VIS and NIR data

Planetary and Space Science 68 (2012) 76–85

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Surface mineralogy and stratigraphy of the lunar South Pole-Aitken basin determined from Clementine UV/VIS and NIR data A.M. Borst a,b,n, B.H. Foing a,b, G.R. Davies a, W. van Westrenen a a b

Faculty of Earth and Life Sciences, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands Research and Scientific Support Department, ESA/ESTEC SRE, Postbus 299, 2200 AG Noordwijk, The Netherlands

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 November 2010 Received in revised form 1 June 2011 Accepted 26 July 2011 Available online 4 August 2011

The South Pole-Aitken (SPA) basin, located on the lunar far side, is one of the oldest and largest recognised impact structures in the solar system. The basin is a proposed site for future sample return missions and human bases due to the unique geological environment and its potential for preservation of water ice in areas of near-permanent shadow. Here, we report surface mineralogy maps of the central and northern parts of the SPA basin, based on Clementine UV/VIS and NIR spectral data. Clementine LIDAR data and SMART-1 AMIE images provide additional geomorphological and stratigraphic information. A noritic mineralogical composition is identified as the deepest stratigraphic unit exposed on the basin floor. Norite is found in nearly all central peaks and in large topographical structures that have punched through an upper, often basaltic or gabbroic layer, including the Leibnitz and Apollo sub-basins. The thin layer of gabbroic/basaltic composition is distributed over large parts of the SPA basin floor and presumably overlays the noritic basement of apparent lower-crustal origin. Our data do not confirm the presence of olivine-rich material in the SPA basin, including at Olivine Hill, suggesting the mantle material was not excavated during the basin-forming impact. & 2011 Elsevier Ltd. All rights reserved.

Keywords: South Pole-Aitken basin Clementine mission Lunar mineralogy Spectroscopy Remote sensing SMART-1 mission

1. Introduction The lunar South Pole-Aitken (SPA) basin is one of the largest identified impact structures in the solar system. It stretches from the lunar South Pole to the Aitken crater, located 151 south of the equator on the 1801 meridian (Garrick-Bethell and Zuber, 2009; Fig. 1), and reaches depths of up to 11 km. The SPA basin serves as a benchmark to models of large impact processes that are thought to have played a key role in the early evolution of Mercury (Benz et al., 1988), Venus (Davies, 2008), the Earth–Moon system (Canup, 2008) and Mars (Andrews-Hanna et al., 2008; Marinova et al., 2008; Nimmo et al., 2008). Remote sensing observations from Clementine, Lunar Prospector and other missions suggest that the SPA basin is a multi-ring elliptical structure with axes measuring 2400 by 2050 km and centred at 531S and 1911E (Garrick-Bethell and Zuber, 2009; Sasaki et al., 2010). The SPA basin has a relatively low albedo and iron, thorium, titanium and magnesium abundances that are different from the rest of the lunar surface (Lucey et al., 1998a,b, 2000; Pieters et al., 1997, 2008; Shevchenko et al., 2007; Stuart-Alexander, 1978; Tompkins and Pieters (1999); Wilhelms and McCauley, n Corresponding author at: Faculty of Earth and Life Sciences, VU University Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands. Tel.: þ31 611213813. E-mail address: [email protected] (A.M. Borst).

0032-0633/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2011.07.020

1987). Based on its unique geochemical characteristics Jolliff et al. (2000) classify the basin as a separate major lunar surface terrain. Recently, a range of remote sensing observations has provided strong evidence for the presence of water and/or hydroxyl in the form of ice or water-bearing minerals near the lunar south pole. Surface neutron flux data from the Lunar Exploration Neutron Detector (LEND) on the Lunar Reconnaissance Orbiter (LRO) suggest multiple areas of elevated hydrogen content near the south pole (Mitrofanov et al., 2010). Measurements of the nearinfrared (NIR) reflectance of the ejecta plume formed by the impact of LRO’s Lunar Crater Observation and Sensing Satellite (LCROSS) in Cabeus crater (Colaprete et al., 2010) confirmed the presence of significant amounts of water ice at or near the surface. A third line of evidence comes from the observation of characteristic absorption features between 2.8 and 3 mm at high lunar latitudes by the Moon Mineralogy Mapper (M3) of the Chandrayaan-1 mission (Pieters et al., 2009). The presence of water provides excellent opportunities for future landing missions and human exploration, as it forms a potential resource for both nourishment and rocket fuel. The scope for simultaneously addressing key lunar science issues related to the composition of the lunar surface and interior as well as key exploration issues related to lunar resources has led to several robotic missions currently being considered to visit and sample the SPA interior (Duke, 2003; Jolliff et al., 2010; Osinski et al., 2010) and its rim (Carpenter et al., 2010).

