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Spectral Properties of Near-Earth Asteroids: Evidence for Sources of Ordinary Chondrite Meteorites Author(S): Richard P

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Spectral Properties of Near-Earth Asteroids: Evidence for Sources of Ordinary Chondrite Meteorites Author(S): Richard P Spectral Properties of Near-Earth Asteroids: Evidence for Sources of Ordinary Chondrite Meteorites Author(s): Richard P. Binzel, Schelte J. Bus, Thomas H. Burbine and Jessica M. Sunshine Reviewed work(s): Source: Science, New Series, Vol. 273, No. 5277 (Aug. 16, 1996), pp. 946-948 Published by: American Association for the Advancement of Science Stable URL: http://www.jstor.org/stable/2891527 . Accessed: 18/12/2012 11:25 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. American Association for the Advancement of Science is collaborating with JSTOR to digitize, preserve and extend access to Science. http://www.jstor.org This content downloaded on Tue, 18 Dec 2012 11:25:44 AM All use subject to JSTOR Terms and Conditions close to the A+ direction obtained from tipodalpositive inclination in the marginof 3. A. Kroner and P. W. Layer, Science 256, 1405 of the to- the in the (1992). the margin Kaap Valley pluton pluton and sedimentsadjacent to 4. E. S. Barton, W. Altermann, I. S. Williams, C. B. nalite and reflects overprinting of the pluton(which is presentto a lesserdegree in Smith, Geology 22, 343 (1994). Moodiesshale duringpluton emplacement. the pluton interior).This directionis signif- 5. L. J. Robb, J. M. Barton Jr., E. J. D. Kable, R. C. Zirconfrom a dikefrom the CluthaMine icantly differentfrom the present-daymag- Wallace, Precambrian Res. 31, 1 (1986). 6. A. R. Tegtmeyer and A. Kr6ner,ibid. 36, 1 (1987); R. (Fig. 1) that crosscutsthe folded Moodies netic field directionand cannot be correlat- A. Armstrong, W. Compston, M. J. de Wit, I. S. sedimentshas a single-grainPb-Pb age of ed with the timing of later thermalevents Williams,Earth Planet. Sci. Lett. 101, 90 (1990); S. L. 3224.5 ? 0.4 Ma (Table3), indicatingthat that could have caused a magnetic over- Kamo, D. W. Davis,:M. J. de Wit, Geol. Soc. Aust. the Moodies was folded before this time. printing.We believe this is a primarymag- Abstr. Ser. 23, 53 (1990). 7. J. L. Kirschvink,Geophys. J. R. Astron. Soc. 62, 699 Becausethe A magnetizationwas acquired netization, acquiredduring cooling of the (1980). afterfolding, it is thereforeonly constrained pluton at 3214 Ma. The data suggestthat a 8. D. R. Van Alstine, ibid. 61, 101 (1979). to be youngerthan 3224 Ma and maycoin- reversal of Earth's magnetic field is pre- 9. P. J. Hattingh, thesis, Universityof Pretoria (1983). cide with the hornblendeage of 3214 Ma. servedin the KaapValley pluton, making it 10. R. B. Hargraves, J. Geol. 78, 253 (1970); 11. P. W. Layer, A. Krbner,M. McWilliams,N. Clauer, J. Our paleomagneticstudy of the Kaap the oldest observedreversal and implying Geophys. Res. 93, 2191 (1988). Valley pluton shows two distinct magnetic that the reversinggeomagnetic dynamo has 12. P. W. Layer, -M. 0. McWilliams,A. Krbner,Eos 45, directions.One is positive inclinationthat been operatingsince the earlyArchean. 689 (1983); P. W. Layer, A. Kr6ner, D. York, M. 0. correspondsto an overprintdirection caused McWilliams,ibid. 70, 1064 (1989). 13. P. W. Layer, A. Kroner, M. McWilliamsA. Burghele, by the intrusionof presumedearly Protero- REFERENCES J. Geophys. Res. 93, 449 (1988). zoic dikes and is seen in both the interior 14. R. A. Fisher, Proc. R. Soc. London Ser. A 217, 295 and marginof the pluton. The other direc- 1. M. W. McElhinnyand W. E. Senanayake, J. Geo- (1953). tion is characterizedby a negative inclina- phys. Res. 85, 3523 (1980); C. J. Hale and D. J. 15. A. Kr6ner, G. R. Byerly, D. R. Lowe, Earth Planet. Dunlop, Geophys. Res. Lett. 11, 97 (1984). Sci. Lett. 103, 41 (1991). tion in the interiorof the pluton (which is 2. P. W. Layer, A. Kroner, D. York, Geology 20, 717 absentfrom the pluton margin)and an an- (1992). 25 March 1996; accepted 13 June 1996 lation of small objects distinct from the Spectral Properties of Near-EarthAsteroids: S-class asteroids(8). Our observationsdo Evidence for Sources of Ordinary not reveal a distinct populationof Q-class objects amongsmall near-Earthasteroids. Chondrite Meteorites We madespectroscopic measurements of RichardP. Binzel, Schelte J. Bus, Thomas H. Burbine, 35 near-Earthasteroids, which we have ob- tainedas target-of-opportunityobservations Jessica M. Sunshine over 1991 through 1996. In making these observations,we useda low-resolutionspec- Although ordinary chondrite (OC) meteorites dominate observed falls, the identification of trographand solid-statecharge-coupled de- near-Earth and