An explanation of bright areas inside Shackleton Crater at the Lunar South Pole other than water-ice deposits
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa, Japan
Corresponding author: J. Haruyama, Institute of Space and Astronautical Science, Space Exploration Center, Japan Aerospace Exploration Agency, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan. (firstname.lastname@example.org).
 Whether water molecules of cometary and/or solar wind origin migrated to and accumulated in cold permanently shadowed areas at the lunar poles has long been debated from the perspective of scientific interest and expectations for future utilization. Recently, high reflectance condition was observed inside the lunar South Pole Shackleton Crater for the 1064.4 nm of the Lunar Orbiter Laser Altimeter on the Lunar Reconnaissance Orbiter, and the high reflectance was explained to perhaps be due to a surface frost layer in excess of 20% water-ice. Here we investigate the crater with the Selenological Engineering Explorer Multi-band imager that has nine bands in the visible to near-infrared range, including a 1050 nm band (62 m/pixel resolution). Part of the illuminated inner wall of Shackleton Crater exhibits high reflectance at 1050 nm but also exhibits the diagnostic 1250 nm spectral absorption, a signature that is consistent with naturally bright purest anorthosite.
 Based on the almost perpendicular rotation axis of the Moon, it has been hypothesized that some water molecules, delivered by comets and/or originating from a combination of hydrogen from solar wind and oxygen of lunar soils, traveled to the cold permanently shadowed areas (PSAs) and accumulated there [Watson et al., 1961; Arnold, 1979; Butler, 1997]. Near-infrared spectroscopic measurements from three spacecraft have detected absorptions near 3 µm on the lunar surface, attributable to hydroxyl and/or water molecules whose strength increases poleward and whose origin may be solar wind-induced [Clark, 2009; Sunshine et al., 2009; Pieters et al., 2009]. Water molecules supplied to the lunar polar regions might be over a short range but can accumulate to a large amount through Giga years of geologic time. A large amount of water might be advantageously utilized in the future development of the Moon and for exploration beyond cis-lunar space. With scientific interest in the origin of water-ice deposits and possibly from the perspective of future resources, the existence of water-ice deposits in the PSAs of the lunar polar regions is heavily debated among scientists and regarded as a high-priority objective of future lunar exploration.
 The permanently shadowed 10.5 km radius inside of the Shackleton crater (Figure 1a) at the lunar South Pole has been a candidate for water-ice since the existence of water-ice was suggested by the Clementine bistatic radar experiment [Nozette et al., 1996] and by high hydrogen concentrations based on Lunar Prospector Neutron Spectrometer observation [Feldman et al., 2000]. In the Shackleton crater, the inferred average abundance of the water-equivalent hydrogen (WEH) is 0.5 wt.% based on the epithermal count rate observed by LPNS, and could be consistent with 20 wt.% WEH if the hydrogen is accumulated over 10% of the crater floor [Elphic et al., 2007]. In addition, there are areas with extended illumination [Noda et al., 2008; Bussey et al., 2010] in the vicinity of Shackleton Crater. Thus, the crater is considered the most advantageous location for possible future lunar base construction.
Haruyama et al. [2008b] first investigated the albedo of the crater floor of Shackleton in the image data from the Terrain Camera (TC) on the Selenological Engineering Explorer (SELENE, nicknamed Kaguya). The inside of Shackleton Crater is occasionally illuminated by scattered sunlight from the upper inner walls of the crater. Based on the estimated albedo inside Shackleton Crater, it was concluded that very little or no water-ice is present on the floor of the crater. This result is consistent with Earth-based radar observations [Campbell et al., 2006] that indicate little evidence of water-ice deposits, although the observations are limited to the upper part of the inner wall of the crater.
