Accretion in the Roche zone: Implications for planetary ring systems and the origin of the Moon

Traditional simulations of accretional growth developed to model the formation of the planets utilize two-body approximations to describe encounters between orbiting bodies. Such approximations break down as orbits approach the classical Roche limit where tidal influences become significant and a three-body approach is required. I have developed a tidal accretion model and identified a dynamical transitional regime surrounding the Roche radius. In the "Roche zone," collisional growth has a unique character: only bodies which differ greatly in mass can remain gravitationally bound, as like-sized bodies are pulled-apart by the differential gravity of the planet. Tidal forces have influenced most circumplanetary systems, as the Roche zone is the locale of planetary rings, inner satellites, and the likely birth place of Earth's Moon.;The co-existence of rings and small satellites around all of the giant planets challenges the premise of the Roche limit as a sharp boundary between accreting and non-accreting regions. Simulations of accretion in the Roche zone show that debris distributions evolve into bi-modal populations, consisting of a swarm of tiny bodies and a small number of moonlets. Tidally modified accretion offers a natural explanation for the formation of systems of co-existing rings and ringmoons.;Tidal accretion also likely influenced lunar formation. The favored theory of lunar origin proposes that the Moon accreted from a debris disk formed when a large body collided with Earth. Simulations of the "Giant-Impact" predict formation of a protolunar disk centered around the Roche limit. Past works propose that multiple moonlets which accreted in the protolunar disk evolved into crossing orbits due to tidal orbital evolution, allowing for their subsequent accumulation into a single Moon. Orbit crossing requires that the innermost moonlet which formed in the disk be the most massive. This requirement, together with modeling of tidal accretion, constrains initial disk conditions which could yield a single moon. The simplest scenario is a disk with a lunar mass of material outside the Roche zone. This corresponds most closely to simulations of impacts with twice the angular momentum of the current Earth/Moon system by a body with twice the mass of Mars.