Impact Melt Breccia and Surrounding Regolith Measured by Chang'e-4 Rover

Earth and Planetary Science Letters 544 (2020) 116378

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Earth and Planetary Science Letters

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Impact melt breccia and surrounding regolith measured by Chang’e-4 rover ∗ Sheng Gou a,b, Zongyu Yue a,c, Kaichang Di a,b,c, , Jia Wang d, Wenhui Wan a, Zhaoqin Liu a, Bin Liu a,b, Man Peng a, Yexin Wang a, Zhiping He e, Rui Xu e a State Key Laboratory of Remote Sensing Science, Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100101, China b State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, China c CAS Center for Excellence in Comparative Planetology, Hefei 230026, China d Science and Technology on Aerospace Flight Dynamics Laboratory, Beijing Aerospace Control Center, Beijing 100094, China e Key Laboratory of Space Active Opto-Electronics Technology, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China a r t i c l e i n f o a b s t r a c t

Article history: Chang’e-4 rover discovered a dark greenish and glistening impact melt breccia in a crater during its Received 15 October 2019 traverse on the ﬂoor of Von Kármán crater within the South Pole Aitken (SPA) basin on the lunar farside. Received in revised form 24 January 2020 The discovered breccia, being 52 × 16 cm, resembles the lunar impact melt breccia samples 15466 and Accepted 28 May 2020 70019 that returned by the Apollo missions. It was formed by impact-generated welding, cementing and Available online xxxx agglutinating of lunar regolith and breccia. Clods surrounds the breccia-hosting crater were crushed into Editor: W.B. McKinnon regolith powders by the rover’s wheels, indicating the regolith may be compacted slightly and becomes Keywords: blocky and friable. Relative mineral fractions are estimated from the in situ measured spectra by a impact melt breccia Hapke model-based unmixing algorithm. Unmixing reveals that plagioclase (PLG, 45 ± 6%) is dominant Hapke model in the regolith, followed by almost equal fractions of pyroxene (PYX, 7 ± 1%) and olivine (OL, 6 ± 2%), relative mineral fractions indicating the regolith is likely related to noritic rocks. The regolith measured by Chang’e-4 rover was noritic rocks actually a highly mixture of multiple sources, with ejecta from Finsen crater being primary and possible differentiated melt pool contributions from Alder crater. Finsen and Alder craters are on the margin of the proposed impact igneous rocks melt pool produced by the SPA basin-forming event. Therefore, the ultimate source of the regolith might originate from a differentiated melt pool or from a suite of igneous rocks. © 2020 Elsevier B.V. All rights reserved.

1. Introduction seismic experiments are about 4.4, 3.7, 8.5, 4.4, 12.2 and 4 m, respectively (Cooper et al., 1974; Nakamura et al., 1975). Recent The lunar regolith mainly consists of unconsolidated rock and in situ penetrating radar measurements revealed that the regolith mineral fragments, breccias, glasses, and agglutinates (e.g., Simon thicknesses at the Chang’e-3 and Chang’e-4 probe landing sites et al., 1981; Houck, 1982; Simon et al., 1982). It was formed by are about 3.2 m and 11.1 m, respectively (Lai et al., 2016, 2019). meteorite impact and gardening on the lunar surface over bil- The composition of lunar regolith, in addition to provide potential lions of years. It covers nearly the entire lunar surface (McKay et resources for future human lunar explorations, records both the lu- al., 1991), except the underlying bedrocks occasionally exposed on nar geological evolution and its interaction with the dynamic space lava channels, the walls of steep-sided craters (e.g., Galilei crater), environment (Lucey et al., 2006). or the central peaks of complex craters (e.g., Copernicus crater). According to the popular crystallization models of the Lunar The average thickness of the lunar regolith, determined by remote Magma Ocean (LMO) hypothesis, olivine and pyroxene are rich sensing techniques or in situ measurements, varies over mare and in the lunar upper mantle due to the cumulate mantle overturn highland areas from 4–5 m to proximately 10–15 m, respectively (Ringwood and Kesson, 1976). In order to constrain the lunar (McKay et al., 1991). For example, the regolith thicknesses at the evolution, it is critically important to investigate the composition Apollo 11, 12, 14, 15, 16 and 17 sites estimated from the in situ and structure of the lunar deep-seated materials. The SPA basin, stretching about 2500 km across and 13 km deep, is the largest, oldest and deepest recognized impact basin on the Moon (Stuart- Alexander, 1978). The SPA basin-forming impact should have sam- * Corresponding author at: P.O. Box 9718, Datun Road, Chaoyang District, Beijing, 100101, China. pled the lunar lower crust and upper mantle (Miljkovic et al., E-mail address: [email protected] (K. Di). 2015; Melosh et al., 2017). The study on the possible mantle- https://doi.org/10.1016/j.epsl.2020.116378 0012-821X/© 2020 Elsevier B.V. All rights reserved. 2 S. Gou et al. / Earth and Planetary Science Letters 544 (2020) 116378

on the unusual substance and its surrounding regolith at Chang’e-4 landing site, and infers the fundamental source of the regolith.