A.M. Borst et al. / Planetary and Space Science 68 (2012) 76–85

Fig. 1. Topography map of the South Pole-Aitken basin from LRO’s Lunar Orbitar Laser Altimeter (LOLA), adapted from http://www.nasa.gov/mission_pages/LRO/ multimedia/lroimages/lola-20100409-aitken.html (credit: NASA/Goddard).

Previous work has shown that rocks exposed in the SPA basin have an overall ultramafic character, and are mainly composed of orthopyroxene, clinopyroxene, olivine, ilmenite and plagioclase (Jolliff et al., 2000; Lucey et al., 1998a,b; Lucey, 2004; Pieters et al., 1997, 2001). SPA rock types include mare basalts from postimpact volcanism, SPA impact melt sheet deposits, parts of the mafic lower crust, possibly upper mantle materials and ejecta blankets from cratering events postdating the main basin forming impact (Nakamura et al., 2009; Pieters et al., 2001). Petro and Pieters (2004) reconstructed the initial stratigraphy of the SPA basin and concluded that material derived from the initial SPA melt sheet comprised between 50% and 80% of the basin floor, suggesting that the initial compositional character has not been completely masked by subsequent cratering processes. One major debate concerning SPA basin geology is whether or not mantle material was exposed during the impact. Several multispectral surveys have previously reported localised occurrences of olivine-rich rocks. One particular area of interest is Olivine Hill, a frequently proposed site for mantle exposure (Duke, 2003; Pieters et al., 2001). Other workers found the detected abundances of olivine (5–10% by volume) too small to be consistent with a mantle origin (Lucey, 2004; Nakamura et al., 2009; Wieczorek et al., 2008; Yamamoto et al., 2010; Yan et al., 2010). Instead, it was suggested that these olivine-bearing rocks represent ultramafic remnants of the impact melt sheet that formed subsequent to the main impact event (Nakamura et al., 2009). Recently, Olivine Hill was renamed Mafic Mound by Petro and Pieters (2010), who argue that the material is most likely gabbroic in composition. Here we reassess the mineralogical and stratigraphic make-up of the SPA basin based on UV/VIS and NIR Clementine data, by combining two analytical approaches derived from Pieters et al. (1997, 2001) and LeMoue´lic and Langevin (2002). A detailed geomorphological and stratigraphic study was also carried out using Clementine Lidar, LRO LOLA and SMART-1 AMIE data to aid in interpretation of the geological evolution of the SPA basin.

2. Methods Clementine’s multispectral data in the ultraviolet/visible (UV/VIS) and near-infrared (NIR) spectral ranges were used for mineralogical

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fingerprinting of the South Pole-Aitken basin rock types, using the UV/VIS algorithm developed by Pieters and co-workers (Pieters et al., 1997, 2001; Tompkins and Pieters, 1999), as modified by Dhingra (2008). This algorithm describes three characteristic parameters in the shape of the absorption spectra of mafic rocks, enabling distinction between anorthosite, norite and basalt/gabbro/troctolite in a false colour composite. We expand their analyses using a band ratio in the NIR spectrum, described by LeMoeulic and Langevin (2002), to distinguish gabbro and basaltic units from olivine-rich troctolites. Due to early instrument calibration problems, the NIR dataset was not released by the USGS until 2007. To date, it has only rarely (Yan et al., 2010) been used for mineral mapping purposes. As shown below, despite their lower spatial resolution compared to UV/VIS, the NIR data provide valuable additional information. Using the USGS Map-a-Planet browser all 11 bands from the UV/VIS and NIR dataset were acquired for analyses on two spatial scales. All images were acquired in a simple cylindrical projection, with radiometrical and empirical corrections as published on the Map-a-Planet browser. The UV/VIS and NIR data were processed using ENVI software. The additional empirical correction factors proposed by P. Lucey (USGS Clementine NIR global mosaic, http:// astrogeology.usgs.gov/Projects/ClementineNIR/, 2007) were not applied. These corrections produce a linear shift in reflectance values for each wavelength, with the largest effect (up to 20%) on the highest reflectance values. The net effect of implementing these corrections would be to increase the UV/VIS wavelength ratios and to decrease the NIR ratios. Since the NIR and UV/VIS ratios are used in separate images, the corrections will solely affect the distribution of the pixel values towards either lower or higher values (for NIR or UV/VIS, respectively). Corrected absorption spectra maintain their characteristic geochemical band shapes. Consequently neither the qualitative colour signatures in the compositional images in this work, nor its main conclusions are significantly affected by not applying the corrections. First, images were produced for the entire SPA basin at a resolution of 1.5 km/pixel. Subsequently, higher resolution data (0.5 km/pixel) were obtained from the centre, the north-western limb and the north-eastern limb of the basin. These three areas represent most of the chemical and mineralogical diversity found in the basin, including signatures related to the global geological structure (Pieters et al., 2001). Additionally, they are located in the northern part of the basin, for which superior Clementine data coverage is available (Nozette, 1995). 2.1. UV/VIS data processing An overview of the band parameters used in this study is provided in Table 1. The first parameter, band strength (bs), is calculated from the 1000/750 nm ratio and assigned to the colour blue in an RGB composite. Band strength values are high for rocks with low ferrous absorption, such as felsic anorthosites (Pieters et al., 1997). Weathered soils also show high bs values as space weathering of surface rocks tends to weaken the ferrous absorption band (Tompkins et al., 1997). The second parameter is band curvature (bc, colour red in RGB) describing the curvature along the 750–900–1000 nm range. A high band curvature characterises materials with abundant low Ca-pyroxene (orthopyroxene). Areas with noritic compositions will appear red or pink. To calculate the band curvature we use the algorithm described by Dhingra (2008): bc¼(750 nm/900 nm)þ (1000 nm/900 nm). The third UV/VIS parameter is band tilt (bt, colour green in RGB), calculated from bt¼ 900/1000 nm intensity ratio. High values of bt reflect units containing abundant clinopyroxene and olivine. Areas that appear green or yellow thus represent gabbroic/basaltic or troctolitic compositions.

