Impact spallation processes on the Moon: A case study from the size and shape analysis of ejecta boulders and secondary craters of Censorinus crater
Abstract 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 3.8 km 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 ejecta (bright rayed craters) The CSFD of rayless craters show a steep power law slope with a b value of 4.0, similar to the secondary craters produced by the impact of ejecta from primary craters. We therefore interpret the rayless craters as the secondary craters of Censorinus. On the other hand, the CSFD of bright rayed craters have smaller power law slope b value 2.7) which is a characteristic of primary craters, and thus provide 3 Ma age for Censorinus crater. When the characteristics of Censorinus boulders are compared with the theoretical spallation models that are sensitive to the petrophysical properties of the target (lunar highland) the models generally agree with the Censorinus boulders. However, the observed shape and size characteristics of the Censorinus boulders are found to be more complex than the theoretical spallation models. The ejecta boulders suffered more complex fragmentation and asymmetric distribution in response to the oblique impact. The spallation models accounting oblique impacts have not yet been developed. Therefore, our Censorinus boulder observations can be used to develop and validate the new theoretical spallation models for the effects of oblique impacts.