Journal of Earth Science, Vol. 27, No. 4, p. 707–715, August 2016 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-015-0655-3 Subsurface Structures at the Chang’e-3 Landing Site: Interpretations from Orbital and In-Situ Imagery Data Le Qiao, Zhiyong Xiao, Jiannan Zhao, Long Xiao* Planetary Science Institute, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China ABSTRACT: The Chang’e-3 (CE-3) spacecraft successfully landed on one of the youngest mare sur- faces on the Moon in December 2013. The Yutu rover carried by CE-3 was equipped with a radar sys- tem that could reveal subsurface structures in unprecedented details, which would facilitate for under- standing regional and global evolutionary history of the Moon. Based on regional geology, cratering scaling, and morphological study, here we quantify the subsurface structures of the landing site using high-resolution orbital and in-situ imagery data. Three layers of lunar regolith, two layers of basalt units, and one layer of ejecta deposits are recognized at the subsurface of the landing site, and their thicknesses are deduced based on the imagery data. These results could serve as essential references for the on-going interpretation of the CE-3 radar data. The ability to validate our theoretical subsurface structure using CE-3 in-situ radar observations will improve the methods for quantifying lunar sub- surface structure using crater morphologies and scaling. KEY WORDS: Chang’e-3, subsurface structure, impact cratering, Lunar Penetrating Radar, lunar ex- ploration. 0 INTRODUCTION the Apollo 17 mission in 1972, this equipment was carried to Subsurface structure of the Moon is a direct record of its explore a portion of the Taurus-Littrow region. The Apollo 17 long and complex geological evolutionary history. Multi- SEP experiments indicated that subsurface boulders of dimension episodes of magmatic activities produced vast volume of ba- in the range of 10–300 m were not present within depths of 1–2 salts with various chemical compositions both on the lunar km from the surface (Simmons, 1974). Besides, the analyses and surface and within the crust. Long-term space weathering and interpretations of these in-situ observation data were usually impact gardening produced a global layer of loose regolith. restricted by certain models, and sometimes it was difficult to While meteoritic impact processes, including both primary and find a model which satisfied most of the observed results. secondary impacts, and tectonic activities would additionally Several indirect techniques have been proposed to quantify complicate the subsurface geology of the Moon. Moreover, the subsurface structures of the Moon. For example, using seis- these geological activities were usually intertwined with and mic experiments (Nakamura et al., 1975), morphologies and influenced by each other, which made the subsurface structure size-frequency distributions of small fresh craters (Fa et al., 2014; further complex. Quantifying the subsurface stratigraphic and Quaide and Oberbeck, 1968), and radar data (Ono et al., 2009; tectonic features of the Moon, including basaltic rocks, lunar Shkuratov and Bondarenko, 2001) to estimate regolith thick- regolith, and impact breccia deposits, with a depth from a few nesses; using crater size-frequency distributions (Hiesinger et al., to hundreds of meters can provide key information for solving 2002), excavation depths of large craters that penetrated (or scientific mysteries concerning regional and global origin and failed to penetrate) through the mare basalts (Thomson et al., evolutionary history. 2009), and gravity data (Talwani et al., 1973) to estimate thick- The most direct and effective technique for determining the nesses of basalt flows; developing empirical scaling laws to es- subsurface structure of the Moon is in-situ investigation. How- timate thicknesses of crater ejecta (McGetchin et al., 1973). All ever, due to the high technical demand and cost for conducting these methods are essential complements for quantifying the lunar surface experiments, few in-situ measurements of the lunar subsurface structure besides in-situ measurements, and they pro- subsurface structure had been carried in human’s lunar explora- vide important references for analyzing and interpreting lunar tion history. So far the only in-situ experiment was the Surface surface experiment observations. Electrical Properties (SEP) experiment carried out during In December 2013, China’s Chang’e-3 (CE-3) spacecraft, carrying the Yutu (Jade Rabbit) rover, successfully landed on *Corresponding author: [email protected] the northern Mare Imbrium at the lunar nearside (Zhao J N et © China University of Geosciences and Springer-Verlag Berlin al., 2014). The landing area is one of the youngest mare basalt Heidelberg 2016 units on the