2. Datasets

2.1. Pancam images

The Pancam system is composed of two cameras with identical functions, performances and interfaces. Each camera has the abil- ity to take color (red: 640 nm, green: 540 nm, blue: 470 nm) and panchromatic (420–700 nm) images with a ﬁeld of view (FOV) of ◦ ◦ 19.6 × 14.5 (Jia et al., 2018). The effective image size is 2,352 × 1,728 pixels in color mode and 1,176 × 864 pixels in panchromatic mode. As the camera has a resolving power of 0.15 mrad (about 120 pixels/degree), the spatial resolution of the Pancam image depends strongly on distance from the camera. The Pancam ◦ can acquire a 360 panoramic view by taking 56 pairs of images (28 different azimuth angles at 2 elevation angles). Pancam has a stereo base of 270 mm and the stereo images can be used to generate high-resolution topographic maps of the lunar surface with photogrammetric techniques (Jia et al., 2018). Pancam imagery is used in this study for visual perception and geometric measurement of the unusual substance (Fig. 2).

2.2. VNIS spectra

The VNIS onboard the Chang’e-4 rover mainly consists of a visible/near-infrared (VIS/NIR) imager with an effective image size of 256 × 256 pixels (∼1 mm/pixel), a shortwave infrared (SWIR) Fig. 1. The regional geologic setting of the Chang’e-4 probe landing site (red star). single-pixel detector, and a white panel for calibration and dust- The base map shown in orthographic projection is Chang’e-2 high resolution (7 m/pixel) digital orthophoto map. The Lunar Orbiter Laser Altimeter (LOLA) topog- proofing (He et al., 2018; Li et al., 2019a). The VIS/NIR imager raphy inset shows extent of the figure within the SPA basin (white box). Colors works from 450 to 945 nm with a spectral resolution of 2–7 nm, represent different heights in the inset, with red being the highest and purple being and the SWIR detector that works from 900 to 2,395 nm has a ◦ the lowest. (For interpretation of the colors in the figure(s), the reader is referred to spectral resolution of 3–12 nm. The FOV of the SWIR ( 3.58 ) the web version of this article.) corresponds to a circular region within the FOV of the VIS/NIR ◦ ◦ (8.5 × 8.5 ). It which has a diameter of 108 pixels on the VIS/NIR derived materials may provide clues to the lunar evolution. Chi- image from a detection distance of 1m (He et al., 2018; Li et al., na’s Chang’e-4 probe successfully landed within Von Kármán crater ◦ ◦ 2019a), with the center being located at the coordinate (96, 128). (186 km in diameter, 176.2 E, 44.5 S) inside the South Pole-Aitken In situ measured VNIS data are preprocessed and released to the (SPA) basin on January 3, 2019 (Fig. 1) (Di et al., 2019). A lunar science community as L2B radiance data by the ground applica- rover was deployed on the same day and conducted a series of tion and research system (GRAS, http://moon .bao .ac .cn/). The L2B in situ measurements during its travels on the lunar farside with radiance datasets are further processed in this study to derive full- onboard science payloads, e.g., a visible and near-infrared imaging range (450–2,395 nm) reflectance (reflectance factor, REFF) spectra spectrometer (VNIS) and a panoramic camera (Pancam) (Jia et al., by the solar irradiance calibration method (Gou et al., 2019). The 2018). Various targets on the lunar surface were measured by the spectra measured on the lunar regolith and the unusual substance VNIS, e.g., plume-disturbed regolith, wheel-trenched regolith, fresh at different rover stations are shown in Fig. 3. crater ejecta and a rock fragment. Initial spectral analysis revealed that Chang’e-4 rover may have discovered material from the lunar 2.3. Hazcam images mantle, or from an impact-produced differentiated melt sheet (Gou et al., 2019; Hu et al., 2019; Li et al., 2019b). These interpretations In addition, images acquired by the hazard avoidance camera perhaps would shed light on the long-standing mysteries about the (Hazcam) are also used in this study to infer the property of the Moon’s formation and evolution, e.g., if the rover measured re- lunar regolith at the Chang’e-4 landing site. The Hazcam is a stereo golith was confirmed to originate from the mantle, the cumulate camera system, and each camera has a fisheye lens that offers a overturn model is supported. 170-degree FOV. The Hazcam acquires 1024 × 1024 pixels stereo Chang’e-4 rover found an unusual substance in the bottom of images, which are used to obtain 3 dimension terrain information a small crater during the eighth lunar day (July 26 to August 7, in front of the rover to detect hazards (e.g., large rocks and craters) 2019). As small pieces of the unusual substance scattered in the along the rover’s pathways. crater, it seems that the substance is actually a distribution of multiple pieces of the same material. Its outermost fragments appear 3. Relative mineral fraction determination fairly similar to that of the surrounding materials. However, the unusual substance differs from the surrounding regolith blocks in The regolith reflectance spectrum measured by VNIS is a highly shape, color and texture. It is dark greenish and glistening. The ap- non-linear combination of the spectra of all components in the parent length and width of its main part, which has an elongated VIS/NIR wavelength range. However, the single-scattering albedo mass-like configuration, are about 52 cm and 16 cm, respectively. (SSA) spectrum of each component, which can be derived from re- Detailed in situ spectral measurements were conducted on the flectance, adds linearly in the VIS/NIR range (Hapke, 1981). There- crater ejecta and the unusual substance during the ninth lunar day fore, the relative mineral fractions of the lunar regolith are re- (August 24 to September 6). This study presents a detailed analysis trieved by unmixing the in situ measured VNIS spectrum through S. Gou et al. / Earth and Planetary Science Letters 544 (2020) 116378 3

