The Big Splat, or How Our Moon Came to Be

  • Dana Mackenzie
John Wiley: 2003. 240 pp. $24.95 (US), $38.95 (Canada), £17.50

Ours is a violent world and always has been. The whole Universe was produced in an unimaginable cataclysm, the Big Bang, about 13.7 billion years ago. Much later, 4.6 billion years ago, a nearby supernova explosion may have triggered the collapse of the protosolar cloud that became our Solar System.

This slim volume, written straightforwardly and engagingly by Dana Mackenzie, a mathematician turned freelance writer, describes how a collision produced Earth and the Moon. The giant-impact hypothesis — the 'Big Splat' of the title — maintains that an object larger than Mars slammed into proto-Earth during the final stages of its accumulation, giving birth to the Moon.

Splat! A giant object crashing into the proto-Earth may have given rise to the Moon. Credit: W. K. HARTMANN

The Big Splat lays out ancient thoughts about the Moon's place in the cosmos, sketches the contributions of the greats of classical physics (Galileo, Kepler, Newton and Laplace), detours into topics such as celestial mechanics and navigation, recalls the Apollo programme, and finally describes the collisional model of the Moon's origin.

This historical tour turns out to be somewhat circular. The first known attempt to explain the Moon's origin occurred in the fifth century bc, when the Greek thinker Anaxagoras, after viewing a meteorite that had been observed falling from the sky, speculated that all celestial objects were glowing 'stone stars' flung off Earth. Apparently he got it right in the case of the Moon, but astronomy textbooks only a generation ago were not so sure. They still listed three scenarios for lunar origin that had been developed in some mathematical detail a century earlier: the reclusive mathematician Edouard Roche contended that the two bodies were siblings, having 'co-accreted' as an orbiting binary; the scholarly George Darwin (son of Charles) promoted the idea that the Moon was our planet's child, having split off when a rotationally distorted primordial Earth became unstable; and later a cantankerous crackpot, T. J. J. See, argued that the Moon formed elsewhere, only to be snared intact by our planet.

These classical hypotheses were still debated vigorously as the space age dawned, even though the flaws of each were well recognized. Co-accretion would yield less angular momentum — the combined 'spin' of the Earth and Moon about one another — than the Earth–Moon system in fact has. Fission would require much more angular momentum than exists now, and there is no plausible explanation for how it could have started off. And the capture of an intact body is ridiculously improbable.

The Nobel Prize-winning chemist Harold Urey believed that the Moon, alone among the terrestrial bodies, was formed cold. To test this hypothesis of the Solar System's formation, Urey used his political influence in 1958 to get NASA's founding mission statement to focus on the origin of the Universe, which was “written plain to our eyes on the surface of the Moon”. So Earth's only satellite became the primary scientific target of the US space programme. The lunar rocks returned by the Apollo and Luna missions showed that the Moon had much lower proportions of iron and volatiles than Earth, but their isotopic signatures had striking similarities to Earth's — as well as occasional differences. As is often the case, no model of origin survived its confrontation with data.

The final third of this book describes the development of the now-preferred scenario for how the Moon came to be. By the mid-1970s the analytical ideas of the Soviet cosmogonist V. S. Safronov about the role of impacts in forming the planets of our Solar System had travelled west, where they were tested and extended numerically by George Wetherill and by the Planetary Science Institute. Bill Hartmann and Don Davis, from the latter group, realized that the final objects to accumulate into the terrestrial planets must have been massive and would have careened through the inner Solar System. They concluded that the Moon could have been born from the final collision of such an object, and argued that the material thrown off proto-Earth would have been from its iron-poor surface layers and that volatiles would have boiled off, explaining the Moon's gross composition.

Independently, Al Cameron and Bill Ward noted that the angular momentum of the entire Earth–Moon system required an impact from at least a Mars-sized projectile, and began to analyse the likely evolution of the flattened, Earth-circling cloud of vaporized material that any giant collision would have generated. The Moon, accumulated from this orbiting debris through much the same processes as the planets themselves, would contain material from both Earth's mantle and the projectile. Despite their plausibility and close match with known facts, these works attracted little attention and languished along with all lunar studies during the early 1980s.

Languished, that is, until a conference in 1984, held in Kona, Hawaii, brought together dynamicists, cosmochemists and geophysicists to consider the Moon's origin. By this time, impacts were accepted as shapers of life, following the proposal of Luis and Walter Alvarez that linked an asteroid collision with the dinosaurs' demise and other mass extinctions at the end of the Cretaceous period. As the conference proceeded, it became clear that a consensus had silently emerged in the various disciplines: each, unaware of the other, favoured a collisional beginning for the Moon.

In the past two decades, there have been increasingly sophisticated simulations of a massive impact and the evolution of the resulting debris disk, and lunar samples and meteorites have been scrutinized. The giant-impact hypothesis does well in explaining the observed physical, thermal and geochemical properties of the Earth–Moon system. Plausible modifications to the original theory may overcome the remaining faults: contemporary models suggest that Earth was not fully grown when it was struck by an even larger projectile than was previously thought.

Besides telling an interesting tale well and elucidating how science progresses, Mackenzie's book emphasizes the fact that impacts have been the primary creative and destructive process throughout the history of the Solar System. Some today wonder whether the final violence that ends civilization will be another such collision — rather than a catastrophe of our own making.