Moon (Morota et al., 2011; Bugiolacchi and Guest, 2008), which has never been explored before. The rover carries Manuscript received November 18, 2014. a set of Lunar Penetrating Radars (LPRs) to detect the subsur- Manuscript accepted March 25, 2015. face information of the Moon (Xiao, 2014). The LPRs consist Qiao, L., Xiao, Z. Y., Zhao, J. N., et al., 2016. Subsurface Structures at the Chang’e-3 Landing Site: Interpretations from Orbital and In-Situ Imagery Data. Journal of Earth Science, 27(4): 707–715. doi:10.1007/s12583-015-0655-3. http://en.earth-science.net 708 Le Qiao, Zhiyong Xiao, Jiannan Zhao and Long Xiao of two types of antennas with different working channels, ena- substantially richer in olivine and thorium (Th) element com- bling resolving the subsurface structure of the landing site at pared with the other lunar basalts (Wu et al., 2014; Lucey, 2004; different depths and vertical resolutions. The Channel 1 an- Lawrence et al., 2000). The basalts at the landing site have a tenna was a monopole mounted on the tail of the rover, oper- higher content of titanium than those ~15 km to the north (TiO2 ated at a central frequency of 60 MHz (VHF band), with over ~1.4 wt.%, Zhao J N et al., 2014), indicating two episodes of 400 m penetration depth and ~1 m thickness resolution. The lava infilling events with different lava compositions. Crater Channel 2 has two dipole bow-tie receiving antennas mounted counting results suggested an Imbrian model age (>3.0 Ga) for at the bottom of the rover, operated at a central frequency of the basalts to the north, and an Eratosthenian model age (~2.5 500 MHz (UHF band), with over 12 m penetration depth and Ga) for the basalts of the CE-3 site (Qiao et al., 2014; Morota et ~30 cm thickness resolution (Zhao N et al., 2014). The LPRs al., 2011; Chen et al., 2010; Bugiolacchi and Guest, 2008; Hi- were scheduled to collect radar echoes of the lunar shallow esinger et al., 2000), which was much younger than the re- subsurface materials along the rover traverse path. Observa- turned lunar basalt samples (most Apollo basalt samples are tions from the CE-3 LPRs will provide an unprecedented op- dated older than 3.0 Ga, Stöffler and Ryder, 2001). So regional portunity to investigate the shallow subsurface structure at the magmatic activities at the CE-3 site generally evolved from landing site. To assist the interpretation of the CE-3 LPRs data, Imbrian-aged low-titanium to Eratosthenian-aged medium- here we quantify the subsurface structures at the landing site of titanium. Featuring one of the youngest basalts on the Moon CE-3 using crater morphologies and scaling on both orbital and that has not been sampled by any previous missions (Giguere et in-situ imagery data. The mutual validation between the result al., 2000; Papike et al., 1976), the CE-3 landing site is indica- of this work and CE-3 LPRs data will improve methods for tive for revealing the nature of late-stage volcanism on the quantifying lunar subsurface structures using crater morpholo- Moon. gies and scaling. 2 METHODOLOGY 1 GEOLOGICAL SETTINGS OF THE CE-3 LANDING Two units of mare basalts that have different ages have SITE been identified at the CE-3 landing site. Due to the ~0.5 b.y.r The CE-3 spacecraft landed on a flat mare surface (cen- age difference between the basalts (Morota et al., 2011; Bugio- tered at 44.12°N, 19.51°W, Liu Z Q et al., 2014, Fig. 1) in the lacchi and Guest, 2008), a regolith layer must have been northern Mare Imbrium. The CE-3 landing site has higher lati- formed on the top of the underlying Imbrian basalts. After the tude than any previous soft landing sites (Fig. 1). Remote sens- formation of the Eratosthenian basalts at ~2.5 Ga, meteorite ing observations proposed a TiO2 content of ~7 wt.% for the impacts and space weathering became the major surface proc- basalts at the landing site (Zhao J N et al., 2014), which is in- esses which have formed a regolith layer on the top of the sur- termediate compared with the high-titanium basalts (Apollo 11 face Eratosthenian basalts. Some large crater with diameters of and 17 basalts) and low-titanium basalts (Apollo 12, 15 and hundreds of meters surrounding the CE-3 landing site might feldspathic basalts). The basalts at the landing site are also have penetrated (or failed to penetrate) through this regolith Figure 1. (a) Landing sites of the Chang’e-3 and previous soft landing exploration missions. The base map is a mosaic of lunar nearside generated from images acquired by the Lunar Reconnaissance Orbiter (LRO) wide-angle camera (WAC). Image credit: NASA/GSFC/Arizona State University. (b) Local detailed image (LRO WAC mosaic image) of the Chang’e-3 landing site and surrounding areas. The dashed line represents the boundary of medium-titanium basalt unit at landing site and northern low-titanium unit.