Compositional evidence for_an_impact_origin_of_moons_procellarum_basin
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Compositional evidence for_an_impact_origin_of_moons_procellarum_basin Document Transcript

  • 1. LETTERS PUBLISHED ONLINE: 28 OCTOBER 2012 | DOI: 10.1038/NGEO1614Compositional evidence for an impact origin of theMoon’s Procellarum basinRyosuke Nakamura1 *, Satoru Yamamoto2 , Tsuneo Matsunaga2 , Yoshiaki Ishihara2 ,Tomokatsu Morota3 , Takahiro Hiroi4 , Hiroshi Takeda5 , Yoshiko Ogawa6 , Yasuhiro Yokota2 ,Naru Hirata6 , Makiko Ohtake7 and Kazuto Saiki8The asymmetry between the nearside and farside of the evolution and thick impact ejecta. On the other hand, we canMoon is evident in the distribution of mare basalt1 , crustal expect surviving compositional evidence of the mantle exposuresthickness2 and concentrations of radioactive elements3 , but and/or large-scale impact melt pool because such an enormousits origin remains controversial. According to one attractive impact must have completely excavated the feldspathic upper crustscenario, a gigantic impact early in the Moon’s history and produced a tremendous amount of impact melt13 . Previousproduced the observed dichotomy; the putative 3,000-km- multispectral global surveys14,15 focused on olivine-rich lithologiesdiameter Procellarum basin has been suggested to be a relic as possible mantle exposures because equilibrium crystallizationof this ancient impact3–5 . Low-calcium pyroxene can be formed and/or density overturn of the lunar magma ocean (LMO) areduring an impact by melting a mixture of crust and mantle supposed to produce olivine-rich mantle16 . The upper mantle,materials6,7 or by excavating differentiated cumulates from the however, would predominantly consist of low-calcium pyroxenelunar magma ocean8 . Therefore, the association of low-calcium (LCP) if the LMO experienced significant fractional crystallizationpyroxene with a lunar basin could indicate an impact origin. before the onset of plagioclace floatation8 . The differentiation ofHere we use spectral mapping data from KAGUYA/SELENE the huge impact melt provides another chance to generate LCP-rich(ref. 9) to show that low-calcium pyroxene is concentrated layers even from olivine-rich mantle6,7,17 . Here, therefore, we havearound two established impact structures, the South Pole– targeted the LCP as a diagnostic mafic mineral linked with hugeAitken and Imbrium basins. In addition, we detect a high impacts and searched for the spectral signature by using the multi-concentration of low-calcium pyroxene at Procellarum, which and hyperspectral data obtained by KAGUYA/SELENE (refs 8,18).supports an impact origin of the ancient basin. We propose Figure 1 indicates the global distribution of the mafic exposuresthat, in forming the largest known basin on the Moon, the dominated by LCP. It should be emphasized that we have selectedimpact excavated the nearside’s primary feldspathic crust, the spectra whose absorption depth is larger than 10% (see Methodswhich derived from the lunar magma ocean. A secondary and Supplementary Fig. S2). This selection rule excludes ubiquitousfeldspathic crust would have later recrystallized from the sea of noritic anorthosite19 and heavily space-weathered materials withimpact melt, leading to two distinct sides of the Moon. weaker absorptions. Localized concentrations do occur around the The dichotomy of the Earth’s Moon was discovered by the two established largest impact structures, the Imbrium basin andfarside images taken by Luna 3 in 1959. Basaltic mare covers only the South Pole–Aitken (SPA) basin. Most of the rest are encirclinga few per cent of the farside, compared with roughly 30% on the the putative Procellarum basin. It has been widely supposed that thenear side1 . Subsequent spaceborne observations revealed the front– lunar lower crust has a globally LCP-rich noritic composition19–21 ,back asymmetry of the crustal thickness2 and the concentration but few points are present on the feldspathic highland terrane22 .of radioactive elements on the nearside3 . Several mechanisms As illustrated in Figs 2 and 3 and Supplementary Fig. S1, most ofhave been proposed to produce this dichotomy, such as spatially the LCP-rich materials are exposed on the inner wall or impactinhomogeneous tidal heating in the Moon-forming stage10 or ejecta of fresh craters. If we apply the empirical algorithm for theseaccretion of a companion Moon11 . Recently, it was found that the multiband images19 , all of the exposures would comprise morenorth–south crustal dichotomy of Mars can be naturally explained than 30% mafic components. Figure 4 compares the spectrum ofby a giant impact in the ancient age12 . Similarly, the Moon’s Apollo sample 14310 (ref. 23) with that of the LCP-rich depositdichotomy could have resulted from a giant impact. The putative