New geological maps and crater size-frequency distribution measurements of the Apollo 15 landing site

NEW GEOLOGICAL MAPS AND CRATER SIZE-FREQUENCY DISTRIBUTION MEASUREMENTS OF THE APOLLO 15 LANDING SITE. W. Iqbal1, H. Hiesinger1, and C. H. van der Bogert1, 1Institut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany, ([email protected]). Introduction: We are testing and improving the the lunar cratering chronology [1,2] within a series of studies [3-8] where we are performing detailed investigations of the geology, sample collection, and crater size-frequency distribution (CSFD) measurements at Apollo landing sites. The correlation of CSFD measurements with radioisotopic and exposure ages of Apollo and Luna samples form the basis for the lunar cratering chronology, which is used to derive absolute model ages (AMAs) of unsampled surfaces of planetary bodies throughout the inner Solar System [e.g., 1-11]. The Apollo 15 landing site is located at the rim of the Imbrian basin, is covered by mare units from late Imbrian to Eratosthenian period, and provides a wide range of samples from various geological periods. Using recent data, we produced a new geological map of this site and updated CSFDs for the described geologic units. The newly derived N(1) (cumulative number of the craters ≥1 km in diameter) values will be correlated with updated sample ages [e.g., 12], resulting in a reevaulated calibration point for the lunar cratering chronology [1]. Methods: The used image data include LROC Wide Angle (WAC; 100 m/pixel) and Narrow Angle Camera (NAC; ~0.5 m/pixel) images [13] with incidence angles between 55-80°, and SELENE (Kaguya) Terrain Camera morning, evening, and ortho mosaics. The topographic features were mapped using a LOLA/SELENE merged digital elevation model (DEM) [14] and LRO NAC-derived DEMs. The spectral boundaries were defined on Clementine data [15] and Kaguya Multiband Imager (MI) data [16]. The geologic units were mapped using ArcGIS. The CSFDs of the mapped units were measured using CraterTools [17] in ArcGIS, and were plotted and fit in cumulative and relative plots using pseudolog binning in Craterstats [18]. Randomness analyses [19] were used to avoid contamination from clusters and chains of secondary craters. Geological Events: The Apollo 15 landing site contains geologic units of Imbrian, Eratosthenian, and Copernican ages. Previous studies [e.g., 20,21] identified the massifs in highlands as preImbrian material, in contrary, we identified them as a part of Imbrium basin rim. The highlands in the mapping area exhibit the Imbrium basin rim (Icr), Imbrium basin ejecta units (Ifm-lt, Ifm-st, and Ifs, which are topographically different), and Imbrian plains (Ip). The mare units in the mapping area belong to the Figure 1. Preliminary geological map of the Apollo 15 landing site showing Imbrium basin units such as: Imbrian basin rim (Icr), Imbrium basin ejecta (Ifm-lt. Ifm-st), a few mare units, young material in Hadley Rille, a network of the rilles and various generations of craters and their materials. The green triangle represents the position of the landing site. Eratosthenian and Imbrian periods [22]. The Copernican-aged Aristillus crater and apparently Eratosthenian Autolycus crater [21,22] rays and secondary craters cover a large area around the landing site. The young Copernican-aged crater to the southwest of the landing site also contributed to the resurfacing of the study area. The section of the Hadley rille to the west of the landing site is morphologically younger than the rille in the north. Other rilles in the mapping area are older than the deposited mare units. CSFD Measurements: Count areas were carefully selected on the basis of our new geological map. First, we modified the areas selected by Neukum (1983), because his areas were found to cross geological unit boundaries in some cases, and discussed the results in [6]. The CSFD of this area (blue, Fig. 2a) was measured using LRO WAC data, which resulted in an N(1) value of 2.98x10-3 km-2 and an absolute model age (AMA) of ~3.22 Ga. Next, we used the Kaguya ortho-image mosaic to measure the CSFD around the landing site (magenta, Fig. 2a), which gives an N(1) value of 3.07x10-3 km-2 and an AMA of ~3.25 Ga. The CSFD measured on LRO NAC data (black, Fig. 2a) resulted in an N(1) value of 2.94x10-3 km-2 and an AMA of ~3.21 Ga. The three measured values are consistent with the N(1) value of 3.2±1.1x10-3 km-2 and age ~3.28 Ga, which was used by Neukum (1983) to calibrate the lunar cratering chronology [3]. Moreover, the recently determined isotopic age for the olivine normative basalts found at the landing site is ~3.26 Ga [12], which is also consistent with the obtained AMAs. Our measured N(1) values and the recently determined isotopic ages of the samples can be used to confirm and slightly update the calibration point for the lunar cratering chronology [3,4]. Acknowledgements: WI and HH were funded by the German Research Foundation (Deutsche Forschungsgemeinschaft SFB-TRR170, subproject A2) and CvdB was supported by EU H2020 project #776276, PLANMAP. References: [1] Neukum (1983) NASA TM77558. [2] Neukum et al. (2001) Space Sci. Rev. 96, 55-86. [3] Iqbal et al (2019) Icarus 333, 528-547. [4] Iqbal et al (2018) LPSC 49, 1002. [5] Iqbal et al (2019) LPSC 50, 1005. [6] Iqbal, et al. (2020) LPSC 51, # 1073. [7] Borisov, et al (2019) LPSC 50, 2323. [8] Gebbing, et al (2019) LPSC 50, 2337. [9] Hartmann (1970) Icarus 13, 299-301. [10] Robbins (2014) EPSL 403, 188-198. [11] Stöffler et al. (2006) Rev. Min. Geochem. 60, 519-596. [12] Snape et al (2019) GCA. 266, 29-53. [13] Robinson et al (2010) Space Sci. Rev. 150, 81-124. [14] Barker et al. (2016) Icarus 273, 346-355. [15] Pieters et al. (1994) Science 266, 1844-1848. [16] Lemelin et al (2016) LPSC 47, 2994. [17] Kneissl et al. (2011). PSS 59, 1243-1254. [18] Michael et al. (2016) Icarus 277, 279-285. [19] Michael et al. (2012) Icarus 218, 169-177. [20] Howard (1971) USGS, I-723 [21] Carr et al (1971) USGS, I-723. [22] Hiesinger et al (2000) JGR 105, 29239-29275. Figure 2. Selected counting areas and the resulting absolute model ages around the Apollo 15 landing site. (a) The blue area was measured on the LRO WAC mosaic, the magenta area on the Kaguya mosaic, and the black area on an LRO NAC image. (b) CSFD measurements and absolute model ages of selected areas are shown in cumulative form. The panel above shows the randomness analysis of the count area, which is used to avoid contamination by clusters of secondary craters at the smallest diameters. a b