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Table 1 Overview of band shape algorithms in UV/VIS and NIR used for determining SPA lithologies. Ratios were calculated using ENVI software. Modified from Dhingra (2008) to include NIR information from LeMoe´ulic and Langevin (2002). Dataset

UV/VIS

NIR

Parameter

Band strength

Band curvature

Band tilt

Olivine/Pyroxene

Colour Wavelength ratio (nm) Olivine Low-Ca pyroxene High-Ca pyroxene Anorthosite

Blue 1000/750 Low Medium Low High

Red (750/900 þ 1000/900) Medium þlow High þhighest High þlow Lowest þlow

Green 900/1000 Highest Lowest Medium Low

Black/White 2000/1250 Highest Medium Lowest Low

2.2. NIR data processing The UV/VIS band tilt parameter does not allow distinction between gabbro/basalt (intrusive vs. extrusive nature) and troctolite. NIR data can be used to distinguish these two chemically distinct rock types, whereas gabbro and mare basalts can be distinguished based on geomorphological characteristics. In order to detect olivine-rich areas we use the intensity ratio between the 2000 nm and 1250 nm bands, modified from the algorithm of LeMoue´lic and Langevin (2002), with a greyscale colour coding (Table 1). The 2000/1250 ratio is high for olivine-dominated rocks and low for orthopyroxene-dominated rocks. Troctolite as well as peridotite, representing possible mantle material, will therefore come out white in NIR ratio greyscale images. Gabbroic and basaltic regions with low olivine contents and noritic units with abundant orthopyroxene will appear dark grey or black in the NIR images. A major limitation in using this method is that no information is available on the effects of weathering on NIR spectra, and no laboratory NIR spectra are available for rock samples that include minerals other than pyroxene and olivine. We therefore focus on NIR band intensity variations in freshly exposed surfaces and in regions previously proposed as olivine and/or pyroxene-rich. In addition, intensity effects due to calibration processes and differences in data resolution in different orbits should be considered when interpreting the data. 2.3. Geomorphological mapping Geomorphological maps were created for each of the selected sub-areas on top of true colour RGB composites (R¼ 1000 nm, G¼900 nm and B¼415 nm) to highlight structural units and assess correlations between lithology and morphology. The maps were used for stratigraphic interpretations of rock units throughout the SPA basin. Major geochemical units and geomorphological features (i.e. crater rims, crater walls, central peaks, smooth plains, pyroclastics) are annotated on the maps. Clementine LIDAR and additional LRO LOLA data were used to aid in the topographic interpretations. In the Bhabha and Bose area (discussed in Section 3.1) SMART-1 AMIE images provided more detailed geomorphological information (Foing et al., 2006). Clementine geochemical maps were also assessed to distinguish volcanic and cryptomare units and the FeO map was found to be of most use in this respect. Transparent colours were used to map zones with a main surface composition of anorthosite, norite or gabbro, as inferred from the UV/VIS and NIR algorithm images. 3. Results Fig. 2 shows the results of the UV/VIS algorithm treatment of data for the complete SPA basin at a spatial resolution of 1.5 km/pixel.