closest to the sampling point. Also plotted are two representativeProcellarum basin, whose diameter is more than 3,000 km with LCP-rich spectra: Plato M on the northern rim of the Imbriumthe centre around (N15, W23), would be the most plausible basin and Antoniadi in the SPA. The rock 14310, collected fromcandidate for the ancient impact event3–5 . The characteristic the Fra Mauro formation as Imbrium ejecta, is supposed to betopographical impact-basin structures must have been obliterated noritic impact melt. Their striking spectral similarities suggest thatby the fluidal nature of the huge impact melt sheet, viscoelastic the composition of 14310 (plagioclace = 59%, LCP = 31%) could1 InformationTechnology Research Institute, National Institute of Advanced Industrial Science and Technology, Umezono 1-1-1, Tsukuba, Ibaraki 305-8568,Japan, 2 Center for Environmental Measurement and Analysis, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506,Japan, 3 Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan, 4 Department of GeologicalSciences, Brown University, Providence, Rhode Island 02912, USA, 5 Department of Earth and Planetary Science, The University of Tokyo, Hongo,Bunkyo-ku, Tokyo 113-0033, Japan, 6 The University of Aizu, Ikki-machi, Aizuwakamatsu, Fukushima 965-8580, Japan, 7 Institute of Space andAstronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshino-dai, Sagamihara, Kanagawa 229-8515, Japan, 8 Department of Earth and SpaceScience, Osaka University, 1-1 Machikaneyama, Toyonaka 560-0043, Osaka, Japan. *e-mail: r.nakamura@aist.go.jp.NATURE GEOSCIENCE | VOL 5 | NOVEMBER 2012 | www.nature.com/naturegeoscience 775 © 2012 Macmillan Publishers Limited. All rights reserved.
  • 2. LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO1614Figure 1 | Location map of the LCP-rich exposures on the Moon. The blue triangles and red crosses denote the sites shown in Figs 2 and 4, respectively,and the rest are shown by yellow diamonds. The olivine-rich exposures18 are represented by green squares for reference. We could confirm the presence ofmany LCP-rich sites in the SPA basin19,25,30 (Apollo, Bhaba, Finsen, Lyman, Antoniadi, Zeeman and Schrödinger). On the nearside, many LCP-rich sites arelocated on the northern Imbrium noritic region31 , Bullialdus30 and Aratus20 , as previously identified by ground-based telescopes and space missions. E A H2 H3 H1Figure 2 | False-colour composite images of four craters encircling the Procellarum basin. The upper panels show an unnamed crater near Aston (left:W85.6, N33.5) and Epigenes F (right: W8.1, N67.0); the lower panels indicate Heinsius (left: W18.0, S39.7) and Hercules (right: E42.0, N50.3). The imagewidths are 8 km and reflectance factors are assigned to red (750 nm), green (900 nm) and blue (1,250 nm). The labels show the locations of the MultibandImager’s spectra shown in Fig. 3.776 NATURE GEOSCIENCE | VOL 5 | NOVEMBER 2012 | www.nature.com/naturegeoscience © 2012 Macmillan Publishers Limited. All rights reserved.
  • 3. NATURE GEOSCIENCE DOI: 10.1038/NGEO1614 LETTERS 0.25 1.0 Reflectance after continuum removal 0.20 0.9 REFF 0.15 0.8 A (Aston) Apollo 14310 E (Epigenes F) Fra Mauro 0.10 H1 (Heinsius) 0.7 Plato M H2 (Hercules) Antoniadi H3 (Hercules) 0.05 0.6 500 1,000 1,500 600 800 1,000 1,200 1,400 Wavelength (nm) Wavelength (nm)Figure 3 | Multiband spectra of the four craters in Fig. 2. Reflectance Figure 4 | Continuum-removed spectra of the Apollo sample 14310factor (REFF) measured by the Multiband Imager onboard (ref. 23) and that of the nearest LCP-rich point in Fra Mauro. Also plottedKAGUYA/SELENE (ref. 28). The pinkish areas (A, E, H1, H2) in Fig. 2 show are spectra for Plato M and Antoniadi25 . Their footprint locations areabsorption minima at 900 nm, representing LCP-dominant compositions, denoted by red crosses in Fig. 1. All of the spectra show short centralwhereas the red H3 region shows an absorption minimum at 1,050 nm, wavelengths (<925 nm) for the 1-µm absorption bands. The 1.3 µmindicative of an exposure of olivine. absorptions are very faint, whereas the 2-µm absorption bands are evident within the limited wavelength coverage. All of these are characteristicbe analogous to that of the LCP-rich deposits in Fig. 1. Model features of LCP (ref. 25). The flat bottom of the 1 mm absorption band forcalculations of radiative transfer also support their compositional 14310 possibly indicates poor data quality of the laboratory measurements.similarity (see Supplementary Information). As the depths of excavation and melting are larger than the complete crystallization of the LMO, the resulting impact meltmaximum lunar crustal thickness, a tremendous amount of impact must have incorporated urKREEP, the late-stage liquid frommelt would be formed by assimilating the feldspathic lunar crust LMO crystallization6,7 . In contrast, a