THE DYNAMICAL EVOLUTION OF LUNAR IMPACT EJECTA

The increasing numbers of known lunar meteorites make it clear that th e delivery of lunar impact ejecta to the Earth is an occurrence much m ore common than previously thought, To better understand the time scal es of the delivery mechanism and to better constrain the launch circum stances, we have conducted a series of numerical simulations of the dy namical evolution of material that is launched off the lunar surface d uring impact events. Launch velocities were chosen between 2.3 and 3.5 km/sec, since 2.38 km/sec is the formal escape speed from the Moon. D uring the first stage of our study we model the physics as a four-body problem consisting of the Sun, Earth, Moon, and impact fragment, The particle is followed until it impacts the Earth or Moon, or it escapes the Earth-Moon system into heliocentric orbit. The fraction of materi al that strikes the Earth or Moon in this stage is a strong function o f the initial ejection velocity, The second stage of our simulation fo llows the swarm of escaping particles during their subsequent evolutio n in the terrestrial planet region. During this stage we include the g ravitational effects of all the planets out to Saturn; the particles, although not interacting with each other, are evolved using a full N-b ody treatment for up to 10 million years, Although differing in some d etails, our results confirm several previous calculations, which used Monte Carlo methods and showed a rapid ( < 1 Myr) accretion of many of the ejected particles by the Earth. We calculate that about one-third of the ejected material reaches the Earth rapidly; in fact, a very la rge fraction of the most slowly ejected material returns in less than 10 kyr. As the particles continue to scatter off the gravitational fie ld of the Earth, their eccentricities and inclina-tions rise while the ir semimajor axes spread until they begin to cross the orbits of other terrestrial planets, Collisions with Venus become common. After about 1 Myr, the particles then settle into an equilibrium state, distribut ed roughly uniformly throughout the inner solar system, with their num bers slowly declining due to continued accretion by the planets and by being driven to a Sun-grazing state. We compare the age spectrum of t he simulated particles that return to reimpact the Earth with the avai lable data from the lunar meteorites. We conclude that the velocity di stribution (in number versus launch speed) of the escaping lunar crate r ejecta must be steep enough that few particles are launched with spe eds greater than 3.0 km/sec. We also show that the returning objects a re delivered uniformly over the surface of the Earth. (C) 1995 Academi c Press, Inc.