Spectroscopic characterization vesta_mineralogy

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Spectroscopic characterization vesta_mineralogy

  1. 1. Spectroscopic Characterization of Mineralogy and Its Diversity Across Vesta M. C. De Sanctis et al. Science 336, 697 (2012); DOI: 10.1126/science.1219270 This copy is for your personal, non-commercial use only. If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here. Downloaded from www.sciencemag.org on May 10, 2012 Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of May 10, 2012 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/336/6082/697.full.html Supporting Online Material can be found at: http://www.sciencemag.org/content/suppl/2012/05/09/336.6082.697.DC1.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/336/6082/697.full.html#related This article cites 27 articles, 8 of which can be accessed free: http://www.sciencemag.org/content/336/6082/697.full.html#ref-list-1 This article has been cited by 4 articles hosted by HighWire Press; see: http://www.sciencemag.org/content/336/6082/697.full.html#related-urls This article appears in the following subject collections: Planetary Science http://www.sciencemag.org/cgi/collection/planet_sciScience (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by theAmerican Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright2012 by the American Association for the Advancement of Science; all rights reserved. The title Science is aregistered trademark of AAAS.
  2. 2. REPORTS structurally by the inward facing scarp, arcuate ridge, 19. W. Bottke, H. J. Melosh, Icarus 124, 72 (1996). and the NASA Dawn at Vesta Participating Scientist or break in slope. 20. If the formation time scale of Veneneia is a few hours, program for support. We thank the Dawn team for the 8. H. J. Melosh, Impact Cratering (Oxford Univ. Press, then to avoid interference with Rheasilvia a binary impactor development, cruise, orbital insertion, and operations Oxford, 1989). would need a separation of at least ~3600 s × 5 km/s ~ of the Dawn spacecraft at Vesta. A portion of this 9. K. Keil, D. Stoffler, S. Love, E. Scott, Meteorit. Planet. Sci. 18,000 km or ~70 Vesta radii. The probability of both work was performed at the Jet Propulsion Laboratory, 32, 349 (1997). binary bodies hit Vesta would then be very small. California Institute of Technology, under contract10. R. Jaumann et al., Science 336, 687 (2012). 21. D. Nesvorny et al., Icarus 193, 85 (2008). with NASA. Dawn data are archived with the NASA Planetary11. S. Marchi et al., Science 336, 690 (2012). 22. F. Marzari, P. Farinella, D. R. Davis, Icarus 142, 63 (1999). Data System.12. M. Jutzi, E. Asphaug, Geophys. Res. Lett. 38, L01102 (2011). 23. F. Marzari et al., Astron. Astrophys. 316, 248 (1996).13. M. C. De Sanctis et al., Science 336, 697 (2012). 24. D. Bogard, Meteoritics 30, 244 (1995).14. V. Reddy et al., Science 336, 700 (2012). 25. D. Bogard, D. Garrison, Meteorit. Planet. Sci. 38, 669 (2003). Supplementary Materials www.sciencemag.org/cgi/content/full/336/6082/694/DC115. Materials and methods are available as supplementary 26. D. Bogard, Chem. Erde Geochem. 71, 207 (2011). materials on Science Online. 27. Ar-Ar age resetting requires an extended time to diffuse Materials and Methods16. B. Kriens, E. Shoemaker, K. Herkenhoff, J. Geophys. Res. Ar from the rocks; there is insufficient time to accomplish Supplementary Text References (28, 29) 104, 18867 (1999). this in just the initial shock.17. T. Kenkmann, Geology 30, 231 (2002).18. P. Allemand, P. Thomas, J. Geophys. Res. 104, 16501 Acknowledgments: The authors thank D. Bogard, E. Asphaug, 12 April 2012; accepted 20 April 2012 (1999). and H. J. Melosh for helpful discussions and comments 10.1126/science.1223272 merous basaltic asteroids