WAS THE MOON INVOLVED IN THE FIRST 650 MILLION YEARS OF EARTH HISTORY?: TWO CONTRASTING SCENARIOS

Zircon crystals yielding dates of crystallization as old as 4.4 Ga are the main documents of early earth history Cavosie et al., 2005, EPSL, 235, p.663). The oxygen isotope ratios recorded in these zircons suggest that many of them were formed in a crustal complex with a granitic component that was cool enough to permit ocean water on the surface (Valley et al., 2002, Geology, 30, p.351). The main question explored here is whether or not our large satellite, the moon, was involved with the earth during this era from 4.4 to 3.85 Ga.

The Giant Impact Model (GIM) (Canup, 2004, Icarus, 168, p.433) features a mars-mass body impacting the primitive earth. The vapor cloud surrounding the earth condenses and forms the moon just beyond 3 earth radii. An orbital evolution scenario for the GIM is presented by Ross and Schubert (1989, JGR, 94, p.9533) in which the moon recedes from about 5 earth radii (e rot=5.3 hr/day) to 40 earth radii (e rot=12.6 hr/day) in 100 to 200 Ma. The equilibrium tidal amplitude would gradually decrease from about 1 km at 4 earth radii to about 1 m at 40 earth radii. A major question is whether or not a stable crustal complex could form under continuous, and gradually decreasing, earth tidal action.

The Tidal Capture Model (TCM) (Malcuit et al., 1992, Proc. Vol., 3rd Archean Symp., p.223) features capture of a lunar-mass body from a near earth-like orbit at about 3.85 Ga. In this model the earth is moonless from the time of formation to the time of capture. A moonless earth, with occasional solid body impacts, would yield ideal conditions for the development of an early earth with a crustal complex containing a zircon-bearing granitic component. In the TCM the rotation rate of the primitive earth is about 10 hr/day from the time of formation until the time of capture. Most of the high earth tides and associated crustal disruption associated with capture are concentrated in the equatorial zone of the planet and the polar regions are sheltered from this rigorous tidal activity. Since most of the energy dissipation for lunar capture (over 90%) must be dissipated in the moon, the surface temperature of earth should be changed very little during the capture and subsequent orbit circularization and ocean water could have a continuous presence on the surface of the planet during this era.