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2014 ; 372
(2024
): 20130256
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Accretion of the Moon from non-canonical discs
#MMPMID25114307
Salmon J
; Canup RM
Philos Trans A Math Phys Eng Sci
2014[Sep]; 372
(2024
): 20130256
PMID25114307
show ga
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
semi-major 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.
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