provide further supportSpectroscopic Characterization of for this hypothesis: Their orbits are distributed Downloaded from www.sciencemag.org on May 10, 2012 from near Vesta to the 3:1 Kirkwood gap andMineralogy and Its Diversity the secular n-6 resonance that results in gravita- tional perturbations. These, combined with colli-Across Vesta sions, provide a convenient mechanism for their delivery to Earth-crossing orbits (5–8). Geochemical, petrologic, and geochronologicM. C. De Sanctis,1* E. Ammannito,1 M. T. Capria,1 F. Tosi,1 F. Capaccioni,1 F. Zambon,1 studies of HEDs have led to the development of models for the magmatic evolution of theirF. Carraro,1 S. Fonte,1 A. Frigeri,1 R. Jaumann,2 G. Magni,1 S. Marchi,3 T. B. McCord,4 parent body. The consensus is that the body wasL. A. McFadden,5 H. Y. McSween,6 D. W. Mittlefehldt,7 A. Nathues,8 E. Palomba,1 substantially melted early in its history throughC. M. Pieters,9 C. A. Raymond,10 C. T. Russell,11 M. J. Toplis,12 D. Turrini1 heating by decay of 26Al and 60Fe, forming a molten core topped by a shell of molten silicates.The mineralogy of Vesta, based on data obtained by the Dawn spacecraft’s visible and infrared Cooling and crystallization of a global magmaspectrometer, is consistent with howardite-eucrite-diogenite meteorites. There are considerable ocean could have produced an olivine-dominatedregional and local variations across the asteroid: Spectrally distinct regions include the south-polar mantle, a lower crust rich in low-Ca pyroxeneRheasilvia basin, which displays a higher diogenitic component, and equatorial regions, which (diogenites), and an upper crust of basaltic flowsshow a higher eucritic component. The lithologic distribution indicates a deeper diogenitic crust, and gabbroic intrusions (eucrites) (9, 10). How-exposed after excavation by the impact that formed Rheasilvia, and an upper eucritic crust. ever, some HEDs are inconsistent with this sce-Evidence for mineralogical stratigraphic layering is observed on crater walls and in ejecta. This is nario, leading to models involving less meltingbroadly consistent with magma-ocean models, but spectral variability highlights local variations, and serial magmatism (11–14). The spatial dis-which suggests that the crust can be a complex assemblage of eucritic basalts and pyroxene tribution of lithologies within the crust of thecumulates. Overall, Vesta mineralogy indicates a complex magmatic evolution that led to a HED parent body would thus provide essentialdifferentiated crust and mantle. elescopic visible and near-infrared spec- Fig. 1. (Top left) Spectra (nor-T troscopy shows that the asteroid Vesta has a basaltic surface dominated by the spec-tral signature of pyroxene. Vesta spectra show malized at 0.7 mm) of regions A and B indicated in the VIR im- age. (Top right) VIR color com-many similarities to those of howardite-eucrite- posite (red = 0.92 mm, green =diogenite (HED) meteorites (1), leading to the 0.62 mm, blue = 0.44 mm). Spa-consensus that Vesta is differentiated and is the tial resolution is ~25 km. Arrowparent body of the HED achondrites (2–4). Nu- indicates the south pole. (Bottom) Average Vesta spectrum with1 Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionale T1 SD of the average. The datadi Astrofisica, Rome, Italy. 2Institute of Planetary Research, between 2.5 and 2.8 mm haveGerman Aerospace Center (DLR), Berlin, Germany. 3NASA Lunar been removed because they areScience Institute, Boulder, CO, USA. 4Bear Fight Institute,Winthrop, WA, USA. 5NASA, Goddard Space Flight Center, not yet fully calibrated in thisGreenbelt, MD, USA. 6Department of Earth and Planetary region.Sciences, University of Tennessee, Knoxville, TN, USA. 7NASAJohnson Space Center, Houston, TX 77058, USA. 8Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Ger-many. 9Brown University, Providence, RI, USA. 10Jet PropulsionLaboratory, California Institute of Technology, Pasadena, CA,USA. 11Institute of Geophysics and Planetary Physics, Universityof California, Los Angeles, CA, USA. 12Observatoire Midi-Pyrenees, Toulouse, France.*To whom correspondence should be addressed. E-mail:mariacristina.desanctis@iaps.inaf.it www.sciencemag.org SCIENCE VOL 336 11 MAY 2012 697
