DOI: 10.1126/science.1219270, 697 (2012);336 Science

et al.M. C. De SanctisAcross VestaSpectroscopic Characterization of Mineralogy and Its Diversity

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structurally by the inward facing scarp, arcuate ridge,or break in slope.

8. H. J. Melosh, Impact Cratering (Oxford Univ. Press,Oxford, 1989).

9. K. Keil, D. Stoffler, S. Love, E. Scott, Meteorit. Planet. Sci.32, 349 (1997).

10. R. Jaumann et al., Science 336, 687 (2012).11. S. Marchi et al., Science 336, 690 (2012).12. M. Jutzi, E. Asphaug, Geophys. Res. Lett. 38, L01102 (2011).13. M. C. De Sanctis et al., Science 336, 697 (2012).14. V. Reddy et al., Science 336, 700 (2012).15. Materials and methods are available as supplementary

materials on Science Online.16. B. Kriens, E. Shoemaker, K. Herkenhoff, J. Geophys. Res.

104, 18867 (1999).17. T. Kenkmann, Geology 30, 231 (2002).18. P. Allemand, P. Thomas, J. Geophys. Res. 104, 16501

(1999).

19. W. Bottke, H. J. Melosh, Icarus 124, 72 (1996).20. If the formation time scale of Veneneia is a few hours,

then to avoid interference with Rheasilvia a binary impactorwould need a separation of at least ~3600 s × 5 km/s ~18,000 km or ~70 Vesta radii. The probability of bothbinary bodies hit Vesta would then be very small.

21. D. Nesvorny et al., Icarus 193, 85 (2008).22. F. Marzari, P. Farinella, D. R. Davis, Icarus 142, 63 (1999).23. F. Marzari et al., Astron. Astrophys. 316, 248 (1996).24. D. Bogard, Meteoritics 30, 244 (1995).25. D. Bogard, D. Garrison, Meteorit. Planet. Sci. 38, 669 (2003).26. D. Bogard, Chem. Erde Geochem. 71, 207 (2011).27. Ar-Ar age resetting requires an extended time to diffuse

Ar from the rocks; there is insufficient time to accomplishthis in just the initial shock.

Acknowledgments: The authors thank D. Bogard, E. Asphaug,and H. J. Melosh for helpful discussions and comments

and the NASA Dawn at Vesta Participating Scientistprogram for support. We thank the Dawn team for thedevelopment, cruise, orbital insertion, and operationsof the Dawn spacecraft at Vesta. A portion of thiswork was performed at the Jet Propulsion Laboratory,California Institute of Technology, under contractwith NASA. Dawn data are archived with the NASA PlanetaryData System.

Supplementary Materialswww.sciencemag.org/cgi/content/full/336/6082/694/DC1Materials and MethodsSupplementary TextReferences (28, 29)

12 April 2012; accepted 20 April 201210.1126/science.1223272

Spectroscopic Characterization ofMineralogy and Its DiversityAcross VestaM. C. De Sanctis,1* E. Ammannito,1 M. T. Capria,1 F. Tosi,1 F. Capaccioni,1 F. Zambon,1

F. Carraro,1 S. Fonte,1 A. Frigeri,1 R. Jaumann,2 G. Magni,1 S. Marchi,3 T. B. McCord,4

L. A. McFadden,5 H. Y. McSween,6 D. W. Mittlefehldt,7 A. Nathues,8 E. Palomba,1

C. M. Pieters,9 C. A. Raymond,10 C. T. Russell,11 M. J. Toplis,12 D. Turrini1

The mineralogy of Vesta, based on data obtained by the Dawn spacecraft’s visible and infraredspectrometer, is consistent with howardite-eucrite-diogenite meteorites. There are considerableregional and local variations across the asteroid: Spectrally distinct regions include the south-polarRheasilvia basin, which displays a higher diogenitic component, and equatorial regions, whichshow a higher eucritic component. The lithologic distribution indicates a deeper diogenitic crust,exposed after excavation by the impact that formed Rheasilvia, and an upper eucritic crust.Evidence for mineralogical stratigraphic layering is observed on crater walls and in ejecta. This isbroadly consistent with magma-ocean models, but spectral variability highlights local variations,which suggests that the crust can be a complex assemblage of eucritic basalts and pyroxenecumulates. Overall, Vesta mineralogy indicates a complex magmatic evolution that led to adifferentiated crust and mantle.

