IRTF Spectra for 17 Asteroids from the C and X Complexes: a Discussion of Continuum Slopes and Their Relationships to C Chondrites and Phyllosilicates ⇑ Daniel R

Icarus 212 (2011) 682–696 Contents lists available at ScienceDirect Icarus journal homepage: www.elsevier.com/locate/icarus IRTF spectra for 17 asteroids from the C and X complexes: A discussion of continuum slopes and their relationships to C chondrites and phyllosilicates ⇑ Daniel R. Ostrowski a, Claud H.S. Lacy a,b, Katherine M. Gietzen a, Derek W.G. Sears a,c, a Arkansas Center for Space and Planetary Sciences, University of Arkansas, Fayetteville, AR 72701, United States b Department of Physics, University of Arkansas, Fayetteville, AR 72701, United States c Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, United States article info abstract Article history: In order to gain further insight into their surface compositions and relationships with meteorites, we Received 23 April 2009 have obtained spectra for 17 C and X complex asteroids using NASA’s Infrared Telescope Facility and SpeX Revised 20 January 2011 infrared spectrometer. We augment these spectra with data in the visible region taken from the on-line Accepted 25 January 2011 databases. Only one of the 17 asteroids showed the three features usually associated with water, the UV Available online 1 February 2011 slope, a 0.7 lm feature and a 3 lm feature, while five show no evidence for water and 11 had one or two of these features. According to DeMeo et al. (2009), whose asteroid classification scheme we use here, 88% Keywords: of the variance in asteroid spectra is explained by continuum slope so that asteroids can also be charac- Asteroids, composition terized by the slopes of their continua. We thus plot the slope of the continuum between 1.8 and 2.5 m Spectroscopy l Meteorites against slope between 1.0 and 1.75 lm, the break at 1.8 lm chosen since phyllosilicates show numer- Mineralogy ous water-related features beyond this wavelength. On such plots, the C complex fields match those of phyllosilicates kaolinite and montmorillonite that have been heated to about 700 °C, while the X complex fields match the fields for phyllosilicates montmorillonite and serpentine that have been similarly heated. We thus suggest that the surface of the C complex asteroids consist of decomposition products of kaolinite or montmorillonite while for the X complex we suggest that surfaces consist of decomposition products of montmorillonite or serpentine. On the basis of overlapping in fields on the continuum plots we suggest that the CI chondrites are linked with the Cgh asteroids, individual CV and CR chondrites are linked with Xc asteroids, a CK chondrite is linked with the Ch or Cgh asteroids, a number of unusual CI/CM meteorites are linked with C asteroids, and the CM chondrites are linked with the Xk asteroids. The associations are in reasonable agreement with chondrite mineralogy and albedo data. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction in characterizing asteroid spectra. In addition, a 0.7 lm feature that has been reported in about half of the C complex asteroids, and As part of a project to characterize the surface of asteroids and correlates with a much stronger water-related absorption feature the asteroid–meteorite linkage, especially for potential mission at 3 lm, is also used in taxonomy (Lebofsky, 1978, 1980; Feierberg targets (Sears et al., 2004), we have measured near-IR spectra for et al., 1985a; Vilas and Gaffey, 1989; Vilas, 1994; Jones et al., 1990; near-Earth and main-belt asteroids using NASA’s Infrared Tele- Fornasier et al., 1999; see Rivkin et al. (2002) for a review). DeMeo scope Facility in Hawaii and the SpeX spectrometer with a range et al. (2009) recently folded near infrared data into the taxonomy, of 0.8–2.5 lm. Here we report data for 17 C and X complex aster- with a few modifications, and their scheme will be used through- oids. Elsewhere we report data for S and V asteroids (Gietzen et al., out this paper. submitted for publication). Because of their low albedos, generally weak features (com- The C, X and related classes were originally defined by Bus and pared, for example, with the S asteroids), and occasional evidence Binzel (2002) using visible spectra, as a development of the Tholen for water-related features, the C asteroids have frequently been (1989) scheme. Their approach is based on a Principal Component linked to carbonaceous chondrites, specifically the CI and CM chon- Analysis that identified absorption in the UV, absorption around drites (Vilas and Gaffey, 1989; Vilas et al., 1993; Vilas, 1994). The CI 0.9 lm, and the continuum slope as the most significant factors chondrites were originally linked to C asteroids by Gaffey and McCord (1978), while the CM chondrites have been linked with the Fortuna Ch asteroid by Burbine (1998). More recently, Carvano ⇑ Corresponding author at: Arkansas Center for Space and Planetary Sciences, University of Arkansas, Fayetteville, AR 72701, United States. et al. (2003) linked the CM chondrites to any asteroid that had a E-mail address: [email protected] (D.W.G. Sears). 