Flaws in the giant impact hypothesis of lunar origin

The currently popular "giant impact hypothesis" attempts to explain both the origin of the Moon and the high angular momentum density of the earth-moon system in terms of a single event in which the growing Earth was struck by a martian-sized planetesimal. This hypothesis is implausible in its present form because: (1) it possesses a low intrinsic dynamical probability; (2) the collision would have melted and differentiated the Earth causing geochemical signatures quite different to those actually observed; and (3) the collision mechanics cause the Moon to be derived mainly from the mantle of the impactor, whereas geochemical evidence strongly indicates derivation of protolunar material mainly from the terrestrial mantle. These problems can be resolved by requiring that a giant impact occurring after the Earth had accreted to ∼ 70% of its present size was responsible for the high angular momentum density of the Earth-Moon system, but did not itself produce the Moon. Collisions by much smaller (e.g. 0.001-0.01 M _E), high-velocity planetesimals at a very late stage of accretion were probably responsible for ejecting protolunar material from the Earth's mantle without causing gross melting and differentiation of the Earth.