Fig. 2. Image of SPA basin produced with the UV/VIS algorithm in ENVI (Gaussian stretching, simple cylindrical projection). The blue parameter represents anorthositic or weathered rocks, yellow/green colours show basalt/gabbro/troctolite and pink colours indicate noritic compositions. This image shows the compositional varieties within the elliptical SPA basin. Three selected areas (a,b,c) that cover the major variations within SPA were used for detailed multispectral and geomorphological analysis that are presented in Figs. 3, 6 and 8, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The outer rims of the basin are characterised by homogeneous faint blue colours indicating low ferrous absorption from the felsic, anorthositic composition of the upper crust (Pieters et al., 2001). In contrast, the SPA basin interior shows large spectral variations visualised in pink, purple, yellow and green colours, indicating great variety in the mineralogy and distribution of mafic rocks in the basin floor. One distinct mineralogical unit is seen in the northeast part of the basin, featuring dark blue to purple areas with bright pink colours in the freshly exposed floors of young craters (Fig. 2, region c). This region is dominated by the large superposed Apollo basin (480 km diameter). The Apollo region appears to be dominated by (low-Ca) orthopyroxene (strong band curvature, Table 1) indicating a noritic composition. Recent analysis of lunar wide distribution of low-Ca pyroxene using M3 data supports this observation (Klima et al., 2011). These workers also report that some noritic rocks in the Apollo basin are more enriched in magnesium (Mg#475) than other low-Ca pyroxene exposures in the SPA basin (Mg# 50–75). This observation may suggest that the Apollo basin tapped a deeper, more meltdepleted source. Other distinctive areas where norite is exposed are the basin floors of various large craters in the south and in the rim of Leibnitz crater (Fig. 2, region b; Fig. 7d) in the northwest of the SPA basin interior. Abundant green and yellow colours, indicative of an elevated 900/1000 ratio, are identified in the northwest and southern part of the SPA basin. This suggests pervasive (high-Ca) clinopyroxene and possibly small amounts of olivine, forming rocks of mixed gabbroic, basaltic and troctolitic compositions. Yamamoto et al. (2010) identified only two very local olivine-rich sites in the southern

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SPA basin (Zeeman and Schrodinger basins), suggesting that the gabbroic, basaltic and troctolitic rocks in the green/yellow areas in Fig. 2 contain little or no olivine. Some of these units represent basaltic mare volcanism, generally identified from dark and topographically smooth plains. Other areas that are more heavily cratered, referred to as ‘non-mare’ regions by Pieters et al. (2001), also exhibit features of a long-wavelength ferrous band suggestive of higher abundances of high-Ca pyroxene and olivine. Pieters et al. (2001) suggested that these regions may represent remnants of the SPA melt sheet or melt-breccia produced during the impact, originally excavated from parts of the lower crust or even the mantle. In order to conclude a mantle origin for any of these ‘non-mare’ materials, one would need to detect high abundances of olivine. Three areas within the SPA basin were selected for higher resolution geomorphology and multispectral analysis. The first area is the central part of the basin and includes Olivine Hill

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(Mafic Mound) and the Bhabha and Bose craters (Fig. 2, region a). The north-western part of the SPA basin is our second region of study and includes the large Leibnitz, Ingenii and Jules Verne basin mare basalts (Fig. 2, region b). The third area covers the deep, multi-ringed Apollo basin in the north-east of the SPA basin (Fig. 2, region c).

3.1. Bhabha–Bose region The central area of the SPA basin (region a in Fig. 2) is depicted in Fig. 3a. The multispectral image of this region is shown in Fig. 3b and our geomorphological map of the area in Fig. 3c. In the centre of the images are two complex craters with central peaks: Bhabha and Bose. Detailed NIR ratio and UV/VIS algorithm images of the Bhabha–Bose area are given in Fig. 4.

Fig. 3. (a) Real colour image (1000–900–415 nm bands from Clementine) of the Bhabha and Bose region in central SPA basin showing the location of Figs. 4 and 5; (b) UV/ VIS algorithm image of the same area; (c) geomorphologic and compositional map on real colour image. Craters are distinguished based on their morphology, size and interpreted age. Light green circles represent small and relatively young bowled-shape craters. Light blue circles are assigned to older partly degraded craters where spectral features are less evident than those of younger craters. Craters in yellow display the larger and complex craters (or basins), some of which have steep and slumped crater walls (yellow), flat crater floors (in red) and significant central peaks (blue). Smooth areas, flooded with low-albedo basaltic lavas, are displayed as purple units. Pyroclastic deposits and small mare ponds are annotated with purple dashed lines. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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SMART-1 AMIE camera images (Fig. 5a) show that Bhabha, 64 km in diameter, has a relatively large (  15 km across) and tall central peak ( 41 km) and steep crater walls. Bose is approximately 90 km in diameter, has smaller central peaks ( o10 km across) and lies  20 km northwest of the Bhabha crater. To the east we find three other significant craters, one of which is the partially flooded crater Baldet. A second volcanic area (annotated purple in Fig. 3c) is the dark region north of Bose crater, where various small craters and depressions are flooded. The crater walls and central peaks of Bhabha, Bose and Stoney expose relatively fresh rocks from the subsurface that record minor space weathering. The UV/VIS algorithm image (Fig. 4a) yields high band curvature (pink colour) for the central peaks of Bose, Bhabha and Stoney indicating a noritic composition with abundant low-Ca pyroxene. No other regions of apparent noritic compositions are identified in Fig. 4a, suggesting that the norite

