THE FIRST 800 MILLION YEARS OF EARTH HISTORY: CAN WE NEGLECT THE TIDAL EFFECTS OF THE MOON?

Zircon crystals yielding dates of crystallization as old as 4.4 Ga are the main documents for the first 500 Ma of earth history (Valley et al., 2002, Geology, v. 30, p. 351). The purpose of this paper is to assess the possible tidal and thermal influence of our large satellite on the primitive earth and its crustal complex. The two models considered here are the Giant Impact Model (GIM) (Canup, 2004, Icarus, v. 168, p. 433) and the Tidal Capture Model (TCM) (Malcuit et al. 1992, Proc. Vol., 3rd Archean Symp., p. 223). Both models have some problems but both appear to be physically possible.

The GIM features a mars-mass body obliquely impacting the earth with an orientation that imparts a rapid prograde rotation to the earth and forms a debris cloud that eventually forms the body of the moon in prograde orbit just beyond 3 earth radii. The moon then recedes rapidly from the earth via rock tidal action as the earth's rotation decreases rapidly. An orbital evolution scenario is presented in Ross and Schubert (1989, JGR, v. 94, p. 9533) but the tidal amplitude and tidally driven convection cell consequences have not been analyzed. In this orbital scenario the moon retreats 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. Tidally driven convection would be active for several 100 Ma.

The TCM features capture of a lunar-mass body at about 3.85 Ga. For this model the earth is moonless from the time of formation (accretion) to the time of capture. This model yields ideal conditions for the formation of a stable crustal complex with a zircon-bearing granitic component. In this model the rotation rate after accretion is about 10 hr/day and changes very little via weak solar tides and occasional impacts until the gravitational capture episode at about 3.85 Ga. During capture and the subsequent orbit circularization, the crust in the equatorial zone is disrupted and subducted by rock tidal action, but the crust in the polar zones is sheltered from the tidal action (and some could survive).

We return to our original question: Can we neglect the tidal effects of the moon in early earth history? For the Giant Impact Model, NO, they are a very important factor. For the Tidal Capture Model, YES, they can be ignored for the first 650 Ma.