RAA 2020 Vol. 20 No. 1, 8(12pp) doi: 10.1088/1674–4527/20/1/8 R c 2020 National Astronomical Observatories, CAS and IOP Publishing Ltd. esearch in Astronomy and http://www.raa-journal.org http://iopscience.iop.org/raa Astrophysics The subsurface structure and stratigraphy of the Chang’E-4 landing site: orbital evidence from small craters on the Von Karm´ an´ crater floor 1,2 1 1 1,3 ¡ï ÞÏ Ü ³Ñ¤ Xiao-Hui Fu (G ) , Le Qiao ( ) , Jiang Zhang ( ) , Zong-Cheng Ling ( ) 1,4 Ç and Bo Li (Ó ) 1 Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, Institute of Space Sciences, Shandong University, Weihai 264209, China; [email protected] 2 State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Taipa, Macau, China 3 CAS Center for Excellence in Comparative Planetology, Hefei 230026, China 4 State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing 100101, China Received 2019 February 14; accepted 2019 August 2 Abstract Chang’E-4 (CE-4) successfully landed on the floor of the Von K´arm´an crater within the South Pole-Aitken basin (SPA). One of its scientific objectives is to determine the subsurface structure and the thickness of lunar regolith at the landing site and along the traverse route of the Yutu-2 rover. Using orbital data, we employed small craters (diameters <1km) on the floor of the Von K´arm´an crater as probes to investigate the subsurface structure and stratigraphy of the CE-4 landing site. In this study, 40 dark-haloed craters that penetrate through the surface Finsen ejecta and excavate underlying mare deposits were identi- fied, and 77 bright ray craters that expose only the underlying fresh materials but do not penetrate through the surface Finsen ejecta were found. The excavation depths of these craters and their distances from the Finsen crater center were calculated, and the thickness distribution of Finsen ejecta on the Von K´arm´an floor was systematically investigated. The boundary between Finsen ejecta and underlying mare basalt at the CE-4 landing site is constrained to a depth of 18m. We have proposed the stratigraphy for the CE-4 site and interpreted the origins of different layers and the geological history of the Von K´arm´an crater. These re- sults provide valuable geological background for interpreting data from the Lunar Penetrating Radar (LPR) and Visible and Near-infrared Imaging Spectrometer (VNIS) on the Yutu-2 rover. The CE-4 landing site could provide a reference point for crater ejecta distribution and mixing with local materials, to test and improve ejecta thickness models according to the in situ measurements of the CE-4 LPR. Key words: Chang’E-4 — dark-haloed crater — ejecta thickness — Moon 1 INTRODUCTION composition and subsurface structure of the landing site and the geological history of this region (Jia et al. 2018). The Von K´arm´an crater is a perfect location to inves- On 2019 January 3, Chang’E-4 (CE-4) safely landed in the tigate both lunar farside volcanism and deep-seated ma- eastern Von K´arm´an crater (186km in diameter; central co- terials possibly from the lunar upper mantle. The SPA is ordinates 44.4◦S, 176.2◦E) within the 2500-km-diameter an ancient impact structure on the lunar farside (Wilhelms South Pole-Aitken basin (SPA) (Li et al. 2019; Di et al. et al. 1979), which shows distinctive geochemical charac- 2019). CE-4 is the first in situ exploration mission on the teristics and has a unique geological history (e.g., Pieters lunar farside as well as in the SPA basin. One scientif- et al. 1997; Lucey et al. 1998; Jolliff et al. 2000). Except ic objective of this mission is to determine the mineral for ultramafic mantle material exposure, previous studies compositions of surface materials and perform geological also suggest the presence of differentiated impact melt- characterization of the landing site (Jia et al. 2018). The s at the bottom of the SPA basin (e.g., Morrison 1998; Visible and Near-infrared Imaging Spectrometer (VNIS) Hurwitz & Kring 2014; Vaughan & Head 2014). The Von and Lunar Penetrating Radar (LPR) aboard the Yutu-2 K´arm´an crater lies within the Mg-pyroxene annulus de- rover are the two key instruments for revealing the surface fined by Moriarty III & Pieters (2018). Its floor was filled 8–2 X. H. Fu et al.: The Subsurface Structure of the CE-4 Landing Site by mare basalts in the Late Imbrian Period (Pasckert et al. excavated by the primarycrater. The other group comprises 2018). The basalt plains were subsequently reshaped by craters surrounded by low-albedo deposits known as dark- ejected materials from surrounding young craters. Both haloed craters (DHCs) (Head & Wilson 