Geological mapping of the South Pole Aitken Basin: A progress report

GEOLOGICAL MAPPING OF THE SOUTH POLE AITKEN BASIN: A PROGRESS REPORT. C. M. Poehler1, H. Hiesinger1, W. Iqbal1, C. Rueckert1, C. H. van der Bogert1, and M. A. Ivanov2. 1Institut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Str. 10, 48149, Münster, Germany, [email protected], 2Vernadsky Inst., RAS, Russia Introduction The South Pole-Aitken (SPA) basin, situated on the lunar farside and centered at 53°S 169°W, is the largest [1-5] and oldest basin [6,7] on the Moon. Due to its morphological appearance it is argued that the SPA basin formed in an already solidified lunar crust [1]. Therefore, the timing of the formation of SPA basin gives valuable information on the evolution of the lunar crust. The large scale of the impact led to the hypothesis that it penetrated the crust and potentially exposed lunar mantle material [e.g., 8]. Thus, it might be possible to observe lunar mantle material or lower crustal material excavated by the impact event [e.g., 9]. Recently, the SPA has been the focus of several ongoing and upcoming missions [10-13]. On January 2, 2019, Chang’e 4 made the first soft-landing on the farside of the Moon in Von Kármán crater (175.9° E and 44.8° S) [11]. Sites in SPA are also being examined as possible landing sites for ESA’s HERACLES mission study [14-16]. Here, we aim to compile a complete new geological map of the SPA basin in the context of PLANMAP [17]. The first part, the NE section surrounding the Apollo basin, has already been published by [12], and we are using the same units and stratigraphic relations to extend the map to the rest of the basin. Method: We are using Lunar Reconnaissance Orbiter (LRO) Wide-Angle Camera (WAC) (100 m/pixel), Narrow-Angle Camera (NAC) (0.5m/pixel) [18], and Kaguya (10 m/pixel) data as base imagery to allow geomorphological and albedo contrast mapping, as well as crater size-frequency distribution (CSFD) measurements. We are also applying a hybrid spectral mapping approach, using Clementine [19], M3 [20], and Kaguya MI [21] data to aid in subdividing geological units. Topographic features are identified using Lunar Orbiter Laser Altimeter (LOLA) digital elevation models and the LOLA/Kaguya merged digital elevation model (DEM) that has the resolution of 512 pixel/degree [22]. We use the standards described in [23] for planetary mapping and the nomenclature is consistent with [24], in addition to project specific recommendations for presentation of nonstandard data sets and mapping products [25]. Unit descriptions: We identified different geological units based on their geomorphological, albedo, and spectral appearances. In general, the units can be divided into plains-forming units and craters/crater-related units. The stratigraphic relations of these units are identified and described by [12]. Several types of crater and crater-related units have been described in the SPA [12]. The specific characteristics of individual impact craters, such as the presence or absence of rays, discernible ejecta blankets and chains of secondarys craters, as well as the degree of degradation of the crater rims are used to classify different crater and crater-related units into a relative stratigraphy (Fig. 1). The ages of specific marker craters are then used to fix the relative ages to an absolute age scale [e.g., 12]. Fig. 1. Examples of crater and crater-related units. (A) Cc with extensive ejecta and ray system, (B) Ec with prominent ejecta but without rays, (C) sharpcrested cratered unit Ic without discernible ejecta or rays, (D) unit with abundant degraded craters (NpNc). Copernican craters (Cc) are the youngest and freshest craters in the study area. They crosscut all other landforms and show extensive ejecta deposits and prominent systems of rays and secondary craters (Fig. 1a). Erathostenian craters (Ec) are characterized by sharp-crested craters with no extensive ray system. They are often small and are numerous throughout the study area (Fig. 1b). However, some are larger craters such as Finsen crater (62 km diameter). Imbrian craters (Ic) show a higher degree of degradion with subdued ejecta that indicates them to be older than Ec and Cc craters (Fig. 1c). Nectarian, pre-Nectarian craters (NpNc) is a unit typically made of large, degraded craters that show mostly complete rims, rarely with visible ejecta deposits. The craters of this unit are abundant and often overlapping (Fig. 1d). The oldest units are associated with the primary features of the SPA basin. These units represent areas of ancient rugged surfaces and consist of chaotically oriented short ridges and equidimensional blocks of several kilometers across. Both the ridges and blocks likely represent remnants of the oldest craters that have been destroyed by the formation of younger craters. According to their locations we divide these pre-Nectarian features into SPA Floor (pNm_SPAf) and rim material (pNm_SPAr). Whithin the SPA rim, materials of the unit pNm_SPAr surround the high-standing massifs of the basin rim (pNrm_SPA) (Fig. 2). Within the study area three types of plainsforming units can be identified (Fig. 3). These units are set apart by their different albedos and morphological surface structures. Upper Imbrian dark plains (UIdp) are identified by the noticeablely lower albedo compared to the surrounding terrains. The low albedo is often overprinted by higher albedo swirls, such as seen in Mare Ingenii. UIdp shows smooth surfaces and is predominantly found as filling in larger craters (Fig. 3a). Imbrian light plains (Ilp) are similar in morphological appearance to UIdp, but differ in albedo. Ilp appear to have the same albedo as the adjacent crater-related landforms (Fig 3b). The light plains are scattered throughout the map area. Lower Imbrian rugged terrain (LIrp) shows a rugged morphology due to numerous low, curvilinear ridges. LIrp appear as larger continuous plains especially towards the center of SPA. Fig. 3. Examples of plains-forming units. (A) UIdp represents low albedo mare units. (B) Ilp is similar in appearance to UIlp but shows higher albedo Acknowledgements: This work receives funding from the European Union Horizon 2020 research and innovation programme under grant agreement No 776276. References: [1] Stuart-Alexander (1978) USGS Map I-1047, 1978. [2] Spudis et al. (1994) Science 266, 1835-1839. [3] Hiesinger and Head (2004) PLPSC 35, 1164. [4] Shevchenko et al. (2007) Solar Sys. Res. 41, 447-462. [5] Garrick-Bethell and Zuber (2009) Icarus 204, 399-408. [6] Wilhelms (1987) USGS SP-1348, 302. [7] Hiesinger et al. (2012) LPSC 43, 2863. [8] Melosh et al. (2017) Geology 45, 1063-1066. [9] Yamamoto et al. (2010) Nature Geoscience 3, 533-536. [10] Hiesinger et al. (2018) LPSC 49, 2070. [11] Huang et al. (2018) JGR 123, 1684-1700. [12] Ivanov et al. (2018) JGR 123, 2585–2612. [13] Joliff et al. (2010) LPI Cont 1595, 3072. [14] Steenstra et al. (2016) Adv Space Res 58, 1050-1065. [15] Allender et al. (2018) Adv Space Res 63, 692-727. [16] Hiesinger et al. (2019) LPSC 50, 1328. [17] Poehler et al. (2019) LPSC 50, 2910. [18] Robinson et al. (2010) Space Sci. Rev 150, 81124. [19] Pieters et al. (1994) Science 266, 184411848. [20] Isaacson et al. (2013) JGR 118, 369381. [21] Ohtake et al (2013) Icarus 226, 364-374. [22] Barker et al. (2016) Icarus 273, 346-355. [23] FGDC (2006) FGDC-STD-013-2016. [24] Blue, J. (1999) Gazette. Planet. Nomen. USGS. [25] wiki.planmap.eu/display/public/D2.1-public Fig. 2. Examples of the oldest morphological units (pNmSPAr rim massif, pNmSPArm rim material, and pNmSPAf floor material) associated with the SPA rim.