Surface Material Analysis of S-Type Asteroids: Removing the Space 1 1 1 Weathering Effect from Reflectance Spectrum

Lunar and Planetary Science XXXIV (2003) 2078.pdf

SURFACE MATERIAL ANALYSIS OF S-TYPE ASTEROIDS: REMOVING THE SPACE 1 1 1 WEATHERING EFFECT FROM REFLECTANCE SPECTRUM. Y. Ueda M. Miyamoto , T. Mikouchi and 2 1 T. Hiroi , Earth Planet. Sci., Univ. of Tokyo, Hongo 7-3-1, Tokyo 113-0033, Japan ( [email protected] 2 tokyo.ac.jp), Geological Sciences, Brown Univ., RI 02912,USA.

Introduction: Recent years, many researchers Data: Reflectance spectra of asteroids in this re- have been observing a lot of asteroid reflectance spec- port were combined from 52 color asteroid survey [1], tra in the UV, visible to NIR at wavelength region SMASSII [2-3]. [e.g., 1-4]. Reflectance spectroscopy of asteroid at this The reflectance spectra of ordinary chondrites Y- range should bring us a lot of information about its 74191 (L3, MB-TXH-084-A) and Y-74646 (LL6, MB- surface materials. Pyroxene and olivine have charac- TXH-085-A) were from RELAB database [14]. They teristic absorption bands in this wavelength range. were pulverized under 25 µm in size. The incident Low-Ca pyroxene has two absorption bands around and emergence angle were 30º and 0º, respectively. 0.9 µm and 1.9 µm. The more Ca and Fe content, the longer both absorption band centers. On the other Results and Discussions: hand, reflectance spectrum of olivine has three com- MGM results: Shown in Figure 1 are the results of plicated absorption bands around 1 µm, and no ab- MGM deconvolution of two S-type asteroids. The sorption feature around 2 µm [5]. asteroid 7 Iris was classified as S(IV) [15], which is In general, reflectance spectra of many asteroids thought to have a similar composition to ordinary that are considered to be silicate rich (i.e., S- and A- chondrite. Olivine, low-Ca pyroxene and plagioclase type asteroids) show redder slope and more subdued are the major minerals in ordinary chondrites. Small absorption bands than those of terrestrial minerals and amount of high-Ca pyroxene also exists as accessory meteorites. These features are now believed to be mineral [e.g., 16]. caused by the space weathering effect [e.g., 6, 7], As shown in Figure 1 (a), the features of broad ab- which is probably caused by micrometeorite bom- sorption band around 1 µm and relatively strong ab- bardment and/or solar wind. This process causes sorption band around 0.9 µm and 1.9 µm suggest the 0 nanophase reduced iron (npFe ) particles near the mixture of olivine and low-Ca pyroxene. Moreover, surface of mineral grains, which leads the optical absorption band around 2.3 µm indicates the presence change. Therefore, the space weathering effect should of high-Ca pyroxene on its surface. Although the ab- be removed from asteroid reflectance spectra to com- sorption band of high-Ca pyroxene is also present pare with those of meteorite and terrestrial minerals. around 1.03 µm, its absorption band could not be de- In this report, we will apply the expanded modified convolved clearly at this position due to the overlap- Gaussian model (MGM) [8] to the reflectance spectra ping of olivine absorption bands around the similar of S-type asteroids 7 Iris and 532 Herculina and com- wavelength position. pare them with those of meteorites. The asteroid 532 Herculina is also classified as S(IV) [15]. The result of its MGM deconvolution Method: As one of the analytical tools for asteroid (Fig1. (b)) was similar to that of Iris, although uncer- surface material, the MGM [9, 10] has often been used. tainty around 1.9 µm still remains. This model has been applied to many minerals and De-reddened results: Figure 2 shows the compari- meteorites and asteroids [e.g., 11, 12]. In this scheme, son of spectra (normalized at 0.55 µm) between de- natural logarithm of a reflectance spectrum is decon- reddened (space weathering term removed) asteroids volved into absorption bands represented by modified and two ordinarily chondrites Y-74191 (L3) and Y- Gaussian distributions in wavelength, and each ab- 74646 (LL6). The reflectance spectrum of Y-74191 sorption band is superimposed onto a background con- matches that of de-reddened Herculina, except for the tinuum. However, in this original model, there are no sharp band of low-Ca pyroxene around 0.9 µm and parameters corresponding to the space weathering shoulder around 0.6 µm. Although Y-74646 does not effect. match the depth of absorption band around 1 µm, its Ueda et al. [8] added a new term based on the overall absorption feature resembles that of both aster- space weathering theory [6] to the original MGM, and oids. Therefore, if both asteroids consist of ordinary succeeded in numerically removing the space weather- chondrite, L or LL chondrites should be the most ing effect from pulse laser simulated reflectance spec- likely candidate. tra of olivine [13]. Lunar and Planetary Science XXXIV (2003) 2078.pdf SURFACE MATERIAL ANALYSIS OF THE S-TYPE ASTEROIDS: Y. Ueda et al.

