THE COOL EARLY EARTH, THE FACE OF THE MOON, AND THE EVOLUTION OF A HABITABLE PLANET

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; Valley, 2005, Sci. Am., 293, p.58).

Casual observation of the face of the Moon reveals that the three largest circular maria, Imbrium, Serenitatis, and Crisium, are located on a straight line and decrease in diameter systematically from west to east. These maria are part of a great-circle pattern of circular maria that was first noted in a publication by a German astronomer (Franz, 1912, Der Mond, 2nd ed.). The Soviets took the first photos of the lunar backside and noted that this great-circle pattern continued onto the lunar backside (Lipskii, 1965, Sky and Telescope, 30, p.338). Malcuit et al (1975, The Moon, 12, p.55) presented a set of numerical simulations showing that this pattern could be generated by way of tidal disruption of the lunar magma ocean during a close gravitational encounter that could be associated with gravitational capture of the Moon at about 3.9 Ga. I have recently completed a sequence of lunar map-view diagrams showing in detail how a tidal disruption sequence of events could yield a great-circle pattern of large circular maria. Thus, this major pattern of the face (and backside) of the Moon could be a major signature related to its origin as a captured planetoid. This model of capture is very compatible with the Cool Early Earth Model bacause the Earth would be moonless until the capture episode at about 3.9 Ga. (Note that the currently very popular Giant Impact Model for lunar origin appears to be incompatible with the Cool Early Earth Model.) Computer simulations of gravitational capture show that the lunar body must absorb over 90% of the energy for capture and that the Earth, although experiencing high rock and ocean tides during the era following capture, would remain cool enough to permit ocean water on the surface. Is it possible, then, that the development of our habitable planet is the result of a "celestial accident" of gravitational capture which may be registered in the form of a Great-Circle Pattern of large circular maria?