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. 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

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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 asteroids 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-

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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 units derived from larger asteroids, that six of our samplednear-Earth objects These 29 near-Earthasteroids display a therebygiving rise to a greaterdiversity of aremuch more likely to be analogousto the continuumof 1-wimband depths spanning mineralogiesand spectralproperties. For ex- OC-like near-Earthasteroid 1862 Apollo. the rangebetween S asteroidsand OC me- ample, largerS asteroidsmay be composed The spectraof these six near-Earthas- teorites (Fig. 3B). Becausethe spectraldis- of heterogeneous units for which hemi- teroidsare similarto the laboratoryspectra tributionis continuousrather than discrete, spherically averaged spectra obtained by of OC meteorites(Fig. 2). There is consid- our results argue against Apollo-like (Q- telescopic techniques cannot reveal the erable variation in measuredband depths class) asteroidsexisting as a groupdistinct presenceof distinct units (10, 18). For this among H, L, and LL chondrites and for fromthe S class (8). However,our observa- scenario, only among a suite of smaller as- metamorphicgrades within each of these tions do not revealthe natureof the factors teroids that individually may be composed OC categories(16). We find that the aver- that give rise to this apparentcontinuum of of discrete units would a continuum of age for H6 chondrites (a subclassof OC spectralproperties. lithologies be possible. Another factor is meteorites) provides the most consistent Most of the observednear-Earth aster- that, for a given mineralogy, variations in match to our observationsas a whole, but regolith particle sizes and in the abundance

-2102 A- O 5660 A A S asteroid 1991 WA S 1.2 _ class_ 1.2 - a 433 v . a 1993U /\ A 1620 * 1995WL8 I . 1627 1862 ,E m*2063 o A 5660 C a)~ ~ ~ ~~~et 0 6053 AA.

> 1.0 /. 2102 A, --'A a)~~~~C A IA A A Z~~~~~~ A A

0 5660A 'A,. 0.8| :),I ,a, 0.8 H6 chondrite axC) , U'+

S - _ 349 Dembowska aD 1991 WA B , ^ asteroid 1 .2 - 1862 Apollo 1.2 a) CU0 0 (D~~~~~~~~~~~' 00~~~ 1993 UB > 1.0--~'x~\' C) 0 1995 WL8 ?1.0A A

1995 YA'

0.8 _

0.4 0.6 0.8 1.0 0.4 0.6 0.8 1.0 Wavelength (gim) Wavelength (,um) Fig. 1. (A and B) Comparison between the spec- 3. reflection for 0.4 0.6 0.8 1.0 Fig. Visible-wavelength spectra tra of six near-Earthasteroids (points) having deep near-Earth asteroids spanning the gulf separating Wavelength (gim) 1 -[Lmabsorption bands and the spectra of possi- the spectra of S asteroids (upper solid line) from bly analogous asteroid types (bold lines). The S Fig. 2. Visible-wavelength reflectance spectra for OC meteorites (lower solid line). (A) Spectra for six asteroid and Dembowska spectra are from (9), six recently observed near-Earth asteroids (plus asteroids are presented to show the range of vari- where the former represents an average of several 1862 Apollo) compared with laboratory measure- ations for individual objects. (B) Spectra for 35 hundred asteroids. The Vesta-like spectrum is ments for OC meteorites. Superimposed on each near-Earthasteroids (23) are indicated by dashed that of the main-belt Vesta fragment 2590 Mourao asteroid spectrum (points) is an OC meteorite lines only. In both (A)and (B), all spectra are nor- (13). The data for 1862-Apollo (21) have a lower spectrum (dashed lines) represented by the aver- malized to unityat 0.55sm but are not offset. The resolution, which gives rise to the poorer match in age for H6 chondrites (16). Allspectra are normal- S-asteroid spectrum represents an average from curvature at 0.8 pLm.All spectra are normalized to ized to unityat 0.55 [Lm and are offset verticallyby main-belt S asteroids (9), and the meteorite spec- unity at 0.55 pLm(22). 0.2 for clarity. Data for 1862 Apollo are from (21). trum is an average for H6 chondrites (16).

