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Liner arrays of small craters
RESEARCH
Linear arrays
of small
craters
Received 23 October 1961.
In discussions of geological processes on the Moon, frequent reference is made to various linear arrays of small craters. Such arrays occur, for example, in the vicinity of Copernicus, as well as at several locations in the highlands, such as the edge of the crater Miiller. It is often pointed out that such linear formations are d~cult to explain as resulting from random impacts. On the basis of this difficulty, the linear arrays are commonly held up as evidence of geologic activity on the Moon(‘.2). That is, some subsurface phenomenon gave rise to a nearly linear crack on the surface, along which volcanic activity occurred, which resulted in craters also arrayed along a line. It appears that an alternate explanation is available to account for such linear arraysan explanation which does not require the hypothesis of volcanic activity or tectonic forces capable of producing such cracks. It is possible that the dynamic behaviour of meteorites on an approach trajectory to the Moon might give rise to linear arrays of impact craters. Like every other body in the solar system, the Moon has its own Roche limit such that when bodies approaching the Moon come within this limit they can no longer hold themselves together by reason of their own gravitational forces. Thus, during a portion of the trajectory of such a body inside a Roche limit of the Moon, the body would disintegrate into pieces whose cohesive force is chemical rather than gravitational; that is, monoliths. Throughout the remainder of the flight, these monoliths would be dispersed from each other, on the
NOTES
I21
basis of such dynamical parameters as the initial spin rate of the original body, the trajectory in the Moon’s field, and the perturbing forces intr~uc~ by the Earth and ~avitational anomalies on the Moon. In addition, bodies which pass close to the Earth and then proceed to strike the Moon would be separated by their passage through the gravitational field of the Earth. Then when finally striking the Moon, this separation distance would be considerably greater than if the separation had been caused only by their entry into the Moon’s gravitational field. Such a disintegration must occur for any large body approaching or passing the Moon or any other planet. The problem is: How far apart would the separate pieces get before impacting the surface ? It would appear that this question could be resolved by calculations which would show at least the range of parameters which would be required in order to obtain separations such as those observed in the linear arrays of craters. This explanation might also account for some of the multiple central peaks observed in some of the large craters. A. R. HIBBS Division of the Space Sciences, Jet ~ru~~is~ora~~or~tor~, California fnstitute of Technology, Pasadena
1. 2.
REFERENCES R. B. BALDWIN, The face of the Moon, University of Chicago Press, Chicago (1949) H. C. UREY, Endeavour, Vol. 29, No. 74, 87 (1960)
Linear arrays
of small
craters
Received 23 October 1961.
In discussions of geological processes on the Moon, frequent reference is made to various linear arrays of small craters. Such arrays occur, for example, in the vicinity of Copernicus, as well as at several locations in the highlands, such as the edge of the crater Miiller. It is often pointed out that such linear formations are d~cult to explain as resulting from random impacts. On the basis of this difficulty, the linear arrays are commonly held up as evidence of geologic activity on the Moon(‘.2). That is, some subsurface phenomenon gave rise to a nearly linear crack on the surface, along which volcanic activity occurred, which resulted in craters also arrayed along a line. It appears that an alternate explanation is available to account for such linear arraysan explanation which does not require the hypothesis of volcanic activity or tectonic forces capable of producing such cracks. It is possible that the dynamic behaviour of meteorites on an approach trajectory to the Moon might give rise to linear arrays of impact craters. Like every other body in the solar system, the Moon has its own Roche limit such that when bodies approaching the Moon come within this limit they can no longer hold themselves together by reason of their own gravitational forces. Thus, during a portion of the trajectory of such a body inside a Roche limit of the Moon, the body would disintegrate into pieces whose cohesive force is chemical rather than gravitational; that is, monoliths. Throughout the remainder of the flight, these monoliths would be dispersed from each other, on the
NOTES
I21
basis of such dynamical parameters as the initial spin rate of the original body, the trajectory in the Moon’s field, and the perturbing forces intr~uc~ by the Earth and ~avitational anomalies on the Moon. In addition, bodies which pass close to the Earth and then proceed to strike the Moon would be separated by their passage through the gravitational field of the Earth. Then when finally striking the Moon, this separation distance would be considerably greater than if the separation had been caused only by their entry into the Moon’s gravitational field. Such a disintegration must occur for any large body approaching or passing the Moon or any other planet. The problem is: How far apart would the separate pieces get before impacting the surface ? It would appear that this question could be resolved by calculations which would show at least the range of parameters which would be required in order to obtain separations such as those observed in the linear arrays of craters. This explanation might also account for some of the multiple central peaks observed in some of the large craters. A. R. HIBBS Division of the Space Sciences, Jet ~ru~~is~ora~~or~tor~, California fnstitute of Technology, Pasadena
1. 2.
REFERENCES R. B. BALDWIN, The face of the Moon, University of Chicago Press, Chicago (1949) H. C. UREY, Endeavour, Vol. 29, No. 74, 87 (1960)