Impact Craters with Circular and Isolated Secondary Craters on the Continuous Secondaries Facies on the Moon

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

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 isolated 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 secondaries 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 secondaries 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 local targets when these craters and basins formed. The high-temperature of the targets could have affected 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 of Mercury order to reduce uncertainties in measuring the perimeters and (Xiao et al., 2014b). The volatile content within the crust of areas of the secondaries. The Integrated Software for Imagers Mercury is far lower than those of Mars and icy satellites, and and Spectrometers (ISIS; http://isis.astrogeology.usgs.gov//) it is not yet known whether or not the low crustal volatile con- developed by the U.S. Geological Survey was employed to run tent had affected the excavation process and caused extraordi- the standard calibration and projection procedure for WAC narily larger ejection angles. Solving this problem will not only images. The open source software ImageJ improve our understanding of the physical process of impact, (http://rsbweb.nih.gov/ij/) was used to fit polylines along the but also help to better understand the distributions of distant rims of the secondaries to measure their rim perimeters and secondaries on different planetary bodies and their effects on areas. All secondaries on the continuous secondaries facies crater counting (Xiao et al., 2013). were included in the measurements to obtain statistically robust The Moon is depleted in crustal volatiles (Heiken et al., results. 1991). If crustal volatiles were the only factor controlling im- We selected three typical Copernican-aged (Stöffler and pact ejection angles, secondaries on the continuous secondaries Ryder, 2001) complex impact craters for comparison to better facies of all lunar complex craters and basins would uniformly show the morphological and distribution differences of secon- have very irregular shapes. However, Xiao et al. (2014b) noted daries between the ICCIS and other typical lunar complex cra- that the Orientale basin and the Antoniadi crater on the Moon ters. The locations of the three Copernican-aged craters, Co- are similar to ICCIS on Mercury, as their secondaries on the pernicus, Tycho and Jackson, are marked in Fig. 1. The secon- continuous secondaries facies are relatively isolated from each daries in their continuous secondaries facies were also studied other and are more circular in shape, although the detailed using the same method. It is worth noting that the morphological characteristics and formation mechanisms of

742 Shangzhe Zhou, Zhiyong Xiao and Zuoxun Zeng

Figure 1. Locations of the three lunar ICCIS (red circles) and the three typical complex craters selected for comparison (blue circles). The base image is the LROC WAC global monochrome mosaic that has an equirectangular projection. secondaries of the three typical lunar craters usually occur in clusters and/or chains, and most of them have very irregular shapes so that their rims could not be confidently determined. Such secondaries were not included in the statistics. Therefore, the average value of the Γ can only serve as a minimum value for the three typical lunar complex craters.

2 RESULTS We discover three lunar ICCIS after a global search: the Orientale Basin (19.87°S, 92.8°W; D=~930 km), the Antoniadi Crater (69.7°S, 172°W; D=~137.91 km), and the Compton Crater (55.3°N, 103.8°E; D=~162 km). We describe the morphological and distribution characteristics of their secondary craters after those of typical lunar complex craters.

2.1 Typical Lunar Complex Craters Secondaries on the continuous secondaries facies of Co- pernicus (9.7°N, 20°W; diameter D=~93 km), Jackson (22.4°N, 163.1°W; D=~71 km) and Tycho (43.31°S, 11.36°W; D=~86.21 km) mostly occur in clusters or chains, and have irregular shapes and herringbone structures, which are typical of normal lunar secondary craters (Fig. 2). Copernicus (Fig. 2a) is a rayed crater located in the east of Oceanus Procellarum. We recognized ~2 000 secondaries on the continuous secondaries facies. There are fewer in the southwestern quarter compared with other regions, and this asymmetric distribution may be caused by the oblique impact that formed the Copernicus crater (Oberbeck and Morrison, 1973). The average degree of irregularity of the secondaries is 1.18, and the standard deviation is 0.12. The Jackson crater is located at the northern highlands of the farside of the Moon (Fig. 2c). We identified ~2 000 secondaries are collected on the continuous secondaries facies. The average degree of irregularity for the measured secondaries is Figure 2. (a) Copernicus crater; (c) Jackson crater; (e) Tycho 1.15, and the standard deviation is 0.10. crater, and their irregular-shaped secondaries (b, d and f) on the The Tycho crater (Fig. 2e) is located in the southern high- continuous secondaries facies. The mosaics are made from lands of the near side of the Moon. We identified ~2 500 LROC WAC images and they are in sinusoidal projections.

