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Distribution of Lunar Craters and Multi-Ring Basins: a Test for Lunar Polar Wandering

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Distribution of Lunar Craters and Multi-Ring Basins: a Test for Lunar Polar Wandering DISTRIBUTION OF LUNAR CRATERS AND MULTI-RING BASINS: A TEST FOR LUNAR POLAR WANDERING. Donald M. Hooper and Ted A. Maxwell, Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, D.C. 20560. Observations based on two independent sets of lunar data, Apollo Subset Magnetometer, and the geometric distribution of lunar multi-ring basins, have been used together to support the hypothesis of lunar polar wandering (1,2); and more recently, the presence of additional moons in Earth orbit (3). Although the past existence of a dynamo process in the lunar core remains uncertain, as does the exact process that caused the magnetic anomalies, it is possible to test the presumed alignment of multi-ring basins. The alignment of Imbrian, Nectarian, and pre-Nectarian age basins is consistent with interpreted paleo-magnetic positions, but in order to produce this consistency, the time-stratigraphic assignments of basin ages must be subdivided further then the observable crosscutting and overlapping relationships allow. Consequently, the thrust of this research has been to determine: 1) Assuming the age subdivisions are correct, how well are the multi-ring basins clustered about the proposed paleo-equatorial positions, 2) Regardless of stratigraphic position, are there any other preferential alignments that might support other preferred orientations of the lunar pole, and 3) Is there any symmetry in the distribution of lunar craters that supports in general the hypothesis of an equatorial symmetry of impacts. Multi-Ring Basin Distribution To analyze the distribution of basins, each group of basins based on the ages and corresponding paleo-poles was plotted on overlays of the lunar 1:5M geologic maps. The distance from each basin to the respective equator was calculated based on the center points of the basins, and the number of basins per unit area was calculated in each 10" latitude band in order to determine basin density. For both lower Nectarian and upper Nectarian-Imbrian age basins, there is a slight positive correlation between basin density and decrease in distance from the equator, but the lrerror bars for these relationships are not sufficient to support the polar wandering hypothesis. Pre-Nectarian basins, in fact, show a negative correlation with the proposed paleo-equator of that period, although this inverse relationship may be due to the Smythii and Marginis basins, which are located near the paleo-poles of that time period. Although both lower Nectarian and Imbrian age basins show the expected correlation, the best fit line for these data are based on only 7 basins for the lower Nectarian paleo-equator, and 8 for the upper Nectarian-Imbrian age equator. The inverse relationship shown by the pre-Nectarian basins is based on 29 basins, and may be more representative of the population. To search for other preferential zones of basin clustering, each basin and corresponding paleo-equator was plotted on an overlay of a O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System LUNAR POLAR WANDERING?? Hooper, D. M. and Maxwell, T. A. Schmidt stereonet. Essentially the same results were found for the two younger groups of basins. Imbrian-upper Nectarian basins are the most aligned along the proposed paleo-equator, and pre-Nectarian basins were the most scattered. The stereonet was rotated to search for possible alternative alignments of crater basin density, but no such zones were found . Crater Distribution Assuming that smaller bodies would also impact preferentially in the equatorial zone, a similar analysis was done using the mapped craters greater than 20 kilometers in diameter. The craters were grouped into pre-Nectarian, Nectarian, Imbrian, Eratosthenian, and Copernican Periods, but the pre-~ectarianand Nectarian craters were dropped from the study due to their susceptibility to later disturbance and obscuration. Imbrian craters were normalized for both obstructions in the post-Imbrian record and area available to impact, and when plotted, the data demonstrated a.increase in density with distance from the paleo-equator - not indicative of equatorial clustering. Craters of the Eratosthenian and Copernican Periods, however, showed different results. When plotted against the present lunar equator, the graph of these craters displayed a significant increase in crater density with decreasing distance from the equator. For the Copernican Period, 49% of the craters lie within the 33% total surface area represented by the first 20° from the present equator. Conclusion The results of this study do not disprove Runcorn's proposals for lunar polar wandering, but they do indicate the need for alternative supportive evidence since the results of the basin distribution are inconclusive. If such a hypothesis of equatorial clustering of craters and basins is found to be valid for the planets, as suggested by the distribution of young lunar craters, this technique would serve as an indication of polar displacement. The moon has probably not always had its present orientation; nevertheless, it is apparent that basin distribution alone cannot be used to support the positions of past lunar axes of rotation. References (l)Runcorn, S. K., Primeval Displacements of the Lunar Pole. --Phys. Earth Planet Inter., 2, 135-147, 1982. (2)Runcorn, S.K., Lunar Magnetism, Polar Displacements and Primeval Satellites in the Earth-Moon System. Nature 304, 589- 596, 1983. (3)Runcorn, S.K. , Lunar ~aleomagnetic Directions and the Existence of Satellites around Satellites. LPSC XVI, p. 718-719, 1985. O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System .
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