  3. 3. REPORTS geological context relevant to the question of its have allowed >65% of the surface to be imaged, tral variations also reflect diurnal changes with formation history. ranging from the south pole up to about 45°N the corresponding surface temperature changes The visible and infrared spectrometer (VIR) (the northern polar region was in shadow). VIR (Fig. 1). on Dawn is a high-resolution imaging spectrom- has acquired about four million spectra of Vesta’s The color composite image of Vesta (Fig. 1) eter with a spectral range of 0.25 to 5.01 mm and a surface under different illumination conditions, demonstrates that there are large-scale variations spatial sampling of 250 mrad (15). It combines with phase angles from 67.8° to 7.9°. in the spectral properties of the surface material two data channels in one compact instrument: the The first data, at a resolution twice that of the and that these variations are greater in magnitude visible-infrared (0.25 to 1.07 mm) and the infrared Hubble Space Telescope, were obtained from a than those described on other asteroids (17, 18). In (0.95 to 5.1 mm) channels, with a spectral sam- distance of ~99,200 km (Fig. 1). The spectra show this image, the reddish color of the northern hem- pling of DlVIS = 1.8 nm per band and DlIR = clear evidence of pyroxene absorption bands at 0.9 isphere indicates greater reflectivity at 0.92 mm, 9.8 nm per band, respectively. and 1.9 mm (hereafter BI and BII). Different re- and hence shallower pyroxene bands compared VIR obtained spatially resolved hyperspectral gions of Vesta are characterized by distinctly dif- with the southern hemisphere. The representa- images of Vesta (fig. S1) with a nominal spatial ferent band depths, widths, shapes, and centers. tive spectrum from region A shows stronger ab- sampling up to ~0.7 km. The orientation of Vesta’s Beyond ~3.5 mm, thermal emission of the surface sorption at 0.92 mm relative to the continuum at spin axis and Dawn’s orbital characteristics (16) becomes increasingly important, and the spec- 0.7 mm than does region B. Fig. 2. Cylindrical and stereo- graphic projections of spectral Downloaded from www.sciencemag.org on May 10, 2012 parameters obtained by VIR. (A) BI depths. (B) BII depths. (C) BI centers. (D) BII centers. Band depths and centers were computed after continuum removal (fig. S2).698 11 MAY 2012 VOL 336 SCIENCE www.sciencemag.org
  4. 4. REPORTS Although the surface of Vesta exhibits spec- whereas in the southern region they are ~0.25 to The global asymmetry evident in the distribu-tral variations at both large and small scales, the 0.3. The depth of an absorption band is mainly tions of pyroxene band depths is also demonstratedmaterials on the surface are always dominated by determined by the abundance of the absorbing by variations in band center wavelengths (Fig. 2, Crocks formed by mafic magmatism, as indicated minerals, the grain size distribution, and the abun- and D). Laboratory studies indicate that band cen-by the ubiquitous BI and BII pyroxene signatures. dance of opaque phases. The process known as ters for BI and BII pyroxene absorptions are sys- These bands are caused by absorption of pho- space weathering also modifies reflectance spectra tematically different for diogenites and eucrites (26).tons, primarily by Fe2+, and their exact position and can make