Bayesian Inference On The Isotopic Building Blocks Of Mars And Earth

By Keith Cowing
Status Report
September 30, 2023
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Bayesian Inference On The Isotopic Building Blocks Of Mars And Earth
Schematic illustration of transport and mixing between different isotopic reservoirs during growth of Mars and Earth. Mars was born near the inner edge of the main asteroid belt, in between the ℰ and � reservoirs. It started accreted an equal mixture of both planetesimals (A) but progressively accreted more ℰ material as it migrated inwards into the ℰ region by type I migration (B). During those times, carbonaceous asteroids from the outer solar system were scattered inwards by the giant planets, but interaction with nebular gas led to their implantation into the outer main belt. Larger carbonaceous objects were less subjected to gas drag and were able to penetrate in the inner solar system. Earth grew later primarily from ℰ -type embryos (C). Several tens of millions of years after solar system formation, one or several interloper carbonaceous embryos implanted in the inner disk during the first phase were stochastically accreted by the Earth (D). — astro-ph.EP

Isotopic anomalies provide a means of probing the materials responsible for the formation of terrestrial planets. By analyzing new iron isotopic anomaly data from Martian meteorites and drawing insights from published data for O, Ca, Ti, Cr, Fe, Ni, Sr, Zr, Mo, Ru, and Si, we scrutinize potential changes in the isotopic composition of the material accreted by Mars and Earth during their formation.

A Principal Component Analysis of isotopic anomalies in meteorites identifies three main clusters (forming the three parts of the isotopic trichotomy): CI, CC=CM+CO+CV+CR, and NC=EH+EL+H+L+LL.

Our results suggest that Earth is primarily an isotopic mixture of ~92% E, 6 % CI, and <2% COCV and O. Mars, on the other hand, appears to be a mixture of ~65% E, 33% O, and <2% CI and COCV. We establish that Earth’s CI contribution substantially increased during the latter half of its accretion. Mars began accreting a mix of O and E but predominantly accreted E later.
Mars’ changing isotopic makeup during accretion can be explained if it underwent gas-driven type I migration from its origin near the O – E boundary to a location well within the E region during the first few million years of solar system history. Earth’s later increased CI contribution may be attributed to the stochastic impact of an interloper carbonaceous embryo that moved inside the inner solar system region while nebular gas was still present, and subsequently participated in the stage of chaotic growth.

The recent findings of Si isotopic anomalies in enstatite chondrites when compared to terrestrial rocks likely stems from insufficient correction for high-temperature equilibrium isotopic fractionation. With appropriate adjustments for this influence, both the silicate Earth and enstatite chondrites exhibit comparable Si isotopic anomalies, reaffirming a genetic link between them.

Nicolas Dauphas, Timo Hopp, David Nesvorny

Comments: In press, Icarus
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2309.15290 [astro-ph.EP] (or arXiv:2309.15290v1 [astro-ph.EP] for this version)
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Submission history
From: Nicolas Dauphas
[v1] Tue, 26 Sep 2023 22:02:25 UTC (3,531 KB)

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