Geological mapping and chronology of lunar landing sites: Apollo 12

Icarus 352 (2020) 113991 Available online 20 July 2020 0019-1035/© 2020 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Geological mapping and chronology of lunar landing sites: Apollo 12 W. Iqbal * , H. Hiesinger, C.H. van der Bogert Institut für Planetologie, Westf€ alische Wilhelms-Universit€ at Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany ARTICLE INFO Keywords: Apollo 12 Crater size-frequency distributions (CSFDs) Absolute model ages (AMAs) Lunar chronology Geological mapping ABSTRACT The lunar cratering chronology is fundamentally important, because it is not just used to determine ages for unsampled lunar surfaces, but is commonly applied to other planetary bodies. As part of a systematic study of the calibration of the lunar cratering chronology at the Apollo landing sites, we produced a new detailed geological map of the Apollo 12 landing site, using recent orbital data including Lunar Reconnaissance orbiter (LRO) Narrow Angle (NAC) and Wide Angle (WAC) images, and a Lunar Orbiter Laser Altimeter (LOLA)/SELENE merged digital elevation model (DEM), as well as Chandrayaan M 3 spectral data. Using high-resolution LRO NAC images and NAC-derived DTMs, new crater size-frequency distributions (CSFDs) were measured on the updated geological units to determine crater retention ages or N(1) values (i.e., the cumulative number of craters with diameters �1 km). Since the Apollo 12 landing site is located in Oceanus Procellarum on a ray of Copernicus crater, it provides two possible calibration points for the lunar cratering chronology: (1) a mare basalt surface age, and (2) an approximate age for Copernicus crater via its ray material. We calibrated our new N(1) values with recently determined radiometric ages of Apollo 12 basalt samples, and compare our results with work done by Hiesinger et al. (2012) for Copernicus crater, and the Neukum (1983) chronology. The updated calibration points are consistent with the lunar chronology of Neukum (1983), which indicates that no updates to this function are currently necessary. 1. Introduction On 24th November 1969, the second manned Apollo mission with astronauts Pete Conrad and Alan Bean landed in Oceanus Procellarum near the site of the unmanned Surveyor 3 lander (e.g., Bean Jr et al., 1970; Shoemaker et al., 1970; Stephenson, 1970), on a ray extending southwest from Copernicus crater (e.g., Bean Jr et al., 1970; Shoemaker et al., 1970; Stephenson, 1970). The pin-point landing was intended to take place within walking distance of the Surveyor 3 lander, to allow the collection of parts from it. The landing site provided an opportunity for the first detailed scientific lunar exploration of Oceanus Procellarum, which has been visited by several robotic missions since. The careful selection of the landing area and subsequent returned samples provided insight into various geological units (e.g., Shoemaker et al., 1970; Warner, 1970; Neukum, 1983; Neal and Taylor, 1992, Neal et al., 1994). After mineralogical and radiometric analysis, the collected samples were classified into pigeonite-, ilmenite-, feldspathic-, and olivine-basaltic suites (e.g., Warner, 1970; Levine et al., 2005; St€ offler and Ryder, 2001, St€ offler et al., 2006; Meyer, 2011; Alexander et al., 2016; Snape et al., 2018). Comparison of the radiometric ages of the collected basalts and impact melt samples with the crater size-frequency distributions of the geological units at Apollo 12 landing site and areas around the Copernicus crater respectively, provided two calibration points for the lunar cratering chronology (e.g., Neukum, 1983, Neukum et al., 2001; Hiesinger et al., 2012; St€ offler and Ryder, 2001, St€ offler et al., 2006; Robbins, 2014; Iqbal et al., 2019; Borisov et al., 2018). At the Apollo 12 landing site, two important calibration points for the lunar cratering chronology (Neukum, 1983; Neukum et al., 2001) include (1) the age of mare basalts from Mare Cognitum, and (2) the approximate age of overlying material from the ray of Copernicus crater. Neukum and Horn (1976) derived two N(1) values for the mare basalts around the Apollo 12 landing site using different area sizes, and discussed the CSFD measurements in the context of the sample ages. In addition, ropy glasses found in the Apollo 12 samples were interpreted to be ray material from Copernicus crater (Eberhardt et al., 1973; Alexander Jr. et al., 1976). Neukum and K€ onig (1976) measured a CSFD on the Copernicus ejecta blanket to use with the radiometric age of the ropy glasses to generate a calibration point for Copernicus crater (Neukum, 1983). Neukum (1983) noted that the N(1) value for Copernicus crater is unexpectedly higher possibly due to secondary crater * Corresponding author. E-mail addresses: [email protected] (W. Iqbal), [email protected] (H. Hiesinger), [email protected] (C.H. van der Bogert). Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus https://doi.org/10.1016/j.icarus.2020.113991 Received 9 April 2020; Received in revised form 30 June 2020; Accepted 14 July 2020 Icarus 352 (2020) 113991 2 contamination. However, the exact area used for measuring the CSFD is not known. Hiesinger et al. (2012) used recently collected lunar datasets to revisit the CSFD measurements around Copernicus crater. After correlation with the measured sample ages, the updated N(1) was found to be more consistent with the lunar cratering chronology. In addition, Hiesinger et al. (2012) observed a slight deflection of the crater population on the Copernicus ray just north of the Apollo 12 landing site (~5.56 �10 4 km 2 ), which is consistent with the approximate age of Copernicus ray (~5.68 �10 4 km 2 ). Hiesinger et al. (2000) used a hybrid mapping approach for mapping the mare basalt units in this region with Galileo spectral data for defining morphologically and spectrally homogeneous units and using crater size-frequency distribution (CSFD) measurements to determine ages of these units. They derived absolute model ages (AMAs) of ~3.0–3.5 Ga for the mare units around the landing site. Robbins (2014) mapped and performed crater size-frequency distribution (CSFD) measurements over an area of 6510 km 2 around the Apollo 12 landing site, excluding areas affected by secondary craters from Copernicus crater around the landing site, which resulted in higher N(1) values conceivably due to unit heterogeneity in the selected area. Given that new lunar data sets, as well as updated and new radioisotopic ages for Apollo 12 samples, are now available, we (1) investigate the Apollo 12 landing site using new high-resolution and spectral data sets to generate new geological maps of the region, (2) revisit the CSFD measurement areas of Neukum (1983) to reproduce his calibration data for the mare basalt units, (3) select a new reference measurement area for the lunar chronology, (4) integrate the improved N(1) values with new and improved radiometric sample ages, and (5) update the lunar cratering chronology with the new data points. Fig. 1. The Apollo 12 landing site (green triangle), located in Oceanus Procellarum southwest of Copernicus crater, was studied using (a) the LRO WAC mosaic and (b) the M 3 false color data, which was used to differentiate different mare units on the basis of spectral differences. The white polygon around the landing site shows the original count area of Neukum and Horn (1976), which was used to measure CSFDs for calibration with Apollo 12 sample ages. At the landing site itself (green triangle in a), (c) LRO NAC mosaic of images M120012135 and M120005333, and (d) the NAC-derived DTM were used for the detailed study of the landing site and CSFD measurements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) W. Iqbal et al. Icarus 352 (2020) 113991 3 2. Methodology 2.1. Data Our detailed geological map of the Apollo 12 landing site is based on Lunar Reconnaissance Orbiter (LRO) Wide Angle Camera (WAC, 100 m/ pixel) and Narrow Angle Camera (NAC) images (Robinson et al., 2010), the Lunar Orbiter Laser Altimeter (LOLA)/SELENE merged digital elevation model (DEM, ~60 m/pixel or 512 pixels per degree) (Barker et al., 2016), Moon Mineralogy Mapper (M 3 ) Reflectance Image (REFIMG) data (Isaacson et al., 2013), and Kaguya Multiband Imager (MI) Mineral Mapper data (Lemelin et al., 2016). The LOLA/SELENE DEM derived hill-shade image was used as a base map for the representation of our geological map (Fig. 2). The selected base map preserves the morphological information and compliments the definition of the mapped geological units. The neutral color of the base map also does not interfere with the representative stratigraphic color scheme (Wilhelms, 1987). The unprocessed LRO NAC images were calibrated and map-projected with ISIS3 (Anderson et al., 2004), while the processed LRO WAC and NAC mosaics and their derived DTMs (Scholten et al., 2012) were acquired from the LRO Science Operations Center Team products (Henriksen et al., 2017). Global M 3 data have a pixel scale of 140 m and show spectral signatures of olivine, pyroxenes, and plagioclase (e.g., Pieters et al., 2009; Isaacson et al., 2013) (The reflectance spectra for the minerals described by Lundeen et al. (2011) are shown in Appendix 1Figure A1:). For mapping the area around the Apollo 12 landing site, we used five M 3 reflectance images (REFIMG) (Fig. 1b): m3g20090610t154042_ v01_rfl, m3g20090610t113334_v01_rfl, m3g20090610t030313_v01_rfl, m3g20090610t200344_v01_rfl, and m3g20090610t070604_v01_rfl. The M 3 REFIMG Band 1 contains the photometrically-corrected albedo, Band 2 contains corrected thermal estimations (in Kelvins), and Band 3 contains radiance (Lundeen et al., 2011). The commercial software packages ENVI and IDL were used for the reduction of the strips and viewing the color composite image of