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Remote detection of_magmatic_water_in_bullialdus_crater_on_the_moon


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Remote detection of_magmatic_water_in_bullialdus_crater_on_the_moon

  1. 1. LETTERS PUBLISHED ONLINE: 25 AUGUST 2013 | DOI: 10.1038/NGEO1909 Remote detection of magmatic water in Bullialdus Crater on the Moon R. Klima1 *, J. Cahill1 , J. Hagerty2 and D. Lawrence1 Once considered dry compared with Earth, laboratory analyses of igneous components of lunar samples have suggested that the Moon’s interior is not entirely anhydrous1,2 . Water and hydroxyl have also been detected from orbit on the lunar surface, but these have been attributed to nonindigenous sources3–5 , such as interactions with the solar wind. Magmatic lunar volatiles—evidence for water indigenous to the lunar interior—have not previously been detected remotely. Here we analyse spectroscopic data from the Moon Mineralogy Mapper (M3 ) and report that the central peak of Bullialdus Crater is significantly enhanced in hydroxyl relative to its surroundings. We suggest that the strong and localized hydroxyl absorption features are inconsistent with a surficial origin. Instead, they are consistent with hydroxyl bound to magmatic minerals that were excavated from depth by the impact that formed Bullialdus Crater. Furthermore, estimates of thorium concentration in the central peak using data from the Lunar Prospector orbiter indicate an enhancement in incompatible elements, in contrast to the compositions of water-bearing lunar samples2 . We suggest that the hydroxyl- bearing material was excavated from a magmatic source that is distinct from that of samples analysed thus far. Hydrogen, hydroxyl (OH− ) and water have been detected on the lunar surface from orbit using a range of different instruments3–7 . Hypotheses for globally distributed surficial H2O/OH− molecules or larger deposits of water ice in the permanently shadowed regions of the Moon include that OH− and H2O molecules are produced in situ through the interaction of solar-wind-derived protons with lunar soils3,5,8 and/or that they are delivered by comets and other impactors9 . Broadly distributed molecular OH− or H2O formed by solar-wind interaction with surface silicates is expected to be loosely bound to the lunar surface8 and may dissociate from surface soils during solar heating, migrating along ballistic trajectories until ultimately becoming cold-trapped in permanently shadowed regions and/or buried10,11 . New research suggests that hydroxyl formed by solar-wind bombardment may also become embedded in agglutinates during micrometeorite impacts as part of the space weathering process12 . Until now, bound, magmatic lunar volatiles have not been detected remotely anywhere on the Moon. The 61-km-diameter Bullialdus Crater, centred at 20.7◦ S, 337.8◦ E in Mare Nubium (Fig. 1), lies along the southern edge of the Procellarum KREEP (potassium, rare earth elements and phosphorus) Terrane (PKT), a region on the nearside of the Moon that is highly enriched in incompatible elements13 . Lunar Prospector measurements suggest that Bullialdus Crater coincides with a localized concentration of thorium14 . Bullialdus Crater is mineralogically distinct and its central peak has long been recognized as exhibiting strong spectral signatures typical of norite, 1Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723, USA, 2US Geological Survey, Astrogeology Science Center, Flagstaff, Arizona 86001, USA. *e-mail: an intrusively formed igneous rock dominated by orthopyroxene (Ca-poor pyroxene) and anorthite (Ca-rich plagioclase feldspar)15 . Stratigraphic interpretations of materials exposed in Bullialdus Crater walls, coupled with the presence of exhumed noritic materials in its central peak, led previous investigators to suggest that the Bullialdus impact excavated a layered mafic intrusion15,16 . Data from M3 provide an opportunity to examine Bullialdus Crater both