Impact spallation processes on the Moon: A case study from the size and shape analysis of ejecta boulders and secondary craters of Censorinus crater

[Display omitted] •We studied 242,000 ejecta boulders and 84,000 secondary craters of Censorinus crater.•Spatial variation of size and shape of the ejecta boulders is characterized.•Spatial variation of secondary and primary craters in the ejecta is characterized.•Oblique impact explains asymmetric...

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Bibliographic Details
Published in:Icarus (New York, N.Y. 1962) N.Y. 1962), 2016-01, Vol.264, p.274-299
Main Authors: Krishna, N., Kumar, P. Senthil
Format: Article
Language:English
Subjects:
Asymmetry
Boulders
Cratering
Craters
Ejecta
Fragmentation
Geological processes
Geophysics
Impact processes
Lunar craters
Moon
Slopes
Spallation
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Summary:[Display omitted] •We studied 242,000 ejecta boulders and 84,000 secondary craters of Censorinus crater.•Spatial variation of size and shape of the ejecta boulders is characterized.•Spatial variation of secondary and primary craters in the ejecta is characterized.•Oblique impact explains asymmetric distribution, size and shape of ejecta boulders.•The boulder characteristics are more complex than the theoretical spallation models. Impact spallation is a fundamental process responsible for formation of ejecta boulders from impact craters. Although theoretical spallation models were developed about three decades ago, only limited geological observations have been made so far to test these models. The 3.8km Censorinus crater on the Moon provides an excellent opportunity for studying the impact spallation processes associated with a fresh simple crater formed by oblique impact. Using the Lunar Reconnaissance Orbiter Narrow Angle Camera images, we prepared the ejecta boulder distribution map of Censorinus crater and measured the boulder sizes and shapes. Mapping of about 242,000 ejecta boulders enabled us to document the size distribution of boulders both radial and concentric to the impact crater. Larger size boulders dominate the crater rim areas, while they become smaller away from the crater. The boulder distribution exhibits a radial asymmetry suggesting Censorinus is a oblique impact, in which the uprange ejecta have smaller ranges with larger concentration of boulders near the southwestern crater rim, while the downrange ejecta are in general characterized by smaller boulders with high spatial dispersion. The cumulative size–frequency distribution (CSFD) of boulders shows a highly variable fragmentation history in which the uprange boulders suffered more complex fragmentation. The ejecta boulders also exhibit a variety of shapes that are gleaned from their axial ratios and edge angle characteristics. There is a general decrease of axial ratios away from the crater rim. Rectangular boulders dominate the crater rim and they become more equant away from the crater. In addition to the boulder sizes, the boulder shape distribution also exhibits a mild asymmetry in response to the oblique impact. Small size fresh impact craters (84,000 craters) are abundant on the Censorinus ejecta and post-date Censorinus. These craters are found in two morphologic types in which a large majority of craters have subdued ejecta (rayless craters), while some possess bright-rayed
ISSN:0019-1035
1090-2643
DOI:10.1016/j.icarus.2015.09.033