Historical review of a long-overlooked paper by R. A. Daly concerning the origin and early history of the Moon
In 1946 the great geologist R. A. Daly published an important paper in which he discussed a great many problems concerning the Moon and its features and origin. His paper was almost completely ignored by the scientists of the day and was ``lost'' for nearly half a century. The present paper marks an attempt to outline Daly's contributions to the interpretation of these lunar problems, in particular the origin of the Moon. One of the major ideas, which probably was the incentive for him to write the paper, was that the Moon was born as a result of a very early glancing collision of the Earth and a planet-sized body. Other subjects covered are the origin of the craters from Earth fragments, although meteoritic impact is also presented; the nature of the maria as lava from the body of the Moon; and origin of the lines of small craters as produced by gas escaping from the Moon. Daly rejected all non impact models for crater origin except for the tiny gas-made aligned pits. He vigorously stated that the Moon was largely created from the body of the Earth and discussed three methods by which this could be accomplished, one internal and two external, before settling on the glancing collision model. Daly clearly was the pioneer in presenting the impact model of the origin of the Moon. Much later, works by others have modified the hypothesis, but this is only natural evolution. Two other ``lost'' papers will be mentioned to show that this is an all too frequent occurrence.In 1893 Gilbert wrote a milestone paper that was generally unrecognized for more than 50 years. He espoused the impact theory of the origin of lunar basins and craters. He was wrong about the mechanism involved, but he was right about the impact idea. Similarly, Öpik in 1916 showed that impact craters must be formed by explosions due to the high energies of striking meteorites. He showed that such impacts, even at low angles of fall would result in circular craters, thus correcting Gilbert, whom he did not mention. His paper also ``disappeared'' for many years. Early recognition and understanding of these three papers would have advanced lunar studies by many years.