at high spatial (∼140 m per pixel) and spectral (20–40 nm sampling) resolution. They also provide, for the first time, the opportunity to characterize visible to near-infrared wavelength reflectance spectra (that is, 0.6–3 µm). The 3 µm region in particular is critical for near-infrared volatile assessment of the Moon or any other airless body because both water and hydroxyl are strongly absorbing in this portion of the electromagnetic spectrum. Previous interpretations of this area suggest that the Bullialdus impactor penetrated the basalt-flooded Nubium Basin, excavating a range of intrusive crustal rocks. The central peaks of Bullialdus are mineralogically diverse, with the westernmost peak exhibiting a more clinopyroxene-rich spectral signature (Fig. 1e, cyan) than the northern peaks. On the basis of radiative transfer models of Clementine reflectance data, this western portion of the peak has been classified as anorthositic gabbronorite, whereas the northern peaks have been classified as anorthositic norite or norite17 . Norite (Fig. 1e, yellow) dominates the bulk of the central peak, though there is a region of anorthositic material (Fig. 1e, dark blue) exposed towards the centre of the peak. Anorthositic material is also exposed in the crater rim and proximal ejecta (Fig. 1d, black). A map of the 2.8 µm band depth (Fig. 1f,g) reveals that hydroxyl is detected in only the central peak and is explicitly found in association with the noritic and anorthositic peak material. In laboratory measurements, bound OH− can be distinguished from adsorbed OH− and H2O molecules by examining the spectral shape. Unfortunately, the specific spectral shape of the hydroxyl absorption band cannot be precisely characterized by M3 because of the 40 nm spectral sampling in the 3 µm region. The surficial OH− and H2O molecules detected previously by M3 typically exhibit a broad absorption beyond 2.8 µm (ref. 8). In contrast, the hydroxyl absorption observed in the central peak of Bullialdus Crater is significantly stronger and sharper than is observed at similar latitudes in other nearby terrains (Fig. 2 and Supplementary Fig. S1). The absorption observed in both the anorthositic and noritic material exhibits a clear band minimum at 2.8 µm. The band shape is significantly sharper than laboratory measurements of agglutinates12 , but is consistent in both energy and band shape with OH− measured in transmission spectra of internally bound hydroxyl in nominally anhydrous terrestrial minerals18 such as anorthite and orthopyroxene, and minor hydrated terrestrial minerals such as apatite19 (Supplementary Fig. S2). NATURE GEOSCIENCE | VOL 6 | SEPTEMBER 2013 | 737
  2. 2. LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO1909 23° W 22° W 21° W 7 6 5 1 2 4 3 b 22° S d f c e g 23° W 22° W 21° W 23° W 22° W 21° W Bullialdus Mare Nubium a 8 20° S 21° S 22° S 20° S 21° S 20° S 21° S 22° S Figure 1 | Context and spectral parameter maps of Bullialdus Crater. a, LROC wide-angle camera (WAC) view of Bullialdus Crater. Scale bar, 100 km. Image credit: NASA/GSFC/ASU. b,c, M3 750 nm albedo map of Bullialdus Crater (80 km image width; b) and central peak (scale bar, 5 km; c). d,e, M3 false colour composite (Supplementary Table S1) highlighting mineralogy of the crater (d) and central peak (e). f,g, 2.8 µm OH− absorption strength in Bullialdus Crater (f) and central peak (g). Absorption strength grades black (0%) to white (4%). Numbers on a–c indicate locations from which Fig. 2 spectra were extracted. If the hydroxyl observed at Bullialdus Crater is solar-wind implanted, it could either be locked stably within agglutinates12 or it could be unstable, adhered loosely to mineral grains but continuously forming in the solar-wind flux10 . The former would imply that the central peak of Bullialdus is more enriched in agglutinates (or older) than any other material within at least 10◦ latitude in any direction, which is inconsistent with surface maturity maps of the region20 . The