main-belt asteroid sources has remained elusive. Telescopic measure- vice (CCD) detectorsattached to the 2.4-m ments of 35 near-Earth asteroids (-3 kilometers in diameter) revealed six that have visible Hiltner telescope of the Michigan-Dart- wavelength spectra similar to laboratory spectra of OC meteorites. Near-Earth asteroids mouth-MassachusettsInstitute of Technol- were found to have spectral properties that span the range between the previously sep- ogy (MDM) Observatoryat Kitt Peak, Ar- arated domains of OC meteorites and the most common (S class) asteroids, suggesting izona (9). Our spectra cover the visible- a link.This range of spectral properties could arise through a diversity of mineralogies and wavelength range from 0.45 to 0.95 pm. regolith particle sizes, as well as through a time-dependent surface weathering process. Over these wavelengths,most of the aster- oids categorizedwithin the S classand most of the near-Earthasteroids we measured displayspectra with a moderateabsorption Of the meteoritesfalling to Earth,80% are magnitudes,and limitedintervals of visibility. band near 1 pLm,which arisesowing to the stones consistingof olivine and pyroxene, Of these measuredasteroids, only one (1862 presenceof olivine and pyroxene(10, 11). which are classifiedas ordinarychondrites Apollo) has spectralproperties widely recog- However,six of the near-Earthasteroids we (OCs).They are thought to representsamples nized as similar to the laboratoryspectral measuredhave spectrathat displayunusu- of the primitivesolar nebula that have under- propertiesof OC meteorites(4-6). We now ally deep I-rim absorptionbands (Fig. 1). gone modestthermal evolution over the age reportthe resultsof a surveyof the visible- Over our measuredwavelength range, the of the solarsystem (1). The most immediate wavelengthspectral properties of near-Earth spectraof these near-Earthasteroids do not sourcesfor meteoritesare likely to be near- asteroids:we find manythat appearto have resemble4 Vesta (Fig. IA), but they do Earthasteroids (2), which in turnare derived spectrasimilar to OC meteorites. appearsimilar to the spectraof 349 Dem- predominantlyfrom the main asteroidbelt There has also been a long-standingde- bowskaand 1862 Apollo (Fig. I B) (l 2, 13). (3). Relativelyfew spectroscopicobservations bate over whetherthe most commonlyob- Although Dembowskaand Apollo clearly of near-Earthasteroids have been obtained served (S class) asteroidsare relatedto the have distinguishablehear-infrared spectra becauseof their small sizes, faint apparent most commonmeteorites, in spite of a mis- (14), such measurementsfor our set of faint match in their red spectralslopes and ab- near-Earthasteroids are not yet available. R. P. Binzel, S. J. Bus, T. H. Burbine, Department of Earth, sorption band depths. One hypothesisfor We makean inferencefor the spectraof our Atmospheric, and PlanetarySciences, Massachusetts In- stitute of Technology, Cambridge, MA 02139, USA. this spectralmismatch is that some "space set of near-Earthasteroids by considering J. M. Sunshine, Department of Earth, Atmospheric, and weathering"process is responsiblefor alter- the mineralogies(based on near-infrared Planetary Sciences, Massachusetts Instituteof Technol- ing the spectralproperties of OC asteroids measurements)and meteorite analogs for ogy, Cambridge, MA 02139, and Science Applications InternationalCorporation, 4501 Daly Drive, Chantilly,VA (7). Another hypothesisis that Apollo-like Dembowska and Apollo. Dembowska is 22021, USA. OC asteroids(the Q class) exist as a popu- consideredto be a pyroxene-olivineassem- 946 SCIENCE * VOL.273 * 16 AUGUST 1996 This content downloaded on Tue, 18 Dec 2012 11:25:44 AM All use subject to JSTOR Terms and Conditions &WREPORTS blagewith little or no metal (14). Apollo is given our spectralcoverage, we are not able oids have estimated diametersof -3 km; interpretedto have a metal, olivine, and to distinguishamong H, L, or LL chondrite these objectsare thus the smallestasteroids pyroxenemineralogy similar to that of OC analogsfor these near-Earthasteroids. that have been spectrallymeasured to date meteorites(5). Given that Dembowskais In addition to identifyingsix potential (17). One possibilityfor the continuumof an outer main-beltasteroid.with no known OC-like near-Earthasteroids (Fig. 2), we spectralproperties is that asteroidsin this dynamicallink to the inner solar system found 29 that have spectralproperties in- size range are more likely than are larger and that no Dembowska-likemeteorites are termediatebetween those of S asteroidsand asteroidsto be composedof distinct litho- present in our collections (15), we infer those of OC meteorites(Fig. 3, A and B). logic
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