0.7 lm band. The CI and CM chondrites are the meteorites that 0019-1035/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2011.01.032 D.R. Ostrowski et al. / Icarus 212 (2011) 682–696 683 are closest to the solar photosphere in composition and they con- Sullivan et al., 2002). Thus Hiroi and Zolensky (1999) performed tain abundant water, up to 10 vol.% for the CM chondrites and up heat treatments on terrestrial phyllosilicates, three members of to 20 vol.% for the CI chondrites (Wiik, 1969; Jarosewich, 1990). the serpentine group (antigorite, lizardite, clinochrysotile), chlo- In addition to the water-rich CI and CM chondrites, other C rite, and saponite. They considered albedo, UV absorption, the chondrite classes include the anhydrous CO and CV chondrites, 0.7 lm band, and the 3.0 lm band and they compared their heated the anhydrous metal-rich CH chondrites, and several others. The phyllosilicate spectra with those of the asteroids. These authors CO and CV chondrites have been linked to K asteroids by Bell concluded that the best fit for the phyllosilicate on the asteroid (1988). The small CR class was linked to C asteroids by Hiroi surfaces was saponite, but were concerned that this mineral had et al. (1996) and equally small CK chondrite class and was linked not been observed in meteorites. to C asteroids by Gaffey and McCord (1978). The CH chondrites Ostrowski et al. (2010) recently reported data for a series of have been linked to C or M asteroids by Burbine et al. (2002). heated phyllosilicates that covered and extended the range of sam- Carvano et al. (2003) linked the CK chondrites to B asteroids (that ples examined by Hiroi and his colleagues and took the experi- are included in the C asteroid complex). ments to higher temperatures where more complete destruction It is very difficult to characterize the mineralogy of CI and CM of the phyllosilicates occurs. In this paper we report results of a de- chondrites since they seem to consist largely of amorphous silicate tailed comparison of spectra for asteroids, meteorites, and heated phases intermixed with microscopic sulfide and metal grains. This phyllosilicates and discuss implications for asteroid–meteorite mixed phase was once called ‘‘PCP’’ (for ‘‘poorly characterized linkage and asteroid surface composition. Preliminary reports have phase’’), but is now termed tochilonite (Barber et al., 1983; been made at various conferences (Ostrowski et al., 2008a,b, Mackinnon and Zolensky, 1984; Tomeoka and Buseck, 1985; 2009a,b; Sears et al., 2008a,b), but the data and conclusions re- Brearley, 2006). It is sometimes possible to find fine-scale regions ported here supersede them. of a recognizable phyllosilicate phase, such as serpentine and chlorite (Brearley, 2006), and indirect inferences based on bulk chemistry have been used to characterize the phyllosilicate phase 2. Methods in meteorites (McSween and Richardson, 1977). Vilas and Gaffey (1989) suggested serpentine and chlorite as possible phyllosili- Our spectra were obtained using the NASA’s Infrared Telescope cates on the surface of low albedo asteroids. In their review of Facility on Mauna Kea, Hawaii, and the SpeX infrared spectrometer spectroscopic information on the nature of the surface of C and X in low-resolution prism mode over 0.8–2.5 lm(Rayner et al. complex asteroids, Rivkin et al. (2002) inferred the presence of 2003), between 2004 August 6 and 2008 April 18 (Table 1). We hydrated phases including phyllosilicates. Rivkin (1997) and Rivkin were able to obtain useful spectra for asteroids as faint as magni- et al. (2006) suggested that the surface of C asteroid Ceres con- tude 17.5. The raw data were reduced by using IRAF and IDL soft- tained an ammoniated smectite, such as ammoniated cronstedtite. ware which eliminated instrumental and atmospheric artifacts, The surface of an asteroid is heavily altered as a result of expo- normalized to solar analogue spectra, and produced wavelength- sure to the space environment. Radiation damage by solar wind calibrated spectra with a resolution of 5 nm. and solar energetic particles, micrometeorite bombardment, and Much of our discussion below will relate to the slope of the major meteorite impacts will have affected the surface. The sur- spectral continuum. There is much precedence for using slope in faces of all asteroids visited by spacecraft to date are heavily im- studies of asteroid spectra. As mentioned above, DeMeo et al. pacted, and are thus covered with unconsolidated regolith (e.g.

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