strongest water/hydroxyl absorption bands are found along and just below the peak ridge on both sides and coincide with bands of large (tens to ∼150-m-diameter) boulders and potentially bedrock (Fig. 3). The coarse texture and high albedo of the OH− -rich regions of the central peak suggest that it has not experienced unusually high degrees of agglutination relative to its surroundings. If OH− in Bullialdus Crater is loosely adhered to the surface, the band depths of the OH− absorption might weaken later in the lunar day, as the surface becomes warmer, or when the Moon is within the Earth’s magnetotail and shielded from the solar wind. The central peak of Bullialdus was imaged by M3 at different times of the lunar day. Hydroxyl absorptions are deepest near the apexes of the peaks and weaken with increasing distance downslope. They occur symmetrically about the central peak ridges, but because of the illumination geometry in each of the optical periods, we compare a slightly weaker absorption on a flatter slope that is not in shadow during any of the three optical periods. After photometric correction, there is little to no change in absorption band depth observed to suggest hydroxyl mobility (Fig. 2 and Supplementary Fig. S3). A previous study10 showed that the likelihood of OH− adsorption may depend on lithology. The central peak of Aristillus Crater (33.9◦ N, 1.2◦ E) has a similar lithology to Bullialdus and is also the focus of a thorium anomaly. Given that Aristillus is more than 10◦ farther from the equator, it should be cooler and more hospitable to OH− adsorption than Bullialdus, resulting in a central peak that is coated with OH− . However, it does not exhibit a prominent regional 3 µm anomaly (Fig. 2). The geologic context coupled with the stability of the 2.8 µm absorption band at Bullialdus Crater leads us to conclude that the water/hydroxyl detected in the central peak is most likely bound within primary magmatic materials rather than implanted by the solar wind or within glassy agglutinates. We can place limits on the amount of water/hydroxyl present by making simplifying assumptions about the nature of the surface and/or mineralogy. On the basis of radiative transfer modelling, another study21 suggested that Bullialdus Crater central peak as a whole contains about 57% mafic minerals and 43% plagioclase. Using this bulk mineralogy, coupled with an orthopyroxene composition of Mg75 (ref. 22), we can derive the single-scattering albedo of the surface, from which the absorbance can be calculated23–25 (Supplementary Fig. S4). If we assume that the local 738 NATURE GEOSCIENCE | VOL 6 | SEPTEMBER 2013 |
  3. 3. NATURE GEOSCIENCE DOI: 10.1038/NGEO1909 LETTERS 1 Central peak 2 Central peak 5 Crater floor 6 Crater wall 7 Mare Nubium soil 6 Highland soil south 3 Central peak 4 Central peak 0.00 1,000 1,500 2,000 Wavelength (nm) 2,500 3,000 1,000 1,500 2,000 Wavelength (nm) 2,500 3,000 1,000 1,500 2,000 Wavelength (nm) 2,500 3,000 1,000 1,500 2,000 Wavelength (nm) 2,500 3,000 0.05 0.10 0.15 0.20 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Scaledreflectance ReflectanceScaledreflectance Scaledreflectance 0.00 0.05 0.25 0.15 0.20 0.10 Optical period 1b Optical period 2a Optical period 2c1 Bullialdus central peak, 20.8° S, 22.3° W Aristillus central peak, 33.9° N, 1.2° E Highlands, 59.0° S, 43.5° W a b c d Figure 2 | Spectra of Bullialdus Crater and the surrounding region. a, Strong absorption bands at 1,000 and 2,000 nm are indicative of pyroxene, whereas flatter, brighter spectra are probably dominated by anorthosite. b, Bullialdus spectra scaled to unity at 2.736 µm. Only the spectra extracted from the central peak exhibit a hydroxyl absorption. c, Spectra from Bullialdus Crater central peak compared with the higher latitude, noritic central peak of Aristillus Crater and a higher-latitude region of highlands exhibiting a typical surficial water signature. d, Repeat spectral imaging during three optical periods. c,d are scaled at 