 Recently, Zuber et al.  analyzed the 1064.4 nm laser beam normal reflection (zero phase angle) [Smith et al., 2010] data of Lunar Reconnaissance Orbiter (LRO) Lunar Orbiter Laser Altimeter (LOLA) for Shackleton Crater and reported the reflectance of both its floor and part of the walls for LOLA beams to be as much as 50%. They stated that the considerably higher reflectance of Shackleton's inner surfaces compared with that of the surrounding terrain could be explained by a dearth of space weathering by micrometeorite bombardment, but they also suggested that possibly up to 22% water-ice deposits inside Shackleton Crater explain the high reflectance. However, such amounts of water-ice on the surface of Shackleton are inconsistent with the results of some previous studies. Thus, we reexamined the high reflectance observed by LRO LOLA using the multiband imager (MI) installed on SELENE, which has a band at 1050 nm (±15 nm) [Haruyama et al., 2008a; Ohtake et al., 2009] and can provide almost the same wavelength observation data as LOLA.
2 SELENE MI Data for Shackleton Crater
 The SELENE MI is a multispectral imager with five color bands in the visible range (415, 750, 900, 950, and 1000 nm) and four bands in the near-infrared range (1000, 1050, 1250, and 1550 nm) [Haruyama et al., 2008a; Ohtake et al., 2009]. The MI-visible pixel resolution was 20 m, and the near-infrared pixel resolution was 62 m at SELENE's nominal altitude of 100 km, one order higher than those of Clementine's similar instruments (UV-VIS and near-infrared) [Pieters et al., 1994].
 SELENE was inserted into a lunar polar orbit with a 90° inclination angle at the end of October 2007. The Lunar Imager/Spectrometer (LISM) onboard SELENE, which consisted of a Terrain Camera (TC), a Spectral Profiler (SP), and an MI, started its operation on 3 November 2007. The inclination of the lunar rotation axis to the Ecliptic is very small (1.62°) [e.g., Vaniman et al., 1991]; therefore, the polar regions experience a small annual summer during which they experience greater illumination and a winter in which they receive less. December 2007, when the SELENE mission began, was south polar summer. June 2009, when the mission ended with a controlled impact on the Moon, was during south polar winter. During the SELENE mission period, the lunar South Pole had two summer seasons; one began in November 2007, and the other began in November 2008. MI acquired the data of the South Pole regions for four periods: every orbit (1) from 18 January to 15 February 2008 (summer) with SELENE revolution numbers 1228–1571, (2) 10–11 August (spring) with revolution numbers 3740–3744, (3) 30 October to 29 November 2009 (summer) with revolution numbers 4724–5097, and (4) one orbit per day from 4 to 16 October 2008 (summer) with revolution numbers 4403–4549. MI was not operated simultaneously with TC because of the resource limit of SELENE (e.g., data storage, data transmission capability, and electric power). Acquired LISM (TC, MI, and SP) data were well calibrated and are in good agreement about the overall spectral properties of lunar materials [Pieters et al., 2013; Besse et al., 2013; Ohtake et al., 2013].
 The Shackleton crater rim inclines a few degrees [Haruyama et al., 2008b]; there is a 590 m difference in elevation between the higher portion of the northeast rim near the South Pole and the lower portion on the opposite rim with a 21 km diameter (the portions are indicated by stars in Figure 1a). In the South Pole midsummer, the upper inner wall on the higher-rim side is illuminated to 1.3 km depth by a slightly elevated Sun. In contrast, even in the summer, the inner wall of the opposite rim of lower elevation is not as illuminated, exhibiting very thin crescents in the TC and MI images.
3 Purest Anorthosite in Shackleton Crater Walls
 For comparison with reflectance data of LRO LOLA beam (1064.4 nm), we investigated SELENE MI 1050 nm reflectance data of SELENE revolution number 4403 taken on 4 October 2009, when an upper inner wall of Shackleton Crater was highly illuminated (Figure 1a).