later impact on the presentand mafic-rich upper mantle13 . The differentiation of this melt SPA region would show a lower KREEP concentration, providedwould have produced the LCP-dominant lithology6,7 . Alternatively, urKREEP had finished lateral migration to the Procellarum and/orLCP-rich exposures could be composed of magnesian-suite vertical settling with mantle overturn16 . The Procellarum impactplutons24 or ultramafic LMO cumulates8,25 excavated by huge itself might have triggered the lateral migration. The front–backimpacts. The magnesian-suite parent magma may have selectively asymmetry of the mare distribution could have resulted fromintruded into the rim of Imbrium and SPA where the crust was the different crustal thickness2 and/or KREEP concentration inthinned by large impacts. The origin of the LCP-rich rocks, in every Procellarum KREEP terrane27 (PKT).case, remains to be related to huge impacts as long as no other According to the present standard impact theories, the Pro-tectonic mechanisms produce the spatial pattern seen in Fig. 1. cellarum and SPA impact should have completely expelled theTherefore, LCP-rich sites surrounding the Procellarum basin could feldspathic crust derived from the LMO (see Supplementary Infor-be linked with the biggest impact on the Moon. mation), whereas gravity data indicate a thin low-density crust in The observed coexistence of LCP and olivine (Fig. 1 and Her- the PKT and SPA (ref. 2). In addition, a previous multiband surveycules crater in Fig. 2) would provide crucial clues to better under- found crater central peaks and inner walls consisting of anorthositicstand their formation process (see Supplementary Information). materials within SPA and PKT (ref. 28). Why does this feldspathicIt should be noted that LCP exposures are not found around crust exist if ancient gigantic impacts removed the primary crustsmaller olivine-bearing basins, such as Crisium and Moscovience18 . crystalized from the LMO? The answer would lie in the vast volumeOne simple interpretation is that those basins could be large of melt sheet produced by the impacts. The thick melt sheets, ex-enough to excavate olivine-rich mantle, but too small to generate tending hundreds of kilometres laterally and a few tens of kilometresLCP through significant differentiation of the huge impact melt in depth, would have differentiated to generate secondary crustsheets6,25 . Another possibility is that LCP is effectively formed in two predominantly consisting of anorthosite6,29 (see Supplementarysuccessive impacts. In fact, Fig. 1 shows a significant concentration Information). It is likely that the crust observed on PKT and SPAof LCP-rich points on the multi-ring basins Apollo, Antoniadi and at present is not a remnant of the primordial crust solidified fromSchrödinger in the SPA. Imbrium could be another example of the LMO, but a secondary product from the impact melt.the second impact following the preceding larger impact, that is,the Procellarum basin. Methods The coincidence between the global thorium concentration Throughout this paper, we focus on the wavelengths between 510 and 1,600 nmand the circular outline of a putative Procellarum basin suggests where we can get good spectra from both the Spectral Profiler18 and the Multibandthat a single giant impact could be responsible for producing the Imager28 onboard KAGUYA/SELENE. From the complete Spectral Profiler data set including about 69 million points, we have selected the spectra with the followingProcellarum basin3 . An old and gigantic impact on the Procellarum five conditions: the absolute radiance at 512 nm is larger than 23 W mm−1 m−2 sr−1 ;region accounts for various aspects of the lunar dichotomy in a the difference in the continuum-removed reflectance Rc is smaller than 0.005consistent way. First, it can produce the observed offset between the between 971 and 980 nm (the continuum is calculated by an automatic algorithmcentre-of-mass and centre-of-figure by stripping off the feldspathic as a tangent line to the target spectrum and continuum removal denotes theupper crust on the impact side5 . As the minimum moment of division of the original spectrum by the continuum line); both the minimum and next-lowest Rc occur below 925 nm; the minimum Rc is smaller than 0.9;inertia would be directed along the Earth–Moon axis by the Rc at 1,403 nm is larger than that at 1,508 nm. The first two conditions rejectspin–orbital evolution, the excavated hemisphere or the antipodal the low-quality data. The rest are the criteria to pick up clear indications of LCPwould eventually face the Earth26 . If the impact occurred before whose central wavelength of the 1 and 2 mm absorptions are located at shorterNATURE GEOSCIENCE | VOL 5 | NOVEMBER 2012 | www.nature.com/naturegeoscience 777 © 2012 Macmillan Publishers Limited. All rights reserved.