◦ Fig. 2. Top panel: The 360 Pancam panchromatic mosaic produced from the left-eye images taken at rover station LE00803. Bottom panel: Pancam color image of the unusual substance. The arrow in the Pancam mosaic marks the location of the crater that hosts the unusual substance.

unmixed using an endmember library that consists of multiple SSA spectra. The main minerals on the Moon, including plagioclase (PLG), olivine (OL), and pyroxene (PYX, consisting of orthopyroxene (OPX), and clinopyroxene (CPX)), as well as one additional dark component (DARK), are selected to construct the endmember library (Supplementary Table 1). VNIS data reduction and unmixing details are presented in the Appendix A that follows the main text.

4. Results and discussions

4.1. Mineral fractions of the lunar regolith

The relative mineral fractions and root-mean-square error (RMSE) retrieved from the spectral unmixing algorithm are sum- marized in Table 1, and the best fitting results for these spectra are shown in Supplementary Fig. 1. Results show the regolith is dominated by PLG (45 ± 6%), followed by PYX (7 ± 1%) and OL (6 ± 2%). These in situ spectral measurements generally are consistent with orbital observations that Chang’e-4 landed in the PYX-abundant zone (Moriarty et al., 2013; Moriarty III and Pieters, 2018). Though the inclusion of a dark endmember precludes this study from determining absolute mineral abundances, the relative abundant PLG and almost equation fractions of PYX (consisting of 6 ± 1% OPX and 2 ± 1% CPX) and OL indicate the regolith is likely related to noritic rocks. Compared with the regolith, various particle sizes (e.g., 10 μm, 20 μm, 30 μm), which have influences on effective optical path, are tested in the unmixing model for the unusual substance spectrum. However, only PLG (38%) and DARK (62%) components are detected. The unmixing result is unlikely in line with the actual situation. Probable physical reasons for the retrieved unrealistic Fig. 3. In situ measured VNIS spectra. The targets of spectra measured at stations mineral component fractions are: (1) The rover measured the un- LE00903 and LE00911 are the regolith in crater interior and the unusual substance, usual substance under a bad illumination condition that shadow respectively. The targets of the rest spectra are the surrounding lunar regolith. The dashed lines located at 750 nm and 1550 nm mark the wavelength range used for accounts for about 42% of the FOV of VNIS (Supplementary Fig. 1), spectral unmixing to exclude thermal effects. resulting in the VNIS measured spectrum is unreliable. (2) The composition of the unusual substance is the same as that of the regolith; however, the formation process changed its spectral prop- a two-step process, which allows nonlinearly mixed spectrum to erties in a way that it becomes insensitive in the VNIS spectral be modeled as linear mixtures. Firstly, the VNIS reflectance (REFF) range, for example, loss of crystal structure through glass forma- spectrum is converted to SSA spectrum using the Hapke (1981) tion; (3) The composition of the unusual substance is different radiative transfer model. Secondly, each SSA spectrum is linearly from that of the regolith but cannot be detected by VNIS, for exam- 4 S. Gou et al. / Earth and Planetary Science Letters 544 (2020) 116378

Fig. 4. Hazcam images show the rover’s locations. (a) Approached the crater rim; (2) Made in situ spectral measurements on the unusual substance; (3) Withdrew from the crater rim. The inset in the upper right of (b) shows powdered regolith particles stick on the wheel.

