ORIGIN OF THE MOON: THE GIANT IMPACT

The startling idea that the Moon may have been born in a gigantic impact between the proto-Earth and a Mars-size protoplanet was first proposed just a few years before the Planetary Geology section of the GSA was founded. This idea grew out of the confusion that arose when the three classical theories of the Moon's origin, fission, capture and co-accretion, all failed in major ways when confronted with the data returned from the Apollo missions. The current theory coalesced out of three lines of evidence: Ringwood, in 1972, suggested that the chemistry of the Moon indicated that it could have been formed from the Earth's mantle, if that mantle had first vaporized, then condensed in vacuum. Hartmann and Davis, in 1975, observed that very large impacts might be able to eject large masses of material into near-orbital trajectories. Cameron and Ward, in 1976, proposed that the large angular momentum of the Earth-Moon system could have originated from the oblique impact of a Mars-size body. These three strands crystallized into the current paradigm at a meeting on the Origin of the Moon 1985 and a subsequent LPSC conference in 1986.

The years subsequent to the founding of the Giant Impact idea have seen the it grow in depth and precision. The Earth is now believed to be 30 to 50 Myr older than the Moon, as required by the theory. Computer simulations of the impact event have achieved greater accuracy and shown more detail, thanks to the continuing growth in computer power and improvement of ancillary fields, such as equations of state of vaporized rock material. The realization of the importance of large impacts late in the growth of all the terrestrial planets has led to new insights into how the planets, their cores and mantles achieved their present form. The inevitable formation of magma oceans from huge impacts on the growing planets has brought new understanding to such apparently unrelated fields as the siderophile element abundances of planetary mantles.

Although great progress has been made, there is still more to do. Application of this idea to other satellite systems, such as the Pluto-Charon double planet are just beginning to yield fruit. Extra-solar planetary system may show signs of such giant impacts. Hopefully, the next few decades will see this startling idea continue to produce new insights into how our solar system evolved.