The dynamical evolution of lunar impact ejecta

Abstract

The increasing numbers of known lunar meteorites make it clear that the delivery of lunar impact ejecta to the Earth is an occurrence much more common than previously thought. To better understand the time scales of the delivery mechanism and to better constrain the launch circumstances, we have conducted a series of numerical simulations of the dynamical evolution of material that is launched off the lunar surface during 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. During 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 material that strikes the Earth or Moon in this stage is a strong function of the initial ejection velocity. The second stage of our simulation follows the swarm of escaping particles during their subsequent evolution in the terrestrial planet region. During this stage we include the gravitational effects of all the planets out to Saturn; the particles, although not interacting with each other, are evolved using a full N-body treatment for up to 10 million years. Although differing in some details, 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 large fraction of the most slowly ejected material returns in less than 10 kyr. As the particles continue to scatter off the gravitational field of the Earth, their eccentricities and inclinations rise while their 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, distributed roughly uniformly throughout the inner solar system, with their numbers slowly declining due to continued accretion by the planets and by being driven to a Sun-grazing state. We compare the age spectrum of the simulated particles that return to reimpact the Earth with the available data from the lunar meteorites. We conclude that the velocity distribution (in number versus launch speed) of the escaping lunar crater ejecta must be steep enough that few particles are launched with speeds greater than 3.0 km/sec. We also show that the returning objects are delivered uniformly over the surface of the Earth. © 1995 Academic Press, Inc.