Table 1 soil sample 74220 returned by Apollo 17 mission in 1972 is rich Relative mineral fractions retrieved from VNIS spectra. in orange glass spheres and fragments, and they are thought to Rover PYX OL PLG DARK RMSE be recrystallized pyroclastic glasses that sprayed from an explo- stations (vol.%) (vol.%) (vol.%) (vol.%) (%) sive volcanic eruption 3.71 ± 0.06 billion years ago (Husain and LE00207 10 5 45 40 0.2 Schaeffer, 1973). The volcanic eruption within Von Kármán crater LE00308 7 7 42 44 0.3 ceased about 3.35 billion years ago (Haruyama et al., 2009). It LE00401 8 6 40 46 0.3 is unlikely that the unusual substance is a volcanic glass. Impact LE00402 7 6 40 46 0.2 LE00504 8 7 57 28 0.3 melts, including glass coatings, glass spherules, glassy breccias, and LE00602 9 7 51 33 0.3 crystalline melt rocks, can be formed by hypervelocity impacts LE00603 8 8 54 30 0.3 (Melosh, 1989). The Pancam color image (Fig. 2) gives clues to the LE00703 6 6 35 53 0.3 genesis of the Chang’e-4 rover discovered unusual substance. In LE00803 7 5 44 43 0.2 appearance, it especially resembles lunar sample 15466 (Fig. 5a), LE00804 5 7 40 48 0.2 LE00903 7 2 47 43 0.2 which was sampled by the Apollo 15 mission from the rim of Spur LE00911 0 0 38 62 0.2 crater (80 m in diameter). Sample 15466 is a collection of breccia fragments cemented together with a vesicular black glass (Meyer, 2011). Due to limited imagery acquired by the Chang’e-4 rover, it ple, incorporation of opaque materials results in spectrally neutral cannot be judged from other morphological evidences that the un- in the VNIS wavelength range. usual substance is an entirely impact melt breccia. It may only be a coating on unmelted rocks that may only account for a small 4.2. Possible formation mechanisms component of the observed total volume. It also resembles lunar The Hazcam images, acquired when the rover approached the sample 70019 (Fig. 5b), which was collected by the Apollo 17 mis- crater rim (Fig. 4a), made in situ spectral measurements (Fig. 4b), sion from the bottom of a 3 m fresh crater. Sample 70019 is a and withdrew from the crater rim (Fig. 4c), revealed that various glass-coated soil breccia, and can be considered as a large “agglu- sizes of blocks around the crater had been crushed into powder tinate” made up of dark regolith microbreccia cemented together particles by the rover’s wheels. The particles can even be clearly with black, shiny glasses (Meyer, 2011). Thus, the best petrologic observed on the wheel (inset in Fig. 4b), indicating these blocks interpretation of the unusual substance discovered by the Chang’e- are clods that composed of fine regolith particles, rather than solid 4 rover in the bottom of the fresh crater is an impact melt breccia rock fragments/breccias. The generation of clods indicates the local or a glass-coated agglutinitic regolith breccia. regolith at Chang’e-4 landing site may be compacted slightly and There may be two sources of the unusual substance: (1) en- becomes blocky and friable; therefore, the regolith may be broken tirely impact melt emplaced in a different event; (2) glass coating up and fractured easily by impacts on the lunar surface. on unmelted rocks emplaced in the crater. In the first scenario, 3 Crater morphology and its surrounding boulder’s size/shape assuming a spherical impactor with density of 3000 kg/m and ve- distribution (if any) reflect the degradation and provide insights locity of 17 km/s vertically impacts the porous lunar regolith that 3 into the relative age dating (Basilevskii, 1973). Due to gravity and has density of 1500 kg/m , according to the scaling laws (Melosh, weathering processes, continued degradation causes the crater rim 1989), the estimated diameter of the impactor that creates the 2 m to lower, infilling produces a broad and flat crater floor, and physi- fresh crater is about 2 cm, and the estimated diameter of the ini- cal weathering disintegrates surrounding boulders. Compared with tial impact melt is about 6 cm (see Supplementary Text 1 for the the very gently sloping craters around, the preservation of the estimation). As the estimation is made on normal conditions, it can clods indicates that the crater didn’t suffer from severe weather- be considered as a minimum estimate only to a limited extent. The ing. The crater’s current depth and diameter are 0.3 m and 2 m, length (∼52 cm) and width (∼16 cm) of the Chang’e-4 discovered respectively. The relative high depth to diameter ratio (0.15) indi- melt breccia are several times larger than the predicted melt diam- cates it’s a rather fresh crater (Pike, 1974). eter even if the melt didn’t disperse in subsequent crater collapse. The dark greenish and shiny features observed from Pancam Hence, the impact melt breccia was not likely formed in situ in the color image (Fig. 2) are signs of possible presence of glasses. crater where it was found, but was very likely emplaced in a dif- Glasses in the lunar regolith are usually sourced from impact ferent event and was ejected to the 2 m fresh crater. However, it is melts or from volcanic eruptions. For example, the orange lunar difficult to determine whether the breccia impacted the lunar sur- S. Gou et al. / Earth and Planetary Science Letters 544 (2020) 116378 5

