Accretion Disk's Magnetic Field Controlled The Composition Of The Terrestrial Planets


Density of the solar system bodies. Uncompressed and solid densities are shown for terrestrial planets and chondrites (grey), respectively. Bulk planetary densities are shown for asteroids (blue). For 1 Ceres, its bulk density is a lower limit of its solid density, given its high ice abundance and porosity. The red line shows a fit curve for the planets (ρ = 4, 100r−0.21). Data sources are given in supplementary materials.

Chondrites, the building blocks of the terrestrial planets, have mass and atomic proportions of oxygen, iron, magnesium, and silicon totaling ≥90% and variable Mg/Si (∼25%), Fe/Si (factor of ≥2), and Fe/O (factor of ≥3).

The Earth and terrestrial planets (Mercury, Venus, and Mars) are differentiated into three layers: a metallic core, a silicate shell (mantle and crust), and a volatile envelope of gases, ices, and, for the Earth, liquid water. Each layer has different dominant elements (e.g., increasing Fe content with depth and increasing oxygen content to the surface). What remains an unknown is to what degree did physical processes during nebular disk accretion versus those during post-nebular disk accretion (e.g., impact erosion) influence these final bulk compositions.

Here we predict terrestrial planet compositions and show that their core mass fractions and uncompressed densities correlate with their heliocentric distance, and follow a simple model of the magnetic field strength in the protoplanetary disk. Our model assesses the distribution of iron in terms of increasing oxidation state, aerodynamics, and a decreasing magnetic field strength outward from the Sun, leading to decreasing core size of the terrestrial planets with radial distance. This distribution would enhance habitability in our solar system, and would be equally applicable to exo-planetary systems.

William F. McDonough (1,2), Takashi Yoshizaki (2) ((1) University of Maryland, College Park, (2) Tohoku Univeristy)
Comments: 23 pages, 4 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2009.04311 [astro-ph.EP] (or arXiv:2009.04311v1 [astro-ph.EP] for this version)
Submission history
From: Takashi Yoshizaki
[v1] Wed, 9 Sep 2020 14:08:06 UTC (2,183 KB)
https://arxiv.org/abs/2009.04311
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