2.736 µm. regolith exhibits a grain size between 15 and 150 µm, we estimate the maximum abundance in the central peak to be 80±40 ppm as water. In principle, one might expect that the bulk hydrogen signature could be detected using neutron spectroscopy in a manner similar to how enhanced hydrogen signatures were detected at the lunar poles6 . However, the small spatial scale of the Bullialdus central peak (∼5 km width) and the low hydrogen concentrations ([H] = (2/18)(80 ± 40) = 9 ± 4 ppm H) preclude the detection of a hydrogen signature from either the Lunar Prospector or Lunar Exploration Neutron Detector data because the hydrogen sensitivity of these data sets is tens of ppm H and >100 ppm H, respectively26,27 . A general Th enhancement (5–6 ppm) focused on Bullialdus Crater suggests that the excavated rocks also contain increased amounts of KREEP. The broad spatial resolution (>30 km) of the Th data14 precludes a unique quantification of the Th content of the central peak. However, using forward modelling techniques that have been validated28 elsewhere on the Moon, the Th concentrations of the central peak region can be non-uniquely estimated. Given the distinct mineralogical character of the central peak, we have carried out forward modelling of the entire Bullialdus region to infer the Th concentration of the central peak region (Supplementary Information). Our modelling (Fig. 4) indicates that the central peak could have a Th concentration as high as 16.5±2.5 ppm. Based on a Clementine-derived FeO estimate, the central peak has FeO abundances ranging from 8 to 10 wt%. When compared with values in the lunar sample suite these Th values are consistent with alkali-suite lithologies such as alkali norites29 , though the composition of the pyroxenes22 is more magnesian than typical for alkali-suite samples. The association of OH− with KREEP is enigmatic. Though it has been suggested that any native lunar OH− , such as KREEP, would be concentrated in late-stage magma ocean fluids8 , studies of hydroxyl in lunar apatites with varying KREEP abundance reveal an anticorrelation between KREEP and OH− , suggesting complexity and heterogeneity in the volatile distribution in the lunar interior2 . Bullialdus may have exposed further source-region heterogeneity by excavating material that is enriched in both volatiles and KREEP. The Bullialdus-forming impact is likely to have excavated about 6–9 km deep into the lunar crust21 . However, the impact occurred near the edge of Mare Nubium, near the inner ring of the basin. This fortuitous placement of Bullialdus Crater may explain its unusual composition. One possible scenario is that material from a pluton, originally residing within the lower crust, was uplifted to 6–9 km beneath the surface during the Nubium basin-forming impact and subsequently excavated by the Bullialdus NATURE GEOSCIENCE | VOL 6 | SEPTEMBER 2013 | 739
  4. 4. LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO1909 Noritic Anorthositic a b c Figure 3 | Geology of Bullialdus Crater central peak. a, Perspective view of central peak, looking towards the south. LROC narrow-angle camera (NAC) images (Supplementary Fig. S5) overlain by 2.8 µm OH− band depth map from Fig. 1 and draped over LROC WAC 100 m digital elevation model30 (vertically exaggerated ×6). The strongest absorptions occur where large boulders are observed on either side of the ridges. Arrows indicate where the images in b,c are located. b,c, Subset from LROC NAC image showing the surface geology in noritic (b) and anorthositic (c) spectral regions. Scale bars, 300 m. 6.24 6.09 5.95 5.80 5.65 5.51 5.36 5.21 5.07 4.92 4.77 Thabundance(ppm) 16.66 15.00 13.33 11.67 10.00 8.33 6.67 5.00 3.33 1.67 0.00 Thabundance(ppm) Th abundance (ppm) 18.18 17.13 16.09 15.04 14.00 12.95 11.91 10.86 9.81 8.77 7.72 FeOabundance(wt%) ¬20 ¬20 ¬20 ¬20 ¬20 ¬20 0.4 12 14 16 18 20 0.6 0.8 1.0 1.2 1.4 1.6 χ2 a c b d Figure 4 | Thorium content of Bullialdus Crater central peak. a, LP-GRS Th map (0.5◦ ×0.5◦) of Bullialdus Crater. Scale bar, 50 km. b, Clementine FeO map of the crater. c, Forward modelling results for the region. Central peak is outlined in black in a–c. d, χ2 error analysis of the central peak, indicating that the optimum Th value for the peak is 16.5±2.5 ppm Th. Crater-forming impact. Alternatively, deep fracturing during and following the Nubium-Basin impact may have facilitated intrusion of hydroxyl-bearing magma into the upper (∼10-km-deep) crust. Our derived water/hydroxyl abundance is within the range of OH− concentrations for nominally anhydrous crustal pyroxenes on Earth18 . High concentrations (>1,000 ppm weight) are strongly a function of orthopyroxene aluminium content and typically form at pressures greater than 1 GPa. A pressure of 1 GPa would imply that any OH− -bearing pyroxenes crystallized at a depth of near to 200 km, well below the excavation depth of Bullialdus or even the Nubium impact. A shallower origin is possible if the OH− is hosted within a minor hydrous mineral, as fluid inclusions, or in a quenched silicate melt. If the minor phase makes up 2% of the bulk rock, it would require ∼0.2–0.6 wt% OH− to account for the observed signature. This concentration is within the range of OH− concentrations measured in apatites from mare basalt samples (some >1.5 wt% OH− ). Methods All M3 and Lunar Reconnaissance Orbiter camera (LROC) data used in this project are available in the NASA planetary data system at Bullialdus Crater was fully imaged by M3 during three optical periods: 1b, which was illuminated from the east (average phase angle 60◦ ) and has a spatial resolution of 140 m per pixel; 2a, which was illuminated from the west (average phase angle 68◦ ) and has a spatial resolution of 140 m per pixel; and 2c1, which was illuminated from the west (average phase angle 40◦ ). Aristillus Crater was imaged in optical periods 1b and 2c1. Data were mosaicked for each of the optical periods and then parameters 740 NATURE GEOSCIENCE | VOL 6 | SEPTEMBER 2013 |
  5. 5. NATURE GEOSCIENCE DOI: 10.1038/NGEO1909 LETTERS were selected to isolate and characterize key absorption features for more easy visualization of spectral variability in its spatial context (Supplementary Table S1). Estimation of hydroxyl abundance in the central peak materials requires conversion of spectra into an absorption coefficient, knowledge of the molar absorption coefficient for the local lithology and an estimate of the path length through the minerals. These calculations further require knowledge of surface temperature, particle size and other scattering properties. We can place limits on the amount of hydroxyl present by making several simplifying assumptions about the nature of the surface and mineralogy. One advantage to using an absorption coefficient as derived from the single-scattering albedo of a particulate surface is that we are not subject to orientation effects, because the observed signal is inherently an integrated absorption average combining all crystallographic directions. Level 2c1 data were specifically selected for modelling hydroxyl abundance because they were obtained at the lowest phase angle, which minimizes shadowing and multiple scattering effects4 on the steep slopes of the central peak. We convert the spectra to single-scattering albedo using the methods of ref. 23 and then solve for the absorption coefficient using a particle size of 45 µm. To derive single-scattering albedo, we calculate the albedo factor23 using the incidence and emission angles and the reflectance values at each wavelength. Albedo factor is then converted to single-scattering albedo23 . If we approximate the mean path ray length (in micrometres) through a single particle as 45 µm, we can approximate absorbance23 . This requires the use of a refractive index (n), which we calculate24 using pyroxene with an Mg# of 0.75 as an estimate for the composition of the central peak of Bullialdus22 . Single-scattering albedo and calculated