Lunar Pyroclastic Deposits and the Origin of the Moon 1

Debate over the origin and evolution of the Moon, and implications relative to the Earth, has remained lively and productive since the last human exploration of that small planet in 1972. More recently, issues related to the evolution of Mars, Venus and Mercury have received increasing attention from the planetary geology community and the public as many robotic missions have augmented the earlier lunar investigations. A number of generally accepted hypotheses relative to the history of the Moon and the terrestrial planets (e.g., Sputis 1996, Canup and Righter 2000, Taylor 2001, Warren 2003, Palme 2004), however, deserve intense questioning as data from many of the lunar samples suggest alternatives to some of these hypotheses. Further, how much do the known, probable and possible events in lunar history constrain what may have occurred on and in Mars? Or, indeed, do such events constrain what may have happened on and in the Earth? Strong agreement exists that at 4.567 ± 0.001 Gyr (T0) (Carlson and Lugmair 2000, Alexander et al 2001, Taylor 2001, Jacobsen 2003, Amelin et al 2004) a portion of an interstellar molecular cloud collapsed to form the solar nebula (Taylor 2001, Hester et al 2004). A sizable consensus also exists that a few tens of millions of years after T0, the Moon belatedly came into existence as a result of a "giant impact" between the very young Earth and a chondritic asteroid that was 11 to 14 percent the mass of the Earth and gave a combined angular momentum of 110-120 percent of the current Earth-Moon system (Hartmann and Davis 1975, Cameron and Ward 1976, Hartmann 1986, Cameron 2002, Canup 2004, Palme 2004). This angular momentum would have been subject to later tidal dissipation to reach today's still unusually high value. Increasingly detailed hydrodynamic simulations (Canup 2004) of such a hypothetical collision indicates that, if it occurred, the impact angle and velocity of the impactor were around 45o and less than