Ding et al. Progress in Earth and Planetary Science (2020) 7:32 Progress in Earth and https://doi.org/10.1186/s40645-020-00340-4 Planetary Science RESEARCH ARTICLE Open Access Compositional variations along the route of Chang’e-3 Yutu rover revealed by the lunar penetrating radar Chunyu Ding1,2, Zhiyong Xiao1,3,4* , Yan Su2,5, Jiannan Zhao6 and Jun Cui1,2,4 Abstract Using the high-frequency lunar penetrating radar data obtained by the Chang’e-3 mission, we apply the frequency- shift method to calculate the decay rate of the electromagnetic wave in the regolith-like ejecta deposits of the Ziwei crater. The radar data are divided into segments according to the navigation points along the traverse route of the Yutu rover. For each segment, we calculate the bulk loss tangent of materials within the top ~ 50 ns of the radar data based on the frequency decreasing rate of the electromagnetic wave. The loss tangent varies from ~ 0.011–0.017 along the route of Yutu, and it is within the range of the measured loss tangent of Apollo regolith samples. Using the empirical relationship between loss tangent and TiO2 + FeO content derived from the Apollo lunar samples, we estimate the TiO2 + FeO content for the bulk regolith along the route of Yutu, which is ~ 23–30 wt.%. This value is comparable with that estimated using both orbital reflectance spectral data and in situ observation made by the Yutu rover. The loss tangent derived along the route of Yutu is larger than the average value of returned lunar samples, which is mainly caused by the larger content of TiO2 + FeO at the landing site compared to the global average. Variations of the TiO2 + FeO content along the route of Yutu are mainly due to the excavation of the Ziwei crater. The TiO2 + FeO content map derived by the radar has a much higher spatial resolution compared to orbital observation, testifying the feasibility of this technique for regional geology study. Keywords: Moon, Regolith, Ground penetrating radar, Loss tangent, Chang’e-3 Introduction et al. 2017). Therefore, there is a gap in the spatial reso- Most of our knowledge about the composition of the lution of interpreted compositions using orbital and la- Moon is derived from orbital observations, since the boratory measurements, and caution should be used number of both returned lunar samples and collected when directly comparing the compositional data ob- lunar meteorites is still rather limited. Orbital observa- tained from orbit and those measured for samples at la- tions using spectrometers that work at various wave- boratories. For example, reflectance spectra obtained lengths have perfect spatial coverage, but they are from orbit (normally > 10 m/pixel) are usually compared usually restricted in the spatial resolution. On the other with those measured at laboratories for typical minerals hand, highly heterogeneous materials exist at any places and lunar samples to determine the possible composi- of the Moon due to intense material transportation, mix- tions (e.g., Pieters et al. 2000), but the two measure- ing, and metamorphism by impact cratering (Huang ments have a scale difference of at least an order of magnitude. Recently, Wu and Hapke (2018) analyzed the * Correspondence: [email protected] visible–near-infrared spectrometer data obtained on the 1Planetary Environmental and Astrobiological Research Laboratory, School of lunar surface by the Chang’e-3 mission (i.e., CE-3), and Atmospheric Sciences, Sun Yat-sen University, Zhuhai, China heterogeneous reflectance spectra were observed both at 3State Key Laboratory of Lunar and Planetary Sciences, Space Science Institute, Macau University of Science and Technology, Macau, China the surface and in the near-subsurface materials. Full list of author information is available at the end of the article © The Author(s). 