Fig. 5. Photos of lunar impact melt breccia samples. (a) S1 face of sample 15466; (b) T1 face of sample 70019. The cubes in (a) and (b) are 1 inch and 1 cm, respectively. (Photo source: NASA/JSC; IDs: S71-44184 and S73-15333). face at relative low velocity and formed the fresh crater, or if the km to 1400 km (Moriarty et al., 2013). Finsen and Alder craters fresh crater formed earlier and the breccia happened to be ejected are just about 370 km and 210 away from the center of the SPA and deposited onto the floor of the crater later. In the second sce- basin, respectively. Therefore, the lunar regolith at Chang’e-4 land- nario, a meteorite (about 2 cm in diameter) impacted the lunar ing site was probably primarily the physical weathering products surface and formed relatively dark, glass-rich impact melt (6 cm in of the noritic rocks that mainly excavated from Finsen crater bil- diameter) in the fresh crater. The glassy melts coated the unmelted lion years ago. rocks, agglutinated the surrounding clods, and formed the current However, as the Finsen and Alder craters are on the margin of observed glass-coated agglutinitic breccia. Nevertheless, on the ba- the proposed impact melt pool, there is another possibility that the sis of the above comprehensive analysis, one most likely scenario regolith measured by Chang’e-4 rover was produced from a suite for the formation of the Chang’e-4 discovered impact melt brec- of igneous rocks. In addition to the relative mineral fractions of the cia was that an impact partially melted local regolith, small drops regolith derived in this study, the rock fragments encountered by of the dark greenish melt cooled rapidly, became shiny and ce- the rover during the third lunar day (Fig. 6) at rover station LE303 mented. The breccia formation process may darken the reflectance provide supplemental evidence. Given that one of the measured spectrum and rendered the regolith insensitive in the VNIS wave- fragment exhibits strong absorption bands (Fig. 6), it is likely that length range. these rock fragments are crystallized. However, it is hard to figure out whether they are aphanitic or phaneritic. They lack discernible 4.3. Fundamental source of the regolith coarse-grained textures, e.g., no grains ≥ 3 mm can be unambigu- ously recognized on the rock fragment surface, indicating the melt Regolith is typically mixtures of materials with many origins, that formed the igneous rocks may cool quickly that crystals did- especially in a complex, ancient terrane such as the SPA basin ◦ ◦ n’t have enough time to grow to sizes easily seen by the unaided (53 S, 169 W). Di et al. (2019)revealedthat the regolith mea- eye. Spectral unmixing reveal the rock fragment contains 40% PYX sured by the Chang’e-4 rover was ejecta primarily from Finsen ◦ ◦ (29% OPX and 11% CPX), 11% OL, and 43% PLG, indicating it can be crater (42.3 S, 177.732 W) with possible contributions from Alder ◦ ◦ classified as norite. crater (48.6 S, 177.9 W), rather than the underlying mare basalt. The regolith measured by Chang’e-4 rover was actually a highly Many hypotheses have been put forward regarding the fundamen- mixture of multiple sources. It was primarily the weathering prod- tal source of the regolith measured by the Chang’e-4 rover, includ- ucts of Finsen crater ejecta. This study supports the hypotheses ing from the lunar upper mantle, or from a differentiated impact that the fundamental source of the regolith might originate from a melt pool (Hu et al., 2019; Li et al., 2019b). differentiated impact melt pool or from a suite of igneous rocks. Mantle materials are expected to be exposed by the basin- forming events (Miljkovic et al., 2015), particularly the SPA basin inevitably spewed them onto the farside highlands (Melosh et al., 5. Conclusion 2017). The LMO hypothesis indicates that the lunar upper mantle is rich in OL and PYX due to cumulate overturn (Ringwood Chang’e-4 rover discovered an impact melt breccia at the bot- and Kesson, 1976). However, recent remote sensing observations tom of a small fresh crater along its traverse within Von Kármán and numeric simulations challenge the current paradigm and argue crater. The length and width of the dark greenish and glistening that the upper mantle is dominated by OPX (Melosh et al., 2017). breccia are about 52 cm and 16 cm, respectively. The discovered For example, strong OPX signatures in the outer basin and rim of breccia resembles the lunar breccia samples 15466 and 70019 that the SPA basin were spectrally observed (Moriarty III and Pieters, returned by the Apollo missions. The breccia may be an entirely 2018), while only mere traces of OL in limited places within the impact melt emplaced in a different event, or it may only be a SPA basin were detected (Yamamoto et al., 2012). Based on the coating on unmelted rocks emplaced in the crater. In either case, relative mineral fractions (abundant PLG) derived in this study, the it was formed by impact-generated welding, cementing and agglu- hypothesis of possible mantle-derived regolith is not supported. tinating of lunar regolith and breccia. Numerical simulations suggested that the SPA basin should A Hapke model-based spectral unmixing algorithm was applied have sampled the lunar lower crust and upper mantle (Miljkovic et on the in situ measured spectra. Unmixing results imply the real., 2015; Melosh et al., 2017). The basin-forming event might gen- golith at the Chang’e-4 landing site is dominated by PLG (45 ± erate an impact melt pond which differentiated and formed noritic 6%), followed by almost equal fractions of PYX (7 ± 1%) and OL layers with a thickness of ∼6 km and possibly as thick as ∼13 (6 ± 2%), indicating the regolith is likely related to noritic rocks. km (Hurwitz and Kring, 2014; Vaughan and Head, 2014). The es- The SPA basin-forming event may generate a differentiated impact timated transient cavity radius of the SPA basin ranges from 840 melt pool via thorough mixing the sampled lunar crust and mantle 6 S. Gou et al. / Earth and Planetary Science Letters 544 (2020) 116378