absorbance spectra for the central peak spectra are shown in Supplementary Fig. S3. The baseline-removed hydroxyl absorption intensity of CP1 was fit by a Gaussian curve and then the centre frequency (in wavenumbers) of the OH− stretching absorption was used to calculate the integrated molar absorption coefficient (εi; ref. 25). As this calibration for εi uses units of water per mol rather than H or OH− , we used the formulation of the Beer–Lambert law shown in equation (1) to determine the concentration of water. The area of the Gaussian was used as the integrated absorption intensity (Ai), the path length of 45 µm was used for thickness (t) and density of 3.0 g cm−3 was used for the bulk rock density (ρ). This results in a concentration (c) of 80 ppm for the chosen particle size of 45 µm. c = Ai t ×p×εi ×1.8 (1) To ensure that this estimate was not highly dependent on the composition of the mineral hosting the hydroxyl, we calculated the absorption coefficient using the refractive index of anorthite and quartz as well. In those cases, the baseline-removed absorption coefficients were indistinguishable from the orthopyroxene and resulted in the same concentration at a fixed grain size. As the scattering properties of the rocks are controlled by the dominant mineral phase(s), we did not carry out an analysis using apatite. For a given concentration of water, the strength of an OH− absorption in apatite is comparable to the OH− strength in a nominally anhydrous mineral18,19 . As any minor phase would comprise only a few per cent of the total rock, the amount of hydroxyl in that phase would be significantly higher (that is, we are calculating the abundance in the bulk path length, not in the host phase per se). In the laboratory, water content calculations based on infrared spectra are subject to typically 10–30% relative uncertainty depending on the strength of the absorption band and the approximation of the molar absorption coefficient12,25 . As the calculation of single-scattering albedo is heavily dependent on grain size, additional uncertainty is introduced. If the average grain size of the surface is closer to 15 µm, the water abundance would be approximately 20% higher, whereas if it is closer to 150 µm the water content would be approximately 20% lower, resulting in a range of ∼45–125 ppm. Received 19 April 2013; accepted 5 July 2013; published online 25 August 2013 References 1. Saal, A. E. et al. Volatile content of lunar volcanic glasses and the presence of water in the Moon’s interior. Nature 454, 192–195 (2008). 2. McCubbin, F. M. et al. Fluorine and chlorine abundances in lunar apatite: Implications for heterogeneous distributions of magmatic volatiles in the lunar interior. Geochim. Cosmochim. Acta 75, 5073–5093 (2011). 3. Pieters, C. M. et al. Character and spatial distribution of OH/H2O on the surface of the Moon seen by M3 on Chandrayaan-1. Science 326, 568–572 (2009). 4. Clark, R. N. Detection of adsorbed water and hydroxyl on the Moon. Science 326, 562–564 (2009). 5. Sunshine, J. M. et al. Temporal and spatial variability of lunar hydration as observed by the Deep Impact spacecraft. Science 326, 565–568 (2009). 6. Feldman, W. et al. Fluxes of fast and epithermal neutrons from lunar prospector: Evidence for water ice at the lunar poles. Science 281, 1496–1500 (1998). 7. Colaprete, A. et al. Detection of water in the LCROSS ejecta plume. Science 330, 463–468 (2010). 8. McCord, T. B. et al. Sources and physical processes responsible for the OH/H2O in the lunar soil as revealed by the Moon Mineralogy mapper (M3 ). J. Geophys. Res. 116, E00G05 (2011). 9. Arnold, J. R. Ice in the lunar polar regions. J. Geophys. Res. 84, 5659–5668 (1979). 10. Hibbitts, C. C. et al. Thermal stability of water and hydroxyl on the surface of the Moon from temperature-programmed desorption measurements of lunar analog materials. Icarus 213, 64–72 (2011). 11. Crider, D. H. & Vondrak, R. R. The solar wind as a possible source of lunar polar hydrogen deposits. J. Geophys. Res. 105, 26773–26782 (2000). 