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Meteorite craters
ICARUS 7 4 , 3 7 4 - 3 7 5
(1988)
BOOK REVIEWS Meteorite Craters. By Kathleen Mark. The University of Arizona Press, Tucson. 1987. 288 pp. $29.95. Science suffers from fashion too, and today meteorite craters are in fashion. But it was not always so. Back in 1665 Robert Hooke was writing about his attempts at producing a laboratory analog of the formation mechanism of lunar craters. First he dropped round bullets into a viscous mixture of clay and water and noticed that the resulting craters were similar to those seen on the Moon. But the formation of lunar craters by impacting objects was ruled out because "it would be difficult to imagine whence these bodies should come." Hooke returned to another experiment in which he heated dry gypsum powder in a pot. This released vapor that rose in bubbles, leaving behind replicas of lunar craters. His conclusion was that the craters were volcanic, and we had to wait for G. K. Gilbert (1893) to propound the first scientific lunar impact theory. Today with disappearing dinosaurs, airborn surveys of the Canadian shield, tektites, and the Barringer Crater in Arizona, impact craters are all the rage. This remarkable change of opinion has occurred since the beginning of the twentieth century. Today more than one hundred meteorite craters have been recognized and we can clearly see the results of the intense extraterrestrial bombardment that we have suffered. Kathleen Mark won the 1975 Nininger Award for her work on meteorite craters and in this book has presented us with a clear, comprehensive, and nontechnical history of their recognition. She writes well and obviously understands the slow and somewhat devious journey that accompanies the development and acceptance of a new scientific finding. Only from about 1960 has the collaboration of geologists, geophysicists, and astronomers led to the production of a clear set of recognition criteria. The nontechnical nature of the book makes it very easy to read, but does not hide the fact that the author has put in an enormous amount of hard work. Every idea, figure, and photograph is clearly referenced, making this book a superb springboard for anyone entering this fascinating field of research. Mark stresses the fact that the main problem was caused by the lack of appreciation of the amount of energy released. In the early 1930s F. R. Moulton and, independently, L. J. Spencer were the first to consider what happened when a meteorite of mass 100 tons hit the Earth at a speed of 40 kilometers per second. It penetrates the surface and in a very small fraction of a second is converted into an explosive "compared with which dynamite and T.N.T. are mild and
harmless . . . . The subjacent materials are forcibly compressed, whilst those above and around are shattered and pulverized and hurled aloft. In a few seconds a vast structure is fashioned which may preserve for untold geological ages the record of its birth." (to quote A. C. Gifford). To begin with, the craters are clearly circular, this being independent of the meteorite entry angle. Rock layers can be exposed which, less than 5 kilometers away, can be found 500 meters below the surface of the ground. For a second or so some of the rock would be subjected to pressures of between 35,000 and 130,000 atmospheres and temperatures between 700 and 1500°C, these pressures and temperatures being much greater than any that occur in volcanic events. Even diamond formation takes place in the meteorite. The effect of these conditions on the rocks led, in places, to the formation of deformed quartz lamellae, coesite, suevite, maskelynite, and shatter-cones. All these processes had to be tested under laboratory conditions before the disbelief drifted away. Kathleen Mark's account reveals the excitement of the subject. Imagine taking high-altitude photographs of parts of Ontario and then discovering the Brent Crater on them. Imagine standing on the rim of the Arizona Meteorite Crater in full view of a host of volcanic peaks, craterlets, and diatremes and finding coesite. Imagine wandering around the Vredefort Ring and through the Bushveld Igneous Complex and slowly realizing that a meteorite had played a part. To quote E. M. Shoemaker, it is now clear that "the impact of solid bodies is the most fundamental of all the processes that have taken place on the terrestrial planets . . . . Collision of smaller objects is the process by which the terrestrial planets were born." We must thank Kathleen Mark for making this subject so accessible to the general reader. DAVID W. HUGHES
University of Sheffield England
Astronomical Observations--An Optical Perspective. By Gordon Walker. Cambridge University Press, Cambridge, 1987. 400 pp., $79.50 hardcover. Gordon Walker has written a valuable book. Astronomical Observations--An Optical Perspective is an overview of modern astronomical instrumentation and techniques. While the emphasis is on optical astron374
0019-1035/88 $3.00 Copyright © 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.
