THE COOL EARLY EARTH AND THE TIDAL CAPTURE MODEL: THERMAL AND TECTONIC EFFECTS ON EARTH AND MOON

There is mounting evidence from zircon crystals that the primitive earth had some granitic crust and was cool enough to have ocean water on the surface from 4.2 Ga or earlier (Valley et al., 2002, Geology, 30, p. 351: Cavosie et al., 2005, EPSL, 235, p. 663). A major question is whether or not the moon was involved in this early history of planet earth. The Giant Impact Model (Canup, 2004, Icarus, 168, p. 433) appears to be incompatible with the concept of a Cool Early Earth because of the disruptive effects of earth tides raised by a lunar-sized body in a small circular orbit throughout this era. In contrast, the Tidal Capture Model (Malcuit et al., 1992, Proc. Vol., 3rd Archean Symp., p. 223), which features capture into a highly elliptical orbit at about 3.9 Ga, is very compatible with the Cool Early Earth Model because the earth would be moonless for the first 650 Ma of earth history. The purpose of this paper is to present information on the thermal and tectonic effects of a lunar capture episode on the primitive earth and moon.

We have done a series of co-planar three-body numerical simulations of capture of a lunar-sized planetoid by an earth-mass planet using a fourth-order Runge-Kutta integration procedure described in Malcuit et al. (1992). The planet is in a circular orbit at 1.0 AU and the planetoid is in a slightly smaller heliocentric orbit (1.5% eccentricity and semi-major axis of 0.972 AU). The important body parameters for the interacting bodies are the displacement Love number (h) and the specific dissipation factor (Q) for each body. Using reasonable h and Q values for the planet and planetoid, we find that well over 90% of the energy for capture must be dissipated in the planetoid. The equilibrium rock tides for the capture encounter are about 20 km for the planet and over 300 km for the planetoid. Thus during any capture scenario, the captured planetoid is severely heated and deformed while the planet is only mildly thermally effected and tidally disturbed only in the equatorial zone.

Crustal zircon crystals could be completely destroyed in the equatorial zone by deep subduction but there would be a transition in a poleward direction to where very little subduction occurs. At some intermediate latitude, subduction would be shallow enough for the zircon crystals to survive and these zircons could record a Cool Early Earth.