Fig. 4. (a) NIR 2000/1500 ratio image of Bose, Bhabha and Olivine Hill (60 px/deg, 0.5 km/pixel). (b) UV/VIS algorithm image of Bose, Bhabha and Olivine Hill (detail from Fig. 3b). Dark central peaks indicate high abundance of orthopyroxene (pink in b). Note the absence of clear indications for olivine-rich areas at Olivine Hill. The bright colours in Bhabha’s northern crater rim are artifacts due to illumination and saturation effects. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

has been uplifted by rebound of the shocked crater floors. Some pink colours are seen in the surrounding crater floors, yet the slumped crater walls of Bhabha, Bose and Stoney all appear green/ yellow lacking a noritic signature. This suggests that the noritic lower rock unit revealed by the central peaks is overlain by a thin (few kilometre) layer of gabbroic composition, poor in low-Ca pyroxene. This overall gabbroic layer is also evident in the fresh surfaces of craters in the surrounding region and can represent a possible remnant of differentiated mafic SPA impact melt sheet, cryptomare or a mafic intrusion such as a pluton formed during initial crustal formation (Pieters et al., 2001). An area in the northeast corner of Fig. 3b is characterised by strong band tilt features (green/yellow) in fresh craters and a mature weathered surface that has retained strong ferrous absorption features. The area is relatively flat and has a markedly lower albedo (Fig. 3a). In the geomorphological map (Fig. 3c) this area is marked with green dashed lines and may represent ancient Pre-Orientale mare basalts covered by regionally derived mafic material and secondary craters. North of the Bhabha, Bose and Stoney craters, another relatively smooth and lightly cratered terrain, also annotated in green in Fig. 3c, shows abundant high-Ca pyroxene, relatively low albedo and may represent ancient basaltic cryptomare. A third smooth plain interpreted as cryptomare is identified below Bhabha and Bose craters (Fig. 3c). All these smooth plains (cryptomare) have FeO contents that are lower than fresh mare basalts but significantly higher than surrounding highlands. The cryptomare units identified in this study spatially coincide with units identified as moderate plains (‘‘Plains South of Apollo’’) in the geomorphological study of Petro et al. (2011), who also interpret the area to represent ancient SPA volcanism. Within the smooth plain north of Bhabha and Bose, low-albedo patches of volcanic rocks filling up irregular depressions exhibit morphological and real colour UV/VIS spectral characteristics similar to those of pyroclastic deposits and mare ponds identified in the Oppenheimer crater by Gaddis et al. (2003) (see Section 3.3). These dark volcanic units, marked with purple lines in Fig. 3c, have high iron contents as indicated by Clementine FeO maps. They show strong band tilt features, indicating a high-Ca pyroxene rich (basaltic) composition, which is compositionally different from the pyroclastic units described in Oppenheimer (noritic, Section 3.3). Close examination of the volcanic area directly north of Bose (Fig. 3c) indicates two mare units with different colours in the UV/VIS image that correspond to two flooding units in the corresponding AMIE SMART-1 image (Fig. 5b). The slightly more

Fig. 5. High-resolution SMART-1 AMIE camera images of Bhabha and Bose area. (a) Detailed image of Bhabha crater with slumped and terraced crater walls, large central peaks and a single bowl-shaped craterlet on its crater floor. The image is a mosaic of two SMART-1 AMIE images, neutral filter, produced in ENVI. (b) SMART-1 AMIE image covering an area of  65 km  65 km, with UV/VIS image detail in inset (c). Note the flooded crater in the volcanic area north of Bose Crater. Two separate flooding events are identified, with the youngest volcanic unit being blue in the UV/VIS image and slightly less cratered in the AMIE image. Subsidence ridge visible in flooded crater interior. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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heavily cratered unit to the right has a yellow/green algorithm signature (basaltic, high-Ca pyroxene) and seems overlain by a second (less cratered) volcanic sheet that appears bright blue (less mafic) in the UV/VIS image (Fig. 5c). The second mare unit, annotated in Fig. 5b, completely flooded the crater north of Bose. The AMIE image also reveals a steep ridge in the flooded interior that is concentric to the crater walls. This ridge may result from loading of the mare basalt on the crater floor. Mare basalts in crater Baldet have a similar spectral character to the overlaying mare unit north of Bose, which could suggest a related volcanic origin. Just south-east of Bhabha lies Olivine Hill, or Mafic Mound, a region previously proposed as a potential sampling site for mantle material (Fig. 4; Duke, 2003; Pieters et al., 2001, Petro and Pieters (2010)). The UV/VIS algorithm image yields a green colour (band tilt parameter) for Olivine Hill, indicating a gabbroic/troctolitic composition, which is in agreement with the findings of Pieters et al., 2001 (Fig. 3b). Additional information on the distribution of olivine and pyroxene is provided by the NIR 2000/1250 ratio image (Fig. 4a). The central peaks from Bhabha and Bose have low 2000/1250 ratios in the NIR data image due to the strong absorption of the 2000 nm wavelength (Fig. 4a), indicating abundant (low-Ca) orthopyroxene. Low values were expected from the noritic composition identified in the UV/VIS image (Fig. 3b, Fig. 4b). Hence, the presence of orthopyroxene is supported by the NIR data. Conversely, olivine-rich mantle material should result in high 2000/1500 ratios. A detailed examination of Olivine Hill in Fig. 4a, however, demonstrates ubiquitous low 2000/1500 ratios. Significantly, the NIR ratio image suggests the absence of olivine-rich compositions in freshly exposed surfaces surrounding Olivine Hill. We are unable to confirm the existence of olivine-rich mantle derived deposits in Olivine Hill and thus support the proposal of Petro and Pieters (2010) to rename the area Mafic Mound, as a gabbroic composition rich in high-Ca pyroxene appears most applicable.