lithological interpretation difficult. To directly compare Vesta band centers with HEDs,and shape are driven by the relative proportion Regolithic howardites show some characteristics we computed the band centers of HEDs by apply-of Fe to Mg in the M1 and M2 sites of pyroxene of exposure to the space environment, such has ing the same method to both data sets (Fig. 3). BIcrystal structures (19, 20). high noble gas and siderophile element contents, and BII centers are at slightly shorter wavelengths VIR spatial resolution allows for the defini- and impact-produced glass, but these character- for diogenites than for eucrites (Fig. 3), a conse-tion of localized mineralogical units: The results istics are not as well developed as in mature lunar quence of more Mg-rich pyroxenes with lower Caindicate a complex geological and collisional his- regolith breccias (24, 25). Vesta retains a reflec- concentrations in the former (26). Howardites, be-tory (21–23) and reveal a crust that was differen- tance spectrum dominated by pyroxene absorp- cause of their intermediate nature, lie between, buttiated before impact bombardment. The spectral tion bands (fig. S5), indicating that the effects of partially overlap the fields of diogenites and eucrites.variations indicate that Vesta’s crust is composi- space weathering are much less pronounced on The BI and BII centers in the VIR spectrationally variable at vertical scales from a few hun- Vesta compared with the Moon or Mercury. form a trend from diogenites to eucrites, anddred meters to 20 km, the depth of excavation of Thus, the VIR data suggest that the region of most plot in the howardites region. Band centerthe southern impact basins (16). the Rheasilvia basin is richer in pyroxene than the values are not uniformly distributed on Vesta, Downloaded from www.sciencemag.org on May 10, 2012 A global-scale spectral difference is observed equatorial regions or that the regolith in this region but they differ systematically between the equa-between the equatorial and southern Rheasilvia has a larger average grain size distribution and/or torial and southern regions, and band center val-regions, as shown on maps of BI and BII depths contains fewer opaque minerals. A larger grain size ues often correlate inversely with band depthsand centers (Fig. 2). In the south pole region, py- would be consistent with less impact comminution (Fig. 2). Equatorial regions are prevalently char-roxene bands are, on average, deeper and wider in the southern region because of the younger age acterized by band centers at longer wavelengthsthan in the equatorial region (fig. S3). In general, of the Rheasilvia basin (23). The lower crust is also (average BI = 0.930 mm and BII = 1.96 mm) andBI depths in the equatorial region are ~ 0.35 to expected to have had a coarser initial grain size be- typically have intermediate to shallow band depths.0.4, whereas those in the Rheasilvia basin are cause pyroxene grain sizes vary from diogenites, In contrast, band centers in the Rheasilvia basincommonly 0.45 to 0.55. Similarly, BII depths in which are much larger than cumulate eucrites, are at shorter wavelengths (average BI = 0.926 mmthe equatorial region are typically ~ 0.15 to 0.2, which are larger than basaltic eucrites (12). and BII = 1.94 mm), and these often correspond to the deepest pyroxene absorption bands (Fig. 2). Overall, the correlations between band depthsFig. 