1979; Salisbury mare basalts and mafic materials with abundant Mg-rich et al. 1968). The DHCs are interpreted as resulting from pyroxene are exposed on the floor of the Von K´arm´an the excavation of low-albedo material from the subsur- crater. The morphology, mineralogy, and geochemistry of face, such mare basalts, glass materials or impact melt- Von K´arm´an crater were investigated (Huang et al. 2018; s, although the nature and origin of DHCs are not com- Ling et al. 2018, 2019; Qiao et al. 2019) using orbital data pletely understood (Head & Wilson 1979; Kaydash et al. because it had been listed as the landing site of the CE-4 2014). The presence of DHCs has been taken as an indica- mission (Jia et al. 2018). tor of ancient lunar volcanic deposits or cryptomaria (Bell The subsurface structure of the CE-4 landing site con- & Hawke 1984; Whitten & Head 2015). tains a direct record of the sequence of major geological The crater excavation method can assess the thickness events and the geological history of this area. The im- of Finsen ejecta and determine the vertical boundary be- pact structures in the regional vicinity of the Von K´arm´an tween Finsen ejecta and mare basalts. On the Von K´arm´an crater span the vast expanse of geological time, from the floor, DHCs are craters that punch through overlying ejec- pre-Nectarian SPA basin (Wilhelms et al. 1979) to the ta to excavate underlying mare material. BRCs expose only Nectarian Von K´arm´an crater (Stuart-Alexander 1978), the the underlying fresh materials but do not exhume the mare Imbrian Alder crater (82km in diameter) (Wilhelms et al. basalt layer below Finsen ejecta. Coordinated analyses of 1987) and the Copernican Finsen crater (73km in diame- the crater excavation depths and their distances from the ter) (Ling et al. 2019). Superpositions among these crater Finsen crater center can constrain the ejecta thickness. materials and mare basalt units establish local stratigraphic To test this hypothesis, we searched the small craters sequences. From lunar surface mineralogy and morpholo- in this area and identified the DHCs and BRCs. Their di- gy analysis (Pasckert et al. 2018; Huang et al. 2018; Qiao ameters (as well as estimated excavation depths) and dis- et al. 2019), it has been widely accepted that the Finsen tances from the Finsen crater were collected. Using these crater is the main source of surface ejecta in the eastern orbital datasets, we investigated the thickness and distri- portion of the Von K´arm´an crater floor as well as at the bution of the Finsen ejecta on the Von K´arm´an floor. This CE-4 landing site (Pasckert et al. 2018; Li et al. 2019). study provides substantial geological background for CE- However, several key questions about the subsurface struc- 4 exploration, such as the stratigraphic sequences and ma- ture and stratigraphyare still open. What is the thickness of jor geological events in the SPA. The thickness estimates Finsen ejecta? How much ejected material did these craters for different geological units at the CE-4 landing site are deliver to the Von K´arm´an crater floor? Is the Alder ejecta robust. Future measurements by the Yutu-2 rover will pro- below or above the mare basalt unit? Stratigraphic inves- vide further insights into the subsurface setting of this area. tigations of the CE-4 landing site allow reconstruction of the general sequence of geological events in the region. 2 DATA AND METHODS Small craters can penetrate through lunar surface ge- 2.1 Data ological units and act as probes into the underlying mate- rials. Such craters have been widely used to determine the We generated regional data mosaics of the Lunar thickness of the surface unit and the compositions of lava Reconnaissance Orbiter (LRO) Wide-Angle Camera flows, impact melt sheets beneath them and other hidden (LROC WAC, 100 m pixel−1, Robinson et al. 2010), layers (Budney & Lucey 1998; Chen et al. 2018). Dozens Narrow-Angle Camera (NAC, up to 0.60m pixel−1, of small craters (less than 2km) are found on the floor of Robinson et al. 2010), and Kaguya Terrain Camera (TC) the Von K´arm´an crater. Based on these small crater stud- Ortho mosaic (10m pixel size; Haruyama et al. 2008) to ies, Huang et al. (2018) reported that the thickness of the perform the identification and topographic analysis of s- ∼ ‘fine grain’ regolith varies from 2.5 to 7.5m using the mall craters on the Von K´arm´an floor. More details about method of Quaide & Oberbeck (1968) in the landing re- WAC and NAC image processing are described in Qiao gion. In the present study, we attempt to use these craters et al.