Telescopic visible and near-infrared spec-troscopy shows that the asteroid Vesta hasa basaltic surface dominated by the spec-

tral signature of pyroxene. Vesta spectra showmany similarities to those of howardite-eucrite-diogenite (HED) meteorites (1), leading to theconsensus that Vesta is differentiated and is theparent body of the HED achondrites (2–4). Nu-

merous basaltic asteroids provide further supportfor this hypothesis: Their orbits are distributedfrom near Vesta to the 3:1 Kirkwood gap andthe secular n-6 resonance that results in gravita-tional perturbations. These, combined with colli-sions, provide a convenient mechanism for theirdelivery to Earth-crossing orbits (5–8).

Geochemical, petrologic, and geochronologicstudies of HEDs have led to the developmentof models for the magmatic evolution of theirparent body. The consensus is that the body wassubstantially melted early in its history throughheating by decay of 26Al and 60Fe, forming amolten core topped by a shell of molten silicates.Cooling and crystallization of a global magmaocean could have produced an olivine-dominatedmantle, a lower crust rich in low-Ca pyroxene(diogenites), and an upper crust of basaltic flowsand gabbroic intrusions (eucrites) (9, 10). How-ever, some HEDs are inconsistent with this sce-nario, leading to models involving less meltingand serial magmatism (11–14). The spatial dis-tribution of lithologies within the crust of theHED parent body would thus provide essential

1Istituto di Astrofisica e Planetologia Spaziali, Istituto Nazionaledi Astrofisica, Rome, Italy. 2Institute of Planetary Research,German Aerospace Center (DLR), Berlin, Germany. 3NASA LunarScience Institute, Boulder, CO, USA. 4Bear Fight Institute,Winthrop, WA, USA. 5NASA, Goddard Space Flight Center,Greenbelt, MD, USA. 6Department of Earth and PlanetarySciences, 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

Fig. 1. (Top left) Spectra (nor-malized at 0.7 mm) of regions Aand B indicated in the VIR im-age. (Top right) VIR color com-posite (red = 0.92 mm, green =0.62 mm, blue = 0.44 mm). Spa-tial resolution is ~25 km. Arrowindicates the southpole. (Bottom)Average Vesta spectrum withT1 SD of the average. The databetween 2.5 and 2.8 mm havebeen removed because they arenot yet fully calibrated in thisregion.

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geological context relevant to the question of itsformation history.

The visible and infrared spectrometer (VIR)on Dawn is a high-resolution imaging spectrom-eter with a spectral range of 0.25 to 5.01 mmand aspatial sampling of 250 mrad (15). It combinestwo data channels in one compact instrument: thevisible-infrared (0.25 to 1.07 mm) and the infrared(0.95 to 5.1 mm) channels, with a spectral sam-pling of DlVIS = 1.8 nm per band and DlIR =9.8 nm per band, respectively.

VIR obtained spatially resolved hyperspectralimages of Vesta (fig. S1) with a nominal spatialsampling up to ~0.7 km.The orientation ofVesta’sspin axis and Dawn’s orbital characteristics (16)

have allowed >65% of the surface to be imaged,ranging from the south pole up to about 45°N(the northern polar region was in shadow). VIRhas acquired about four million spectra of Vesta’ssurface under different illumination conditions,with phase angles from 67.8° to 7.9°.

The first data, at a resolution twice that of theHubble Space Telescope, were obtained from adistance of ~99,200 km (Fig. 1). The spectra showclear evidence of pyroxene absorption bands at 0.9and 1.9 mm (hereafter BI and BII). Different re-gions of Vesta are characterized by distinctly dif-ferent band depths, widths, shapes, and centers.Beyond ~3.5 mm, thermal emission of the surfacebecomes increasingly important, and the spec-

tral variations also reflect diurnal changes withthe corresponding surface temperature changes(Fig. 1).