Impact Craters with Circular and Isolated Secondary Craters on the Continuous Secondaries Facies on the Moon 743 secondaries on the continuous secondaries facies. The average The morphologic irregularities of the secondaries on the degree of irregularity of the measured secondaries is 1.15, and continuous secondaries of the three ICCIS and three typical the standard deviation is 0.11. lunar complex craters are compared in Fig. 4. Secondaries on the continuous secondaries facies of the ICCIS are more circu- 2.2 ICCIS on the Moon lar than those of the typical complex craters. Their spatial dis- Compared with the typical lunar craters shown in Fig. 2, tributions are more isolated as shown in Fig. 2 versus Fig. 3. secondaries on the continuous secondaries facies of the three ICCIS craters are more isolated from each other and more cir- 3 DISCUSSION AND CONCLUSIONS cular in shape (Fig. 3). On airless bodies such as the Moon and Mercury, the The Orientale Basin (Fig. 3a) is one of the youngest mul- morphology and distribution of secondary craters are mainly ti-ring basins on the Moon. The continuous secondaries facies controlled by ejection angles and velocities (Xiao et al., 2014b; is up to ~2 000 km in radial dimension. We identified over 2 Schultz and Singer, 1980). Xiao et al. (2014a) found that while 100 secondaries on the western side of the continuous seconda- some Mercurian craters have more circular and isolated secon- ries facies. The average degree of irregularity of the seconda- daries on the continuous secondaries facies, other similar-sized ries is 1.03, and the standard deviation is 0.02. craters on Mercury typically have irregularly-shaped seconda- The Antoniadi crater (Fig. 3c) is located within the ries in the same facies. This finding shows that greater ejection Aitken Basin near the lunar south pole. We identified ~1 200 velocities from Mercurian craters compared with those from secondaries on the continuous secondaries facies. The average similar-sized lunar craters are not the reason for the more cir- degree of irregularity of the secondaries is 1.03, and the stan- cular and isolated secondaries on Mercury (Xiao et al., 2014b). dard deviation is 0.02. The ICCIS on the Moon are large complex craters and basins The Compton crater (Fig. 3e) is located in the northern (Fig. 3), but the typical lunar craters selected for comparison highlands on the farside of the Moon. The preservation state is (Fig. 2) are smaller in size, so the secondaries around these not as good as Antoniadi or Orientale. We identified ~800 se- lunar ICCIS must have had greater ejection velocities on for- condaries on the continuous secondaries facies. The average mation. Other similar-sized lunar basins also have typical irre- degree of irregularity of the secondaries is 1.04, and the stan- gular-shaped secondary chains and clusters (e.g., the Lagrange dard deviation is 0.02. Crater (32.3°S, 72.8°W; D=~225 km)) suggesting that compared with the three selected typical lunar complex craters (Fig. 2), the greater ejection velocities formed during the formation of the three lunar ICCIS are not the reason for the more circular and isolated secondaries. Higher ejection angles are the reason for the special secondaries of the lunar ICCIS instead. Ejection angles are mainly controlled by target properties in cratering processes. Previous studies found that secondaries on the continuous secondaries facies of some Mar- tian craters are more isolated from each other and are more circular in shape than those on the Moon and Mercury, and argued that crustal volatiles on Mars (especially water ice) had affected the impact excavation process and caused higher ejection angles (Schultz and Singer, 1980). Xiao et al. (2014a) proposed that target properties at some places on Mercury are special compared with the global average, producing higher ejection angles during the excavation stage. It is still not known whether or not the limited content of crustal volatiles on Mercury is the reason causing this target inhomogeneity. Schaber et al. (1977) suggested that ~4 Ga, the crust of ancient Mercury had higher temperature than at present, so that the viscosity of crustal materials was less. However, many Mercu- rian ICCIS are fresh morphological Class 1 craters, and many older craters on Mercury do not have circular or isolated secondaries. This indicates that the presumed past higher crustal heat flux on Mercury was not the reason for the formation of ICCIS, or that the heat flux on Mercury has never been uniform across the planet. However, quantitative and qualitative rela- Figure 3. (a) Orientale Basin; (c) Antoniadi crater; (e) Compton tionships between target temperature (or viscosity) and ejection crater, and their circular and isolated secondaries (b, d and f). The angles remain unknown. black seam in panel (e) is a mosaic seam caused by images taken Although recent studies suggest that the hydrogen content at different incidence angles. The mosaics were compiled from in the lunar mantle is up to thousands of ppm, greater than LROC WAC images and are in sinusoidal projections.