2020 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Ding et al. Progress in Earth and Planetary Science (2020) 7:32 Page 2 of 11 Lunar penetrating radar (i.e., LPR) can also be used to (Irving and Knight, 2003). The high-frequency LPR was deduce the composition of surface materials, especially operated with different gain values during the mission the content of TiO2 + FeO (Schaber et al. 1975). The (Feng et al. 2017), and a constant gain of 0 dB was used rate of energy decay of electromagnetic waves in lunar from the navigation point N105 to N208 (Fig. 1). To regolith is mainly controlled by the content of TiO2 + avoid uncertainties raised by the different gain values FeO (Strangway et al. 1977), so that the loss tangent of (Feng et al. 2017), we use the radar data from N105 to lunar regolith derived from the energy decay rate of N208 here, which contain ~ 1600 valid traces after re- electromagnetic waves can be used to estimate the moving the redundant data. amount of TiO2 + FeO (Campbell et al. 1997). The radar data are processed following the routine Pommerol et al. (2010) noticed that radar echoes procedure (e.g., Su et al. 2014). The Yutu rover was returned from high-Ti regions on the Moon were low, driven with different speeds from the navigation points which were detected by the lunar radar sounder onboard N105 to N208 (Feng et al. 2017). To remedy distortions the Selenological and Engineering Explorer Kaguya mis- in the radargram that are caused by the different moving sion. Applying this method, Campbell et al. (1997) esti- speeds (e.g., data sections N202–203 and N204–206 mated the bulk composition of regolith in the lunar shown in Fig. 1c), we perform a trace equivalent calcula- mare using Earth-based radar data. The LPR onboard tion for the data based on the average rover speed (i.e., the Yutu rover of the CE-3 mission obtained the first in 0.055 m/s). Afterward, the standard processing proced- situ radar profile on the surface of the Moon (Su et al. ure (e.g., remove direct current component and back- 2014). Compared to the radar data obtained by Earth- ground, and band-pass filtering) is applied to obtain the based telescopes (e.g., Campbell et al. 1997), the LPR final LPR radargram (Fig. 2a). data obtained by CE-3 have a much shorter wavelength We use the top ~ 50 ns of the radargram for this (“Data and method” section) and higher spatial reso- study. There is a general consensus that this part of the lution. This work utilizes the CE-3 LPR data to derive radargram corresponds to the continuous ejecta deposits the composition along the route of Yutu, which will fill of the Ziwei crater (Fig. 1a, e.g., Xiao et al. 2015; Fa et al. the observation gap in terms of the spatial resolution be- 2015; Zhang et al. 2019), which is supported by both the tween the orbital (e.g., Zhao et al. 2014) and in situ mea- geological context (Qiao et al. 2016) and the signal tran- surements (e.g., Wu and Hapke, 2018). Furthermore, sition of radar echoes (Xiao et al. 2015). Data at larger instead of probing only the top-most materials, the LPR- depths are less clear in terms of the geological interpret- derived contents of TiO2 + FeO represent the average ation (Xiao et al. 2015; Zhang et al. 2015; Zhang et al. value of materials within the radar detection ranges. 2019), and the signal-to-noise ratio is lower (Xing et al. We introduce the data obtained by the CE-3 high- 2017). To avoid possible controversies regarding the reli- frequency LPR in the “Data” section, the frequency-shift ability of both the data and geological interpretation at technique used to estimate the loss tangent of lunar ma- larger depths, we focus on the ~ 50 ns of the radargram terials and the method used to derive the TiO2 + FeO here. Previous studies have found that materials within content are introduced in the “Estimate of loss and tan- this depth (i.e., the continuous ejecta deposits of Ziwei) gent”. The derived loss tangent values and the TiO2 + have similar properties with typical regolith materials FeO contents are discussed in the “Estimate of loss and (Lai et al. 2016; Xiao et al. 2015). tangent” and “Bulk TiO2 + FeO content” sections, re- spectively, and the reliability of these results is verified Estimate of loss tangent in the “Reliability of the loss tangent estimated” and “Re- We focus on the imaginary part of the relative permittiv- liability of the TiO2 + FeO contents estimated”. Indica- ity (i.e., loss tangent) to invert the content of TiO2 + tions of the results are discussed in “Indications to FeO. Radar permittivity consists of real and imaginary regional geology” section. parts. For the top ~ 50 ns LPR radargram, the real part of the relative permittivity has been estimated using Data and method hyperbolic reflectors in the radargram, resulting in Data values below 5 (Feng et al.