The color composite image of Vesta (Fig. 1)demonstrates that there are large-scale variationsin the spectral properties of the surface materialand that these variations are greater in magnitudethan those described on other asteroids (17, 18). Inthis image, the reddish color of the northern hem-isphere indicates greater reflectivity at 0.92 mm,and hence shallower pyroxene bands comparedwith the southern hemisphere. The representa-tive spectrum from region A shows stronger ab-sorption at 0.92 mm relative to the continuum at0.7 mm than does region B.

Fig. 2. Cylindrical and stereo-graphic projections of spectralparameters obtained by VIR.(A) BI depths. (B) BII depths.(C) BI centers. (D) BII centers.Band depths and centers werecomputed after continuumremoval (fig. S2).

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Although the surface of Vesta exhibits spec-tral variations at both large and small scales, thematerials on the surface are always dominated byrocks formed by mafic magmatism, as indicatedby the ubiquitous BI and BII pyroxene signatures.

These bands are caused by absorption of pho-tons, primarily by Fe2+, and their exact positionand shape are driven by the relative proportionof Fe toMg in theM1 andM2 sites of pyroxenecrystal structures (19, 20).

VIR spatial resolution allows for the defini-tion of localized mineralogical units: The resultsindicate a complex geological and collisional his-tory (21–23) and reveal a crust that was differen-tiated before impact bombardment. The spectralvariations indicate that Vesta’s crust is composi-tionally variable at vertical scales from a few hun-dred meters to 20 km, the depth of excavation ofthe southern impact basins (16).

A global-scale spectral difference is observedbetween the equatorial and southern Rheasilviaregions, as shown on maps of BI and BII depthsand centers (Fig. 2). In the south pole region, py-roxene bands are, on average, deeper and widerthan in the equatorial region (fig. S3). In general,BI depths in the equatorial region are ~0.35 to0.4, whereas those in the Rheasilvia basin arecommonly 0.45 to 0.55. Similarly, BII depths inthe equatorial region are typically ~0.15 to 0.2,

whereas in the southern region they are ~0.25 to0.3. The depth of an absorption band is mainlydetermined by the abundance of the absorbingminerals, the grain size distribution, and the abun-dance of opaque phases. The process known asspaceweathering alsomodifies reflectance spectraand can make lithological interpretation difficult.Regolithic howardites show some characteristicsof exposure to the space environment, such hashigh noble gas and siderophile element contents,and impact-produced glass, but these character-istics are not as well developed as in mature lunarregolith breccias (24, 25). Vesta retains a reflec-tance spectrum dominated by pyroxene absorp-tion bands (fig. S5), indicating that the effects ofspace weathering are much less pronounced onVesta compared with the Moon or Mercury.

Thus, the VIR data suggest that the region ofthe Rheasilvia basin is richer in pyroxene than theequatorial regions or that the regolith in this regionhas a larger average grain size distribution and/orcontains fewer opaque minerals. A larger grain sizewould be consistent with less impact comminutionin the southern region because of the younger ageof the Rheasilvia basin (23). The lower crust is alsoexpected to have had a coarser initial grain size be-cause pyroxene grain sizes vary from diogenites,which are much larger than cumulate eucrites,which are larger than basaltic eucrites (12).

The global asymmetry evident in the distribu-tions of pyroxene band depths is also demonstratedby variations in band center wavelengths (Fig. 2, Cand D). Laboratory studies indicate that band cen-ters for BI and BII pyroxene absorptions are sys-tematically different for diogenites and eucrites (26).To directly compareVesta band centerswithHEDs,we computed the band centers of HEDs by apply-ing the same method to both data sets (Fig. 3). BIand BII centers are at slightly shorter wavelengthsfor diogenites than for eucrites (Fig. 3), a conse-quence of moreMg-rich pyroxenes with lower Caconcentrations in the former (26). Howardites, be-cause of their intermediate nature, lie between, butpartially overlap the fields of diogenites and eucrites.

The BI and BII centers in the VIR spectraform a trend from diogenites to eucrites, andmost plot in the howardites region. Band centervalues are not uniformly distributed on Vesta,but they differ systematically between the equa-torial and southern regions, and band center val-ues often correlate inversely with band depths(Fig. 2). Equatorial regions are prevalently char-acterized by band centers at longer wavelengths(average BI = 0.930 mm and BII = 1.96 mm) andtypically have intermediate to shallow band depths.In contrast, band centers in the Rheasilvia basinare at shorter wavelengths (average BI = 0.926 mmand BII = 1.94 mm), and these often correspondto the deepest pyroxene absorption bands (Fig. 2).

