The Moon is generally believed to have formed from the debris ejected when a body (nicknamed Theia) about the size of Mars struck the partly formed Earth a glancing blow. That cataclysmic event can be considered to have marked the start of geochemical evolution of both Earth and Moon. From a purely mechanical standpoint, it seems almost inevitable that the Moon is made mainly from debris supplied by the offending small planet. Yet Earth and Moon have some profound geochemical similarities, the most remarkable being their now similar blend of oxygen isotopes. Meteorite studies suggest that oxygen isotopes varied widely in the early Solar System, probably differing according to distance from the Sun. That suggests that the Earth-Moon similarity is somewhat odd, unless the impacting planet formed in the same part of space as the Earth itself, i.e. in a very similar orbit. However, that is as mechanically unlikely as the Moon being a chunk of Earth flung off by the impact.
A new explanation for shared oxygen isotopes is based on a model for the collision that involves the vaporisation of most of the Earth and Theia (Pahlevan, K. & Stevenson, D.J. 2007. Equilibration in the aftermath of the lunar-forming giant impact. Earth and Planetary Science Letters. v. 262, p. 438–449). High temperature vapour would have involved sufficient turbulence for the geochemical signatures of both Earth and Theia to have been mixed efficiently. The Moon would then have condensed from a disk of orbiting vapour of this mixed composition, most of the Earth re-accreting in a molten state too. Thus both bodies would have begun their evolution with deep magma oceans. The light-coloured, highland part of the Moon is thought to be a relic of the flotation of plagioclase crystals that floated to the top of its magma ocean as it began to cool; the lunar highlands are made of anorthosite and are at least 4.4Ga old. So far no tangible sign of such relics of early fractionation have appeared in the Earth’s geological record. Pahlevan and Stevenson’s model indicates that only between 100 to 1000 years would have elapsed from impact to appearance of the moon as a tangible body.
Another angle on the mysteries of the Hadean
Geochemists will be celebrating the end of 2007 after a steady growth in knowledge about times before formation of the first real rocks, albeit of a proxy nature. The latest addition stems from the isotopes of the rare-earth element neodymium. Its heaviest isotope 144Nd is a direct product of nucleosynthesis in supernova star explosions The middleweight isotope 143Nd is well-known as the daughter product of the decay of one unstable isotope of a sister element, samarium (147Sm, half-life 1.06 x 105 Ma). The Sm-Nd dating method, based on this decay, has been an important means of dating ancient mafic and ultramafic rocks and examining the geochemistry of their source rocks in the mantle for over 20 years. The lightest isotope is also a daughter of radioactive decay but would have formed from short-lived 146Sm (108 Ma half life). Potentially, 142Nd in old rocks can be used to judge processes in the Hadean mantle as 146Sm would have declined rapidly in the early Solar System – none is detectable nowadays. In meteorites it reveals complexities in the early differentiation of their parental planetesimals, and lunar studies show that too was subject to fractionation. That something odd happened in the early Earth became apparent when it was discovered that modern crust and mantle had more radiogenic 142Nd than the chondritic meteorites thought to have been the building blocks for the Earth. A study of neodymium isotopes in the two largest old chunks of continental crust – the Archaean gneisses of SW Greenland and Western Australia – revealed yet more (Bennett, V.C. et al. 2007. Coupled 142Nd-143Nd isotopic evidence for Hadean mantle dynamics. Science, v. 318, p. 1907-1910). The two blocks are different as regards their neodymium, and this suggests that a fundamental chemical division of the Earth’s mantle took place during the Hadean, which lasted for the next billion years at least. Yet another long-held idea about the Earth’s origin seems condemned to the status of myth. It had been assumed that the early Earth was well-mixed as a result of its accretion from countless planetesimals – it doesn’t really matter if they included different varieties because accretion would have been such a chaotic process. Discovering whether the now-established mantle fractionation resulted during accretion or after a cataclysmic collision with another world formed the Earth-Moon system is set to be the next challenge for students of the Hadean. It will probably be argued that this requires yet more samples to be brought from the Moon…