 The MI reflectance (I/F) data are photometrically normalized to conditions of 30°, 0°, and 30° for incidence, emission, and phase angles using Digital Terrain Model (DTM) generated from MI nadir (1000 nm) and slant (900 nm) band sets that have a 5.5° parallax [Haruyama et al., 2008a; Ohtake et al., 2009, 2013]. Figure 1b presents the normalized reflectance images of Shackleton Crater. Most of the Shackleton interior portions exceed 30% reflectance. The highest reflectance values are nearly 50%. Here we note that some patchy distributions of 1050 nm high reflectance mainly on the northeast outer rim are on steep curved surfaces of small craters and are close to the shadowed regions (less than a few MI pixels). In such areas, the computation of MI DTM and the vector perpendicular to the surface may be incorrect, resulting in the wrong normalized reflectance. Indeed, such patchy, bright small areas are not observed in the LOLA measurements [see Zuber et al., 2012, Figure 1f]. In contrast, widely distributed higher reflectance portions (indicated by a green circle) on the Shackleton Crater inner wall are probably real and similar to the location where high reflection of the LOLA beam was observed.
 At the brightest parts on the Shackleton Crater wall, the solar elevation angle from the slope surface is nearly 30°. The illuminated surface with a solar elevation angle of 30° on the Moon would produce a temperature exceeding 300 K in a few days [see Vasavada et al., 1999]. Thus, the brightest parts are not cold enough to keep any water-ice deposit on the surface. While the SELENE TC was able to provide broadband reflectance information of the permanently shadowed floor by acquiring faint secondary scattered light from there, the MI sensitivity was not sufficient to provide narrowband reflectance information for the Shackleton's inside, even when the Shackleton's upper rim was maximally illuminated in the high summer season.
 The brightest Shackleton interior portions revealed by MI images are the locations where Yamamoto et al.  found the purest anorthosite (PAN) (see their Figure 2), a rock of high plagioclase abundance (approaching 100 vol.%) in the SP data, based on the diagnostic spectral absorption band seen at 1250 nm. Previously, SP identified the unambiguous absorption at 1250 nm, indicating the existence of PAN at several locations on the Moon (e.g., the central peak of Tsiolkovsky crater) [Matsunaga et al., 2008], and subsequently, a wide distribution of PAN over the lunar surfaces was confirmed based on the SELENE MI [Ohtake et al., 2009] and a global survey of PAN by SP performed by Yamamoto et al. . PAN has very high reflectance in the visible to near-infrared region [Matsunaga et al., 2008; Ohtake et al., 2009], and the high reflectance of the Shackleton inside surfaces in the SP data could be explained by PAN. However, SP is a line profiler with a footprint of 500 m [Haruyama et al., 2008a; Matsunaga et al., 2008]. In order to evaluate the spatial extent of PAN, we used an MI reflectance ratio of 1050/1250 nm across the region as in Figure 1b. A larger ratio indicates deeper absorption at 1250 nm compared to 1050 nm, consistent with an increased abundance of PAN. The brightest points (50% reflectance) at MI 1050 nm in Figure 1b correspond well to the red areas in Figure 1c, where the ratio is high, indicating deep absorption at 1250 nm, and are interpreted to be rich in PAN. The small, brighter patchy areas in Figure 1b that may be affected by incorrect topographic reconstructions do not exhibit the 1250 nm absorption.
 The outcrop locations at the inner wall of Shackleton Crater with possible PAN that exhibit relatively deep 1250 nm absorption in the MI data are consistent with highest reflectance areas of the LOLA 1064.4 nm measurements. Such high reflectance areas occur in PSAs of the Shackleton Crater interior [Zuber et al., 2012]. MI and SP could not provide direct evidence of PAN within the PSAs due to their lack of sensitivity. However, it is most reasonable that the other surfaces of the crater's inner wall expose fresh PAN or high-reflectance plagioclase. The inner walls of Shackleton have distinctly steep slopes (30°) [Campbell et al., 2006; Haruyama et al., 2008b; Zuber et al., 2012] that would promote near-continuous small, impact-triggered sloughing of materials, exposing fresh surfaces. In fact, in the TC images of the inside of the crater that was lit by secondary scattered sunlight from the upper parts of the illuminated inner walls, we observed brighter streaks on the inner wall and a mound on the floor (Figure 2c), possibly caused by landslides.