12. Liu, Y. et al. Direct measurement of hydroxyl in the lunar regolith and the origin of lunar surface water. Nature Geosci. 5, 779–782 (2012). 13. Jolliff, B. L., Gillis, J. J., Haskin, L. A., Korotev, R. L. & Wieczorek, M. A. Major lunar crustal terranes: Surface expressions and crust–mantle origins. J. Geophys. Res. 105, 4197–4216 (2000). 14. Lawrence, D. J. et al. Small-area thorium features on the lunar surface. J. Geophys. Res. 108, JE002050 (2003). 15. Pieters, C. M. Bullialdus—strengthening the case for lunar plutons. Geophys. Res. Lett. 18, 2129–2131 (1991). 16. Tompkins, S., Pieters, C. M., Mustard, J. F., Pinet, P. & Chevrel, S. D. Distribution of materials excavated by the lunar crater Bullialdus and implications for the geologic history of the Nubium region. Icarus 110, 261–274 (1994). 17. Cahill, J. T. & Lucey, P. G. Radiative transfer modeling of lunar highlands spectral classes and relationship to lunar samples. J. Geophys. Res. 112, 10007 (2007). 18. Johnson, E. A. in Water in Nominally Anhydrous Minerals Vol. 62 (eds Keppler, H. & Smyth, J. R.) 117–154 (Mineralogical Society of America, 2006). 19. Wang, K. L., Zhang, Y. X. & Naab, F. U. Calibration for IR measurements of OH in apatite. Am. Mineral. 96, 1392–1397 (2011). 20. Lucey, P. G., Blewett, D. T., Taylor, G. J. & Hawke, B. R. Imaging of lunar surface maturity. J. Geophys. Res. 105, 20377–20386 (2000). 21. Cahill, J. T. S., Lucey, P. G. & Wieczorek, M. A. Compositional variations of the lunar crust: Results from radiative transfer modeling of central peak spectra. J. Geophys. Res. 114, 09001 (2009). 22. Klima, R. L. et al. New insights into lunar petrology: Distribution and composition of prominent low-Ca pyroxene exposures as observed by the Moon Mineralogy Mapper (M3 ). J. Geophys. Res. 116, E00G06 (2011). 23. Hapke, B. Theory of Reflectance and Emittance Spectroscopy 2nd edn (Cambridge Univ. Press, 2012). 24. Lucey, P. G. Model near-infrared optical constants of olivine and pyroxene as a function of iron content. J. Geophys. Res. 103, 1703–1713 (1998). 25. Libowitzky, E. & Rossman, G. An IR absorption calibration for water in minerals. Am. Mineral. 82, 1111–1115 (1997). 26. Lawrence, D. J. et al. Improved modeling of Lunar Prospector neutron spectrometer data: Implications for hydrogen deposits at the lunar poles. J. Geophys. Res. 111, E08001 (2006). 27. Mitrofanov, I. G. et al. Lunar exploration neutron detector for the NASA lunar reconnaissance orbiter. Space Sci. Rev. 150, 183–207 (2010). 28. Hagerty, J. J. et al. Refined thorium abundances for lunar red spots: Implications for evolved, nonmare volcanism on the Moon. J. Geophys. Res. 111, E06002 (2006). 29. Papike, J. J., Ryder, G. & Shearer, C. K. in Planetary Materials Vol. 36 (ed. Papike, J. J) 1–189 (Mineralogical Society of America, 1998). 30. Scholten, F. et al. GLD100: The near-global lunar 100 m raster DTM from LROC WAC stereo image data. J. Geophys. Res. 117, E00H17 (2012). Acknowledgements We thank the NASA Lunar Advanced Science and Engineering Program (NNX10AH62G to RK/JHUAPL), the NASA National Lunar Science Institute Polar Exploration Node (NNA09DB31A to JHUAPL) and the NASA Planetary Mission Data Analysis Program (NNH09AL42I to JH/USGS) for supporting this research. We are also grateful to the NASA Discovery Program, Indian Space Research Organization and M3 team. Author contributions All authors contributed extensively to this work. R.K. wrote the main manuscript with comments and feedback from the whole team. R.K. led analysis and modelling of M3 data, J.C. led processing and analysis of LROC data and J.H. and D.L. contributed the thorium analyses and forward modelling. Additional information Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at Correspondence and requests for materials should be addressed to R.K. Competing financial interests The authors declare no competing financial interests. NATURE GEOSCIENCE | VOL 6 | SEPTEMBER 2013 | 741