From: arXiv.org e-Print archive
Posted: Friday, January 23, 2015
Julien Salmon, Robin M. Canup
(Submitted on 27 Dec 2014)
Impacts that leave the Earth-Moon system with a large excess in angular momentum have recently been advocated as a means of generating a protolunar disc with a composition that is nearly identical to that of the Earth's mantle. We here investigate the accretion of the Moon from discs generated by such "non-canonical" impacts, which are typically more compact than discs produced by canonical impacts and have a higher fraction of their mass initially located inside the Roche limit.
Our model predicts a similar overall accretional history for both canonical and non-canonical discs, with the Moon forming in three consecutive steps over hundreds of years. However, we find that, to yield a lunar-mass Moon, the more compact non-canonical discs must initially be more massive than implied by prior estimates, and only a few of the discs produced by impact simulations to date appear to meet this condition. Non-canonical impacts require that capture of the Moon into the evection resonance with the Sun reduced the Earth-Moon angular momentum by a factor of 2 or more.
We find that the Moon's semimajor axis at the end of its accretion is approximately 7R, which is comparable to the location of the evection resonance for a post-impact Earth with a 2.5 h rotation period in the absence of a disc. Thus, the dynamics of the Moon's assembly may directly affect its ability to be captured into the resonance.
Comments: 14 pages, 5 figures, 2 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Journal reference: Published in book "Origin of the Moon: Challenges and Prospects" (2014), Ed. David Stevenson and Alex Halliday, The Royal Society Publishing
Cite as: arXiv:1412.8080 [astro-ph.EP]
(or arXiv:1412.8080v1 [astro-ph.EP] for this version)
From: Julien Salmon
[v1] Sat, 27 Dec 2014 21:07:52 GMT (784kb)
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