3. BI center versus BII center. Green, and band centers can be interpreted in terms ofyellow, and violet ovals are the distribution diogenite/eucrite content of the different terrains.of howardites, eucrites, and diogenites,respectively. The scatter plot represents Diogenites contain ~90 to 95 volume % (vol %)the distribution of the VIR BI and BII centers pyroxene (27), whereas basaltic eucrites containacquired during the Survey phase. ~50 vol % pyroxene (28), implying that, for a given grain size, the diogenites spectra have stronger bands with respect to eucrites, as con- firmed by HED spectra (table S1). The corre- spondence of stronger pyroxene absorptions with shorter BII and BI centers in the Rheasilvia basin is consistent with a greater proportion of dioge- nite on the surface in this deeply excavated region. Spectra from the equatorial regions have band centers shifted to longer wavelengths, indicat- ing more Fe-rich pyroxenes, and intermediate or shallow band depths, indicating lower pyroxene abundance, both consistent with a greater eucrite component. However, the equatorial region is not spectrally uniform. An extensive area at about 40°E has measurably deeper absorption bands and shorter wavelengths, suggesting a lower proportion of eucritic material in this region, possibly related to the influence of Rheasilvia ejecta (23–25). Over- all, the mineralogical north-south diversity indi- cates that the lower crust exposed in Rheasilvia is dominated by pyroxene-rich, diogenitic material. Although the difference between the south polar and equatorial regions is the dominant first-order feature (Fig. 2), VIR data also demon-Fig. 4. The vestan surface near Oppia crater. E, Oppia ejecta; F, Oppia crater floor; H, Oppia crater walls; strate that Vesta’s surface and subsurface showS, small crater near Oppia. (A) False-color image in the visible continuum. (B) Lithologic diversity around variations at local scales, that is, bright and darkthe central crater with copious “red” ejecta (shallower absorption bands) and deeper pyroxene bands on localized areas (fig. S4). Study of geologicalthe crater walls. (C) Image made by combining colors defining the 930-nm band depth, where the small structures at scales of tens of kilometers, in par-crater near the Oppia rim (S) is clearly seen. ticular impact craters with copious ejecta and www.sciencemag.org SCIENCE VOL 336 11 MAY 2012 699
  5. 5. REPORTS mass movements, often show associated spec- evolutionary history, more similar to that of the 15. M. C. De Sanctis et al., Space Sci. Rev. (2010). tral differences. For example, the Oppia region’s terrestrial planets than to other asteroids visited 16. C. T. Russell et al., Science 336, 684 (2012). 17. A. Coradini et al., Science 334, 492 (2011). surface exhibits variations in albedo and spec- by spacecraft (17, 18). The occurrence of a greater 18. J. Veverka et al., Science 289, 2088 (2000). tral slope that indicate differences in surface ma- proportion of diogenite at depth is a critical finding, 19. R. G. Burns, Mineralogical Applications of Crystal Field terials (Fig. 4A). Moreover, the area around the not demonstrated by data from the Hubble Space Theory (Cambridge Univ. Press, Cambridge, 1993). (Fig. 3) fresh Oppia crater (E) and the crater Telescope or telescopic observations (30, 31), 20. L. A. McFadden, T. B. McCord, C. Pieters, Icarus 31, 439 (1977). floor (F) have shallower BI depths (Fig. 4B), and broadly consistent with magma ocean mod- 21. P. Schenk et al., Science 336, 694 (2012). revealing material poorer in pyroxene. The els for Vesta’s differentiation. On the other hand, 22. R. Jaumann et al., Science 336, 687 (2012). cratering process here results in inverted stratig- the fact that mixtures of diogenite and eucrite 23. S. Marchi et al., Science 336, 690 (2012). raphy of roughly the upper third of the target appear ubiquitous in all regions, coupled with the 24. P. H. Warren, G. W. Kallemeyn, H. Huber, F. Ulff-Møller, W. Choe, Geochim. Cosmochim. Acta 73, 5918 lithology in the ejecta blanket nearest the rim occurrence of smaller-scale variations in mineralogy, (2009). [e.g., (29)]. The crater floor and material part- make it premature to distinguish between a simple 25. L. Wilkening, D. Lal, A. M. Reid, Earth Planet. Sci. Lett. way up the walls have a reddish hue similar to layered crust of eucrite and underlying dioge- 10, 334 (1971). the ejecta just outside the rim, consistent with nite (32) or a complex eucrite crust with 26. M. J. Gaffey, J. Geophys. Res. 81, 905 (1976). 