From analyses of lunar crater counts, Canadian impact crater counts, and current meteoritic fluxes, combined with absolute datings keyed to Apollo 11 and Apollo 12 ages for the maria, it is concluded that: The flux of impacts on the Moon has declined steadily from a high value at the earliest observable times. Superimposed on the smooth decline are fluctuations. The observed rate of recent infalls on the Canadian Shield is too high to be consistent with the lunar counts. It is suggested that this may be a result of a major asteroidal collisiom 4 × 108 years ago. The mass distribution of crater-forming planetesimals has been fairly constant throughout geologic time. Probably the present flux of meteoritic particles is largely of asteroid belt origin. The extrapolation of the observed rates of fall of bodies on the Moon backward in time does not give sufficient material to build the Moon. It falls short by several orders of magnitude. This is interpreted to mean that the Moon accreted from some other source of material. The source of the early crater-forming planetesimals does not appear to be the asteroid belt because the collisions there could not have produced enough bodies quickly enough to yield the observed flux of impacts on the Moon very early in its history, and even if there were sufficient asteroid belt bodies existing at the beginning of the solar system, they could not have been ejected quickly enough to reach the Moon at such an early time. The source of early collisions is suggested to be a halo of small to moderate size planetesimals distributed in eccentric orbits around the Sun between Mercury and Saturn. These bodies were probably formed at the same time as the Moon and planets.
- 1 - natural sciences ; 2 - physics & astronomy ; 3 - astronomy & astrophysics
- 1 - sciences appliquees, technologies et medecines ; 2 - sciences exactes et technologie ; 3 - terre, ocean, espace ; 4 - geophysique externe
- 1 - Physical Sciences ; 2 - Earth and Planetary Sciences ; 3 - Space and Planetary Science
- 1 - Physical Sciences ; 2 - Physics and Astronomy ; 3 - Astronomy and Astrophysics