the data. The red hues in the M 3 color composite image (Fig. 1b) are due to a strong 1 μ m (band area between 0.789 and 1.308 μ m) integrated band depth, green hues are due to strong 2 μ m integrated band depth (band area 1.658–2.498 μ m), and blue hues are due to strong reflectance at 1.58 μ m (Pieters et al., 2009; Staid et al., 2011). Spectral data for the highlands, which exhibit lower mafic contents, show blue hues, while maria with high mafic contents show ranges of yellow/ green to orange/red, depending on the iron content and optical maturity of the material (Staid et al., 2011). Thus, the red hues (strong, asymmetric 1 μ m absorptions) are possibly due to high olivine contents (Staid et al., 2011). Similarly, the blue hues may represent high plagioclase contents, while green hues may represent high pyroxene contents. M 3 reflectance data (Fig. 1b) are well-suited for delineating spectral mare boundaries, and Kaguya Multiband Imager (MI) Mineral Mapper data (Fig. 3a-e, Appendix A3) provide detailed information about the abundances of various minerals, including olivine, clinopyroxene, orthopyroxene, plagioclase, and iron oxide (Lemelin et al., 2015, 2016). MI data have a pixel scale of ~60 m in the NIR from an altitude of ~100 km (Lemelin et al., 2015). The well-calibrated concentration values in the Kaguya MI spectral data (Lemelin et al., 2015, 2016) can be compared with the concentrations of the minerals in the returned samples (e.g., Neal et al., 1994; Snape et al., 2018). For the detection of the TiO 2 abundances, we used the LRO WAC-derived TiO 2 map (Fig. 3f), with a pixel scale of ~75 m (Sato et al., 2017). We used LRO WAC data for remeasuring and validating the CSFD for the area defined by Neukum and Horn (1976) (Fig. 1a). In addition, we used LRO NAC data (Fig. 1c) to measure new CSFDs on the updated geological unit immediately surrounding the landing site, as well as other units within the original Neukum and Horn (1976) area. The LRO SOC team (Henriksen et al., 2017) produced a DTM (Fig. 1d), a slope map, and hill-shade images from LRO NAC stereo image pairs M120012135 (pixel scale 0.519 m, incidence angle 53.05�) and M120005333 (pixel scale 0.518 m, incidence angle 52.11�) (Fig. 1c). We used the data to aid in the accurate measurement of the crater diameters. We also processed a few LRO NAC image pairs including M104662862 (pixel scale 1.05 m, incidence angle 49.56�), M117643719 (pixel scale 0.896 m, incidence angle 80.44�), and M165985425 (pixel scale 0.484 m, incidence angle 63.17�). For areas where LRO NAC images were not available, we used Kaguya Terrain Camera evening mosaic TCO_MAPe04_N00E336S03E339SC (Haruyama et al., 2008) (TC, ~10 m/pixel) for our CSFD measurements. 2.2. Mapping technique For the detailed geological mapping of the area around the Apollo 12 landing site, we combined the albedo, morphological, topographic, and spectral information to produce a new geological map. First, obvious albedo and morphological differences were mapped on the LRO WAC mosaic (Robinson et al., 2010). For example, highland material with higher albedo and distinctive morphology is clearly distinguishable from the lower albedo, low-lying mare units. Second, different mare unit boundaries were mapped on the M 3 color composite image derived from the M 3 reflectance data (Lundeen et al., 2011), giving hybrid morphological and spectral mare units, following the technique of Hiesinger et al. (2000). The extent of ejecta materials from Copernicus crater can also be clearly observed in both albedo and spectral contrast. The abundances of different minerals were determined using the Kaguya MI mineral data products. The morphology of geological features was also mapped using topographic information from LOLA/SELENE DEM (scale in meters) (Barker et al., 2016). We follow the stratigraphy of Wilhelms (1987), which was updated and digitized by Fortezzo and Hare (2013). The Fortezzo and Hare (2013) lunar geological map has a scale of 1:5 M (https://astrogeology. usgs.gov/search/map/Moon/Geology/Lunar_Geologic_GIS_Renovation_ March2013). The map symbology follows the standards of the Federal Geographic Data Committee (2006) with incorporating standards defined by the PlanMap (Rothery et al; 2018) (https://wiki.planmap. eu/display/public/D2.1-public) where additional guidance was needed for detailed geological mapping. The Gazetteer of Planetary Nomenclature (Blue, 1999) proposed nomenclature was used for the regions and crater names (planetarynames.wr.usgs.gov). 