Fig. 6. (a) Pancam image of rock fragments encountered by Chang’e-4 rover; (b) VIS/NIR image (∼1 mm/pixel) @ 750 nm, the black circle corresponds to the field of view of the SWIR spectrometer; (c) VNIS spectrum of the rock fragment. materials. The regolith measured by Chang’e-4 rover was actually a Therefore, only the absorption feature around 1000 nm is essen- highly mixture of multiple sources, with ejecta from Finsen crater tially interpreted in this study. This feature is highly nonunique being primary and possible contributions from Alder crater. Both and can be attributed to numerous lunar materials, including vol- Finsen and Alder craters are on the margin of the proposed impact canic glass, olivine, crystalline plagioclase, and pyroxene mixtures. melt pool. Therefore, the fundamental source of the regolith might Usually, these minerals can be spectroscopically distinguished by originate from a differentiated melt pool or from a suite of igneous their diagnostic absorption centers. Orthopyroxene (OPX) spectrum rocks. exhibits a relatively narrow and symmetrical absorption near 900 nm, and a broad absorption near 1900 nm; Clinopyroxene (CPX) Declaration of competing interest spectrum exhibits a narrow, symmetrical absorption near 1000 nm, and a broad absorption near 2200 nm; Olivine (OL) spectrum ex- The authors declare that they have no known competing finan- hibits three overlapping absorption features at approximately 850, cial interests or personal relationships that could have appeared to 1050, and 1150 nm, forming what appears to be a single, broad, influence the work reported in this paper. and asymmetric absorption near 1050 nm; Iron-bearing crystalline plagioclase (PLG) spectrum has a weak and broad absorption near Acknowledgements 1250 nm; Iron-bearing Glass (GLS) spectrum exhibits two wide absorptions centered near 1060 nm and 1900 nm (Horgan et al., The authors gratefully thank the whole Chang’e-4 team for 2014). making the mission a great success and providing the data (http:// moon .bao .ac .cn). This research was supported by the Strategic Pri- A.2. Spectral unmixing ority Research Program of Chinese Academy of Sciences (grant No. XDB41000000), National Natural Science Foundation of China (grant Nos. 41702354 and 41941003), Macao Young Scholars Pro- A.2.1. Spectral endmember selection gram (grant No. AM201902), and Science and Technology Devel- The primary minerals on the lunar surface are pyroxene (PYX), opment Fund of Macau (grant No. 131/2017/A3). Thanks for Kevin olivine (OL) and plagioclase (PLG) (Spudis and Davis, 1986). It Cannon and Honglei Lin for discussions about Hapke model. Con- is understood from Apollo samples that natural lunar materials structive reviews by Daniel P. Moriarty III and Axel Wittmann im- exhibit a multitude of diverse PYX compositions even within in- proved the quality of this manuscript substantially, and are greatly dividual rocks and soils (Isaacson et al., 2011; Moriarty III and appreciated. Pieters, 2016). Though spectral measurements cannot sufficiently reflect their diversity, PYX is roughly classified as orthopyroxene Appendix A. Methods (OPX) and clinopyroxene (CPX) based on chemistry and crystallog- raphy (Burns, 1993). And the relative abundances of OPX and CPX A.1. VNIS data reduction in the lunar regolith can be distinguished appropriately by spectral deconvolution (Noble et al., 2006). OL and PLG also have diverse Major steps for the VNIS data reduction includes: (a) extrac- compositions. tion of a VIS/NIR average spectrum, (b) elimination of spectrum The aim of this study is to make relative, qualitative assess- discontinuity, (c) construction of a full-range spectrum, and (d) ments of general mineralogical trends, therefore, SSA spectra of spectrum smoothing. More details on the data processing can be four lunar components, including OL, OPX, CPX and crystalline found in Gou et al. (2019). To avoid any thermal emission effects, PLG, as well as one additional dark component (DARK) that is the wavelength range is limited from 750 to 1550 nm in this study. physically represented by opaque materials (e.g., ilmenite), and S. Gou et al. / Earth and Planetary Science Letters 544 (2020) 116378 7 actual shade in the FOV of VNIS, are selected as spectral endmem- surface scatters light equally in all directions (isotropic), the single- bers in this study. All the mineral endmember spectra are from particle phase function P (g) = 1.0. The SSA spectrum can thus be reflectance experiment laboratory (RELAB, http://www.planetary. solved from REFF spectrum by the simplified equation (3). brown .edu /relab) and their details are listed in Supplementary ωavg Table 1. Note that ilmenite is not a generic opaque material. REFF i, e, g = H , H , (3) ( ) + μ0 ωavg μ ωavg It is highly absorbing and strongly affects the reflectance spec- 4 (μ0 μ) trum when its abundance is high, especially for the fine particle A.2.3. Space weathering simulation size samples (Moriarty III and Pieters, 2016). TiO abundance at 2 To consider the space weathering effect, spectra of endmembers Chang’e-4 landing site estimated from Kaguya multiband imager with different weathering levels are simulated by adding different data is about 1.42 ± 0.24 wt.%. Thus the influence of ilmenite is mass fraction of submicroscopic iron (SMFe) to the host materials not so strong on the overall continuum and pyroxene absorption of the lunar regolith (Hapke, 1981, 1984, 1986, 2001) (equations band properties as that of the ilmenite-rich (>17%) Apollo 17 bulk (4)–(12)). The major steps are stated below. rock sample. a) Resample the endmember spectra to the VNIS bands. The The shiny features of the breccia (Fig. 2) show the possible spectral resolution of the endmember and VNIS spectra are differ- presence of glass (GLS). However, the broad spectral absorption of ent, thus the endmember spectra are resampled to the VNIS bands GLS at approximately 1.06 μm is easily suppressed by mafic min- to ensure both have the same spectral resolution (band number). erals and nanophase iron that is ubiquitous in the lunar regolith The spectra are limited to the wavelength range from 750 to 1,550 (Cannon and Mustard, 2015). To test the sensitivity of the inclusion nm to avoid any thermal emission effects. of a GLS component to the unmixing results, the VNIS measured b) Derive SSA from reflectance. SSA ω is derived from VNIS breccia spectrum is modeled with and without a GLS endmember, multiple GLS endmembers (RELAB IDs: KC-JFM-038, KC-JFM-039, reflectance (REFF) spectrum (equations (4)-(5)). √ KC-JFM-040, KC-JFM-041, KC-JFM-042, KC-JFM-043) are tested in 1 − ω the unmixing model to check whether one composition of GLS pro- 1/2 vides a better fit. However, none of them can provide reasonable (μ + μ)22 + (1 + 4μ μ) (1 − ) − (μ + μ) = 0 0 0 result and F-test result indicates the inclusion of a GLS component (4) 1 + 4μ0μ does not have significant influence on the modeled spectrum at μ0 + μ 1 the 95% confidence level (See Supplementary Text 1 for the intro- = 4 REFF (5) + + duction of F-test). Hence, a GLS component is not included in the μ0 (1 2μ0)(1 2μ) final endmember library. where μ0 and μ are the cosine of the incidence and emission angles, respectively. A.2.2. Single-scattering albedo determination c) Derive absorption coefficient from SSA. Absorption coeffi- The core of the unmixing algorithm is the Hapke model (Hapke, cient α is derived from optical constants of each endmember 1981), which offers an approximate solution to the radiative trans- (equation (6)). fer theory. Due to the definite physical meaning of the model parameters, the simplicity of model expression, and the fast com- 1 (1 − Se)(1 − Si) α = ln Si + (6) putation, Hapke model is widely applied in the planetary science D ω − Se community to characterize the properties of planetary surface ma- where D is the average distance traveled by all rays during a sin- terials, e.g., grain size, morphology, internal structure and surface roughness (Schmidt and Fernando, 2015). When the particle sizes gle transit of the particle. It has various approximation expressions of all components in a mixture are known or fixed (similar as- (Hapke, 2001). Area weighted mean size (equation (7)) is adopted sumption is made in this study), the uncertainties of relative min- in this study and the derived D is listed in Supplementary Ta- eral fractions estimated from reflectance spectrum by the simpli- ble 1. Se and Si are the average Fresnel reflection coefficients for fied Hapke model are commonly <5% (Mustard and Pieters, 1987). externally and internally incident light, respectively. For k2 1 With the assumptions that all components within the lunar re- and 1.2 ≤ n ≤ 2.2, where n and k are the real and imagery parts golith are mixed intimately and the regolith particles are much (refractive index and extinction coefficient) of the endmembers’ larger than the spectral wavelength, the relationship between VNIS complex refractive index (also known as optical constants), respec- reflectance (REFF) and single-scattering albedo (SSA) can be ex- tively, Se and Si are empirically approximated by equations (8) and pressed in equation (1) (Hapke, 1981). (9) (Hapke, 2012).

