Journal of Earth Science, Vol. 26, No. 5, p. 740–745, October 2015 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-015-0579-y Impact Craters with Circular and Isolated Secondary Craters on the Continuous Secondaries Facies on the Moon Shangzhe Zhou, Zhiyong Xiao*, Zuoxun Zeng School of Earth Sciences, China University of Geosciences, Wuhan 430074, China ABSTRACT: On airless bodies such as the Moon and Mercury, secondary craters on the continuous secondaries facies of fresh craters mostly occur in chains and clusters. They have very irregular shapes. Secondaries on the continuous secondaries facies of some Martian and Mercurian craters are more iso- lated from each other in distribution and are more circular in shape, probably due to the effect of target properties on the impact excavation process. This paper studies secondaries on the continuous seconda- ries facies of all fresh lunar complex craters using recently-obtained high resolution images. After a global search, we find that 3 impact craters and basins on the Moon have circular and isolated secon- daries on the continuous secondaries facies similar to those on Mercury: the Orientale basin, the Anto- niadi crater, and the Compton crater. The morphological differences between such special secondaries and typical lunar secondaries are quantitatively compared and analyzed. Our preliminary analyses suggest that the special secondaries were probably caused by high temperature gradients within the lo- cal targets when these craters and basins formed. The high-temperature of the targets could have af- fected the impact excavation process by causing higher ejection angles, giving rise to more scattered circular secondaries. KEY WORDS: Moon, impact cratering, secondary craters, comparative planetology. 0 INTRODUCTION parabolic profile featuring steep slopes. Impact craters are the most common morphologic units on c. Modification stage: the transient crater is a theoretical the surface of Solar System bodies, and impact cratering is the transitional stage in the formation of an impact crater. It col- most important geological process in the formation and evolu- lapses quickly in the modification stage because of its steep tion of the surfaces of all solid celestial bodies (Melosh, 1989). gravitationally unstable crater walls, enlarging the crater di- The impact cratering process is usually broadly divided into ameter and reducing the crater depth by filling with slumped three stages (Melosh, 1989; Gault et al., 1974). deposits. During this stage, ejecta fall back to the target surface. a. Contact-compression stage: shock waves are generated Some ejecta have large ejection velocities so that they form when an impact body contacts a target surface at high speed. secondary craters (i.e., secondaries) when landing on the target During this stage, the kinetic energy of the impactor is trans- surface. ferred to the target. Both the impactor and parts of the target Many details of the physical process of crater formation material are highly compressed during impact events with large are still not understood, including the effect of target properties enough energy, inducing impact melting and perhaps vaporiza- on ejecta distribution (Xiao and Komastu, 2013), and the for- tion. mation mechanism of central pits within impact craters (Xiao et b. Excavation stage: outside the melting and vaporization al., 2014a; Xiao and Komastu, 2013). A better understanding zone, shock waves generated by the impactor accompanied by the physical laws of cratering processes can be obtained by rarefaction waves that formed when shock waves are reflected studying the controlling factors during different stages of im- from free surfaces destroy and eject target materials, forming pact cratering (Xiao et al., 2014b), such as the effects of gravity, an excavation cavity within the target. Ejecta are thrown out of target properties, and impact velocity. Both internal (e.g., cen- the cavity during this stage, and the energy of shock waves is tral peaks, crater terraces) and external structures (e.g., conti- attenuated. The excavation stage ceases when the shock energy nuous ejecta blankets and secondary crater fields) of impact is not sufficient to eject materials outwards. The excavation craters are key features for understanding the controlling fac- cavity at this time is called a transient crater, and has a tors of different stages in the impact cratering process (Schultz, 1988; Pike, 1980; Gault et al., 1975). *Corresponding author: [email protected] Ejecta of fresh complex craters of diameter ~15–300 km © China University of Geosciences and Springer-Verlag Berlin (Pike, 1980) and impact basins of diameter >~300 km on the Heidelberg 2015 Moon (Spudis, 1993; Wilhelms et al., 1987; Hartmann and