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3.2. Leibnitz–Ingenii region The north-western rim of the South Pole-Aitken basin is marked by several large impact craters including Leibnitz, Von Karman, Finsen, Jules Verne and the Ingenii basin (Fig. 6 a–d). The UV/VIS image (Fig. 6b) defines three geochemical regions. The north-west corner of the image is characterised by a high-albedo, anorthositic area that coincides with the rise of the SPA basin rim (blue in Fig. 6b and d). In this region we find the strongly eroded crater Jules Verne, which measures 143 km in diameter and is flooded with low-albedo lavas in its interior (Fig. 6d and Fig. 7). In a large area below the SPA rim, band tilt is the dominant parameter (high Ca-pyroxene). The green band tilt signature is best seen at the rims of 20 to 100 km sized craters and in the interior of Mare Ingenii (Fig. 6d and Fig. 7), indicating an overall gabbroic composition. On the right side of Fig. 6 a large pink region of noritic rocks is exposed in the rim and interior of the 245 km diameter Leibnitz crater. Leibnitz has an impact eroded rim and central peaks. The interior has been resurfaced by mare basalts leaving a flat featureless floor with two small craterlets, one of which is flooded (Fig. 6a and d). In Fig. 6b one can identify clear band curvature features (pink colours) in the rim of Leibnitz and in the interior and central peak of Finsen. The rim and ejecta sheets of Finsen crater, however, retain a strong green colour, reflecting the local occurrence of basaltic and gabbroic rocks, which is in contrast with the stratigraphically underlying layers (central peak and floor) of noritic composition. High band tilt signatures, resulting in scattered green and yellow colours in the interior of Leibnitz, are likely to reflect basaltic lavas emplaced on top of a noritic basin floor. Towards the SPA rim, where band tilt (green) is the more dominant parameter, pink colours suggesting abundant low-Ca pyroxene are still common in small freshly excavated surfaces. Hence, in this region of overall basaltic/gabbroic composition, norite again appears to be the underlying unit. This observation is

Fig. 6. (a) Real colour image of Leibnitz region, (b) UV/VIS algorithm image. Central Peaks (CP) 1 and 2, of noritic composition, are indicated with arrows, (c) NIR 2000/1250 ratio image and (d) geomorphologic and compositional map on real colour image, see Fig. 3 for legend. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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supported by the multispectral signature of the central peaks of two smaller complex craters, cp 1 and cp 2 (Fig. 6b and d), of 50 km and  65 km diameter, respectively. Both central peaks exhibit strong spectral band curvature, as they have tapped noritic rocks from the subsurface in a similar fashion as Bhabha, Bose and Finsen craters in the central SPA (Section 3.1). The NIR data support the UV/VIS detection of norite, with low 2000/1250 ratios in Leibnitz’s northern rim, central peaks 1 and 2 and Finsen crater floor (Fig. 6c). Very high 2000/1250 ratio values (white) are seen in the central part of the image, where a gabbroic or basaltic surface composition was identified with UV/VIS data. In particular, the floors of both flooded and non-flooded craters have a very high 2000/1250 nm reflectance ratio, contrasting sharply with values obtained for their direct surroundings. We observe a striking contrast in NIR signature between volcanic units on the SPA basin floor (Mare Ingenii) and at the rise of the SPA basin rim (floor-fractured crater Jules Verne). In Fig. 7 three composite images for both volcanic regions are shown to indicate their distinct spectral features. The NIR data reveal contrasting spectra, with low 2000/1500 nm ratios for the low-albedo