ωavg DU REFF (i, e, g) = [(1 + B (g))P (g) D = D ln (7) 4 (μ + μ) L 0 DL + − ] H μ0, ωavg H μ, ωavg 1 (1) where DU and D L are the effective upper and lower limit of the particle size where μ0 and μ are the cosine of the incidence angle i and emission (viewing) angle e, respectively; g is phase angle. ωavg is the (n − 1)2 average single scattering albedo (SSA) of all the components. B(g) Se ≈ + 0.05 (8) (n + 1)2 is the backscatter function that describes opposition effect. P (g) is the single-particle phase function. H(x, ω ) is the isotropic scat- 4 avg Si ≈ 1 − (9) tering function approximation (equation (2)) (Hapke, 2002). n (n + 1)2 + d) Simulate different space weathering level. Assuming the = 1 2x H x, ωavg (2) space weathering levels are proportional to the mass fraction of 1 + 2x 1 − ωavg SMFe, the effect of SMFe that produced by space weathering on Because the phase angle of each VNIS spectrum analyzed in spectrum is simulated by modiﬁcation on the absorption coeﬃ- ◦ this study is larger than 15 (see supplementary material for VNIS cient (equation (10)). measure parameters at each station), the opposite effect is negligi- ble and B(g) = 0 (Mustard and Pieters, 1987). Assuming the lunar αw = αh + αFe (10) 8 S. Gou et al. / Earth and Planetary Science Letters 544 (2020) 116378

