Dynamical Origin Of Theia, The Last Giant Impactor On Earth

Cosmochemical studies have proposed that Earth accreted roughly 5-10% of its mass from carbonaceous (CC) material, with a large fraction delivered late via its final impactor, Theia (the Moon-forming impactor). ‘

Here, we evaluate this idea using dynamical simulations of terrestrial planet formation, starting from a standard setup with a population of planetary embryos and planetesimals laid out in a ring centered between Venus and Earth’s orbits, and also including a population of CC planetesimals and planetary embryos scattered inward by Jupiter.

We find that this scenario can match a large number of constraints, including i) the terrestrial planets’ masses and orbits; ii) the CC mass fraction of Earth; iii) the much lower CC mass fraction of Mars, as long as Mars only accreted CC planetesimals (but no CC embryos); iv) the timing of the last giant (Moon-forming) impact; and v) a late accretion phase dominated by non-carbonaceous (NC) bodies.

For this scenario to work, the total mass in scattered CC objects must have been ~ 0.2 – 0.3 M⊕ , with an embryo-to-planetesimal mass ratio of at least 8, and CC embryos in the ~ 0.01 – 0.05 M⊕ mass range.

In that case, our simulations show there are roughly 50-50 odds of Earth’s last giant impactor (Theia) having been a carbonaceous object – either a pure CC embryo or an NC embryo that previously accreted a CC embryo. Our simulations thus provide dynamical validation of cosmochemical studies.

Duarte Branco, Sean N. Raymond, Pedro Machado

Comments: 20 pages, 11 figures, to be published in Icarus
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2507.01826 [astro-ph.EP] (or arXiv:2507.01826v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2507.01826
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Submission history
From: Duarte Branco
[v1] Wed, 2 Jul 2025 15:41:34 UTC (2,574 KB)
https://arxiv.org/abs/2507.01826

Astrobiology,