52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 1917.pdf
INVESTIGATING A POTENTIAL SOURCE OF YOUNG AGES AT APOLLO 15 LANDING SITE. W. 1 1 1 2 1 Iqbal , H. Hiesinger , C. H. van der Bogert and J. W. Head , Institut für Planetologie, Westfälische Wilhelms-Uni- versität, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany, ([email protected]), 4Department of Earth, En- vironmental and Planetary Sci-ences, Brown University, Providence, RI 02912 USA.
Introduction: The lunar cratering chronology is be- high ambiguity due to secondary cratering. We intend ing widely used to derive absolute model ages (AMAs) to continue this investigation to test other candidate cra- of geological units on various planetary bodies [e.g., 1- ters e.g., Aristillus crater as sources of these ages. 6]. In our previous series of studies [7-10], we per- Methods: LROC Wide Angle (WAC; 100 m/pixel) formed detailed investigation of the Apollo landing sites and Narrow Angle Camera (NAC; ~0.5 m/pixel) data to test the calibration points for the lunar chronology [14] with incidence angles between 55-80°, and [3]. Here, we investigate the potential source of young SELENE (Kaguya) morning and evening image data reseted radiometric ages recorded in Apollo 15 landing were used for geological mapping and CSFD measure- site samples [e.g., 11,12]. The Apollo 15 landing site is ments. Apart from the image data, various digital eleva- surrounded by ejecta material from many young local tion models (DEMs), including LOLA/SELENE and distant craters, including Autolycus and Aristillus merged DEM [15], and LRO NAC derived DEMs, as craters. The young ages of ~ 2.1 Ga [11,12] found in the well as spectral data, including Clementine [16] and Ka- samples were linked to Autolycus crater. The deter- guya Multiband Imager (MI) data [17], were used. mined isotopic age can be potentially used to derive a CSFDs of the mapped geological homogeneous units new calibration point for the lunar cratering chronology were measured using CraterTools [18] in ArcGIS, and curve [11], if the source crater for this age can be clearly the measurements were plotted and fitted by using Cra- recognized. The new calibration point will be obtained terstats [19]. Randomness analyses [20] were used to by correlating N(1) (cumulative number of the craters further recognize secondary craters. ≥1 km in diameter) values of the mapped areas on the Observations: The Apollo 15 landing site is sur- source crater with the determined sample ages. A previ- rounded by craters of various ages. The large craters ous study [13] thoroughly investigated N(1) values of around the landing site include Archimedes crater, Au- Autolycus crater and found a wide range of values with
Figure 1. Regional geological map of the Apollo 15 landing site. Autolycus crater and Aristillus crater are mapped as Erathosthenian craters. The ejecta material and rays from Aristillus crater resurface Autolycus crater. The ray material extends to the Apollo 15 landing site shown with green triangle.
52nd Lunar and Planetary Science Conference 2021 (LPI Contrib. No. 2548) 1917.pdf
tolycus crater, and Aristillus crater. Small craters in- the alternative interpretation that the source craters are clude St. George crater, Hadley C crater, and Bridge located closer to the landing site can not be completely crater. discarded. On the basis of morphological evidence, Archime- des and St. George are mapped as Imbrian craters, whereas Autolycus, Aristillus, and Hadley C craters are Eratosthenian craters (Figure 1) and Bridge crater was mapped as a Copernican crater. Thus, we considered the later four craters as candidate sources of young ejecta material around the landing site. The investigation of Autolycus crater [13] did not provide consistent N(1) values and AMAs, most likely due to high resurfacing from the Aristillus crater as mapped in Figure 1. Our study yielded CSFD measurements on the rim-terraces and floor of Aristillus crater (Figure 2). The preliminary results show an AMA of ~2 Ga with large errors. These errors are most likely due to the population of self-sec- ondary craters present in our selected count areas.
Figure 3. Thorium concentration of the area around the Apollo 15 landing site shows higher concentrations (blue) for Aristillus crater.
Acknowledgements: WI was funded by the German Research Foundation (Deutsche Forschungsgemein- schaft SFB-TRR170, subproject A2) and CvdB was supported by EU H2020 project #776276, PLANMAP. References: [1] Hartmann (1970) Icarus 13, 299- 301. [2] Neukum et al. (1975) The Moon 12, 201-229. [3] Neukum (1983) NASA TM-77558. [4] Neukum et al. (2001) Space Sci. Rev. 96, 55-86. [5] Robbins (2014) EPSL 403, 188-198. [6] Stöffler et al. (2006) Rev. Min. Geochem. 60, 519-596. [7] Iqbal et al. (2019) Icarus 333, 528-547. [8] Iqbal et al. (2020) Icarus 3352, 113991. [9] Iqbal et al. (2019) LPSC 50, 1005. [10] Iq- bal et al (2020) LPSC 49, 1073. [11] Ryder et al. (1991) Geology 19, 143-146. [12] Bogard et al. (1990) Geo- chim. Cosmochim. Acta 54. [13] Hiesinger, et al (2016) LPSC 47, 1879. [14] Robinson et al (2010) Space Sci. Rev. 150, 81-124. [15] Barker et al. (2016) Icarus 273, 346-355. [16] Pieters et al. (1994) Science 266, 1844- Figure 2. Preliminary CSFD measurements on the crater rim 1848. [17] Ohtake et al (2013) Icarus 226, 364-374. [18] (black) and floor (blue) of the Aristillus crater. Kneissl et al. (2011). PSS 59, 1243-1254. [19] Michael Discussion: The three samples (15434,25, 15434,29 et al. (2016) Icarus 277, 279-285. [20] Michael et al. and 15358) collected near Spur crater contains KREEP (2012) Icarus 218, 169-177. [21] Jolliff et al. (2000) basalts of ~3.8 Ga and contain yellow glass in a matrix JGR 105, 4197-4216. of ~2.1 Ga [11,12], which lead to the conclusion the
source crater of these samples lies in the KREEP-rich
basalts. The thorium map (Figure 3) of Lunar Prospec-
tor shows that the Aristillus crater exposed thorium-rich
material, possibly from KREEP-rich basalts [21]. This supports the interpretation that Aristillus crater is a likely candidate for the source of the samples. However,