M/ρiDi where αw, αh, αFe are the absorption coefficients of the weath- F = (15) i M ered host materials, unweathered host materials (equation (6)) and i Mi/ρiDi SMFe (equation (11)), respectively. M/ρi Fi = (16) 3 M M /ρ 36πfρh n nFekFe i i i = · h (11) αFe 2 λρFe 2 − 2 + 2 + 2 2 nFe kFe 2nh 4nFekFe Appendix B. Supplementary material where f is mass fraction of SMFe; λ is wavelength; n∗, k∗, ρ∗ Supplementary material related to this article can be found on- (∗ denotes h or Fe) are the real and imagery parts of the complex line at https://doi.org/10.1016/j.epsl.2020.116378. refractive index (also known as optical constants), and densities of host materials or SMFe, respectively. The optical constants of Fe are cited from Johnson and Christy (1974), and that of OL, OPX References and CPX are determined by Dorschner et al. (1995), and that of Anthony, J.W., Bideaux, R.A., Bladh, K.W., Nichols, M.C., 2003. Handbook of Miner- PLG are from Anthony et al. (2003). alogy. Mineralogical Society of America, Chantilly, VA, USA. 20151-1110. http:// e) Derive SSA from modified absorption coefficient. The rela- www.handbookofmineralogy.org/. tionship between absorption coefficient and SSA is expressed in Basilevskii, A.T., 1973. On the evolution rate of small lunar craters. In: 7th Lunar Science Conference. Pergamon Press, Houston, TX, pp. 1005–1020. equation (12) (See supplementary Fig. 2 for the simulated end- Burns, R.G., 1993. Mineralogical Applications of Crystal Field Theory, 2nd edition. member spectra with the inclusion of different SMFe mass frac- Cambridge University Press, New York. tion). Cannon, K.M., Mustard, J.F., 2015. Preserved glass-rich impactites on Mars. Geol- ogy 43, 635–638. (1 − Se)(1 − Si) Cooper, M.R., Kovach, R.L., Watkins, J.S., 1974. Lunar near-surface structure. Rev. = S + (12) ω e − Geophys. 12, 291–308. 1 Si Di, K., Zhu, M.-H., Yue, Z., Lin, Y., Wan, W., Liu, Zhaoqin, Gou, S., Liu, B., Peng, Man, where = exp (−α D). 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