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This content downloaded on Tue, 18 Dec 2012 11:25:44 AM All use subject to JSTOR Terms and Conditions of opaque materials can give rise to a range ous small main-belt asteroids having the same classi- C. R. Chapman, in preparation. fication appear to be related to Vesta (13). (A second 21. B. Zellner, D. J. Tholen, E. F. Tedesco, Icarus 61, of absorption band depths similar to the related set of small asteroids related to Vesta, denot- 355 (1985). range observed (14). ed the J class, have the same mismatch seen in Fig. 22. The dispersion within our spectrograph is about 25 As a final possibility for interpreting the 1A at 0.8 aLm.)Asteroid 349 Dembowska is the only A per pixel. To achieve a higher signal-to-noise observed range of spectral properties span- object currentlyassigned to taxonomic class R (4). ratio for compact plotting in this report, we further 13. R. P. Binzel and S. Xu, Science 260, 186 (1993). binned our data in 10-pixel increments with the ning the divide between S asteroids and OC 14. M. J. Gaffey, J. F. Bell, D. P. Cruikshank, in (4), pp. resulting points and error bars representing the meteorites, we would suggest that asteroids 98-1 27. weighted least squares average of each indepen- in the observed size range have the most 15. M. J. Gaffey, T. H. Burbine, R. P. Binzel, Meteoritics dent 10-pixel sample. 28, 161 (1993). 23. The following near-Earth asteroid spectra are pre- diverse range of ages of any asteroid popula- 16. M. J. Gaffey, J. Geophys. Res. 81, 905 (1976). We sented in Fig. 3B: 433, 1036, 1620, 1627, 1864, tion sampled to date. Because significant account for the 0.025-[Lm wavelength calibrationer- 1866, 2062, 2063, 2102, 4179, 4954, 5143, 5626, collisions occur much more frequently for rorin these data [as reported on p. 91 of M. J. Gaffey, 5660, 6053, 6489, 6455, 6569,1989 VA, 1991 BB, Icarus 60, 83 (1984)]. 1991 VK, 1991 WA, 1991 XB, 1992CC1, 1993TQ2, small objects, "young" fragments or "fresh" 17. Notable exceptions in our data set are the two larg- 1993 UB, 1993 WD, 1993 XN2, 1994 AB1, 1994 surfaces are most likely to be found among est near-Earth asteroids, 433 Eros and 1036 AW1, 1994 EF2, 1994 TW1, 1995 BL2, 1995 WL8, small asteroids (19). However, because col- Ganymede, which have estimated diameters of >20 and 1995 YA3. and >50 km, respectively. Three other sampled ob- 24. Data reported here were obtained at the MDM Ob- lisions are a stochastic process, the sampled jects, 1627 Ivar,1866 Sisyphus, and 4954 Eric, may servatory. This work was supported by NASA grants population will contain a wide range of sur- have diameters of >10 km. For the remaining 30 NAGW1450, NAGW3901, and NASW5026 and by a face ages. If a time-dependent weathering objects in our sample, the estimated mean diameter National Science Foundation Presidential Young In- is 3 km. Because the bulk of our observations span vestigator Award (R.P.B.). A grant from the Planetary process (7) is active, as has been proposed to such a narrow size range, the observed variations in Society provided additional access to the telescope explain surface variations measured in Gali- band depth are not part of a diameter-dependent essential for the success of this project. We thank B. leo spacecraft images of 243 Ida (20), aster- trend reported in (10). G. Marsden, G. V. Williams,and E. L. G. Bowell for oids most closely resembling OC meteorites 18. M. J. Gaffey, Lunar. Planet. Sci. Cont. XXVII,391 assistance with ephemerides for newly discovered (1996). objects and M. Gaffey and an anonymous referee for would be those with the youngest surfaces. 19. D. R. Davis, S. J. Weidenschilling, P. Farinella, P. helpful reviews. For this scenario, a wide range of surface Paolicchi, R. P. Binzel, in (4), pp. 805-826. ages could give rise to a continuum of spec- 20. C. R. Chapman et al., Nature 374, 783 (1995); 26 April1996; accepted 21 June 1996 tral properties, such as we have observed. REFERENCESAND NOTES Arabidopsis AUX1 Gene: A Permease-Like 1. H. Y. McSween, Meteorites and TheirSource Bodies (Cambridge Univ. Press, New York, 1987). Regulator of Root Gravitropism 2. We define near-Earth asteroids on the basis of their orbital characteristics. For this work, we specifically MalcolmJ. Bennett,*Alan Marchant,Haydn G. Green, refer to asteroids categorized as Aten, Apollo, and Amor objects. Aten and Apollo asteroids have orbits Sean T. May, Sally P. Ward, Paul A. Millner,Amanda R. Walker, that cross the orbit of