A PROGRADE PLANETOID CAPTURE EPISODE ABOUT 3.95 GA AGO: IS THIS MODEL COMPATIBLE WITH THE INFORMATION FROM HADEAN- AND ARCHEAN-AGE DETRITAL ZIRCON CRYSTALS?

The discovery and subsequent study of detrital zircon crystals from the Jack Hills Formation of Western Australia is recasting our concept of the Primitive Earth. It has changed from “hell-like” (Cloud, 1972, Am. Jour. Sci, 272, p. 537) to “A Cool Early Earth” (Valley et al., 2002, Geology, 30, p. 351). The Giant Impact Model for the origin of the Moon and the Earth-Moon System is more consistent with the “hell-like” model. The purpose of this paper is to present a Tidal Capture Model that is compatible with the concept of “A Cool Early Earth”.

This version of a Tidal Capture Model (Malcuit et al., 1992, Proc. Vol., 3^rd Archean Symp., p. 223) features capture of a lunar-sized body at about 3.95 Ga. Before the capture episode there was NO MOON. Assumptions for the model are that the earth accreted from meteoritic debris, underwent core formation and magma ocean development early in earth history (Harrison, 2009, Ann. Rev. Earth Planet. Sci., p. 479). A chill crust developed as the magma ocean crystallized. Water vapor condensed eventually to form primitive oceans as the earth continued to cool. Occasional impacts would disturb this scene locally.

Tidal capture of a lunar-mass body would cause significant and predictable disturbances to the surface of the planet but well over 90% of the energy for capture must be absorbed by the body being captured (i.e., the Moon). A calculated capture scenario suggests that the equilibrium rock tides as well as the tidally generated thermal energy decrease rapidly after capture. For capture the rock tides on earth are up to 18 km; within 1 Ka after capture, the tidal amplitudes are below 3 km; after 25 Ka, the tidal amplitudes are below 1 km. With radioactive element heat production at about 3E21 joules/yr at 3.95 Ga, the surface tidal energy flux would be down to about 10x R-A heat production after 5 Ka and about equal to R-A heat production after about 30 Ka. The main effect of the high rock tidal amplitudes in the equatorial zone is the establishment of major, unidirectional convection cells which would recycle a large portion of the original Hadean-age crust back into the earth’s mantle via a process akin to plate tectonics. Since most of the severe tidal action would be in the equatorial zone, Hadean-age crustal complexes in the polar zones could survive and these then could be source regions for the Hadean-age zircons.