Overall, the correlations between band depthsand band centers can be interpreted in terms ofdiogenite/eucrite content of the different terrains.Diogenites contain ~90 to 95 volume % (vol %)pyroxene (27), whereas basaltic eucrites contain~50 vol % pyroxene (28), implying that, for agiven grain size, the diogenites spectra havestronger bands with respect to eucrites, as con-firmed by HED spectra (table S1). The corre-spondence of stronger pyroxene absorptions withshorter BII and BI centers in the Rheasilvia basinis consistent with a greater proportion of dioge-nite on the surface in this deeply excavated region.

Spectra from the equatorial regions have bandcenters shifted to longer wavelengths, indicat-ing more Fe-rich pyroxenes, and intermediate orshallow band depths, indicating lower pyroxeneabundance, both consistent with a greater eucritecomponent. However, the equatorial region isnot spectrally uniform. An extensive area at about40°E has measurably deeper absorption bands andshorter wavelengths, suggesting a lower proportionof eucritic material in this region, possibly relatedto the influence of Rheasilvia ejecta (23–25). Over-all, the mineralogical north-south diversity indi-cates that the lower crust exposed in Rheasilvia isdominated by pyroxene-rich, diogenitic material.

Although the difference between the southpolar and equatorial regions is the dominantfirst-order feature (Fig. 2), VIR data also demon-strate that Vesta’s surface and subsurface showvariations at local scales, that is, bright and darklocalized areas (fig. S4). Study of geologicalstructures at scales of tens of kilometers, in par-ticular impact craters with copious ejecta and

Fig. 4. The vestan surface near Oppia crater. E, Oppia ejecta; F, Oppia crater floor; H, Oppia crater walls;S, small crater near Oppia. (A) False-color image in the visible continuum. (B) Lithologic diversity aroundthe central crater with copious “red” ejecta (shallower absorption bands) and deeper pyroxene bands onthe crater walls. (C) Image made by combining colors defining the 930-nm band depth, where the smallcrater near the Oppia rim (S) is clearly seen.

Fig. 3. BI center versus BII center. Green,yellow, and violet ovals are the distributionof howardites, eucrites, and diogenites,respectively. The scatter plot representsthe distribution of the VIR BI and BII centersacquired during the Survey phase.

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mass movements, often show associated spec-tral differences. For example, the Oppia region’ssurface exhibits variations in albedo and spec-tral slope that indicate differences in surface ma-terials (Fig. 4A). Moreover, the area around the(Fig. 3) fresh Oppia crater (E) and the craterfloor (F) have shallower BI depths (Fig. 4B),revealing material poorer in pyroxene. Thecratering process here results in inverted stratig-raphy of roughly the upper third of the targetlithology in the ejecta blanket nearest the rim[e.g., (29)]. The crater floor and material part-way up the walls have a reddish hue similar tothe ejecta just outside the rim, consistent withthe lower layers in this crater being composedof rock poorer in pyroxene. The cyan color in-dicates that the soils just below the rim (H) havestronger BI absorption and thus have higherpyroxene content or different grain size (Fig. 4C).The small crater (S) is surrounded by a halo ofbright and green materials, similar to the layer ex-posed in Oppia (H), suggesting a similar compo-sition. VIR thus reveals that the Oppia impactexposed different kinds of materials, suggestingcomplex, small-scale crustal stratigraphy onVesta.

At all scales, pyroxene absorptions are themost prominent spectral features on Vesta and,on average, the spectral parameters of Vesta re-semble those of howardites (fig. S5). The VIRspectra are thus consistent with a surface coveredby a howardite-like regolith containing varyingproportions of eucrite and diogenite at differentlocations. This firmly supports the link betweenVesta and the HEDs, providing geologic contextfor these samples, which furthers our understand-ing of the formation and evolution of Vesta.

