IGPP Seminar Series

The Late Heavy Bombardment (LHB) of the Moon and Tidal Evolution of its Orbit

by Paul Davis
Department of Earth and Space Science


Melt breccia samples returned from the Apollo mission have dates that suggest that the impacts that formed major basins on the Moon (e.g., Nectaris, Serenitatis, Imbrium, Orientale) occurred between 3.8 and 4.0 Ga i.e., about 0.6 Gy after Lunar formation. Three models have been proposed to explain the LHB: (1) The period marked the end of large-scale impacts associated with planetary formation. (2) It corresponded to a spike in impacts associated with major reorientation of the solar system (the Nice model), when the orbits Jupiter and Saturn became resonant, causing the orbits of Uranus and Neptune to become unstable and grow, scattering cometary and asteroidal fragments into Earth-Moon crossing orbits. (3) It was a local phenomenon in which the last of a series of moonlets formed during Earth accretion was swept up by tidal regression of a large Moon that had been formed near the Earth by a giant impact. While there is no smoking gun for any of these scenarios I will discuss (3). Numerical calculations show that tidal regression of a large inner Moon sequentially traps exterior smaller moonlets into 2:1 resonance. Resonant trapping rapidly increases the eccentricity of their orbits causing them to become Moon-crossing. If the orbital radii of the moonlets had a Bode's law distribution, for the last collision to take place at 0.6 Gy, the Moon would have been at ~40 RE. One of the implications is that the associated LHB impacts would have significantly less relative velocity than those derived from asteroidal or cometary distances associated with (1) or (2). This may explain the low content of vapor condensate in the Lunar breccias. The tidal evolution from ~40 RE at 0.6 Gy is consistent with rhythmite data but requires a lower tidal friction than at present.
Tuesday, 28 April 2009
3853 Slichter Hall
Refreshments at 3:45 PM
Lecture at 4:00 PM