basalts in Jules Verne and high 2000/1500 nm ratios in Mare Ingenii (Fig. 7c). In the UV/VIS algorithm image (Fig. 7b) Mare Ingenii has strongly varying spectral features (high band tilt, band strength and band curvature) resulting in bright green, pink and blue colours, whereas Jules Verne only shows minor signatures of band curvature (pink). This indicates a different mineralogy (and source) of the extruded melts, with (ortho) pyroxene-dominated lavas on the SPA rim and more mafic olivine-rich basalts in the basin interior. The implied difference in composition of the extruded melt volumes may originate from the significant difference in crustal thickness between Jules Verne at the SPA rim and Mare Ingenii in the SPA interior (estimated to be  50 km and 30 km, respectively, Ishihara et al., 2009). The difference in crustal thickness between the two regions would probably have led to different degrees of partial melting and different cooling rates in the resultant magmas that in turn will result in different amounts of fractional crystallisation as the magmas ascended. As such, a thicker crust will lead to more evolved and less mafic magmas. The apparent differences in magma compositions between the interior and margin of the SPA basin are therefore not unexpected.

Fig. 7. Three high-resolution composites (a: Real colour, b: UV/VIS, c: NIR) of flooded regions Mare Ingenii and Jules Verne, showing clear differences in spectral signatures between volcanic units on the SPA basin interior and on the rim. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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3.3. Apollo–Oppenheimer region The north-eastern edge and rim area of the SPA basin are dominated by the large, 480 km diameter, multi-ringed Apollo basin (Fig. 8). The inner ring of the Apollo basin contains low-albedo mare basalts. Additional patches of basaltic lava are emplaced in the southern and western parts of the second ring depression (Fig. 8). To the west lies the 205 km diameter, floor-fractured Nectarian aged Oppenheimer crater, overprinting the high-albedo Apollo rim (Head III et al., 2000). Oppenheimer’s crater floor is filled with low albedo rocks (Fig. 9), referred to as dark-haloed craters by Head et al. (2000) and identified as pyroclastic deposits (Petro et al., 2001; Gaddis et al., 2003) of Imbrium age (Stuart-Alexander, 1978). Sill emplacement underneath the crater floor was modelled and proved consistent with the location of floor fractures and their associated darkhaloed craters (Head et al., 2000). Another crater flooded by Imbrium-age mare basalt is the lower-Imbrium aged Maksutov crater south-west of Oppenheimer (Wilhelms and McCauley, 1987). Maksutov is 83 km wide and has a well-defined outer rim, largely unaffected by impact erosion (Fig. 9). UV/VIS data (Fig. 8b) reveal a large noritic province comprising the Apollo basin and its surroundings. Surface rocks and regolith have undergone severe space weathering and alteration and lost their spectral absorption features, whereas the freshly excavated rocks in younger craters show a clear noritic composition with strong band curvature. A good example of this is the central peaked crater Dryden overprinting the inner ring of Apollo in the north-western quadrant, which has exposed fresh norite both in its crater wall and central peaks (Fig. 8b). One particularly mafic area identified in the area between the craters White and Grissom (Fig. 8c) was described in Section 3.1 above and was interpreted as cryptomare, consistent with the findings of Pieters et al. (2001) and Petro et al. (2011). In the southwest corner of Fig. 8b we identify an even stronger signature of

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high-Ca pyroxene, in bright green colours. This region is connected to the extensive gabbroic zone in the Bhabha and Bose region. The Oppenheimer and Maksutov craters both contain Imbrium-age volcanic deposits with different mineralogical signatures (Fig. 9). With the exception of very small craters in the mare basalt in the Maksutov crater, all freshly exposed rocks of superposed craters in Maksutov and Oppenheimer have a noritic composition. Both crater walls and central peaks of Maksutov have a strong band curvature, in contrast to the band tilt signatures in green and yellow on the basaltic floor. The dark-toned pyroclastic deposits in Oppenheimer, however, do not exhibit the mineral signatures characteristic of crystalline basalt as recorded in other fresh craters. This indicates that these deposits are either enriched in low-Ca pyroxene, or texturally different from the basaltic plains in Maksutov crater, possibly containing a significant glassy component (Pieters et al., 2001; Gaddis et al., 2003).

