Wednesday’s Book Review: “The Big Splat: Or How Our Moon Came to Be”


the-big-splatThe Big Splat: Or How Our Moon Came to Be. By Dana Mackenzie. Hoboken, NJ: John Wiley & Sons, 2003.

If there is one dramatic moment—as opposed to myriad important but mundane events—in the history of lunar science it is the 1984 conference in Kona, Hawaii, in which scientists around the world presented papers on the sole topic of how the Moon originated. What made this conference so remarkable, however, was that a new consensus on the subject emerged through this process of presentation and discussion. Usually, positions are well known prior to any scientific meeting and few scientists change their minds right away. As the author of this outstanding popular history phrased it, “other specialists have to go home and process the new information. Old theories have to be sifted through and reappraised. More papers come out in favor of the new hypothesis, and others come out against it. Eventually, sometimes after many years, a new consensus emerges” (p. 167). Not so at Kona. The consensus on the origins of the Moon that came about there has enjoyed remarkable exceptional staying power since.

The Big Splat: Or How Our Moon Came to Be by Dana Mackenzie is a concise and exceptionally readable account of how a significant but divisive scientific question came to be settled through the investigation of the Moon made possible by sending human and robotic missions there in the 1960s and 1970s. The Kona conference established a consensus in favor of a theory of origins known as the “big whack,” or “big splat.” Two scientists working independently, William Hartmann and Alastair Cameron, first advanced the theory in 1974 that the Moon had been formed by debris from a massive collision with the Earth about 4.6 billion years ago. This theory was predicated on the study of lunar rock and soil samples returned from the Moon by the Apollo astronauts, and over the course of the next decade further analysis allowed scientists to resolve most of the questions plaguing other theories of lunar origin by applying the “big splat” hypothesis.

So contentious had the question of lunar origins been prior to the Apollo program, as Mackenzie shows, that many scientists just threw up their hands in frustration at ever being able to develop a reasonable hypothesis. Confusion ruled among scientists about the Moon’s origin as competing schools battled among themselves for dominance of their particular viewpoint in the textbooks. Indeed, some expressed concern that determining the Moon’s origins should be the single most significant scientific objective of Project Apollo, thinking of it as a hopeless objective.

Their concern was legitimate based on what had gone before. Prior to the Apollo missions the origin of the Moon had been a subject of considerable scientific debate and careers had risen and fallen on championing one or another theory. Prior to the 1960s there had been three principal theories:

  1. Co-accretion—a theory which asserted that the Moon and the Earth formed at the same time from the Solar Nebula.
  2. Fission—a theory that asserted that the Moon split off from the Earth.
  3. Capture—a theory that held that the Moon formed elsewhere and was subsequently drawn into orbit around the Earth.

The data supporting these various theories had been developed to an amazingly fine point over time but none of these theories actually explained enough open questions to convince a majority of planetary scientists.

As Mackenzie recounts in The Big Splat, the new and detailed information from the Moon rocks pointed toward an impact theory–which suggested that the Earth had collided with a very large object (as big as Mars and named after the fact “Theia”)–and that the Moon formed from the ejected material. This proved to be a theory that fit the fact that although the Earth has a large iron core the Moon does not, because the debris blown out of both the Earth and the impactor would have come from iron-depleted, rocky mantles. Also lending credence to this theory, although the Earth has a mean density of 5.5 grams/cubic centimeter the Moon’s density is only 3.3 g/cc, which would be the case were it to lack iron, as it does. The Moon has exactly the same oxygen isotope composition as the Earth, whereas Mars rocks and meteorites from other parts of the Solar System have different oxygen isotope compositions. While there were some details to this theory that have yet to be worked out, the impact theory came out as the consensus at the Kona conference and is now widely accepted. In the end, further research will be required but all evidence to date seems to fit into the confines of this giant impact theory.

The Big Splat also offers a wonderful affirmation of the scientific method as a self-correcting system of knowledge. Clinging to the marketplace of ideas, it insists that practitioners explicate their theories in a manner that is rigorous, peer-reviewed, and replicable. In all cases, the mode of science is to seek to disprove or at least modify these new theories. Doing so helps to self-correct the state of knowledge, and there is no higher calling in science. Of course, this road to scientific understanding is rugged and winding, and The Big Splat states this well in the context of lunar origins. What we learn is that scientific understanding is infinitely more complex, convoluted, interesting, and significant than most popular conceptions allow. Dana Mackenzie is to be commended for showing this process in detail and in so doing restates the positive nature of the process. Apply this case study to the major scientific debates of the present, of which there are many, and it is apparent that there are few easy answers.

Dana Mackenzie has written as fascinating detective story in which scientists act as Sherlock Holmes deciphering discreet but imperfect clues to piece together the set of incidents that led to the formation of the Moon. The Big Splat is a wonderfully written science story. It will be of interest to historians, non-specialist readers, and students of all types.

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