  • 4. LETTERS NATURE GEOSCIENCE DOI: 10.1038/NGEO1614wavelengths compared with LCP (ref. 25). This screening procedure results in 19. Tompkins, S. & Pieters, C. M. Mineralogy of the lunar crust: Results from528 data points in Fig. 1. The Spectral Profiler covered more than 20% of the Clementine. Meteor. Planet. Sci. 34, 25–41 (1999).lunar surface even at the equator18 during the total 1.5-year mission period. 20. Spudis, P. D., Hawke, B. R. & Lucey, P. G. Proc. of 18th Lunar and PlanetarySubsequently, we have conducted an areal survey around the Spectral Profiler Science Conference 155–168 (Cambridge Univ. Press, 1988).detection points by using Multiband Imager multispectral images with the same 21. Pieters, C. Noritic anorthosite is the most common highland rock type seen onalgorithm for Clementine UVVIS (ref. 31). the surface of the lunar nearside. Rev. Geophys. 24, 57–588 (1986). The original data can be found online at the SELENE data archive 22. Jolliff, B. L. et al. Major lunar crustal terranes: Surface expressions and(http://l2db.selene.darts.isas.jaxa.jp). crust-mantle origins. J. Geophys. Res. 105, 4197–4216 (2000). 23. Gancarz, A. J., Albee, A. L. & Chodos, A. A. Comparative petrology of ApolloReceived 15 May 2012; accepted 21 September 2012; 16 sample 68415 and Apollo 14 samples 14276 and 14310. Earth Planet. Sci. Lett.published online 28 October 2012 16, 307–330 (1972). 24. Shearer, C. K. & Papike, J. J. Early crustal building processes on the moon:References Models for the petrogenesis of the magnesian suite. Geochim. Cosmochim. Acta1. Head, J. W. III & Wilson, L. Lunar mare volcanism: Stratigraphy, eruption, 69, 3445–3461 (2005). conditions, and the evolution of secondary crusts. Geochim. Cosmochim. Acta 25. Nakamura, R. et al. Ultramafic impact melt sheet beneath the South Pole-Aitken 56, 2155–2174 (1992). basin on the Moon. Geophys. Res. Lett. 362, L22202 (2009).2. Ishihara, Y. et al. 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Geophys. Res. Lett. 35, L23201 (2008). KAGUYA/SELENE. We thank Fujitsu and the JASCO Corporation for their dedicated10. Garrick-Bethell, I., Nimmo, F. & Wieczorek, M. A. Structure and formation of efforts in developing the Spectral Profiler. the lunar farside highlands. Science 330, 949–951 (2010).11. Jutzi, M. & Asphaug, E. Forming the lunar farside highlands by accretion of a Author contributions companion moon. Nature 476, 69–72 (2011). R.N. and S.Y. performed the spectral analysis and model calculations. S.Y., T.M., Y.Y.12. Nimmo, F. et al. Implications of an impact origin for the martian hemispheric and Y.O. carried out the data reduction and instrument calibration. S.Y. and N.H. dichotomy. Nature 453, 1220–1223 (2008). contributed to the qualitative estimate of the impact melt production. T.H. contributed to13. Cintala, M. J. & Grieve, R. A. F. 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Hess, P. C. & Parmentier, E. M. A model for the thermal and chemical Additional information evolution of the Moon’s interior: Implications for the onset of mare volcanism. Supplementary information is available in the online version of the paper. Reprints and Earth Planet. Sci. Lett. 134, 501–514 (1995). permissions information is available online at www.nature.com/reprints. Correspondence17. Spray, J. G., Thompson, L. M., Biren, M. B. & O’Connell-Cooper, C. The and requests for materials should be addressed to R.N. Manicouagan impact structure as a terrestrial analogue site for lunar and martian planetary science. Planet. Space Sci. 58, 538–551 (2010).18. Yamamoto, S. et al. Possible mantle origin of olivine around lunar impact Competing financial interests basins detected by SELENE. Nature Geosci. 3, 533–536 (2010). The authors declare no competing financial interests.778 NATURE GEOSCIENCE | VOL 5 | NOVEMBER 2012 | www.nature.com/naturegeoscience © 2012 Macmillan Publishers Limited. All rights reserved.