Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p13d1689s&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P13D-1689
Physics
[6250] Planetary Sciences: Solar System Objects / Moon
Scientific paper
We perform numerical simulations of the formation of the Moon from an impact-generated disk, with a new hybrid model: material within the Roche-limit is represented by a fluid inner disk with a vapor atmosphere, and we track outer solid bodies with an N-body code. We include: (1) viscous spreading of the inner disk, with either an instability driven viscosity (Ward & Cameron 1978), or a radiation-limited viscosity (Thompson & Stevenson 1988); (2) accretion of moonlets when material crosses the Roche limit; (3) tidal accretion criteria (Canup & Esposito 1995) to determine whether collisions rebound or merge; and (4) interactions of orbiting bodies with the inner disk at 0-th order resonances. While pure N-body simulations show accretion timescales of less than a year (e.g., Ida et al. 1997), the slow spreading of the inner disk due to its radiation-limited viscosity can delay the final accretion of the Moon by up to hundreds of years. Such long timescales might allow the disk to compositionally equilibrate with the Earth (Pahlevan & Stevenson 2007), providing an explanation for, e.g., the identical O-isotope compositions of the Earth and Moon. Our typical simulation shows a three-step accretion process: (1) bodies outside the Roche limit collide, accrete and scatter until only a few massive objects remain; (2) the inner disk is confined below the Roche limit by the outer bodies, which in turn recede away, eventually allowing the inner disk to spread outward; and (3) as the inner disk reaches the Roche limit, new moonlets are spawned and collide with the outer bodies. This process continues until the inner disk is depleted (see Figure). Resonant confinement of the inner disk is very efficient, so that most of its mass is lost onto the planet: a ~2 lunar mass disk with 70% of the mass initially in the inner disk leads to the formation of a 0.8 lunar mass Moon, with ~25% of its mass originating from the inner disk, where material may have compositionally equilibrated with the Earth. We will discuss the influence of the disk's initial configuration on the properties of the final Moon, as well as processes that could allow a larger fraction of the Moon's final mass to be derived from the inner disk.
Canup Robin M.
Salmon Joseph
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