2.3. CSFD measurements and lunar cratering chronology Crater size-frequency distribution (CSFD) measurements were used to determine the relative and absolute ages of the geological units via their crater spatial densities. The two basic steps for the method are: (1) selection and measurement of a geologically homogeneous areas, and (2) measurement of the diameters of the primary craters in the area (e.g., Hartmann, 1970; Neukum et al., 1974; Neukum et al., 1975, Neukum, 1983, Neukum et al., 2001; Hiesinger et al., 2012 reference here within). We imported the LRO WAC and NAC data into ArcGIS and used the CraterTools (Kneissl et al., 2011) for measuring the crater diameters on the homogeneous areas. Although NAC images have a pixel scale of ~0.5 m, we only measured the diameters of craters >~ 10 m, because this is the minimum crater diameter valid for the production function of Neukum et al. (2001). Crater chains and clusters were carefully identified by their morphology (McEwen and Bierhaus, 2006) and avoided in the CSFD measurements. A randomness analysis (Michael et al., 2012) was used to identify secondary chains and clusters that were not obvious to the eye, and exclude these if necessary. The CSFDs were exported to Craterstats for plotting and fitting (Michael and Neukum, 2010; Michael et al., 2012; Michael, 2013; Michael et al., 2016) as a Spatial Crater Count (SCC) summary file. Usually, the numbers and diameters of the craters are distributed into diameter bins as described by e.g., Neukum (1983), Neukum et al. (2001), Crater Analysis Techniques Working Group (1979), and Michael and Neukum, 2010, Michael, 2013, Michael et al., 2016. We used W. Iqbal et al. Icarus 352 (2020) 113991 4 pseudolog binning (Michael and Neukum, 2010; Michael, 2013; Michael et al., 2016) on cumulative plots and R-plots to determine the fit range, as recommended by the Crater Analysis Techniques Working Group (1979). The binned CSFDs are fit with a production function (Neukum, 1983; Neukum and Ivanov, 1994; Neukum et al., 2001), which gives the cumulative number of craters with diameters larger than the reference diameter (D) per unit area. As a result, we can acquire crater frequencies N(D) for a fixed D (generally D ¼1 km or D ¼10 km), representing crater retention ages. We did not use the newer Poisson fitting method (Michael et al., 2016), rather the standard cumulative fitting approach from Michael and Neukum (2010), so that we could compare the N(1) values determined during our study directly with previously determined values. The lunar cratering chronology function allows the assignment of an absolute model age (AMA) to a surface unit (e.g., Hartmann, 1970; Neukum, 1983; Neukum et al., 2001; Marchi et al., 2009; Robbins, 2014) by calibrating radiometric and exposure ages of returned samples with the N(1) values measured at the respective landing sites. The details on the development of the production and chronology functions are well described by e.g., Hartmann (1970), Hartmann and Gaskell, (1997) Neukum et al. (1975, 2001), Neukum, 1983, Hiesinger et al. (2000), St€ offler and Ryder, 2001, St€ offler et al., 2006, Marchi et al. (2009), and Robbins (2014). The Neukum et al. (2001) production function (PF) was used for the determination of N(1) values. However, we also compared these results to those derived with the Neukum (1983) production function. Finally, recently determined radiometric ages (Snape et al., 2016, 2018) were used to evaluate and update the calibration point for the lunar chronology function (Neukum, 1983). 3. Geological mapping The lunar module Intrepid landed at 3.2�S, 23.4�W in Mare Cognitum, in the eastern part of Oceanus Procellarum on a ray of Copernicus crater, which is located ~370 km to the northeast (e.g., Bean Jr et al., 1970; Shoemaker et al., 1970; Stephenson, 1970). Geological units and features previously mapped around the landing site using spectral, topographic, and albedo contrasts include mare basalts, graben, wrinkle ridges, highlands terrains, plains, and differently aged craters with their related materials (Wilhelms, 1987; Fortezzo and Hare, 2013). The extent of our new mapping area lies between 0�-8�S and 18�-26�W, and has a 1:50 K scale (Fig. 2). The marked vertex distance in digital geological mapping depends on the quality of the data, as well as on the details of the features. Here, the small scale features have a vertex distance of ~400 m and larger scale features have a vertex distance of ~10 km. We mapped various mare units around the landing site using M 3 REFIMG data (e.g., Pieters et al., 2009; Isaacson et al., 2013) and compared the mapping units with Kaguya MI derived abundances of clinopyroxene, orthopyroxene, plagioclase, olivine, and iron oxide (Lemelin et al., 2015, 2016). Abundances are represented in wt% similar to those described by Lemelin et al. (2015, 2016). Olivine (Fig. 3a) is higher at ~7 to 14% in highlands compared to mare units with values of <3%. Young Copernican crater materials show the highest abundances of ~21 to 25% of olivine in the study area. Clinopyroxene (Fig. 3b) in mare units ranges from 21 to 28%, whereas it is depleted in highlands with abundances of <7%. Orthopyroxene (Fig. 3c) is slightly higher (~28 to 35%) in the highlands, especially around crater rims (~42 to 50%), compared to the mare