1.4 Conclusions: We analyzed two S(IV) type aster- 7 Iris oids and pointed out the resemblance of L and LL chondrite. So far, it was difficult to compare reflectance spectrum of asteroid directly with that of mete- 1.2 orite or mineral. Using this expanded MGM in con- sideration of the space weathering effect, it became 532 Herculina much easier for comparing between reflectance spectrum of asteroid and that of meteorite and mineral. 1 The de-reddened reflectance spectra of two S-type asteroids appeared to have ordinary chondrites-like spectra, as shown in Fig. 2. The S-type asteroids con- 0.8 sist of one of the largest groups in the inner main belt

[17]. Some of them have been assumed to be the par- ReflectanceNormalized (Shifted) 7 Iris 532 Herculina ent body of ordinary chondrite. To know the accurate 0.6 De-reddened spectrum composition of S-type asteroids will be the clue for our Y-74646 (LL6) <25 µm µ understanding of the connections between asteroids Y-74191 (L3) <25 m and meteorites and of the nature of the solar system. 1 2 Wavelength (µm) RMSD: 2.21E-3 Fig. 2. De-reddened reflectance spectra (open 0 symbol) of 7 Iris and 532 Herculina. Each spectrum was normalized at 0.55 µm. Each reflectance spectrum of asteroid was shifted for clarity. Solid lines -0.2 (red) are the de-reddened spectra of each asteroid. Broken lines (green and blue) are the reflectance spectra of ordinary chondrites Y-74191 (L3, MB-TXH- 084-A) and Y-74646 (LL6, MB-TXH-085-A) from -0.4 RELAB database. Both meteorites were pulverized Natural Log Reflectance Log Natural under 25 µm in size. The incident and emergence (a) 7 Iris angle are 30º and 0º, respectively. -0.6 1 2 References: [1] Bell J. F. et al. (1988), LPSC XIX, RMSD: 4.30E-3 57-58. [2] Bus S. J. and Binzel R. P. (2002), Icarus, 0 158, 105-145. [3] Bus S. J. and Binzel R. P. (2002), Icarus, 158, 146-177. [4] Burbine T. H. and Binzel R. P. (2002), Icarus, 159, 468-499. [5] Burns R.G. -0.2 (1993), Mineralogical applications of crystal field theory, pp551. [6] Hapke (2002), JGR, 106, 10039- 10073. [7] Pieters C. M. et al. (2000), M&PS, 35, -0.4 1101-1107. [8] Ueda et al. (2002), LPSC, XXXIII,

Natural Log Reflectance Log Natural Abstract #1950. [9] Sunshine J. M. et al. (1990), JGR , 6955-6966. [10] Sunshine J. M. and Pieters C. M. (b) 532 Herculina 95 (1993), JGR 98, 9075-9087. [11] Hiroi T. and Sasaki -0.6 1 2 S. (2001), M&PS, 36, 1587-1596. [12] Binzel R. P. Wavelength (µm) et al. (2001), M&PS, 36, 1167-1172. [13] Yamada et Fig. 1. The results of MGM deconvolution of as- al. (1999), EPS, 52, 1255-1265. [14] Pieters C. M. teroid (a) 7 Iris and (b) 532 Herculina. Spectral data (1983) JGR, 88, 9534-9544. [15] Gaffey et al. (1993), are from the combination of SMASS II [2-3] and 52 Meteoritics, 28, 161-187. [16] McSween H. Y. Jr. et color survey [1]. Natural log reflectance spectrum of al. (1991) Icarus, 90, 107-116. [17] Gradie J. C. et al. each asteroid is the open symbols. Solid line (red) and (1989), Asteroids II, pp316-335. broken line (blue) are the fitted spectrum and background continuum calculated from MGM scheme, Acknowledgment: Several reflectance spectra respectively. Residual error spectrum is shown in a were obtained by T. Hiroi with the NASA RELAB solid line in the top part, offset by 0.03 for clarity. multiuser facility at Brown University, under NASA grant NAG5-3871.