On the origin of the lunar smooth-plains |
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| Authors: | V R Oberbeck F Hörz R H Morrison W L Quaide D E Gault |
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| Institution: | (1) NASA-AMES Research Center, Space Science Division, Moffett Field, Calif., USA;(2) NASA Johnson Space Center, Planetary and Earth Sciences Division, Houston, Tex., USA;(3) LFE Corporation, Richmond, Calif., USA;(4) NASA-AMES Research Center, Space Science Division, Moffett Field, Calif., USA |
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| Abstract: | Before the Apollo 16 mission, the material of the Cayley Formation (a lunar smooth plains) was theorized to be of volcanic
origin. Because Apollo 16 did not verify such interpretations, various theories have been published that consider the material
to be ejecta of distant multiringed basins. Results presented in this paper indicate that the material cannot be solely basin
ejecta. If smoothplains are a result of formation of these basins or other distant large craters, then the plains materials
are mainly ejecta of secondary craters of these basins or craters with only minor contributions of primary-crater or basin
ejecta. This hypothesis is based on synthesis of knowledge of the mechanics of ejection of material from impact craters, photogeologic
evidence, remote measurements of surface chemistry, and petrology of lunar samples. Observations, simulations, and calculations
presented in this paper show that ejecta thrown beyond the continuous deposits of large lunar craters produce secondary-impact
craters that excavate and deposit masses of local material equal to multiples of that of the primary crater ejecta deposited
at the same place. Therefore, the main influence of a large cratering event on terrain at great distances from such a crater
is one of deposition of more material by secondary craters, rather than deposition of ejecta from the large crater.
Examples of numerous secondary craters observed in and around the Cayley Formation and other smooth plains are presented.
Evidence is given for significant lateral transport of highland debris by ejection from secondary craters and by landslides
triggered by secondary impact. Primary-crater ejecta can be a significant fraction of a deposit emplaced by an impact crater
only if the primary crater is nearby. Other proposed mechanisms for emplacement of smooth-plains formations are discussed,
and implications regarding the origin of material in the continuous aprons surrounding large lunar craters is considered.
It is emphasized that the importance of secondary-impact cratering in the highlands has in general been underestimated and
that this process must have been important in the evolution of the lunar surface. |
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