units (~14 to 28%). The data show plagioclase (Fig. 3d) to be present at a higher percentage of ~64 to 74% in highlands as compared to mare units with <57%. The highlands Fig. 2. New geological map of the area surrounding the Apollo 12 landing site in Oceanus Procellarum. The map shows various geological units around the landing site, including mare units, highlands, the Fra Mauro and Cayley Formations, a few wrinkle ridges, rilles, differently aged craters, and ray material. The LOLA/SELENE DEM derived hill-shade product is used as the base map. W. Iqbal et al. Icarus 352 (2020) 113991 5 contain <10% FeO, whereas maria contain >10%. In the data varying abundances of FeO in different mare units is also seen. In addition, we used a LRO WAC-derived TiO 2 abundance map (Sato et al., 2017), which reveals TiO 2 variations in different mare units (Fig. 3f); highlands are depleted in TiO 2 . 3.1. Mare basalt units The mare basalts units around the landing site had been mapped previously using photographic and multispectral data (e.g., Eggleton, 1965; Fortezzo and Hare, 2013; Hiesinger et al., 2000, 2003). Eggleton (1965) mapped the region at a scale of 1:1 M, and interpreted the region as consisting of late Imbrian mare units of various thicknesses. Later maps defined the mare basalts as both Erathosthenian and Imbrian in age (Wilhelms and McCauley, 1971; Fortezzo and Hare, 2013). Hiesinger et al. (2000, 2003) mapped several spectral units in the mapping area using Galileo data, and assigned them absolute model ages of 3.0 to 3.5 Ga. We mapped four different mare units (Fig. 2) around the landing site based on their morphological and stratigraphic characteristics, where Em1 and Em2 belong to the Eratosthenian Period and Im1 and Im2 are Imbrian in age. We performed CSFD measurements only on unit Em2 and Im1 as these units are directly located at the landing site making them relevant for comparison to Apollo 12 sample ages and the calibration of the lunar cratering chronology. 3.1.1. Em1-Eratosthenian Mare 1 M 3 data show that the Em1 basalt unit is mixed with feldspathic highland material (Fig. 1b) either due to its contamination by lateral transport from the adjacent highlands (Mustard et al., 1998; Li and Mustard, 2000) or via contamination by crater ejecta and ray materials. In Kaguya MI data (Fig. 3a-e), the abundance of olivine is ~8%, clinopyroxene ~14%, orthopyroxene ~25%, and plagioclase <57% except for a few places with slightly higher concentration of plagioclase due to mixing. The FeO abundance of Em1 ranges from 14.5 to 16 wt%, and in the LRO TiO 2 map (Fig. 3f) is lower than 2 wt%. The lower density of craters and the fresh morphology of Em1 suggests it belongs to the Eratosthenian Period and is younger than Em2. 3.1.2. Em2-Eratosthenian Mare 2 The Em2 unit appears dark red in M 3 data (Fig. 1b), indicating the presence of highly mafic and ferrous materials, compared to the surrounding units. Apollo 12 landed on this unit, within an area covered by thin rays of ejecta from Copernicus crater (Fig. 2). The abundance of olivine is <3%, except for the materials ejected by young craters, which show higher abundances of olivine. The clinopyroxene abundance is ~27%, orthopyroxene is ~25%, and plagioclase is <50% (Fig. 3a-d). The Copernicus crater rays show higher plagioclase contents. Fresh craters near the landing site excavate materials with higher olivine contents. Em2 has a slightly higher abundance of FeO (~19%) (Fig. 3e) and TiO 2 (~4%) (Fig. 3f) compared to the Em1 unit. The crater spatial Fig. 3. Kaguya Multiband Imager data (Lemelin et al., 2015, 2016) superposed on the LRO WAC mosaic show different abundances of (a) olivine, (b) clinopyroxene, (c) orthopyroxene, (d) plagioclase, and (e) iron oxide in the vicinity of the Apollo 12 landing site (white triangle). (f) TiO 2 abundance is derived from LRO WAC data (Sato et al., 2017). The white polygon indicates the original CSFD count area of Neukum and Horn (1976). White dashed lines shows the spectral boundaries mapped on the M3 data. W. Iqbal et al. Icarus 352 (2020) 113991 6 density observed through CSFD measurements shows the material to be Eratosthenian in age, thus being consistent with Wilhelms and McCauley (1971) observations. 3.1.3. Im1-Imbrian Mare 1 This unit shows the most prominent red tone in the M 3 data, indicating a higher abundance of mafic material than other units (Fig. 1b). The Im1 unit has very similar abundances of clinopyroxene (~27%), orthopyroxene (~25%), and plagioclase (<50%) compared to Em2 (Fig. 3b-d). However, this unit has a slightly higher abundance of olivine (~5%) (Fig. 3a). The FeO (Fig. 3e) abundance of the unit ranges from 15% to ~20%. The TiO 2 (Fig. 3f) abundance is ~4%, with a few patches as high as ~7%. The unit was previously interpreted as an Imbrian unit on the basis of albedo contrasts (e.g., Wilhelms and McCauley, 1971). 