(1988)
BOOK REVIEWS Meteorite Craters. By Kathleen Mark. The University of Arizona Press, Tucson. 1987. 288 pp. $29.95. Science suffers from fashion too, and today meteorite craters are in fashion. But it was not always so. Back in 1665 Robert Hooke was writing about his attempts at producing a laboratory analog of the formation mechanism of lunar craters. First he dropped round bullets into a viscous mixture of clay and water and noticed that the resulting craters were similar to those seen on the Moon. But the formation of lunar craters by impacting objects was ruled out because "it would be difficult to imagine whence these bodies should come." Hooke returned to another experiment in which he heated dry gypsum powder in a pot. This released vapor that rose in bubbles, leaving behind replicas of lunar craters. His conclusion was that the craters were volcanic, and we had to wait for G. K. Gilbert (1893) to propound the first scientific lunar impact theory. Today with disappearing dinosaurs, airborn surveys of the Canadian shield, tektites, and the Barringer Crater in Arizona, impact craters are all the rage. This remarkable change of opinion has occurred since the beginning of the twentieth century. Today more than one hundred meteorite craters have been recognized and we can clearly see the results of the intense extraterrestrial bombardment that we have suffered. Kathleen Mark won the 1975 Nininger Award for her work on meteorite craters and in this book has presented us with a clear, comprehensive, and nontechnical history of their recognition. She writes well and obviously understands the slow and somewhat devious journey that accompanies the development and acceptance of a new scientific finding. Only from about 1960 has the collaboration of geologists, geophysicists, and astronomers led to the production of a clear set of recognition criteria. The nontechnical nature of the book makes it very easy to read, but does not hide the fact that the author has put in an enormous amount of hard work. Every idea, figure, and photograph is clearly referenced, making this book a superb springboard for anyone entering this fascinating field of research. Mark stresses the fact that the main problem was caused by the lack of appreciation of the amount of energy released. In the early 1930s F. R. Moulton and, independently, L. J. Spencer were the first to consider what happened when a meteorite of mass 100 tons hit the Earth at a speed of 40 kilometers per second. It penetrates the surface and in a very small fraction of a second is converted into an explosive "compared with which dynamite and T.N.T. are mild and
harmless . . . . The subjacent materials are forcibly compressed, whilst those above and around are shattered and pulverized and hurled aloft. In a few seconds a vast structure is fashioned which may preserve for untold geological ages the record of its birth." (to quote A. C. Gifford). To begin with, the craters are clearly circular, this being independent of the meteorite entry angle. Rock layers can be exposed which, less than 5 kilometers away, can be found 500 meters below the surface of the ground. For a second or so some of the rock would be subjected to pressures of between 35,000 and 130,000 atmospheres and temperatures between 700 and 1500°C, these pressures and temperatures being much greater than any that occur in volcanic events. Even diamond formation takes place in the meteorite. The effect of these conditions on the rocks led, in places, to the formation of deformed quartz lamellae, coesite, suevite, maskelynite, and shatter-cones. All these processes had to be tested under laboratory conditions before the disbelief drifted away. Kathleen Mark's account reveals the excitement of the subject. Imagine taking high-altitude photographs of parts of Ontario and then discovering the Brent Crater on them. Imagine standing on the rim of the Arizona Meteorite Crater in full view of a host of volcanic peaks, craterlets, and diatremes and finding coesite. Imagine wandering around the Vredefort Ring and through the Bushveld Igneous Complex and slowly realizing that a meteorite had played a part. To quote E. M. Shoemaker, it is now clear that "the impact of solid bodies is the most fundamental of all the processes that have taken place on the terrestrial planets . . . . Collision of smaller objects is the process by which the terrestrial planets were born." We must thank Kathleen Mark for making this subject so accessible to the general reader. DAVID W. HUGHES
University of Sheffield England
Astronomical Observations--An Optical Perspective. By Gordon Walker. Cambridge University Press, Cambridge, 1987. 400 pp., $79.50 hardcover. Gordon Walker has written a valuable book. Astronomical Observations--An Optical Perspective is an overview of modern astronomical instrumentation and techniques. While the emphasis is on optical astron374
0019-1035/88 $3.00 Copyright © 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.