New geological maps of the Apollo 11 and 12 landing sites

NEW GEOLOGICAL MAPS OF THE APOLLO 11 AND 12 LANDING SITES. 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: Geological mapping combined with crater size-frequency distribution (CSFD) measurements are important tools for understanding geological processes through out the Solar System [e.g., 1-8]. The Apollo missions played a vital role in deciphering the lunar geological history via the collection of samples that serve as important groundtruth for remote observations [e.g., 5,9]. By combining detailed geological mapping, CSFD measurements and sample analyses, we can determine absolute model ages (AMAs) on various planetary bodies [e.g., 6]. We used recent Lunar Reconnaissance Orbiter Camera (LROC) data [10] along with Clementine [11] and Moon Mineralogy Mapper (M3) [12] spectral data to reanalyze and map the geology of the Apollo 11 and 12 landing sites, and we produced new geological maps to provide an updated context for landing site-related sample and remote sensing studies. Methods: ISIS3 [10] was used to calibrate and mapproject the LROC Wide Angle (WAC; 100 m/pixel) and Narrow Angle Camera (NAC; ~1.2 m/pixel) data with incidence angles of 73-75°. The geological units were mapped on the basis of their morphology and albedo contrasts. The LOLA/Kaguya digital elevation model (DEM) [13] aided us in defining the units and related structures. Spectral data, including Clementine [11] and Chandrayaan-1 M3 [12], were used to subdivide units in the mare basalts around the landing sites. We mainly used the stratigraphic scheme proposed by Wilhelms (1987) [1]. Thus, the mapped craters were classified as Cc-Copernican, Ec-Erathosthenian, Ic-Imbrian, and pIc-Pre-Imbrian craters. The symbology of the mapping units follows the standards of the Federal Geographic Data Committee (2006) [14], and the nomenclature used for the mapped craters and regions is consistent with the Gazetteer of Planetary Nomenclature (1999) [15]. Apollo 11 Landing Site Geology: The map covers the southwestern edge of Mare Tranquillitatis basin [16]. Based on variations in the iron and titanium concentrations observed in spectral data, we mapped five different mare units; Im1, Im2, Im3, Im4 and Im5 (Fig. 1). We observed rays (Ccr) around the landing site, which are directed towards Theophilus crater. However, a few rays probably belong to other young craters (Cc) (Fig. 1), such as Moltke, Alfraganus, Dionysius, and Tycho. We mapped few plains units in the highlands near the landing site on the basis of albedo and topography contrast, including: Ifs-Imbrian Fra Mauro smooth plains, Ip-Imbrian plains, and IpIm-undivided Imbrian or pre-Imbrian material observed in topographical depressions in the highlands. The mapped units in the highlands terrains are: Ifm-Imbrian Fra Mauro formation (Imbrium basin ejecta), and IpIt-undivided Imbrian or pre-Imbrian terrain, which does not have the northwest-southeast striation characteristic for Ifm. We observed a concentric pattern of graben (rilles) around the southwestern edge of Mare Tranquillitatis, which are mostly perpendicular to the radial pattern of the wrinkle ridges in the southwestern Mare Tranquilitatis. Apollo 12 Landing Site Geology: The landing site is situated in Mare Cognitum on a southwest trending ray (Ccr) from Copernicus crater [17], in the Oceanus Procellarum. We observed mare units with Erathosthenian (Em1, Em2) and Imbrian (Im1, Im2) ages on the M3 data spectral contrast and crater density differences. Most rays (Ccr) in the mapping area point toward Copernicus crater, while a few may also belong to other young Copernican-aged craters (Cc), such as Gambert A and Lansberg B. Wrinkle ridges show a half concentric orientation in the east and south of the landing site. A few north-south and east-west trending graben are mapped to the west of the landing site. The highlands are covered with Imbrian ejecta (Fra Mauro formation; Ifm), while in the south and southwest of the landing site, a few areas in the highlands are mapped as undivided Imbrian and pre-Imbrian material (IpIt). Implications: Detailed geological mapping supports not only a better understanding of the geological processes active at the landing sites, but is also necessary for defining homogeneous areas for the accurate measurement of CSFDs and deriving AMAs. We used our new maps to check and improve the previously selected count areas [e.g., 3,4] for determination of the N(1) values at the Apollo 11 and Apollo 12 landing sites [16,17]. Our new values [16,17] compared with recently determined radiometric ages via modern sample analyses are consistent with those of [3,4,6,8], supporting the calibration of the lunar cratering chronology curve purposed by [3,4] via our updated landing site studies. References: [1] Wilhelms (1987) USGS Prof Paper 1348. [2] Neukum et al. (1975) The Moon 12, 201-229. [3] Neukum (1983) Habil. thesis, U. of Munich. [4] Neukum et al. (2001) Space Sci Rev 96, 55-86. [5] Stöffler et al. (2006) Rev Min Geochem 60, 519-596. [6] Hiesinger et al. (2000) JGR 105, 2923929275. [7] Hiesinger et al. (2003) JGR 108, E001985. [8] Hiesinger et al. (2012) JGR 11, E00H10. [9] Meyer (2012) Lunar Sample Comp, ARES. NASA. [10] Robinson et al. (2010) Space Sci Rev 150, 81-124. [11] Pieters et al. (1994) Science 266, 1844-11848. [12] Isaacson et al. (2013) JGR 118, 369-381. [13] Scholten et al. (2012) JGR 117, E00H17. [14] FGDC (2006) FGDC-STD-0132016. [15] Blue (1999) Gazette of Plan Nom USGS. [16] Iqbal et al (2017) LPSC 48, #1258. [17] Iqbal et al (2018) LPSC 49, #1002. Acknowledgement: W. I. and H.H. were funded by the German Research Foundation (Deutsche Forschungsgemeinschaft SFB-TRR170, subproject A2) and CvdB was supported by EU H2020 project #776276, PLANMAP. Figure 1: New geological map of the southwestern part of the Mare Tranquillitatis around the Apollo 11 landing site (green triangle), showing mare units, various highland units (including the Fra Mauro and Cayley Formations), a radial pattern of wrinkle ridges, a concentric pattern of rilles, as well as different generations of craters and rays. Figure 2: New geological map of the Apollo 12 landing site (green triangle) in Oceanus Procellarum, which shows mare units, differently aged craters, rays crossing the landing site, highland units (including Fra Mauro Formation and older material), as well as wrinkle ridges and rilles.