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M-Asteroids: Searching for Weak Silicate Features on Potentially Differentiated Objects

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M-Asteroids: Searching for Weak Silicate Features on Potentially Differentiated Objects Lunar and Planetary Science XXXIII (2002) 1148.pdf M-Asteroids: Searching for Weak Silicate Features on Potentially Differentiated Objects. P.S. Hardersen1,2, M.J. Gaffey1,2 and P.A. Abell1, 1Department of Earth and Environmental Science, Rensselaer Polytechnic Institute, Troy, New York 12180; 2Current Affiliation: Space Studies Department, University of North Dakota, Grand Forks, North Dakota 58202 ([email protected], [email protected], [email protected]). Introduction: The detection of weak mafic sili- zero in other specimens from silicate-bearing iron me- cate features in the near-infrared spectra (NIR, ~0.8 to teorite types [8]. The major mineral phases in silicate 2.5 µm) of M-type asteroids can help to constrain the inclusions are generally olivine ± clinopyroxene ± likely mineralogies and petrologic histories of this orthopyroxene [7], but other silicate inclusions are group of asteroids. The absence of spectral features poor in silicates and are dominantly composed of vary- can also help to constrain potential surface composi- ing amount of minerals such as schreibersite, troilite, tions. A subset of the M-type asteroids was observed graphite, metal and different phosphates [11]. Silicate in April and May 2001 using the SpeX [1] instrument inclusions also tend to be Fe-poor (<Fs~20 and <Fo~20) at the NASA Infrared Telescope Facility on Mauna [7,8,9,10,12,13,14]. Kea, Hawaii. Of the asteroids observed, 69 Hesperia, Although the detection of weak silicate features on 110 Lydia, 201 Penelope and 216 Kleopatra were all asteroids would be consistent with the iron meteor- observed over the course of multiple nights and nearly- ite/silicate inclusions analogue, another potential ana- full rotational coverage was obtained for all four aster- logue is with the Bencubbin-type CH/CB chondrites. oids. 325 Heidelberga and 413 Edburga were also This undifferentiated meteorite type, similar in a num- observed, as were the S-asteroids 295 Theresia and ber of ways to the CR chondrites, contains ~60 vol% 305 Gordonia, but they received only limited rotational FeNi metal and ~40 vol% iron-poor silicate clasts [15]. coverage. Thus there are at least four meteoritic analogues for Meteorite Analogues: The M-type asteroids are the M-type asteroids whose spectra are dominated by compositionally ambiguous due to their general lack of NiFe metal; two differentiated (iron meteorites, sili- spectral features at visible and NIR wavelengths [2,3]. cate-bearing iron meteorites) and two undifferentiated Assignment to the “M” taxonomic class is based on (enstatite chondrites, CH/CB/Bencubbinites). albedo, the lack of identified spectral features and a Distinguishing between these four potential mete- flat or slightly red spectral curve across the 0.3 - 1.1 oritic analogues for individual M-asteroids requires µm wavelength range. Gaffey [4] and Cloutis et al. [5] spectra sufficient to detect weak mafic silicate features, showed that enstatite chondrites and FeNi metal have and to determine the mineralogically diagnostic pa- albedos in the range of M-type asteroids and display rameters of those features if present. The derived sili- featureless spectra lacking obvious absorption features, cate mineralogy would then be compared to each rele- although their spectral slopes vary. Resolving this vant meteorite group and the thermal history of the compositional ambiguity is important because it would individual M-asteroid constrained from that relation- have a significant impact on constraining the thermal ship. histories of these asteroids. Understanding of the com- The ability to detect mafic silicate features on the positional nature of individual M-asteroids is impor- M-type asteroids will depend on the abundance of sili- tant to the effort to better define the thermal history of cates on their surfaces, as well as on the grain size dis- the asteroid belt, as indicated by the ongoing effort of tribution. Cloutis et al. [5] estimate that detection re- Hardersen and Gaffey [6]. quires ~20 wt% olivine in metal-olivine mixtures and Iron meteorites with silicate inclusions, almost cer- ~10 wt% orthopyroxene in metal-orthopyroxene mix- tainly derived from the main belt, are one potential tures. Grain size distributions on asteroids, while also meteoritic analogue for the M-type asteroids. (Most not well constrained, can be estimated from spacecraft M-objects have been classified based on spectra which study of 433 Eros by the Near-Earth Asteroid Rendez- were inadequate to detect weak silicate features near 1 vous (NEAR) mission [16,17]. µm.) Silicate inclusions have been found in the types 69 Hesperia: This M-type asteroid has been ob- IAB, IIICD, IIE, IVA, IIB and IIIAB iron meteorites served by ECAS [18], the 24-color survey [19], the as well as in some ungrouped irons [7,8,9]. The vol- Seven Color Asteroid Survey (SCAS) [20] and the 52- ume percentages and the major mineral components of color survey [21]. 