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P~~ PAGE Blxnlc NOT Fltbaa .*. ..... ' "7 ~78-29014 ,+*:,# b. ASTEROID SURFACE MINERALOGY : EVIDENCE FROM EARTH-BASED TELESCOPE OBSER'JATIONS THOfiMS B. McCORD Ine&itute fop Astm Mvsreitl( of &codi, Bonohtac, &cod$ 96828 P~~ PAGE BlXNlC NOT Fltbaa Surface mineralogy of asteroids can be inferred in many cases using a variety of spectroscopic rmte sensing techniques. Through the appl ication of these techniques, mainly over the past ten years, mineral assemblages analogous to most meteor- ite types have been found as surface materials of asteroids. Conspicuously raw or absent from the main asteroid be1t are ordinary chondri te-1 ike assemblages, whi le carbonaceous mate- rials are common as are metal-silicate assemblages. The dis- tribution of mineral assemblages with asteroid size and distance from the Sun reveals heterogeneity which is surely infomtive of early accretionary and evolution processes, but the precise meanings are yet to be agreed on. Low temperature assemblages are relatively more abundant with increasing distance fm the Sun. A1 1 assemblages generally can be found inside 3.0 AU and metal plus orthopyroxene assem- blages are concentrated inside 2.5 AU. INTRODUCTION Surface mineralogy is one of the most revealing types of information obtainable about asteroids, since direct evidence of the compositf@nal and thermal evolution of the objects is derivable. The mineral assemblages present are more infomtive than elemental abun- dances, for the exact combinations of elements in a mineral and the crystal structure are very sensitive to the composition of the parent materlal and to the temperature and pres- sure history. Near ultraviolet, visible, and near-infrared reflectance spectroscopy is the most definitive avai lable technique 'or the mwte determination of asteroid surface miwral- ogles and petrologies. Electronic absorption features present in the reflectance spectra of asteroids (Figuw 1) are directly related to the mineral phases present (Adams, 1975; McCord et al. , 1978). Polarimetry, infrared radiometry and several other techniques (Morrison, 1978) provide complementary information, such as albedo, which, although not a unique function of minerat- ogy, is very useful in differentiating between mineralogic groups and in resol ing ambigu- ities. Y PREVIOUS CHARACTER1ZATIONS OF ASTER01 D SURFACE MATERIALS kCord et al. (1970) measured the first 0.3-1.1 um spectrum of an asteroid, 4 '.'orta, and identified an absorption feature near 0.9 um (see Figure 1) as due to the mineral py- roxene. Thej suggested that the surface materlal was similar to basa; ~icachondri tic WAVELENGTH (+m) WAVENUMBER (m10,000 cm") WAVENUMBER (mt0.600 c6') ORIGmAt PBGE IS OF POOR QUALITY meteorites. Hapke (1971) comp~wdthe UBV colors of a number of asteroids to a variety of lunar, meteoritic and terrestrial rocks and rock powders. He concluded that the surface material cf these asteroids could be matched by powders similar to a range of the compari- son materials but not by metallic surfaces. Chapman and Salisbury (1973) indicated that some matches between asteroid and meteorite spectra were found for several meteorite types including enstati te chondri tes, a basal tic dchondri te, an optically unusual ordinary chon- d-i te and, possibly, a carbonaceous chondrite. Johnson and Fanale (1973) showed that the a1:~edo and spectral characteristics of some asteroids are similar to C1 and C2 carbonaceous chondrites and others to iron meteorites. The latter two sets of authors noted the problem of defining precisely what constituted a 'match,' and both raised the question of subtle McCoM and Gaffey (1974) utilized absorption features and qenerai spectral properties I to characterize surface materials of 14 asteroids. They identified minera' assemblages similar to carbonaceous chondri te, stony-i ron, iton, basal tic achond-i te and si1 icate-metal meteorites. At that time it was possible to establish the general identity of the spec- Chapman et at. (1975) utilized spectral, albedo and polarization parameters to define two major asteroid groups. The first group was characterized by having low albedos (20.09), strong negative polarizations (>I. 1%) at small phase angies and relatively flat, feature- less spectral reflectance curves. These parameters were similar to those for carbonac~ous chondrites and these asteroids were designated as 'carbonaceous' or C type. The second group was characterized as having higher albedos (3.09). weaker negative polarizations (0.4-1.03 and reddish, sometimes featured spectral curves. These parameters were compara- ble to those for most of the meteorites which contain relatively abundant silicate minerals so this group was designated 'siliceous or stony-iron' or S types. A small minority ' -_ (-10%) of the asteroids could not be classified in this system and were designated 'unclas- 4 .