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J.E. Chambers, J.J. Lissauer (NASA Ames Research Center), A. Morbidelli (Observatoire de la Cote d'Azur)
Lunar samples from the Apollo and Luna missions show a cluster of impact melt ages around 3.9 Gyr ago, and many of the large impact basins appear to have formed at this time. In contrast, the younger lunar maria contain few impact craters, and no large basins have formed within the last 3.8 Gyr. These data suggest that either (a) 3.8 Gyr ago marked the end of a steadily declining impact rate, or (b) there was a ``spike'' in the cratering rate at this time, with relatively quiet periods before and after.
Dynamical models for a spike in the cratering rate are hard to devise since the impacting material must be stored in a safe place for 600 Myr without causing a high impact rate in the meantime. Models proposed to date include the (unlikely) collisional disruption of a main-belt asteroid larger than Ceres, and the rapid accretion of Uranus and Neptune 600 Myr after the formation of the Solar System.
Here we will describe a new model in which the Solar System originally contained a fifth terrestrial planet (``Planet V'') with an orbit between Mars and the asteroid belt. Initially, Planet V had a stable orbit, but 600 Myr of perturbations from the other planets drove it onto an unstable orbit that crossed the asteroid belt. Subsequently, close encounters with Planet V scattered many asteroids into the inner Solar System via resonances, temporarily enhancing the flux of impactors on the Moon until Planet V was itself removed by a resonance.
We will present results of N-body integrations examining this scenario, placing constraints on the possible mass and orbit of Planet V, and estimating the resulting flux of lunar impactors before and during the cratering spike.