A GRAVITATIONAL CAPTURE MODEL FOR THE ORIGIN OF THE EARTH-MOON SYSTEM: IS IT COMPATIBLE WITH THE COOL EARLY EARTH MODEL?

Valley et al. (2002, Geology, 30:351) suggest that a constant range of oxygen isotope values in Hadean and Archean-age zircons implies somewhat temperate conditions on Earth during that time. They question whether the Giant-Impact Model (GIM) for the origin of the Earth-Moon system is compatible with this information because the putative earth-shattering impact which results in for formation of the Moon must happen and the Earth must cool to a condition where oceans can form before 4.45 Ga. They then suggest that maybe a planetoid capture model should be considered for the origin of the Earth-Moon system.

A Gravitational Capture Model (GCM) (Malcuit et al., 1992, Proc., 3^rdIAS, p.223; Malcuit, 2015, Springer, Ch. 4) appears to be compatible with the Cool Early Earth Model. The capture occurs at about 3.95 Ga. To explain the volatile-poor nature of lunar rocks, the suggested place of origin of the Moon, and sibling planetoids, is inside the orbit of Mercury. Such a place of origin is compatible the X-Wind Model for the origin of calcium-aluminum inclusions (CIAs) and the composition of the Moon is very similar to the composition of a composite CAI (Gast, 1972, Moon, 5:121; Anderson, 1973, Moon, 8:33).

In the GCM the Earth is moonless from the end of the accretion process until the sharply defined capture event at about 3.95 Ga. The Earth begins to develop a geochemically enriched crust with a felsic component as early as 4.45 Ga. During capture the planetoid undergoes a “major thermal episode” featuring remelting, via tidal energy dissipation, of a large portion of the magma ocean zone of the lunar body. In contrast, Earth absorbs only a few percent of the energy for capture and remains “cool” during the capture episode and the subsequent geocentric orbit circularization era.

The circularization sequence to 40% eccentricity is estimated to be ~300 Ma. During this time subduction of surface rocks on Earth is facilitated by a combination of mantle convection and unidirectional rock tidal action. The scenario for surface and atmospheric conditions on Earth presented by Van Kranendonk (2014, GSA-SP 504, p.105) is compatible with the GCM if the GIM episode is eliminated and the Late Heavy Bombardment episode is replaced with a Gravitational Capture Scenario which features limited tidal disruption of the lunar body.