Moonfalls: Collisions between the Earth and its past moons

Status Report From: e-Print archive
Posted: Wednesday, May 2, 2018

Uri Malamud, Hagai B. Perets, Christoph Schafer, Christoph Burger
(Submitted on 30 Apr 2018)

During the last stages of the terrestrial planet formation, planets grow mainly through giant-impacts with large planetary embryos. The Earth's Moon was suggested to form through one of these impacts. However, since the proto-Earth has experienced many giant-impacts, several moons are naturally expected to form through a sequence of multiple (including smaller scale) impacts. Each impact potentially forms a sub-Lunar mass moonlet that interacts gravitationally with the proto-Earth and possibly with previously-formed moonlets. Such interactions result in either moonlet-moonlet mergers, moonlet ejections or infall of moonlets on the Earth. The latter possibility, leading to low-velocity moonlet-Earth collisions is explored here for the first time. We make use of SPH simulations and consider a range of moonlet masses, collision impact-angles and initial proto-Earth rotation rates. We find that grazing/tidal-collisions are the most frequent and produce comparable fractions of accreted-material and debris. The latter typically clump in smaller moonlets that can potentially later interact with other moonlets. Other collision geometries are more rare. Head-on collisions do not produce much debris and are effectively perfect mergers. Intermediate impact angles result in debris mass-fractions in the range of 2-25% where most of the material is unbound. Retrograde collisions produce more debris than prograde collisions, whose fractions depend on the proto-Earth initial rotation rate. Moonfalls can slightly change the rotation-rate of the proto-Earth. Accreted moonfall material is highly localized, potentially explaining the isotopic heterogeneities in highly siderophile elements in terrestrial rocks, and possibly forming primordial super-continent topographic features. Our results can be used for simple scaling laws and applied to n-body studies of the formation of the Earth and Moon.

Subjects:    Earth and Planetary Astrophysics (astro-ph.EP)
Cite as:    arXiv:1805.00019 [astro-ph.EP] (or arXiv:1805.00019v1 [astro-ph.EP] for this version)
Submission history
From: Uri Malamud
[v1] Mon, 30 Apr 2018 18:00:01 GMT (687kb)

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