Unlocking the Moon’s secrets: from Galileo to giant impact

It is a curious fact that some of the most obvious questions about our planet have been the hardest for scientists to explain. Surely the most conspicuous mystery in paleontology was “what killed the dinosaurs.” Scores of hypotheses were proposed, ranging from the sublime to the ridiculous and including such delusions as “senescent overspecialization, lack of standing room in Noah’s Ark, and paleoweltschmerz.” Not until 1980 did a hypothesis catch on—the Alvarez Theory of dinosaur extinction by meteorite impact—and it is still not fully accepted today, 182 years after Richard Owen discovered the terrible lizards.

In the nineteenth century, scientists were preoccupied with explaining the ice ages, when vast sheets of ice marched from the poles, obliterating everything in their path, only to retreat—and repeat. Not until the 1970s did scientists discover that cyclical changes in Earth’s orientation in space and distance from the Sun caused the great glaciers to wax and wane. The first maps of the Atlantic Ocean in the sixteenth century showed that the facing coastlines of Africa and South America fit together like two pieces of a jigsaw puzzle. Geologists rejected Alfred Wegener’s 1912 explanation that continental drift had ripped a giant protocontinent asunder. It took until the mid-1960s and the discovery of plate tectonics to explain the jigsaw fit. This led to the realization that the continents and ocean basins do not stay in one place, but move constantly, albeit at the minuscule rate at which our fingernails grow. Or consider one of the most conspicuous features of our planet, America’s Grand Canyon, plainly visible from space. In the nineteenth century, its cause seemed obvious: the land underneath the Colorado River had risen, causing the muddy stream to incise itself ever deeper in its channel in order to keep up with the uplift. But by mid-twentieth century, newly discovered facts about the age of the canyon led scientists to finally reject the old hypothesis.

Every field of science has puzzles that despite being obvious, are exceedingly difficult to solve. Witness the Moon, the most viewed object in the sky, whose largest features we can see with the naked eye. Where did it come from and why does it hang there in space, the same face always turned toward us? The very first person to view the Moon through a telescope, the great Italian father of science, Galileo, saw that it’s surface was not smooth and regular, as the Greeks had supposed, but is marked by pits that came to be called craters. Another great scientist, Robert Hooke, unaware of the existence of meteorites, conducted experiments that indicated that the forces that had created the craters had come from below, from volcanism, a view that persisted for centuries.

Isaac Newton was able to answer the question of why the Moon neither crashes into Earth nor flies off into space: the Moon’s velocity in space gives it a momentum that exactly balances the gravitational pull of the Earth. Another great of the Enlightenment, the German philosopher Immanuel Kant, pointed out that the Moon’s gravity would slow the Moon’s rotation until eventually it kept the same face turned to the Earth.

Sometimes the answer to a scientific question cannot even be conceived because it depends on some fact yet to be discovered or some research method yet to be invented. Hooke realized that an object descending from above could have created lunar craters, but as far as he knew, the sky contained no such objects. Another reason scientists are slow to adopt a new idea is that they have become wedded to a particular dogma or theory and are unwilling to change their minds. Meteorite impact on the Moon was proposed in the 1870s, but it took nearly a century and spacecraft voyages to the Moon before scientists were willing to entertain it. Both meteorite impact and continental drift were resisted in part because they violated uniformitarianism: the belief that geological processes are gradual rather than catastrophic. Most geologists were unwilling to give up this fundamental principle, learned at their professor’s knee.

I have always been fascinated by how scientists have handled these great questions, both because I would like to know the answers myself, but also because they reveal so much about how science really works, as opposed to the idealized scientific method that we learned about in high school. I hope that readers will be interested in my latest effort to make science accessible: Unlocking the Moon’s Secrets: From Galileo to Giant Impact. You will learn how our seemingly placid and unchanging heavenly companion was born in the most colossal act of violence in the history of the solar system.

_{Feature image by Helen Field, via iStock.}