3.1.4. Im2-Imbrian Mare 2 Unit Im2 is defined by hues of yellow in the M 3 data (Fig. 1b). This mafic unit shows a higher density of the craters with relatively wide ranging abundances of iron compared to unit Im1. The unit has a similar range of clinopyroxene abundance (~27%) as Em2 and Im1. However, the orthopyroxene abundance is a bit lower (~21%) (Fig. 3b, c). Similar to unit Im1, the plagioclase abundance is <50% and olivine is ~5%, except around the craters, which have ~22% concentrations of olivine (Fig. 3a, d). The TiO 2 range varies from ~2–4 wt% (Fig. 3f). The higher density of the craters indicates the unit may belong to the Imbrian Period. Fig. 4. Examples of the CIdmm, Idm, Ip, Ifm, Ifs, and IpIt units that we mapped on the basis of morphological and albedo differences in the study area using an LRO WAC mosaic and LOLA/Selene merge DTM (reference map in Appendix A2Figure A2:): (a) The Ip unit shows distinctive albedo contrast from the surroundings and exhibits a flat topography with small-scale roughness. Low albedo patches are mapped as dark mantle deposits (CIdmm), which are interpreted as pyroclastic deposits, and isolated topographical high in the 6 km north of the patches is interpreted as a dome. The dome is possibly associated with Ip unit, as it is stratigraphically older than CIdmm unit. (b) the Ifm unit, which is much hummockier than Ip, shows a few lobate-like features at the edges. (c) Ifs is smoother than Ifm, and has a slightly lower albedo than Ip. (d) The IpIt unit represents probable pre-Imbrian terrains, but it may consist of more than one geological unit. (e) The wrinkle ridges are long linear thrust faults that are clearly visible in the DTM. (f) Graben are long narrow depressions commonly related to the normal faulting. W. Iqbal et al. Icarus 352 (2020) 113991 7 3.2. Dark mantle material and mare dome material In the southwest of Riphaeus Boreus (Fig. 2), we mapped two small patches of ~13 km 2 and ~ 45 km 2 of low albedo dark mantle material of unidentified age (CIdmm) (Fig. 4a). We interpreted these deposits as pyroclastic material on basis of the high iron content and low albedo. The unit shows high abundances of FeO (~21%) and olivine (~23%) in Kaguya MI data. The material appears to originate from graben oriented in northeast and southwest directions. About 6 km north of the CIdmm, we also observed a ~ 50 km 2 dome (Idm) in LRO WAC and LOLA/SELENE Terrain Camera DEMs (Fig. 4a). The dome is stratigraphically older than the graben, as observed by their cross-cutting relationships. Thus, unit CIdmm is stratigraphically younger than unit Idm. Both unit CIdmm and Idm are surrounded by Imbrian plains (Ip) material (Fig. 2, 4a). The unit CIdmm has lower albedo, and Idm material shows slightly higher albedo (Fig. 4a), compared to unit Ip in LRO WAC UVVIS image data. We did not see any other dome structures in our study area. Neither Eggleton (1965) nor Wilhelms and McCauley (1971) mapped or defined either of these units in their geological maps (at the scale of 1:1000,000). These maps used telescopic observations, which had much lower resolution than the recent LRO image data. 3.3. Highland units The highlands around the lower-lying landing site area are different in albedo, topographic, and spectral characteristics and mainly consist of Imbrian material (e.g., Eggleton, 1965; Wilhelms and McCauley, 1971). At few places, kipukas and hills occur, which we mapped as undifferentiated material of Imbrian or pre-Imbrian age. The units Ip, Ifm, Ifs and IpIt are distinguished in the mapping area on the basis of albedo, roughness, and stratigraphic differences (Fig. 2). 3.3.1. Ip-Imbrian plains An Imbrian-aged plains unit in this region is also known as the Cayley Formation (Wilhelms and McCauley, 1971). The unit usually is less hummocky and has slightly higher albedo compared to other highlands units in this region (Fig. 4a). Although the unit was not initially mapped by Eggleton (1965) in the study area, it was commonly observed and defined in later maps (e.g., Wilhelms and McCauley, 1971; Fortezzo and Hare, 2013; Meyer and Boyd, 2018). We observed the unit to the west of Riphaeus Boreus and on the crater floor of Lansberg crater (Fig. 2), which is consistent with Fortezzo and Hare (2013) and Meyer and Boyd (2018) mapped units. The unit was previously described as accumulated volcanic ash or ballistically deposited ejecta layers (Wilhelms, 1987) from surrounding craters, and has also been interpreted as deposited ejecta material from the Orientale basin (e.g., Meyer and Boyd, 2018). 3.3.2. Ifm-Imbrian Fra Mauro Hills The Fra Mauro Formation (Fig. 2) is the geological unit defined to represent ejecta material from the Imbrium basin (Wilhelms, 1987). The unit is distinguished by lineations that radiate from the Imbrium basin. In our study area, these lineations do not show very clear orientations due to the overprinting of ray material from young Copernicus crater (Fig. 4b). 