Geological mapping and absolute model ages around the Apollo 17 landing site

GEOLOGICAL MAPPING AND ABSOLUTE MODEL AGES AROUND THE APOLLO 17 LANDING SITE. W. Iqbal1, C. H. van der Bogert1, and H. Hiesinger1, 1Institut für Planetologie, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany, ([email protected]). Introduction: The crater size-frequency distributions (CSFD) and their related N(1) values of the area around the Apollo and Luna landing sites are calibrated against the radiometric ages [e.g., 3] of the selected samples and provide important calibration points for the lunar cratering chronology [4,5], which were later modified to derive the absolute model ages on the various terrestrial bodies in the Solar System [e.g., 4-9]. Thus, the accuracy of the lunar cratering chronology can be reexamine by using up-to-date data [10-13] and sample analyses [e.g., 3]. The diverse geology [e.g., 1,2] around the Apollo 17 landing site provide insights for several key lunar geological events that feed into the lunar cratering chronology. Thus, we produced new geological maps at scales of 1:800,000 and 1:143,000 around the Apollo 17 landing site, and measured new CSFDs for the various units using data collected by recent missions. Subsequently, we compared our results with the previous analyses of [4] and [14] for the evaluation of lunar cratering chronology through the calibration points gained from the landing site. Methods: We used Lunar Reconnaissance Orbiter Camera (LROC) images [10] for defining albedo contrasts, the LOLA/Kaguya merged digital elevation model (DEM) [11] for topographic analysis, and Clementine [12] and Kaguya Multiband Imager (MI) [13] data for investigating spectral differences. The used LRO Wide Angle Camera (WAC) data have pixel scales of 100 m and Narrow Angle Camera (NAC) data have a pixel scale of ~1.3 m. The data we used have incidence angles ranging from 69-76°. The WAC and NAC data was calibrated and map-projected in ISIS3 [15]. CraterTools [16] in ArcGIS was used to measure the CSFDs of the different units. We used CraterStats [17] for plotting the data in cumulative and relative plots with pseudolog binning. Although the obvious secondary crater chains and cluster were avoided during area definition and measurement, we still used randomness analysis [18] to avoid fitting diameter ranges where clustering of the craters may indicate the presence of secondary craters. Geological Mapping: The geological events observed in Taurus Littrow Valley are divided into four time periods: Copernican, Post-Imbrian. Imbrian, and Pre-Imbrian (Fig. 1 and 2). The Copernican events (Figure 1, 2a) include the deposition of light mantle material or LMD (Cls1 and Cls2) and secondary crater material from the Copernican craters (Csc, Cdec, Cld and Ccr). These events have been widely considered to be Tycho-triggered [1,2,14]. However, due to spectral [1] and albedo [19] differences within the landslide deposits, it has been proposed that the landslide material may have been deposited during seismic shaking associated with the formation of the Lee Lincoln scarp. The ages determined around the scarp [20,21] are very similar to the ages determined for the light mantle deposit [14]. We also mapped and determined ages for the Paint-Splatter [1,2] feature and light mantle deposit (Cld), which is also considered to be the material related to the Tycho crater [2]. A variety of mare basalt units (Im1, Im2, Im3, Im4, Im5, Im6, Im7, Im8, Im9 and Im10) were deposited in the post-Imbrian period, which were mapped on the basis of Clementine data (shown in Fig 1). The mare units in the mapping region was previously mapped by [22] on the Galileo data, which has slight differences in the mapped boundaries, due to different pixel scales of the both data set. The Taurus Littrow valley floor is covered with mare basalts and pyroclastic materials of similar ages (CIdm) [22,23]. Through observations of the albedo and spectral contrasts, we identified pyroclastic mantling in the depressions of the highlands (CIdmh) [1] around the Taurus Littrow valley. Potentially the pyroclastic material (CIdm and CIdmh) noticed in Clementine data, may originated from the rilles (Normal Faults in Fig. 1) in the west of the Taurus Littrow, and deposited in the valley