- sifid' or U types. This classification system has been redefined recently by Bowel1 et at. ' -7 (1978). (In this volume, see Morrison, 1978.) ! :,I: This simple classificaticn scheme can be quite useful since it does seem to often separate these two major types of objects and the obse-va tional parameters on which the scheme is based can be measured at present for objects fainter than those for which com- plete spectra can be obtained. The choice of terminology is unfortunate, however, since it implies a specific definition of surface materials in meteoritic terms, which was not intended. Any 'flat-black' spectral curve would be designated C type whether or not the surface material would be characterized as carbonaceous by any other criteria. A similar objection can be raised with respect tc th? 'silSceous' tevminology since it implies a degree of specificity not present in the classification criteria. Thus, while the C and S classification of asteroids cannot be viewed as a description of mineralogy or petrology, it does provide valid characterization with respect to the chosen parameters. Since the groups appear in each of the parameters used (albedo, polari- zation, color!, a single measurement such as UBV color can be used to classify the asteroid (Zellner et at. , 1975; Zcllner et aZ. , 1977b; Zellner and Eowell , 1977; Florrison, 1977a,b). Fig. 1. Typical spectral reflectance curves for the various asteroid spectral groups : I 3 Juno, RA-1; 8 Flora, RA-2; 16 Psyche, RR; 9 Metis, RF; 4 Vesta, A; 349 Dembowska, A; 1 Ceres, F; 141 Luwn, TA; 10 Hygiea, TB; 51 Nemausa, TC; 80 Sappho, TD; and 532 Herculina, TE. Spectral curve for each asteroid is displayed in sevcral formats: left--normalized reflectance versus wavelength (pm); center--normalized reflectance versus energy (wave- number, cm'l; and right--di fference between spectral curve and a l inear 'continuum' fitted through 0.43 pm and 0.73 pm points. (From Gaffey and McCord, 1978. ) ..I ,\ - , . -4*s 8 : i .'\~ '-4 ' I. *; : b I I. , '.. ,G.\; 1. t . j '.- ! .! ! ; I : 't i , , , . This approach can also be utilized to identify anomalous objects (Zellner. 1975; Zellner et at. , 1977a) or to establish possible genetic telationships between ~nell~bersof asteroid dynamical fami 1 ies (Cradle and Zellner, 1977). Chapman (1976) uti1 ired the basic CSM clas- sification system hut identified subdivisions based on ddditional spectral criteria ('slope.' ( 'bend' and 'band depth' ; McCord and Chaplnan. 1975a.b) which are n~ineralogically signi f icant. (I I Johnson ot al. (1975) ~neasuredthe near-infrared reflectance of three asteroids , , through the broad bandpass H, and K filters (1.24, 1.65 and 2.2 11111) and coricluded that J, ! I these were consistent with the infrared reflectdnce of suggested n~eteoritic aaterials. dt H Matson al. (1977a.b) utilized infrared and K reflectances to infer ttut space wedther- , ing processes were relatiiely irioctive on asteroid surfaces in contrast to the surfdces of i1 the Moon and Mercury. A very favorable apparition in early 1375 penllitted the llleasurelllent of a vdriety of spectral data sets for the Earth-approaching asteroid 433 E1.o~. Pieters ,.t ,z!. (1476) measured the 0.33-1.07 IIIII spectral reflectance uf Cros tl!rough 25 ndrrow bdndpdss flltet's. This curve was interpreted as ir~dicatingan asselllhldge of olivine. pyroxene and metal. with metal abundance equal to or greater than that in the H-type chondriteq. Veeder ,*t ,I:. (1976) measured the spectrum of Eros through 11 filters from 0.65-2.2 UIII and cor~cludedthdt their spectral data indicated a mixture of olivine and pyroxene with a nletdl-like phase. Uisniewski (1976) concluded from a hiqher resolution spectrunl (0.4-1.0 ~1111) that this sur- face was best nlatched by a nli~tureof iron or stony-iron n~atcrialwith ordinary chondri tic material (ri'.,~. iron + pyroxene + olivine). but sugq~stedthdt olivine is absent or r'dre. Larson ct dl. (1976) measured the 0.9-2.7 L~III spectral ref!ectdnce curve for Eros and iden- tified Ni-Fe and pyroxene. but found no evidence of olivine or feldspar. The dispute over the olivine content arises bcra8rse of slight differences in the observed spectrn near 1 ~1111, and the uncertainty in the nletal abundance is due to i ncoll~pletequantitative understanding of the spectral contribution of metal in a mixture witr~silicdtes. In d comprehensive article, Gaffey and McCord (1977) presented d detailed n~ineraloq- .t ., ical analysis of 65 dsteroid reflectance spect1.d and drrived at the n~ostcolllplete descrip- tion ,-xisting of the n~ineralassenlbldges present on asteroid surfaces. They also gave d , 'I* review of the field and a detailed discussion of the interpretive techniques apolied to I A derive n~ineralogy. Much of the 111ateria1in the present drticle is derived froni this pd~er t and the reader is referred to it for nore detai led and c~nipreh~nsive infotls,~tion. .; . I .. .-.i f The evolving characterization of the surface mineralogy of the asteroid 3 Vestd is , i illustrative of the in~provingsophistication of the interpretative ~)t'occss. a.
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