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 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 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