27. L. E. Bowman, M. N. Spilde, J. J. Papike, Meteorit. Planet. the lower layers in this crater being composed intruded diogenitic plutons (14). The Dawn mis- Sci. 32, 869 (1997). of rock poorer in pyroxene. The cyan color in- sion provides the first spatially detailed view of 28. J. S. Delaney, M. Prinz, H. Takeda, J. Geophys. Res. 89, dicates that the soils just below the rim (H) have the distribution of the rock types, allowing insight (suppl.), C251 (1984). stronger BI absorption and thus have higher into the magmatic processes that formed the solar 29. H. J. Melosh, Impact Cratering: A Geologic Process pyroxene content or different grain size (Fig. 4C). system’s “smallest planet.” (Oxford Univ. Press, Oxford, 1989). Downloaded from www.sciencemag.org on May 10, 2012 30. Telescopic data suggested a prominent diogenite region. The small crater (S) is surrounded by a halo of References and Notes In the adopted coordinate system, this diogenite spot was bright and green materials, similar to the layer ex- 1. T. B. McCord, J. B. Adams, T. V. Johnson, Science 168, moved to the northern hemisphere, where VIR does not posed in Oppia (H), suggesting a similar compo- 1445 (1970). find this evidence. sition. VIR thus reveals that the Oppia impact 2. M. A. Feierberg, M. J. Drake, Science 209, 805 (1980). 31. J. Y. Li et al., Icarus 208, 238 (2010). 3. G. J. Consolmagno, M. J. Drake, Geochim. Cosmochim. 32. H. Takeda, Icarus 40, 455 (1979). exposed different kinds of materials, suggesting Acta 41, 1271 (1977). complex, small-scale crustal stratigraphy on Vesta. 4. M. J. Drake, In Asteroids, T. Gehrels, Ed. (Univ. Arizona Acknowledgments: VIR is funded by the Italian Space Agency At all scales, pyroxene absorptions are the Press, Tucson, AZ, 1979), pp. 765–782 and was developed under the leadership of INAF-Istituto most prominent spectral features on Vesta and, 5. R. P. Binzel et al., Icarus 128, 95 (1997). di Astrofisica e Planetologia Spaziali, Rome, Italy. The 6. M. C. De Sanctis et al., Astron. Astrophys. 533, A77 (2011). instrument was built by Selex-Galileo, Florence, Italy. The on average, the spectral parameters of Vesta re- authors acknowledge the support of the Dawn Science, 7. N. A. Moscovitz et al., Icarus 208, 773 (2010). semble those of howardites (fig. S5). The VIR 8. M. C. De Sanctis et al., Mon. Not. R. Astron. Soc. 412, Instrument, and Operations Teams. This work was supported spectra are thus consistent with a surface covered 2318 (2011). by the Italian Space Agency, and NASA’s Dawn at Vesta by a howardite-like regolith containing varying 9. K. Righter, M. J. Drake, Meteorit. Planet. Sci. 32, 929 (1997). Participating Scientists Program. A portion of this work was 10. P. H. Warren, Meteorit. Planet. Sci. 32, 945 (1997). performed at the Jet Propulsion Laboratory under contract proportions of eucrite and diogenite at different 11. R. C. Greenwood, I. A. Franchi, A. Jambon, P. C. Buchanan, with NASA. locations. This firmly supports the link between Nature 435, 916 (2005). Vesta and the HEDs, providing geologic context 12. D. W. Mittlefehldt et al., in Planetary Materials: Reviews Supplementary Materials www.sciencemag.org/cgi/content/full/336/6082/697/DC1 for these samples, which furthers our understand- in Mineralogy 36, J. J. Papike, Ed. (Mineralogical Supplementary Text ing of the formation and evolution of Vesta. Society of America, Chantilly, VA, 1998), pp. 4-1–4-195. 