Astrogeology

Constructing Earth Formation History Using Deep Mantle Noble Gas Reservoirs

By Keith Cowing

Status Report

astro-ph.EP

November 25, 2025

Filed under astro-ph.EP, astrogeology, Earth, geology, planetary embryos, primordial gas, Protoplanetary Disk

Constructing Earth Formation History Using Deep Mantle Noble Gas Reservoirs — Schematic illustration of the favored Earth formation scenario implied by our results. Top: From left to right, we show the different formation stages of the Earth from a side-view of a truncated solar nebula, which is initially rich in gas (yellow) and dust (gray dots). As the disk progressively dissipates, the coagulation of solids (see Section 4.1.1) leads to the formation of a set of three ∼ 0.3M_⊕ embryos, displayed with cyan, orange and red circles representing the layered embryo structure of Figure 1. Following the dispersal of the disk, embryo mergers are enabled, resulting in the formation of the proto-Earth via the doubling (orange arrow) of two ∼ 0.3M_⊕ embryos. The proto-Earth later collides (blue arrow) with the remaining embryo (i.e. Theia) to form the final Earth-Moon system. Bottom: Time evolution (yellow curve) of the volume gas density ρdisk of the solar nebula beginning from a full minimum-mass solar nebula density ρ_MMSN, as parametrized by the parameter fdep of Equation (8). Key values of fdep = 10⁻² and f_dep = 10⁻⁴ setting the boundaries between the gas-rich, gas-poor and dispersed stages of the disk are shown with dash horizontal lines. — astro-ph.EP

Noble gases are powerful probes of the Earth’s early history, as they are chemically inert. Neon isotopic ratios in deep mantle plumes suggest that nebular gases were incorporated into the Earth’s interior.

This evidence implies the Earth’s formation began when there was still gas around, with Earth embryos accreting primordial gas and a fraction of that gas dissolved into molten magma.

In this work, we examine these implications, simulating the growth of primordial envelopes using modern gas accretion schemes, and computing the dissolution of nebular Ne into magma oceans following chemical equilibrium.

We find that the embryo mass that reproduces the deep mantle concentration of primordial Ne is tightly constrained to ∼0.3M_⊕, within a solar nebula depleted by ≥100× in gas density. Embryos of smaller masses cannot accrete enough gas to allow the mantle to reach the melting temperature of basalt.

Embryos of larger masses accrete way too much gas, producing excessive Ne concentrations in the deep mantle. Based on our calculations, we suggest that the Earth’s formation began with the assembly of ∼0.3M_⊕ embryos during the dispersal of the solar nebula. Light noble gases (He, Ne) in the deep mantle reflect the primordial gas accretion history of the Earth, while heavy noble gases (Ar, Kr, Xe) probe early solid accretion processes.

Our results are consistent with the final assembly of the Earth through at least two giant impacts after the dispersal of the nebula.

Vincent Savignac, Eve J. Lee

Comments: 27 pages, 9 figures, 1 table. Submitted to the Planetary Science Journal (PSJ)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2511.16859 [astro-ph.EP] (or arXiv:2511.16859v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2511.16859
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Submission history
From: Vincent Savignac
[v1] Thu, 20 Nov 2025 23:55:57 UTC (816 KB)
https://arxiv.org/abs/2511.16859
Astrobiology,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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