earth-like planets. However, to effectively utilize this tool, it is essential to understand the fractionation mechanisms of nitrogen isotopes during the early stages of planetary evolution. Using first-principles computational methods, Wang Wenzhong studied the nitrogen isotope fractionation during the condensation of nebular material to form protostars, including the stages of volatile melting and core-mantle differentiation. The research revealed that volatile melting enriched 14N in protostars while core-mantle differentiation led to the enrichment of 15N in silicate melts.
By combining first-principles computational results with actual observational data, the research team found that the early evolution of protostars alone is insufficient to explain the nitrogen isotope composition of the current silicate Earth. It is necessary to introduce a certain amount of material rich in volatile components, such as carbonaceous chondrites, in the late accretion phase to account for the observed nitrogen isotope characteristics. Therefore, the nitrogen abundance in the silicate Earth is the result of the combined effects of early protostar evolution and late accretion stages.
The researchers noted that while late accretion significantly influences the nitrogen abundance in the silicate Earth, the contribution of volatile-rich materials added is minimal due to their extremely low mass to the overall abundance of other volatiles in the silicate Earth.
For more information, refer to the related paper: https://doi.org/10.1038/s41467-024-48500-0