13. A. Beck, H. Y. McSween Jr., Meteorit. Planet. Sci. 45, Figs. S1 to S5 Furthermore, Vesta exhibits large color and 850 (2010). Table S1 spectral variations that often reflect geological 14. J.-A. Barrat, A. Yamaguchi, B. Zanda, C. Bollinger, 17 January 2012; accepted 16 April 2012 structures, indicating a complex geological and M. Bohn, Geochim. Cosmochim. Acta 74, 6218 (2010). 10.1126/science.1219270 Color and Albedo Heterogeneity 1 Max Planck Institute for Solar System Research, Max-Planck- Strasse 2, 37191 Katlenburg-Lindau, Germany. 2Department of Space Studies, University of North Dakota, Grand Forks, ND of Vesta from Dawn 58202, USA. 3Department of Astronomy, University of Mary- land, College Park, MD 20742, USA. 4Planetary Science Vishnu Reddy,1,2* Andreas Nathues,1 Lucille Le Corre,1 Holger Sierks,1 Jian-Yang Li,3 Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, USA. 5Department of Mineral Sciences, Smithsonian National Robert Gaskell,4 Timothy McCoy,5 Andrew W. Beck,5 Stefan E. Schröder,1 Carle M. Pieters,6 Museum of Natural History, 10th and Constitution NW, Kris J. Becker,7 Bonnie J. Buratti,8 Brett Denevi,9 David T. Blewett,9 Ulrich Christensen,1 Washington, DC 20560–0119, USA. 6Department of Geologi- Michael J. Gaffey,2 Pablo Gutierrez-Marques,1 Michael Hicks,8 Horst Uwe Keller,10 cal Sciences, Brown University, Providence, RI 02912, USA. 7 Thorsten Maue,1 Stefano Mottola,11 Lucy A. McFadden,12 Harry Y. McSween,13 Astrogeology Science Center, U.S. Geological Survey, Flagstaff, AZ 86001, USA. 8Jet Propulsion Laboratory, California In- David Mittlefehldt,14 David P. O’Brien,4 Carol Raymond,8 Christopher Russell15 stitute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA. 9Johns Hopkins University Applied Physics Multispectral images (0.44 to 0.98 mm) of asteroid (4) Vesta obtained by the Dawn Framing Cameras Laboratory, Laurel, MD 20723, USA. 10Institut für Geophysik reveal global color variations that uncover and help understand the north-south hemispherical und extraterrestrische Physik, TU Braunschweig Mendelssohn- dichotomy. The signature of deep lithologies excavated during the formation of the Rheasilvia basin on strasse 3, DE 38106 Braunschweig, Germany. 11Deutsches the south pole has been preserved on the surface. Color variations (band depth, spectral slope, and Zentrum für Luft und Raumfahrt (DLR)–German Aerospace Center, Institute of Planetary Research, Rutherfordstrasse 2, D-12489 eucrite-diogenite abundance) clearly correlate with distinct compositional units. Vesta displays the Berlin, Germany. 12NASA/Goddard Space Flight Center, Mail greatest variation of geometric albedo (0.10 to 0.67) of any asteroid yet observed. Four distinct color Code 160, Greenbelt, MD 20771, USA. 13Department of Earth units are recognized that chronicle processes—including impact excavation, mass wasting, and and Planetary Sciences, University of Tennessee, 1412 Circle space weathering—that shaped the asteroid’s surface. Vesta’s color and photometric diversity are Drive, Knoxville, TN 37996–1410, USA. 14Astromaterials Research Office, NASA Johnson Space Center, Mail Code KR, indicative of its status as a preserved, differentiated protoplanet. Houston, TX 77058, USA. 15Institute of Geophysics and Plan- etary Physics, University of California Los Angeles, Los Angeles, he Dawn spacecraft rendezvoused with in seven colors (0.44 to 0.98 mm) and one broad- T CA 90024–1567, USA. the asteroid Vesta on 16 July 2011, and the band clear filter, mapping the entire sun-lit surface *To whom correspondence should be sent. E-mail: reddy@ Framing Cameras (FCs) (1) acquired images at a detail of ~9 to ~0.016 km/pixel. We used mps.mpg.de700 11 MAY 2012 VOL 336 SCIENCE www.sciencemag.org

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