Thomson et al.  also suggested a rough surface of the Shackleton Crater wall based on the Lunar Reconnaissance Orbiter Mini-RF observation; that may be evidence of landslides at the Shackleton wall. The authors pointed out that the rough portions at the wall based on Mini-RF observation are slightly inconsistent with the brightness distribution observed by the LOLA measurement. However, even a superficial sloughing of materials that might not result in the rough surfaces observed by Mini-RF may expose fresh materials providing the bright reflectance in the visible to near-infrared range on the permanently shadowed Shackleton wall.
 PAN or high-reflectance plagioclase that covers the Shackleton's floor would produce bright reflectance as measured by the LOLA beam. At higher latitudes, surface conditions appear less mature [Yokota et al., 2011], perhaps because of less solar wind, which may also contribute to the high brightness of Shackleton Crater since it is located at the highest latitude, the lunar South Pole. The flat floor where PAN or high-reflectance plagioclase may be exposed or covered would produce brighter reflectance than the exterior of the Shackleton but less than the continually resurfaced steep walls. The MI reflectance of 30%–50% at 1050 nm, which corresponds to 20%–30% in the TC visible range (430–850 nm) in the plagioclase-rich areas [Ohtake et al., 2013], is not so high as to require bright water-ice [Haruyama et al., 2008b].
 The morphology of Shackleton Crater with its steep inner wall and slightly inclined rim causes high reflection of solar light on the inside floor of the crater for several days in midsummer. On those particular days, the floor temperature of Shackleton rises to 100 K from a low temperature of 40 K in a few days [Siegler et al., 2011]. Large temperature oscillations that are a unique characteristic of Shackleton Crater compared to other PSAs [Sefton-Nash et al., 2013] may result in effective diffusion of water molecules from the surface into the deep subsurface [Schorghofer and Taylor, 2007].
 Some recent observations limit water-ice deposits inside Shackleton Crater to be less than a few percent. Earth-based radar observations indicate little evidence of water-ice deposits [Campbell et al., 2006], although only the upper part of the Shackleton's inner wall could be observed. Visible wavelength observation at 430–850 nm by SELENE TC [Haruyama et al., 2008b] and far ultraviolet wavelength at 155–190 nm by LRO LAMP [Gladstone et al., 2012] indicate that surface water-ice could be disseminated and mixed with soil over only a small area. Neutron flux mapping by LRO Neutron Detector Experiment (LEND) [Mitrofanov et al., 2012] detected numerous local spots of suppression of epithermal neutron emission at the lunar poles, indicating a possible concentration of hydrogen or water-ice, but such a peculiar feature has not been observed in Shackleton Crater. The LRO Mini-RF orbital radar experiment [Thomson et al., 2012] provided no evidence of thick, slab-like ice that would be consistent with some other observation results. While its radar polarization signature may be consistent with a small percentage (5%–10%) of ice mixing into the uppermost meter or two of regolith, this would be an upper bound. Surface roughness alone may not be ruled out as the sole cause of this observed radar signature.
 In conclusion, the brighter inside of Shackleton Crater could be explained by the natural distribution of very bright, fresh plagioclase, probably PAN, rather than by water-ice frost exceeding 20%. We do not deny (or confirm) possible water-ice deposition on some other areas in the lunar polar regions, such as Cabeus LCROSS impacts points, where Colaprete et al.  reported 5% frost and inside Shoemaker Crater where a hydrogen concentration is suggested by LRO LEND data [Mitrofanov et al., 2012].
 The authors thank all the contributors to the SELENE (Kaguya) project, especially members of the project management group (Y. Takizawa, M. Kato, S. Sasaki, R. Nagashima, K. Tsuruda, and H. Mizutani) and the Lunar Imager/Spectrometer (LISM) working group. The authors thank B. J. Thomson and C. M. Pieters for comments, which led to significant improvements in the manuscript.
 The Editor thanks Bradley Thomson and Carle Pieters for their assistance in evaluating this paper.