3.3.3. Ifs-Imbrian Fra Mauro smooth plains This unit was not mapped by Eggleton (1965) as a separate unit from Ifm, however, Wilhelms and McCauley (1971) mapped this unit as undifferentiated Imbrian terrain. We observe that this unit is less hummocky than Ifm and hummockier than Ip. Although the unit appears very similar to Imbrian plains, it shows a comparatively lower albedo (Fig. 4c). The unit appears mostly in topographically lower areas compared to the Ifm (Fig. 2). 3.3.4. IpIt-Imbrian pre-Imbrian terrain The IpIt unit includes generally Nectarian highland terrains. However, the terrain might also be partially covered by thin Imbrian material. These Imbrian and pre-Imbrian terrains (Fig. 4d) are interpreted to consist of the ejecta material from various old craters. In our mapping area, this terrain appears as small occurrences in the Riphaeus Mountains, as well as some isolated kipukas, that are not similar to other Imbrian units: Ip, Ifm, and Ifs (Fig. 2). 3.4. Structures The area around the lunar module is mostly devoid of any major fault network, except a small wrinkle ridge in the northeast of the landing site (Fig. 2). In the mapping area, we observed a semicircular pattern of wrinkle ridges (Fig. 2) that are oriented in a north-south direction to the east of the landing site and a northeast-southwest direction to the south of the landing site (Fig. 2). The eastern part of our mapping area has a larger number of wrinkle ridges compared to the western part. Similar observations were previously made by Eggleton (1967). Most of the graben structures or rilles (Fig. 4f) in the area are also oriented in a north-south direction (Fig. 2). The graben observed to the east of Riphaeus Boreus appear younger than and restricted to the Imbrian plains (Ip) unit (Fig. 4f). A few graben to the east of this area are oriented in a northeast-southwest direction and do not appear to be related to unit Ip. These graben are shorter than 30 km. Another set of graben are mapped in the east of Lansberg crater (Fig. 2), where they follow a similar orientation as the one observed in the east of Riphaeus Boreus. However, these graben are more than 100 km in length. A single sinuous rille to the east of Fra Mauro A crater was observed (Fig. 2), which is restricted to the mapped extent of mare unit Im2. No associated pit craters or skylights were observed. 3.5. Craters We used the Wilhelms (1987) stratigraphic scheme to classify the mapped craters into different chronological periods such as the Copernican (Cs), Eratosthenian (Ec), Imbrian (Ic), and Nectarian or preImbrian (pIc) on the basis of their morphological characteristics. Generally, our crater classifications agree with those of previous authors (e.g., Eggleton, 1965; Wilhelms and McCauley, 1971; Fortezzo and Hare, 2013). 3.5.1. Cc-Copernican craters These craters have sharp rims and prominent ejecta (Ccm) (Fig. 5a), which are defined by the young crater Copernicus. The extent of the ejecta is easily traceable on basis of albedo and spectral properties. Rays (Ccr) are the most prominent feature of the Copernican-aged ejecta (Ccm). In our map, Lansberg B, a crater to the southwest of Lansberg crater, and Gambert A crater, in the northeast corner of the map, are the most prominent craters of this period (Fig. 2). In our study region, we also observed many secondary crater chains and clusters (Csc) of Copernican age. Most of these chains and clusters (Csc) are oriented radially from Copernicus crater itself (Fig. 2). 3.5.2. Ec-Eratosthenian craters These craters (Fig. 5b) have sharp crater rims with slightly degraded ejecta (Ecm), compared to the Copernican craters ejecta (Ccm). These craters are distinguished from Copernican craters due to the absence of rays. In our map (Fig. 2), Fra Mauro A and Fra Mauro B are the most prominent Eratosthenian craters (Ec). The extent of their ejecta (Ecm) was identified through spectral data. Small Eratosthenian craters (Ec) do not show traces of ejecta in spectral data, in contrast to Copernican craters (Cc). 3.5.3. Ic-Imbrian craters These craters have degraded crater rims and ejecta (Icm) (Fig. 2). In W. Iqbal et al. Icarus 352 (2020) 113991 8 many areas, these craters are filled with mare units, which is the characteristic which distinguishes them from Eratosthenian craters. In our map, Lansberg crater (Fig. 5c) is a well preserved Imbrian crater (Ic), with slightly degraded crater rim. The ejecta near the crater rim is still traceable, but the extent of the ejecta is debatable. Possibly, a few highland (IpIt) kipukas mapped around the crater are also part of its ejecta (Fig. 2). Lansberg crater is younger than the Fra Mauro Formation (Ifm) and older than the Imbrian plains (Ip), as small patches of Ip occur on the crater floor (Fig. 2). 