744 Shangzhe Zhou, Zhiyong Xiao and Zuoxun Zeng

Figure 4. Morphological differences of secondaries on the continuous secondaries facies between typical complex craters (Fig. 2) and ICCIS (Fig. 3) on the Moon. previously thought (Boyce et al., 2014), the volatile content in (Th) on the floor of Antoniadi is higher than the average lunar the lunar crust is far less than those of Mars and Mercury (Hei- surface (Jollif et al., 2000), suggesting an enrichment of heating ken et al., 1991). Xiao et al. (2014a) suggested that crustal vo- by radioactive elements within this region and thus a relatively latile contents were not the cause of central pits in impact cra- higher thermal gradient within the crust. Therefore, it is certain ters on the Moon, because the crustal volatiles on the Moon are that the Antoniadi crater formed in target material with a higher not sufficient to affect the cratering process. Therefore, it is thermal gradient than the present, which might possibly have possible that the relatively circular and isolated secondaries of affected the impact excavation process. the three ICCIS on the Moon are due to crustal volatiles. The crustal thickness of the background highlands of the The Orientale Basin formed at the end of the late heavy Compton Crater is ~28 km (Wieczoreck et al., 2013), while the bombardment (Strom et al., 2005), when the Moon was still in excavation depth of Compton was about 11–15 km (Losiak et al., the thermal expansion stage (Solomon and Head, 1979). The 2009). Compton formed in late Imbrian times (Losiak et al., 2009), lunar crust was thinner at that time compared with the present and the local crustal thickness at that time is not known. It is dif- and the thermal gradient within the crust was higher. According ficult to determine whether or not the Compton impact has exca- to cratering scaling laws (Melosh, 2011), the excavation depth vated mantle materials. Recent studies have found that this region of the Orientale Basin was ~90 km, while the average crustal is unique compared with the rest of the lunar highlands because it thickness of the lunar highlands is less than 60 km at present now exhibits a concentration of Th, suggesting historically higher (Wieczoreck et al., 2013). This comparison suggests that the thermal gradients (Chauhan et al., 2014). This finding indicates impact that formed the Orientale Basin penetrated through the that the target materials in which Compton formed were hotter ancient lunar crust and reached the upper mantle, but whether than the rest of the lunar highlands. or not the lower crust and/or upper mantle were molten at that The geological backgrounds of the three lunar ICCIS sug- time was not confirmed. Nevertheless, the excavation stage in gest that penetrating into lunar mantle is not a certain or essen- forming the Orientale Basin occurred in target material that had tial mechanism to explain the larger ejection angles or the more a higher thermal gradient than the present. circular and isolated secondaries on the continuous secondaries The Antoniadi crater is located on the floor of the ~2 500 facies. On the contrary, perhaps, the only one common charac- km diameter lunar South Pole-Aitken Basin. Large volumes of teristic associated with the three ICCIS is that they all formed lower crust and upper mantle materials must have been melted in areas that had higher heat fluxes than the global average. during the cratering process, and most of the melted materials This observation suggests that the hotter target materials when were located at the bottom of the basin. Differential crystalliza- impact occurred may be the reason for the more circular and tion of the impact melts may have lasted for millions of years. isolated secondaries on the continuous secondaries facies zone The Antoniadi crater formed at in Late Imbrian times of the Orientale Basin, the Antoniadi and Compton craters. (~4.2–3.92 Ga; Losiak et al., 2009), and the time relationship Recent studies have pointed out that higher target temper- between its formation and full crystallization of the impact melt ature would cause larger impact craters because high target is not clear. The present lowest elevation of the Moon’s surface temperature enhances the modification stage of the cratering is located at the floor of the Antoniadi crater and the excavation process (Miljković et al., 2013). But current physical and nu- depth of Antoniadi was more than 10 km (Losiak et al., 2009). merical simulations of impact cratering have not investigated Whether or not the Antoniadi impact penetrated into the lunar the effect of target temperature on the impact ejection process. mantle is not clear due to the complicated background of the Our observations and preliminary studies set observational South Pole-Aitken Basin, but the present abundance of thorium constraints for future physical and numerical simulations of the

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