Geologic Map of the Lalande Crater Region

GEOLOGIC MAP OF THE LALANDE CRATER REGION L. Daszkiewicz, W. Iqbal, C. H. van der Bogert, and H. Hiesinger. Institut für Planetologie, Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany ([email protected]) Introduction: The lunar crater Lalande has a diameter of 23.4 km and is located in the equatorial region of the Moon (4.45°S 8.60°W) in the Procellarum KREEP Terrane [1]. The area is widely covered by ejecta and shows a distinct system of lunar rays. The formation age of Lalande is still under debate. Crater size-frequency distribution (CSFD) measurements based on Lunar Orbiter images dated Lalande’s formation to between 2.2 and 2.8 Ga, which would imply that Lalande formed in the Eratosthenian Period [2]. However, the distinct ray system and the sharp crater rim of Lalande support the theory that Lalande’s formation age is Copernican [3,4] Indeed, while it is known that lunar rays can be older than ~ 1.0 Ga due to compositional distinct feldsparrich ejecta [5], more recent CSFD measurements indicate a Copernican formation age of 410 ± 20 Ma for Lalande and an Imbrian age of 3.8-0.06+0.04 Ga for the mare basalts that are now covered by Lalande’s ejecta blanket [6]. The newest version of the unified global lunar geologic map [7] also depicts Lalande as a Copernican crater. Our new geologic map has a special focus on the rays and ejecta distribution of Lalande crater. Geological units: Mapping at a scale of 1:200,000, we find different terra, plains, crater, mare and basin units surrounding Lalande. In addition, we divided the Lalande ejecta materials into different units. Terra units are often characterized by elevated terrain, a hummocky appearance, and a relatively high density of craters. Nt (Nectarian terra) are characterized by a moderately high relief and rugged surface with a complex mixture of degraded materials and ejecta burying the underlying craters of varying ages [7] It (Imbrian terra) exhibits a low relief and a moderate to high albedo. A lot of small-sized craters with Imbrian age can be found in the moderately smooth surface. Plains units are found in topographic lows such as crater floors and show flat and smooth surfaces. Ip (Imbrian plains) are smooth units observed in the low-lying areas, for example the crater floors of the ancient Nectarian craters. The crater density is moderate in comparison to the other units. The formation of this unit thought to be due to the emplacement of ejecta from older impact events. Im (Imbrian mare) is a dark, smooth plains unit, interpreted to be mare basalt deposits. These deposits have a lower crater density and show a flat and smooth surface with the lowest albedo in the mapping area. Basin units are remnants of basin rims or bedrock that was uplifted when impact basins formed. These basin units are often rugged with high relief. Nbm (Nectarian basin, massif) are rugged blocks or raised ridges, which were uplifted during the formation of Nectarian-aged basins [7]. Nbl (Nectarian basin, lineated) are sharp raised ridges with a lineation pointing towards Mare Imbrium. [7]. Crater materials are associated with impact basins and large craters. The different units cover craters, crater rims, ejecta deposits, secondary materials, and crater rays. Nc (Nectarian craters) exhibit a lower relief than Ec and Cc due to their greater extent of degradation. Thus, the crater rims are highly degraded and are partially superposed by other units. Additionally, there is little to no identifiable ejecta present. Ec (Eratosthenian craters) show a lower albedo and less sharp rims compared with Cc. These craters are circular and the ejecta material can be partially discerned. However, rays are absent. Cc (Copernican craters) are characterized by well-defined rim, wall and floor deposits. The crater rims are sharp and circular. Often the craters have a high albedo and are typically surrounded by bright ray material. Csc (Copernican secondary craters) are composed of small craters, which are densely spaced and located on or near the ejecta materials. Secondary craters can also form as chains and clusters outside the continuous ejecta. Ccr (Copernican crater ray material) are filamentous high albedo features that show a radial or subradial pattern around fresh impact craters and consist of ejecta material. The rays are often discontinuous, and their size correlates with the size of the parent crater. The rays do not change the topographic relief noticeably [5]. Ccrh (Ejecta, hummocky) describes the ejecta material near the crater rim. The topography is hummocky and is also characterized by big boulders. Ccrr (Ejecta, radial) is described as the most extensive part of the ejecta materials. The most prominent feature is a radial pattern pointing towards the parent crater similar to Ccr. Conclusion: The detailed geologic map of the Lalande crater region depicts the distribution of the ejecta from Lalande with a special focus on the crater rays and ejecta. The map provides a first step to understanding the geological history and processes active in the region. Future work for the Lalande crater area will make further investigations of La- lande’s formation age. CSFD measurements on welldefined areas will help to check and refine the model ages of the mapped units. Furthermore, the area is characterized by very thorium-rich ejecta and a high silica anomaly. This raises the question of whether Lalande crater excavated upper crustal or lower crustal material, which was then distributed in the bright rays. References: [1] Jolliff et al. (2000) JGR, 105, 4197-4216. [2] Baldwin (1985) Icarus 61, 63-91. [3] Wilhelms et al. (1987) USGS. [4] Stöffler and Ryder (2001) Space Science Reviews 96, 9–54. [5] Hawke et al. (2004) Icarus 170, 1-16. [6] Xu, Le Qiao et al. (2022) Icarus 386, 115166. [7] Fortezzo et al. (2020) USGS.