Wood, 1971) can be divided into three concentric radial impact Manuscript received July 19, 2014. facies surrounding crater rims (Xiao et al., 2014b; Schultz and Manuscript accepted November 07, 2014. Singer, 1980) identified outwards from the crater rim as follows: Zhou, S. Z., Xiao, Z. Y., Zeng, Z. X., 2015. Impact Craters with Circular and Isolated Secondary Craters on the Continuous Seconda- ries Facies on the Moon. Journal of Earth Science, 26(5): 740–745. doi:10.1007/s12583-015-0579-y. http://en.earth-science.net Impact Craters with Circular and Isolated Secondary Craters on the Continuous Secondaries Facies on the Moon 741 (1) The continuous ejecta facies adjacent to the crater rim, these secondaries are still not known. Whether or not more characterized by continuous ejecta deposits. No secondary ICCIS exist on the Moon is not determined but this question clusters or chains are visible in this facies. would shade light on the origin of these circular secondary (2) The continuous secondary crater facies consists of craters. secondary crater chains and/or clusters. These secondaries have In this paper we describe a global survey of lunar impact extensive herringbone morphologies, downrange ejecta fans, craters and collect craters that have similar circular and isolated ridge-like rims, and floor mounds (Oberbeck and Morrison, secondaries on the continuous secondaries facies. Their mor- 1974, 1973). phological and distribution characteristics were quantitatively (3) The discontinuous secondary crater facies where the and qualitatively analyzed. Their possible formation mechan- secondaries are isolated from each other (described as distant isms are discussed based on a comparison with ICCIS on Mer- secondaries by Xiao and Strom, 2012). This facies covers a cury and Mars. much larger area compared with the continuous ejecta facies zone andcontinuous secondaries facies, and those of large im- 1 DATA AND METHOD pact craters and basins may cover the entire planetary surface We used global mosaics of the Moon acquired by the Lu- (Melosh, 1989). nar Reconnaissance Orbiter Camera Wide-Angle Camera Compared with older craters, both the morphology and (LROC WAC) (Robinson et al., 2010) to study the morphology sizes of external structures of fresh impact craters are directly of all fresh lunar complex craters. A detailed description of the controlled by the impact excavation process and less affected mosaic is at http://wms.lroc.asu.edu/lroc/global_product by the following modification stage (Xiao et al., 2014b) and /100_mpp_global_bw/about. Morphological Class 1 craters therefore these structures are ideal objects for studying the (Xiao et al., 2013) and impact basins contain circular and iso- main controlling factors of the impact excavation stage (Xiao et lated secondaries are identified as lunar ICCIS on the conti- al., 2014b). Recently, Xiao et al. (2014b) found that secondary nuous secondaries facies. We omitted primary craters that impact craters on the continuous secondaries facies of some formed by impacts of higher velocity asteroids or comets from Mercurian craters are different from those of typical lunar cra- the study. We studied all craters whose secondaries are clearly ters. These secondaries are more isolated from each other in resolved in the LROC WAC global mosaic. The results were distribution and are more circular in shape compared with typ- crosschecked among the co-authors. In order to further quanti- ical lunar continuous secondaries. tatively constrain the morphological characteristics of the se- We have previously named this type of Mercurian impact condaries, we calculate the degree of irregularity (Γ) (Kargel, crater as ‘Impact Craters with Circular and Isolated Seconda- 1989) for each secondary crater on the continuous secondaries ries’ (ICCIS). On the contrary, most complex craters and basins facies. Γ was defined as follows: on Mercury are similar to those on Moon, as secondaries on 0.5 their continuous secondaries are very irregular and complex in Γ= PP/(4πAP) shape, but the sizes of both the continuous ejecta facies and PP is the rim perimeter of the secondary crater; AP is the areal continuous secondaries facies are comparable. This finding size. Γ=1 indicates that a secondary has a perfect circular shape suggests that the ICCIS on Mercury formed ejection angles and more irregular shapes of secondaries would cause larger larger than normal during the impact excavation process (Xiao values of Γ. et al., 2014b). The substrates of ICCIS on Mercury are exclu- We made regional mosaics for each of the collected ICCIS, sively low-reflectance materials which have a higher content of we found and set the projection centers at the crater centers in crustal volatile compared with the global average