69 Hesperia has a rotation period of the silicate inclusions vary significantly. For example, 5.655 hours [22], an IRAS albedo of 0.14, and an ef- silicates can compose up to ~40 vol% of IAB iron me- fective IRAS diameter of 138 km. The spectrum of 69 teorites [8] and the IVA iron, Steinbach, contains Hesperia from the 52-color survey shows a broad, about 50% silicate inclusions [10]. However, the shallow absorption feature beyond ~1.6 µm, as men- abundances of silicate inclusions can be very low or tioned by Gaffey et al. [23]. However, the average Lunar and Planetary Science XXXIII (2002) 1148.pdf M-ASTEROIDS AND SILICATE FEATURES: P.S. Hardersen et al. spectrum from Clark et al. [20] does not show any the April/May 2001 observing run will be presented. A absorption feature and displays a featureless, reddened key focus of this effort is to develop methodologies spectrum at longer wavelengths. The results from the and calibrations for analysis of “featureless” asteroid present observing run should be able to determine if spectra that can provide diagnostic information about one, or both, spectra of Hesperia are accurate and if the the histories and meteorite affinities of these asteroids. two disparate spectra are caused by surface heteroge- Integration of asteroid data obtained in different wave- neities or other effects. length intervals is needed to derive consistent, rigorous 110 Lydia: This asteroid has a rotation period of compositional interpretations, increasing the confi- 10.9 hours [22], an IRAS albedo of 0.18, and an effec- dence in our understanding of the nature of any given tive IRAS diameter of 86 km. Lydia was observed as asteroid. High quality spectra of M-type asteroids in a part of the 24-color survey [19] and some research the 10 µm region could detect the presence of silicates, suggests that some silicates are present on the surface and potentially identify the mineral type. This would of Lydia [29]. Work by Rivkin et al. [24] place Lydia be critical to identifying M-type asteroids composed of in their W class of asteroids based on their detection of enstatite chondrite assemblages. absorption features in the 3 µm region of the spectrum. Accomplishing these objectives can greatly expand Although these authors consider Lydia to be a hy- our understanding of the asteroids and the complex drated assemblage, they also calculate the equivalent conditions they experienced in the early solar system. water content on the asteroid’s surface to range from Acknowledgements: Various aspects of this work 0.14-0.27 wt% [24]. The calculated wt% of water on were supported by NASA Planetary Geology and Geo- Lydia is not reconcilable with its interpretation as a physics Program Grant NAG5-10345 and NASA Exo- true hydrated (e.g., phyllosilicate-rich) assemblage. biology Program Grant NAG5-7598 (NSCORT: New Results from the recent observing run, with nearly full York Center for Studies on the Origin of Life). rotational coverage of Lydia, should provide additional References: [1] Rayner J.T. et al. (1998) Proc. constraints on the nature of this asteroid. SPIE, 3354, 468-479. [2] Tholen D.J. (1984) PhD 201 Penelope: This asteroid has an effective IRAS Thesis. [3] Burbine T.H. (1991), Master’s Thesis. [4] diameter of 68 km, a rotation period of 3.75 hours Gaffey M.J. (1976) JGR, 81, 905-920. [5] Cloutis E. [22], and an IRAS albedo of 0.16. It has been previ- A. et al. (1990) JGR, 95, 281-293. [6] Hardersen P.S. ously investigated by Rivkin et al. [25] and Busarev and Gaffey M.J. (2001) LPSC XXXII, Abst. 1103. [7] [26]. In Rivkin et al. [25], 201 Penelope, which they Bunch T.E. (1970) Contr. Mineral. And Petrol., 25, place in their W class, was found to display an absorp- 297-340. [8] Prinz M. et al. (1982) LPSC XIII, 632- tion feature ~5% deep in the 3 µm region correspond- 633. [9] Prinz M. et al. (1991) LPSC XXII, 1101. [10] ing to a calculated water content ranging from 0.13- Prinz M. et al. (1984) Meteoritics, 19, 291-292. [11] 0.15 wt%. Busarev [26] obtained 0.34-0.76 µm spec- McCoy T.J. (1993) Meteoritics, 28, 552-560. [12] tra of Penelope over ~20% of its rotational period. Prinz M. et al. (1983) LPSC XIV, 616-617. [13] These data were interpreted to indicate some combina- McCoy T.J. et al. (1996) Meteoritics & Planet. Sci., tion of metals, mafic silicates and phyllosilicate miner- 31, 419-422. [14] Olsen E.J. and Schwade J. (1998) als. New data will be presented that will further con- Meteoritics & Planet. Sci., 33, 153-155. [15] Weis- strain the nature of this asteroid. berg M.K. et al. (1990) Meteoritics, 25, 269-279. [16] 216 Kleopatra: This well studied asteroid has Veverka, J.
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