AMHERST, Mass. – “Everyone knows” our moon formed when a Mars-sized body crashed into the young Earth and knocked off part of its outer mantle. Even grade school kids learn this “giant impact” scenario of our moon’s origin. But wait, perhaps it isn’t so, says Donald Wise, professor emeritus of geosciences at the University of Massachusetts Amherst.
Writing in the January issue of Physics Today, Wise points out that new models proposed in 2012 assume “starting conditions” similar to those in a long-abandoned lunar origin model he and several others put forward in the 1960s. These conditions eliminate objections that once seemed insurmountable and explain how the moon formed directly from Earth’s mantle with no need for a giant impact. It’s time, Wise suggests, that “our quest to answer one of mankind’s oldest questions should expand to include this simpler hypothesis.”
Wise says underpinnings of currently accepted “giant impact” lunar origin models are far shakier than generally recognized because they all fail a major test: No evidence of impact contamination by a non-Earth body has ever been found on the moon. He notes that increasingly precise isotopic and chemical analyses developed over the last 40 years show lunar rocks are identical with Earth’s mantle, in some cases to parts per million.
Nevertheless, he adds, “Even though there is no trace of contamination of lunar rocks by some foreign body of the solar system, the impact model has so permeated our understanding that most people believe the origin of the moon was by giant impact. Until two years ago, no computer simulation could explain the composition of real lunar samples.”
In the early 1960s, he was one of several geologists who proposed a “fission” origin for the moon. It posited that settling of the dense core of the rapidly spinning early Earth caused acceleration beyond stability, so part of the outer mantle spun off to form the moon. Other competing models said the moon was either a minor partner of an original double planet system or a foreign body “captured” by gravity from elsewhere in the solar system. By decade’s end, the capture hypothesis was widely accepted.
During the lead-up to Apollo 11’s 1969 moon landing, Wise was working in NASA’s Lunar Exploration Office helping to plan the first extra-terrestrial field trip and looking forward to rock samples that would help test the capture and fission hypotheses. In a 1969 publication he had just pointed out major problems in the capture hypothesis while answering the main criticism of fission models, that high-speed rotation left far too much unexplained rotational energy and angular momentum in the Earth-moon system.
He hypothesized that year that extreme heat vaporized part of Earth’s mantle into an incandescant atmosphere. Its escape into outer space took the excess angular momentum with it and slowed the system toward observed modern values.
As Wise recalls, Apollo’s moon samples and discovery of the remarkable similarity between those rocks and Earth’s mantle doomed the capture models. “The evidence was clear, that in some way or another the moon was a child of the earth’s mantle,” he notes. “But by that time criticisms of early fission models were so pervasive and generally accepted that the proposed 1969 answer was generally ignored and forgotten.”
Then in 1975, the giant impact hypothesis opened a new era of lunar origin research, Wise says. At last, it was a hypothisis capable of being tested by computer simulations. Increasingly detailed analyses showed Earth’s mantle and the moon’s composition are not only similar but for some isotopes, identical. After a third of a century of failed attempts to simulate those compositions with more and more sophisticated giant-impact simulations, “it began to look like this hypothesis had begun to approach a dead end,” he notes.
Wise points out that just two years ago, yet another era of lunar origin models began when Harvard astrophysicists Cuk and Stewart unveiled a mechanism of tidal transfer of Earth’s angular momentum from the moon to the sun. With it they proposed a successful giant impact model that also has relevance for fission models, he says. Acceptance of one validates use of the same mechanism by the other.
In his letter this month, Wise upgrades the older model and suggests that a giant impact is an unnecessary complication, because a simpler scenario can use similar conditions to produce the same results. Thus a revitalized version of core-driven mantle spin-off is a robust alternate candidate to explain the moon’s origin. He now suggests, “the scientific community needs to reconsider the hypotheses of lunar origin and its bandwagon should be considering a much richer array of exploration possibilities.”