3.5.4. pIc-Pre Imbrian crater The highly eroded Nectarian craters, that belong to the pIc unit, are traceable in the highlands units (Fig. 2), otherwise they are buried under maria. The crater walls and ejecta of these craters are highly degraded (Fig. 5d). Mostly the crater rims can be identified with the digital terrain model (DTM). The pIc craters are commonly covered by Imbrian materials. Most of the mapped craters are either filled by mare units or other crater materials (Fig. 2). The extent of the ejecta for these craters is not traceable in the data. 3.6. Rays We mapped rays (Ccr) (Fig. 2) as a separate unit in our geological map due to their albedo and spectral contrast. Hawke et al. (2004) proposed that this contrast is due to material immaturity or compositional differences. Sabuwala et al. (2018) pointed out that the formation and propagation of the rays depends on the topography and properties of the target material. Hawke et al. (2004) also suggested that compositional rays can remain visible for longer than 1.1 Ga on the lunar surface. Thus, rays might not be the best characteristic to define the CopernicanEratosthenian boundary. Nevertheless, they are a morphological feature that can be mapped. Rays from Copernicus crater, which are oriented north-south, are one of the major geological effects in our mapping area (Fig. 2). The high albedo, fine-grained ray material was also seen and reported during the second EVA period of the Apollo 12 mission (Shoemaker et al., 1970). Thus, some of the collected samples were interpreted to represent material from both the mare and Copernicus crater. The rays of Lansberg B and Gambert A craters do not appear to extend far enough to reach the landing site (Sabuwala et al., 2018). 4. Crater size-frequency distribution (CSFD) measurements Our new geological map allows the reassessment of the geological units from which the Apollo 12 samples were collected. Thus, we measured new CSFDs using LRO NAC data, particularly for the geological unit that contains the landing site. We compared our results (Fig. 6) with CSFD measurements on LRO WAC data for the area used by Neukum and Horn (1976). The comparison was made to test and possibly improve the calibration points of lunar chronology by Neukum (1983). The crater diameters used range from 10 m to 4 km. Table. 1 provides a summary of results gained from these measurements. The Neukum and Horn (1976) area (Fig. 6a) yields an N(1) value of 6.65 �0.225 �10 3 km 2 , and an AMA of ~3.60 Ga (Fig. 6b). Our new geological map shows that the count area covers various geological units (Fig. 6a). As a consequence, it cannot be correlated with any representative sample collected from the landing site. Since the landing region is situated on the mare unit Em2 and is covered by ray material (Ccr) from Copernicus crater (Fig. 6a), we investigated smaller areas on homogeneous parts of these units within the original count area. The CSFD measurements on LRO NAC images show two crater retention ages or N(1) values (Fig. 6b, c), i.e., 6.67 � 0.531 �10 4 km 2 for the ray material (Ccr in Fig. 2) and 2.81 �1.02 � 10 3 km 2 for the underlying mare unit (Em2 in Fig. 2). Hiesinger et al. Fig. 5. Examples of craters with morphologies indicative of different chronological periods. (a) Cc, Copernican craters with well-defined crater rims and ejecta (Ccm and Ccr) like Lansberg B crater, and (b) Ec, Eratosthenian craters, which appear similar to Cc but have degraded ejecta material, Fra Mauro A crater. (c) Ic, Imbrian craters with eroded crater rim and ejecta (Icm) like Lansberg crater, and (d) pIc, preImbrian craters, which have highly eroded crater rims and are buried beneath younger units (LRO WAC mosaic, reference map in Appendix A2Figure A2:). W. Iqbal et al. Icarus 352 (2020) 113991 9 Fig. 6. (a) CSFD measurement areas superposed on the new geological map: The area mapped by Neukum and Horn (1976) is shown in blue; CSFDs were measured using LRO WAC data. The area selected by Neukum and Horn (1976) shows heterogeneities in the geological map. Thus, we used LRO NAC and Kaguya data to measure new CSFDs around the landing site and other units (areas in black and red). (b) The CSFDs from the Neukum and Horn (1976) area, mare units Em2 and Im1 are compared in a single plot. The Neukum and Horn (1976) area shows a slightly higher crater retention age compared to the other areas. (c) The Em2_LS area around the landing site shows two retention ages representing ray and mare material, and (d) Em2_N is the ray free area in the northeast of the landing site and shows an N(1) value only for the mare unit. (e) The CSFD measurements of the three areas selected on the unit Im1 show very similar results. The plots for Fig. 6c, d, and e are presented in cumulative form and as R-plot. The randomness analysis in the panel above shows some degree of clustering in the craters with diameters smaller than 63 m on LRO NAC data and about 600 m on LRO WAC data. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) W. Iqbal et al. 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