with associated mare material as well as in the surrounding highland area via pyroclastic surges [2]. On the contrary, [1] argues the pyroclastic material deposited in the valley may originated from the “pyroclastic fissure” noticed on the Sculptured Hills. The highlands around the valley mostly consists of Imbrian terrain (It), likely ejecta material from the Imbrium basin. Though, the North and South massifs may belong to the pre-Imbrian Serenitatis crater wall or ejecta material [1,2], superimposed by Imbrian material (IpIr). CSFD Measurements: CSFDs for the different geological units were determined using LRO NAC images and DTMs [7]. The CSFD measurements on the Paint Splatter (unit Cld) Figure 1. New geological map of the region surrounding Taurus Littrow valley. The units include Imbrian and pre-Imbrian highlands, Imbrian basalts and plains, and different generations of the craters and structural features. Inset shows the area covered by a local geological map shown in Figure 2a. 2.a show an N(1) of 2.72x10-5 km-2 with a model age of 32.4±5 Ma, which is consistent with AMAs (~32-38 Ma) and N(1) (~2.74x10-5 km-2) values determined by [14] for Tycho impact melt pools. Although the nature of this unit is still unclear, it was previously interpreted as a possible impact melt deposit due to its relationship to other secondary materials from the Tycho crater [2,24]. However, the young age and the lobate features around the Paint Splatter may instead be related to seismic events. We also determined an N(1) value of 8.59x105 km-2 and the age of 103±3.5 Ma on the Central Cluster (CC or Cc in the map), which is also consistent with the determined age of the Tycho crater [14]. Nevertheless, the values determined on the units Cld and Cc may be influenced by resurfacing, undetectable secondary craters contamination, and the small sizes of the selected areas. On the basis of albedo contrast, we mapped and measured CSFDs for two units on the LMD; Cls1 shows an N(1) value of 7.04x10-5 km-2 and AMA of 84.0±4.5 Ma, and Cls2 shows two N(1) values of 8.69x10-5 km-2 and 6.34x10-4 km-2 and AMAs of 104±4.9 Ma and 757±180 Ma, respectively. [14] determined similar N(1) values and AMAs on the unit Cls1 for the determination of the calibration point for the Tycho crater on the lunar chronology curve [4,5]. The values determined on the Cls2 unit are similar to the values determined by [20, 21] for the Lee Lincoln scarp. The origin of the older age appearing in the Cls2 unit is not clear. The CSFD measurements on the CIdm unit show an N(1) value of 1.03x10-2 km-2 and model age of 3.70±0.037 Ga, which are consistent with values determined by [22,23], as well as radiometric ages of high Ti mare basalts [9]. In the future, we will compare our new geological maps and measured CSFD values with the most recent radiometric and exposure ages of the collected samples to evaluate and improve the lunar cratering chronology [4,5]. Acknowledgements: W. I. and H.H. were funded by the German Research Foundation (Deutsche Forschungsgemeinschaft SFB-TRR170, subproject A2) and C.vdB. was supported by EU H2020 project #776276, PLANMAP. 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(2000) JGR 105, 29239-29275. [23] van der Bogert et al. (2016) LPSC 47, 1616. [24] Lucchitta (1977) Icarus 30, 80-96. Figure 2. (a) New geological map of the area around Apollo 17 landing site in Taurus Littrow valley. The map shows preImbrian North and South Massifs (IpIr), Imbrian Sculptured Hills (It), valleys filled with pyroclastic and basalts material (CIdm), pyroclastic mantling (CIdmh) on the highland units, light mantle deposits (Cls1 and Cls2), secondary craters cluster (Cc), and other young crater materials (Csc and Ccr). (b) CSFD measurements and absolute model ages of selected geological units at the Apollo 17 landing site: the Paint-Splatter (unit Cld in map), the Central Cluster (unit Cc in map), the light mantle deposits (unit Cls1 and Cls2 in map), and the underlying pyroclastic unit (CIdm in map). All results are shown in cumulative form with cumulative age fits. The randomness analysis of the count area shows potential secondary crater contamination at crater diameters smaller than 10 m (panel above each plot), which were avoided while fitting the AMAs for each unit. a b