4. Discussion Based on the results discussed above, cryptomare and other igneous rock units represent the youngest and uppermost unit in the SPA basin interior stratigraphy. Igneous rock units mostly consist of basalt and gabbro, some containing a significant glass component, which were emplaced by volcanic outflows or as plutonic intrusions. Multispectral signatures differ depending on age, style of eruption, rate of cooling and crystallisation and the composition of the source melt. Lava compositions on the SPA rim differ from lavas that extruded in the SPA interior, with the latter containing more olivine and clinopyroxene than lavas on the rim. The volcanic mare basalts and gabbros were most likely deposited in the Pre-Orientale and Late Imbrium periods. The non-mare gabbroic/basaltic units were generally deposited at an earlier stage and may represent (i) mafic differentiation of the SPA impact melt sheet; (ii) mafic plutons

Fig. 8. (a) Real colour image of Apollo region, (b) UV/VIS algorithm image, (c) geomorphologic and compositional map on real colour image, see Fig. 3 for legend. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 9. Maksutov and Oppenheimer in real colour (a) and UV/VIS algorithm (b) composites.

produced during crustal formation; (iii) stalled dikes and sills of basaltic composition (iv) or ancient basalts that are extensively weathered and covered with regolith (Pieters et al., 2001). The deepest crustal layer exposed in the basin, directly underneath the anorthite-rich upper crust, is norite. The pervasive nature of noritic materials in the SPA basin suggests a low-Ca pyroxene lithology for the bulk composition of the excavated lower crustal material. The noritic lithology may occur in the form of lower crustal basement or differentiated impact melt sheet and breccia. Contrary to the findings of Pieters et al. (2001), NIR ratio images do not indicate olivine-rich or troctolitic lithologies at the Olivine Hill. Our data suggest norite is exposed in all large subbasins (Leibnitz, Apollo, Oppenheimer) and in the central peaks of smaller craters (Bhabha, Bose, White and others). The widely distributed gabbros, with abundant clinopyroxene, seem to overlay the noritic layers and may represent remnants of a mafic impact melt sheet, which differentiated shortly after the SPA impact. Later impacts may have re-exposed the orthopyroxenerich noritic lower crust from beneath a differentiated gabbroic melt sheet. Although minor areas of felsic composition are identified within the SPA basin, the general lack of plagioclaserich rocks in the inner part of the basin indicates that the upper crust was effectively removed by the basin-forming impact.

5. Conclusions We have used a combination of Clementine UV/VIS and NIR data to assess the mineralogy and stratigraphy of the SPA basin. UV/VIS data suggest the presence of a pervasive noritic composition in the north-east quadrant of the SPA basin that contrasts with a gabbroic-basaltic signature in the north-west quadrant of the basin. The presence of central peaks with a noritic composition and a largely noritic Leibnitz crater wall, however, suggests that norite is a substantial (4km thick) stratigraphic unit, which we postulate to stratigraphically underlie most of the SPA basin floor. NIR ratio images confirm the presence of norite, with low 2000/1250 ratio values indicating abundant orthopyroxene. Furthermore, the NIR data suggest a different mineralogical composition in volcanic features on the rim of the SPA basin compared to the SPA interior. High NIR ratio values indicate more mafic volcanism in the SPA basin floor, where we propose that the

significantly thinner crust ( 20 km difference, Ishihara et al., 2009) influenced both partial melting and fractional crystallisation processes. Consistent with the work of Yan et al. (2010) and Yamamoto et al. (2010) our results do not support the presence of olivine-rich materials in Olivine Hill. Moreover, we find no evidence in any of the areas we studied in detail to suggest that the SPA impact excavated down to the upper mantle. We agree with Pieters et al. (2001) and Petro and Pieters (2010) that the wide distribution of norite throughout the SPA basin suggests that low-Ca pyroxene dominated rocks are the primary component of the impact debris and/or impact melt. They likely represent the composition of the lower crust. As higher resolution multispectral and hyperspectral data from the Kaguya, Chandrayaan-1, Chang’e 1 and LRO missions have become available, more detailed studies of the SPA basin will further help elucidate the geological evolution of this unique lunar feature.

Acknowledgments This study was performed as part of AMB’s bachelor’s thesis at VU University Amsterdam. We are grateful to our colleagues in RSSD at ESTEC-ESA in Noordwijk for providing the materials and working space. We also thank the Clementine and SMART-1 teams for the use of the data. In particular, we thank Detlef Koschny and Bjorn Grieger for help with software issues and providing SMART-1 AMIE images, respectively. We acknowledge two anonymous reviewers for their very constructive criticism of the manuscript. Inclusion of their suggestions has undoubtedly improved our study. WvW acknowledges financial support from an ESF EURYI award. References Andrews-Hanna, J.C., Zuber, M.T., Banerdt, W.B., 2008. The Borealis basin and the origin of the martian crustal dichotomy. Nature 453, 1212–1215. Benz, W., Slattery, W.L., Cameron, A.G.W., 1988. Collisional stripping of Mercury’s mantle. Icarus 74, 516–528. Canup, R.M., 2008. Lunar-forming collisions with pre-impact rotation. Icarus 196, 518–538. Carpenter, J., Fisackerly, R., Espinasse, S., the Lunar Exploration Definition Team, 2010. Lunar Exploration Definition Team Lunar Exploration Objectives and Requirements Definition. ESA Document LL-ESA-ORD-413.

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