Furthermore, Vesta exhibits large color andspectral variations that often reflect geologicalstructures, indicating a complex geological and

evolutionary history, more similar to that of theterrestrial planets than to other asteroids visitedby spacecraft (17, 18). The occurrence of a greaterproportion of diogenite at depth is a critical finding,not demonstrated by data from the Hubble SpaceTelescope or telescopic observations (30, 31),and broadly consistent with magma ocean mod-els for Vesta’s differentiation. On the other hand,the fact that mixtures of diogenite and eucriteappear ubiquitous in all regions, coupled with theoccurrence of smaller-scale variations inmineralogy,make it premature to distinguish between a simplelayered crust of eucrite and underlying dioge-nite (32) or a complex eucrite crust withintruded diogenitic plutons (14). The Dawn mis-sion provides the first spatially detailed view ofthe distribution of the rock types, allowing insightinto the magmatic processes that formed the solarsystem’s “smallest planet.”

References and Notes1. T. B. McCord, J. B. Adams, T. V. Johnson, Science 168,

1445 (1970).2. M. A. Feierberg, M. J. Drake, Science 209, 805 (1980).3. G. J. Consolmagno, M. J. Drake, Geochim. Cosmochim.

Acta 41, 1271 (1977).4. M. J. Drake, In Asteroids, T. Gehrels, Ed. (Univ. Arizona

Press, Tucson, AZ, 1979), pp. 765–7825. R. P. Binzel et al., Icarus 128, 95 (1997).6. M. C. De Sanctis et al., Astron. Astrophys. 533, A77 (2011).7. N. A. Moscovitz et al., Icarus 208, 773 (2010).8. M. C. De Sanctis et al., Mon. Not. R. Astron. Soc. 412,

2318 (2011).9. K. Righter, M. J. Drake, Meteorit. Planet. Sci. 32, 929 (1997).10. P. H. Warren, Meteorit. Planet. Sci. 32, 945 (1997).11. R. C. Greenwood, I. A. Franchi, A. Jambon, P. C. Buchanan,

Nature 435, 916 (2005).12. D. W. Mittlefehldt et al., in Planetary Materials: Reviews

in Mineralogy 36, J. J. Papike, Ed. (MineralogicalSociety of America, Chantilly, VA, 1998), pp. 4-1–4-195.

13. A. Beck, H. Y. McSween Jr., Meteorit. Planet. Sci. 45,850 (2010).

14. J.-A. Barrat, A. Yamaguchi, B. Zanda, C. Bollinger,M. Bohn, Geochim. Cosmochim. Acta 74, 6218 (2010).

15. M. C. De Sanctis et al., Space Sci. Rev. (2010).16. C. T. Russell et al., Science 336, 684 (2012).17. A. Coradini et al., Science 334, 492 (2011).18. J. Veverka et al., Science 289, 2088 (2000).19. R. G. Burns, Mineralogical Applications of Crystal Field

Theory (Cambridge Univ. Press, Cambridge, 1993).20. L. A. McFadden, T. B. McCord, C. Pieters, Icarus 31, 439

(1977).21. P. Schenk et al., Science 336, 694 (2012).22. R. Jaumann et al., Science 336, 687 (2012).23. S. Marchi et al., Science 336, 690 (2012).24. P. H. Warren, G. W. Kallemeyn, H. Huber, F. Ulff-Møller,

W. Choe, Geochim. Cosmochim. Acta 73, 5918(2009).

25. L. Wilkening, D. Lal, A. M. Reid, Earth Planet. Sci. Lett.10, 334 (1971).

26. M. J. Gaffey, J. Geophys. Res. 81, 905 (1976).27. L. E. Bowman, M. N. Spilde, J. J. Papike, Meteorit. Planet.

Sci. 32, 869 (1997).28. J. S. Delaney, M. Prinz, H. Takeda, J. Geophys. Res. 89,

(suppl.), C251 (1984).29. H. J. Melosh, Impact Cratering: A Geologic Process

(Oxford Univ. Press, Oxford, 1989).30. Telescopic data suggested a prominent diogenite region.

In the adopted coordinate system, this diogenite spot wasmoved to the northern hemisphere, where VIR does notfind this evidence.

31. J. Y. Li et al., Icarus 208, 238 (2010).32. H. Takeda, Icarus 40, 455 (1979).

Acknowledgments: VIR is funded by the Italian Space Agencyand was developed under the leadership of INAF-Istitutodi Astrofisica e Planetologia Spaziali, Rome, Italy. Theinstrument was built by Selex-Galileo, Florence, Italy. Theauthors acknowledge the support of the Dawn Science,Instrument, and Operations Teams. This work was supportedby the Italian Space Agency, and NASA’s Dawn at VestaParticipating Scientists Program. A portion of this work wasperformed at the Jet Propulsion Laboratory under contractwith NASA.

