The Case for a Geocentric rather than Heliocentric Origin of the Late Stage Heavy Bombardment (LHB) of the Moon and Tidal Evolution of its Orbit

Details The Case for a Geocentric rather than Heliocentric Origin of the Late Stage Heavy Bombardment (LHB) of the Moon and Tidal Evolution of its Orbit The Case for a Geocentric rather than Heliocentric Origin of the Late Stage Heavy Bombardment (LHB) of the Moon and Tidal Evolution of its Orbit

Dec 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p23c1288d&link_type=abstract

American Geophysical Union, Fall Meeting 2009, abstract #P23C-1288

Physics

[6250] Planetary Sciences: Solar System Objects / Moon

Scientific paper

Melt breccia samples returned from the Apollo mission have dates that suggest that the impacts that formed major basins on the Moon occurred between 3.8 and 4.0 Ga i.e., about 0.6 G years after Lunar formation. Three models have been proposed to explain the LHB. Heliocentric models including (1) The period marked the end of large-scale impacts associated with planetary formation and (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, and a geocentric model (3) It was due to collision with the last of a series of moonlets formed during Earth accretion which were 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 we will discuss a possible scenario for (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 resonance or Bode's law-type distribution, for the last collision to take place at 0.6 Gy, the Moon would have been at ~40 RE when it took place. 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 requires a lower tidal friction than at present, but this has been evident for many years from tidal rhythmite data.

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