Earth. Amor asteroids ap- proach within 0.3 astronomical unit of Earth's orbit. BurkhardSchulz,t Kenneth A. Feldmann 3. G. W. Wetherill,Philos. Trans. R. Soc. London Ser. A 323, 323 (1987); J. Wisdom, Icarus 56, 51 (1983); Nature 315, 731 (1985); R. P. Binzel, S. Xu, S. J. Bus, The plant hormone auxin regulates various developmental processes including root E. Bowell, Science 257, 779 (1992). formation, vascular development, and gravitropism. Mutations within the AUX1 gene 4. D. J. Tholen and M. A. Barucci, in Asteroids I1,R. P. confer an auxin-resistant root growth phenotype and abolish root gravitropic curvature. Binzel, T. Pehrels, M. S. Matthews, Eds. (Univ. of Arizona Press, Tucson, 1989), pp. 298-315. Polypeptide sequence similarity to amino acid permeases suggests that AUX1 mediates 5. L. A. McFadden, M. J. Gaffey, T. B. McCord, Sci- the transport of an amino acid-like signaling molecule. Indole-3-acetic acid, the major ence 229, 160 (1985). form of auxin in higher plants, is structurally similar to tryptophan and is a likely substrate 6. Asteroid 1862 Apollo is the only object currentlyassigned to the taxonomic class Q, which is the OC for the AUX1 gene product. The cloned AUX1 gene can restore the auxin-responsiveness analog case we consider here. Two other potential of transgenic auxl roots. Spatially, AUX1 is expressed in root apical tissues that regulate OC-likeasteroids have been reported, but both have root gravitropic curvature. spectra that differfrom that of Apollo. Asteroid 6611 (1993 VW) is a near-Earthasteroid measured by M. Di Martino,A. Manara,and F. Migliorini[Astron. Astro- phys. 302, 609 (1995)]. A potential main-belt asteroid OC analog, 3628 Boznemcova, is discussed by R. P. Binzel et al. [Science 262, 1541 (1993)]. Near-Earth Auxins regulate many aspects of plant shoot apex to other tissues (3), where it asteroids found to have spectra revealingother spec- growth and development (1). Indole-3-ace- influe'ncesmany cellularprocesses, includ- tral types (for example, C types) will be discussed tic acid (IAA), the major form of auxin in ing elongationgrowth (4). Our knowledge elsewhere (R. P. Binzel et a/., in preparation). 7. C. M. Pieters, Meteoritics 19, 290 (1984); G. W. higher plants, is synthesized from an indole of the cellularmachinery that mediatesthe Wetherilland C. R. Chapman, in Meteorites and the precursorof the tryptophan amino acid bio- transportand perceptionof IAA is limited. Early Solar System, J. F. Kerridge and M. S. Mat- synthetic pathway within shoot apical tis- Several auxin-bindingproteins have been thews, Eds. (Univ. of Arizona Press, Tucson, 1988), pp. 35-67; C. M. Pieters, E. M. Fischer, 0. Rode, A. sues (2). IAA is then redistributed from the identified, including putative carriersand receptors(5), but their physiologicalimpor- Basu, J. Geophys. Res. 98, 20817 (1993). M.J. Bennett,A. Marchant,H. G. Green,S. T. May,S. P. 8. J. F. Bell, D. R. Davis, W. K. Hartmann;M. J. Gaffey, Ward,Department of BiologicalSciences, Universityof tance remains unclear. Moleculargenetic in (4), pp. 921-945. Warwick,Coventry, CV4 7AL, UK. approachesin Arabidopsisthaliana have pro- 9. S. Xu, R. P. Binzel, T. H. Burbine, S. J. Bus, Icarus P. A. Millner,Department of Biochemistryand Molecular vided new opportunitiesto identify and 115, 1 (1995). Biology,University of Leeds,Leeds, West Yorkshire, UK. 10. S-class asteroids display a diverse range of mineralo- A. R.Walker, Department of PlantSciences, Universityof characterizenovel auxin signalingcompo- gies. One subset, denoted as S(IV),appears to have Cambridge,Cambridge, UK. nents (6). We selected root gravitropismas a silicate most to OC meteor- mineralogy analogous B. Schulzand K.A. Feldmann,Department of PlantSci- our genetic model for auxin signaling. ites [M. J. Gaffey et al., Icarus 106, 573 (1993)]. ences, Universityof Arizona,Tucson, AZ 85721, USA. 11. R. G. Burns, Mineralogical Applications of Crystal Roots use specializedgravity-sensing col- Field Theory (Cambridge Univ. Press, New York, ed. *Towhom correspondence should be addressed.E-mail: umellacells located in the root cap to mon- 2, 1983). The slope shortward of 0.7 atm results from [email protected] the charge transfer 0 -O Fe2 . tPresent address: Botany Institute,University of Co- itor root orientation.After a gravistimulus, 12. The taxonomic classification for 4 Vesta is V. Numer- logne,50931 Cologne,Germany. the columellacells direct actively growing

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