Supplementary Materialswww.sciencemag.org/cgi/content/full/336/6082/697/DC1Supplementary TextFigs. S1 to S5Table S1

17 January 2012; accepted 16 April 201210.1126/science.1219270

Color and Albedo Heterogeneityof Vesta from DawnVishnu Reddy,1,2* Andreas Nathues,1 Lucille Le Corre,1 Holger Sierks,1 Jian-Yang Li,3

Robert Gaskell,4 Timothy McCoy,5 Andrew W. Beck,5 Stefan E. Schröder,1 Carle M. Pieters,6

Kris J. Becker,7 Bonnie J. Buratti,8 Brett Denevi,9 David T. Blewett,9 Ulrich Christensen,1

Michael J. Gaffey,2 Pablo Gutierrez-Marques,1 Michael Hicks,8 Horst Uwe Keller,10

Thorsten Maue,1 Stefano Mottola,11 Lucy A. McFadden,12 Harry Y. McSween,13

David Mittlefehldt,14 David P. O’Brien,4 Carol Raymond,8 Christopher Russell15

Multispectral images (0.44 to 0.98 mm) of asteroid (4) Vesta obtained by the Dawn Framing Camerasreveal global color variations that uncover and help understand the north-south hemisphericaldichotomy. The signature of deep lithologies excavated during the formation of the Rheasilvia basin onthe south pole has been preserved on the surface. Color variations (band depth, spectral slope, andeucrite-diogenite abundance) clearly correlate with distinct compositional units. Vesta displays thegreatest variation of geometric albedo (0.10 to 0.67) of any asteroid yet observed. Four distinct colorunits are recognized that chronicle processes—including impact excavation, mass wasting, andspace weathering—that shaped the asteroid’s surface. Vesta’s color and photometric diversity areindicative of its status as a preserved, differentiated protoplanet.

The Dawn spacecraft rendezvoused withthe asteroid Vesta on 16 July 2011, and theFramingCameras (FCs) (1) acquired images

in seven colors (0.44 to 0.98 mm) and one broad-band clear filter, mapping the entire sun-lit surfaceat a detail of ~9 to ~0.016 km/pixel. We used

1Max Planck Institute for Solar System Research, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany. 2Departmentof Space Studies, University of North Dakota, Grand Forks, ND58202, USA. 3Department of Astronomy, University of Mary-land, College Park, MD 20742, USA. 4Planetary ScienceInstitute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719,USA. 5Department of Mineral Sciences, Smithsonian NationalMuseum of Natural History, 10th and Constitution NW,Washington, DC 20560–0119, USA. 6Department of Geologi-cal Sciences, Brown University, Providence, RI 02912, USA.7Astrogeology Science Center, U.S. Geological Survey, Flagstaff,AZ 86001, USA. 8Jet Propulsion Laboratory, California In-stitute of Technology, 4800 Oak Grove Drive, Pasadena, CA91109, USA. 9Johns Hopkins University Applied PhysicsLaboratory, Laurel, MD 20723, USA. 10Institut für Geophysikund extraterrestrische Physik, TU Braunschweig Mendelssohn-strasse 3, DE 38106 Braunschweig, Germany. 11DeutschesZentrum für Luft undRaumfahrt (DLR)–GermanAerospace Center,Institute of Planetary Research, Rutherfordstrasse 2, D-12489Berlin, Germany. 12NASA/Goddard Space Flight Center, MailCode 160, Greenbelt, MD 20771, USA. 13Department of Earthand Planetary Sciences, University of Tennessee, 1412 CircleDrive, Knoxville, TN 37996–1410, USA. 14AstromaterialsResearch Office, NASA Johnson Space Center, Mail Code KR,Houston, TX 77058, USA. 15Institute of Geophysics and Plan-etary Physics, University of California Los Angeles, Los Angeles,CA 90024–1567, USA.

*To whom correspondence should be sent. E-mail: reddy@mps.mpg.de

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