Numerous Chondritic Impactors And Oxidized Magma Ocean Set Earth's Volatile Depletion

By Keith Cowing
October 26, 2021
Filed under
Numerous Chondritic Impactors And Oxidized Magma Ocean Set Earth's Volatile Depletion
Cartoon of element partitioning processes during Earth’s accretion according to our model. Accreting planetesimals and giant impactors deliver volatiles and simultaneously form a vapour plume eroding the atmosphere. a: Model for the main accretion stage (10% to 99.5% of the Earth’s mass). Equilibration among the magma ocean (silicate melt), liquid metal droplets transiting to the core, and the overlying atmosphere are achieved according to each metal-silicate partitioning coefficient and solubility. b: Model for the late accretion stage after the solidification of the magma ocean (the last 0.5%). We consider the liquid water oceans and the carbonate-silicate cycle to be driven by plate tectonics on the surface. In this stage, most H and C on Earth are stored in the oceans and carbonate rocks, respectively. Numerous impactors can selectively erode N.

Earth’s surface environment is largely influenced by its budget of major volatile elements: carbon (C), nitrogen (N), and hydrogen (H).

Although the volatiles on Earth are thought to have been delivered by chondritic materials, the elemental composition of the bulk silicate Earth (BSE) shows depletion in the order of N, C, and H. Previous studies have concluded that non-chondritic materials are needed for this depletion pattern. Here, we model the evolution of the volatile abundances in the atmosphere, oceans, crust, mantle, and core through the accretion history by considering elemental partitioning and impact erosion.

We show that the BSE depletion pattern can be reproduced from continuous accretion of chondritic bodies by the partitioning of C into the core and H storage in the magma ocean in the main accretion stage and atmospheric erosion of N in the late accretion stage. This scenario requires a relatively oxidized magma ocean (log10fO2 ≳ IW−2, where fO2 is the oxygen fugacity, IW is log10fIWO2, and fIWO2 is fO2 at the iron-wüstite buffer), the dominance of small impactors in the late accretion, and the storage of H and C in oceanic water and carbonates in the late accretion stage, all of which are naturally expected from the formation of an Earth-sized planet in the habitable zone.

Haruka Sakuraba, Hiroyuki Kurokawa, Hidenori Genda, Kenji Ohta

Comments: 17 pages, 4 figures (comprising 11 panels in total), 1 table, Methods, and Supplementary Information (available online: this https URL)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Geophysics (physics.geo-ph)
Journal reference: Scientific Reports, volume 11, Article number: 20894 (2021)
DOI: 10.1038/s41598-021-99240-w
Cite as: arXiv:2110.12195 [astro-ph.EP] (or arXiv:2110.12195v1 [astro-ph.EP] for this version)
Submission history
From: Haruka Sakuraba
[v1] Sat, 23 Oct 2021 11:03:06 UTC (1,854 KB)

SpaceRef co-founder, Explorers Club Fellow, ex-NASA, Away Teams, Journalist, Space & Astrobiology, Lapsed climber.