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This chapter takes a look at various theories that had been offered to explain lunar craters and their relevance to meteorite impact. In 1921 Alfred Wegener published the results of his investigations of the Moon in a pamphlet titled Die Enstehung der Mondkrater, or The Origin of Lunar Craters. Wegener had carefully read Grove Karl Gilbert's The Moon's Face. Like Gilbert, he pointed out that terrestrial volcanic craters and lunar craters differ in size, shape, and features. On Earth, volcanoes typically display a “steep, cone-shaped mountain with a crater opening on the top.” Yet “on the Moon, the normal shape of a crater, especially the fresh ones, is that of a plate whose floor lies kilometers lower than the surrounding terrain.” Wegener realized that if impact craters and their debris blanket the Moon, so must they have once blanketed the Earth. This chapter also considers the work of other scientists such as Robert Sinclair Dietz, Walter Bucher, John Daniel Boon Sr., and Claude C. Albritton Jr.

Until the lunar explorations began in earnest in the 1960s, the Barringer crater in Arizona was believed to be one of the few, if not the only, impact crater on earth. Before the moon landings, many scientists thought that lunar craters were volcanic in origin and that the moon might be covered in a layer of volcanic dust meters thick so that astronauts would sink up to their eyeballs when disembarking from their space capsules. A pleasant sense of relief greeted the news that the first unmanned lunar spacecraft did not disappear into the dust. For a century or more it was doubted that lunar craters were produced by impacts because it was assumed that such craters would seldom be circular. It seemed obvious that circular craters could only be produced by objects falling straight down, a rare situation, since meteorites are likely to approach from random directions, especially on the moon where there is no atmosphere to slow them down before impact. W. M. Smart in 1928 stated this explicitly: “Objections to lunar craters being caused by meteors is that the craters are round and there is no a priori reason why meteors should fall vertically and in no other direction.” He also shuddered at the notion that the impactors would have to be as large as asteroids to create the lunar basins. At about the same time, Thomas Chamberlin ruled out impacts on the moon because there was no evidence for an appropriate population of objects anywhere in the solar system that could have made the craters That was in 1928 when near-earth asteroids had not yet been found, and when little was known about the history of the moon or the formation of the solar system. Richard A. Proctor in 1896, however, had concluded that because so many meteors continued to fall to earth that the planet and the solar system were still forming. To him, this made more sense than to blame the formation of the planets on “the creative fiats of the Almighty.” There is merit to his point of view, because today’s bombardment merely represents a faint, ongoing manifestation of the process of accretion that assembled the planets in the first place.

This chapter examines early scientific ideas that opened the door to the theory of meteorite impact. Isaac Newton showed that the Earth and the Moon are bound by an invisible yet irresistible force from which neither will ever escape: gravity. One might say that the relationship resembles that of the members of a family, which include spouses, siblings, and sons and daughters, whose bonds not even death can part. No wonder then that scientists came to nickname the three prominent explanations of the Moon's origin the daughter, sister, and spouse theories. Although we see tides ebbing and flowing in the oceans, Newtonian mechanics requires that the solid Earth and Moon also pull tides in each other. The effect slows the Earth's rotation while causing the Moon to recede. This chapter considers the theories of Thomas Chamberlin, Forest Ray Moulton, Edwin Hubble, and Thomas Jefferson Jackson See, as well as scientists who attributed lunar craters to meteorite impact.

This chapter looks at space missions that shed more light on the nature of the Moon. On July 20, 1969, Apollo 11 landed on the surface of Mare Tranquillitatis. When Apollo 11 returned samples to Earth, scientists saw what looked at first glance like dusty charcoal briquettes and at second glance like basalt. Microscopic and chemical analysis showed that the Apollo 11 rocks indeed were basalt. When scientists measured the age of the Tranquility Base basalts using radiometric methods, they got 3.6 billion years. Once the Moon rocks had been thin-sectioned, geologists could detect craterlets as small as a few microns (one-millionth of a meter) across. The difference in size between the largest impact basins like Orientale and these minuscule pits was over one trillion times, yet the same process, meteorite impact, created each of them. This chapter also considers the theories of Reginald A. Daly and Bill Hartmann concerning lunar craters.

The Moon is no longer the “in” thing. We see it as often as the Sun and give it little thought—we’ve become indifferent. However, the Moon does reflect more than just sunlight. The nomenclature of lunar craters holds up a mirror to an important aspect of human history. Of the 1586 lunar craters that have been named honoring philosophers and scientists, only 28 honor a woman. These 28 women of the Moon present us with an opportunity to meditate about this gap, but perhaps more significantly, they offer us an opportunity to talk about their lives, mostly unknown today. The women of the moon tell us stories of love, sorrow, and courage, of remarkable scientific achievements realized through perseverance, and of tragedies triggered by circumstances.

For each of the “Women of the Moon”, a biography and the reason for receiving the honor are described. The women included are Hypatia of Alexandria, Catherine of Alexandria, Nicole-Reine de la Briere Lepaute, Caroline Lucretia Herschel, Mary Fairfax Greig Somerville, Anne Sheepshanks, Catherine Wolfe Bruce, Maria Mitchell, Agnes Mary Clerke, Sofia Vasílyevna Kovalévskaya, Annie Scott Dill Russell Maunder, Williamina Paton Fleming, Annie Jump Cannon, Antonia Caetana de Paiva Pereira Maury, Henrietta Swan Leavitt, Mary Adela Blagg, Mary Proctor, Marie Skłodowska-Curie, Lise Meitner, Amalie Emmy Noether, Louise Freeland Jenkins, Priscilla Fairfield Bok, Gerty Theresa Radnitz Cori, and the astronauts/cosmonauts Judith Arlene Resnik, Sharon Christa McAuliffe, Kalpana Chawla, Laurel Blair Salton Clark, and Valentina Vladímirovna Nikolayeva Tereshkova.

As we apprehend the likelihood of an almost inconceivable cosmic impact occurring again at some time in the future, it is worth considering how we got to be here in the first place. The quest for an explanation of our origins is, of course, as old as the ability of humans to conceptualize questions and consider answers. Our species has probably been able to do that for hundreds of thousands of years, since well before evidence of its ability to comprehend was etched in cave paintings, perhaps back in an age when stone tools began to be patiently chipped out of flint rock. But when questions about origins were first hesitatingly formulated, answers could only be invented. There was no way any human beings could have known back then what we know now about the nature of the universe and its contents. Our collective ability to understand the world in which we live received an enormous impetus starting about 400 years ago when the scientific method for approaching reality was first practiced. That was when it was discovered that through experiment and observation, and above all through measurement, it became possible to unravel the secrets of the universe. That was when Galileo first pointed a telescope at the heavens, William Gilbert experimented with natural magnets, and Johannes Kepler discovered the laws of planetary motion. Since then, our species has gathered a stunning new perspective on the nature of this universe and its origins, a perspective that has relegated to the back burner of human thought most of the fantasies that have so long held sway over the human mind. As a result of the high technology that has emerged during this century, scientists have learned to probe into the depths of matter and into the farthest reaches of space. In the course of this exploration, astronomers, in particular, have learned that the universe has its roots in awesome violence and that the birth of the earth and moon were accompanied by what, from our perspective, would be considered catastrophic events. Were anything remotely similar to occur today, all life on earth would be instantly terminated.