UV to far-IR reflectance spectra of carbonaceous chondrites - I. Implications for remote characterization of dark primitive asteroids targeted by sample-return missions

MNRAS 437, 227–240 (2014) doi:10.1093/mnras/stt1873 Advance Access publication 2013 November 5 UV to far-IR reflectance spectra of carbonaceous chondrites – I. Implications for remote characterization of dark primitive asteroids targeted by sample-return missions Josep M. Trigo-Rodr´ ıguez,1‹Carles E. Moyano-Cambero,1Jordi Llorca,2 Sonia Fornasier,3Maria A. Barucci,3Irina Belskaya,3Zita Martins,4Andy S. Rivkin,5 Elisabetta Dotto,6Jos´ eM.Madiedo 7,8 and Jacinto Alonso-Azc´ arate9 1Institute of Space Sciences (CSIC-IEEC), Facultat de Ci` encies, Campus UAB, Torre C-5, Parells, 2aPlanta, E-08193 Bellaterra (Barcelona), Spain 2Institut de T` ecniques Energ` etiques i Centre de Recerca en Nanoenginyeria, Universitat Polit` ecnica de Catalunya, Diagonal 647, ETSEIB, E-08028 Barcelona, Spain 3LESIA, Observatoire de Paris, CNRS, UPMC Univ Paris 06, Univ. Paris-Diderot, 5 Place Jules Janssen, F-92195 Meudon Principal Cedex, France 4Department of Earth Science and Engineering, Imperial College London, London SW7 2AZ, UK 5John Hopkins University Applied Physics Laboratory, MP3-E196, 11100 Johns Hopkins Road, Laurel, MD 20723, USA 6INAF-Osservatorio Astronomico di Roma, I-00136 Rome, Italy 7Facultad de Ciencias Experimentales, Universidad de Huelva, E-21071 Huelva, Spain 8Departamento de F´ ısica At´ omica, Molecular y Nuclear, Facultad de F´ ısica, Universidad de Sevilla, E-41012 Sevilla, Spain 9Universidad de Castilla-La Mancha, Campus F´ abrica de Armas, E-45071 Toledo, Spain Accepted 2013 October 2. Received 2013 October 1; in original form 2013 February 11 ABSTRACT We analyse here a wide sample of carbonaceous chondrites from historic falls (e.g. Allende, Cold Bokkeveld, Kainsaz, Leoville, Murchison, Murray, Orgueil and Tagish Lake) and from NASA Antarctic collection. With the analysis of these meteorites we want to get new clues on the role of aqueous alteration in promoting the reflectance spectra diversity evidenced in the most primitive chondrite groups. The selected meteorite specimens are a sample large enough to exemplify how laboratory reflectance spectra of rare groups of carbonaceous chondrites exhibit distinctive features that can be used to remotely characterize the spectra of primitive asteroids. Our spectra cover the full electromagnetic spectrum from 0.2 to 25 µmbyusing two spectrometers. First one is an ultraviolet (UV)–near-infrared (NIR) spectrometer that covers the 0.2–2 µm window, while the second one is an attenuated total reflectance infrared spectrometer covering the 2–25 µm window. In particular, laboratory analyses in the UV–NIR window allow obtaining absolute reflectance by using standardized measurement procedures. We obtained reflectance spectra of specimens belonging to the CI, CM, CV, CR, CO, CK, CH, R and CB groups of carbonaceous chondrites plus some ungrouped ones, and it allows identifying characteristic features and bands for each class, plus getting clues on the influence of parent body aqueous alteration. These laboratory spectra can be compared with the remote spectra of asteroids, but the effects of terrestrial alteration forming (oxy)hydroxides need to be considered. Key words: instrumentation: spectrographs – techniques: imaging spectroscopy – meteorites, meteors, meteoroids – minor planets, asteroids: general – comets: general. 1 INTRODUCTION In the past few decades, parallel studies of the composition, petrology and reflectance spectra of meteorites have allowed the identification of achondrite parent bodies and clues to the physical properties of asteroids to be inferred (Johnson & Fanale 1973), offering E-mail: [email protected] a perspective on the complex collisional and dynamical history and evolution of asteroids (Gaffey 1976). A clear example of the relevance of obtaining reflectance spectra of asteroids was the successful identification of the Howardite, Eucrite and Diogenite (HED) suite of achondrites as free-delivered samples from the asteroid 4 Vesta. Such significant success was achieved due to the characteristic olivine and pyroxene absorption bands (for a review see e.g. Pieters & McFadden 1994). Despite such progress, the identification of other asteroidal classes has remained controversial due to C 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society Downloaded from https://academic.oup.com/mnras/article-abstract/437/1/227/996705 by UNIVERSIDAD DE SEVILLA user on 21 March 2018 228 J. M. Trigo-Rodr´ ıguez et al. several factors: first, the intrinsic spectral diversity of some asteroid surfaces due to compositional differences across their surfaces. Secondly, changes in reflectance spectra produced by space weathering, caused by cosmic rays and meteoroid bombardment and resulting in slope and band depth changes that are still being explored (Gaffey 2010; Clark et al. 2002). Thirdly, and sometimes overlooked, asteroid spectra are usually taken in non-ideal observing circumstances, subjected to geometrical and phase angle effects that make absolute calibration not always feasible (Sanchez et al. 2012). Some of the above-mentioned problems are a direct consequence of the intrinsic heterogeneity in the surface of undifferentiated bodies that can exhibit averaged spectra significantly different from the one exhibited by a specific meteorite coming from them. In fact, recent evidence suggests that most chondrites are complex mixtures of different materials, or breccias, produced by the progressive impact gardening of asteroidal surfaces by projectiles of very different composition (Bischoff et al. 2006). Due to all these circumstances, meteorite spectra obtained in laboratories might differ significantly from the spectral appearance of their respective parent asteroids, increasing the magnitude of this puzzle (see e.g. Burbine et al. 2002). Significant progress has been made since pioneering work concerning the characterization of asteroids in distinctive classes according to their spectra (Tholen 1984, 1989; Barucci et al. 1987; Bus & Binzel 2002; Clark et al. 2011; Ostrowski et al. 2011). Guided by accurate meteorite spectra obtained in terrestrial laboratories, direct comparisons were made and potential matches identified. From the identified groups of chondrites and achondrites, the spectroscopists were then able to find distinctive features, bands and slopes that help them to remotely identify the presence of certain minerals and also define asteroid classes. Significant differences between the defined asteroid classes and other asteroid classes or likely meteorite analogues were found and they were later explained as a consequence of different processes: space weathering, parent body aqueous alteration, different regolith sizes, etc. Some dark and primitive asteroids are future targets of spacebased missions like OSIRIS-REx or Marco Polo-R (Barucci et al. 2012a; Lauretta et al. 2012). However, their successful characterization and association with meteorite analogues remain in most cases elusive. Asteroids of the C class (or the C complex in the Bus-DeMeo taxonomy), which are the targets of the next round of sample-return missions, are mostly characterized by their low albedos, and flat, mostly featureless spectra. The C class was first temptatively associated with the CI and CM groups of carbonaceous chondrites (hereafter CCs; Vilas & Gaffey 1989). Later on, the presence of water absorption features in about half of the C complex asteroids was found, particularly at 0.7 and 3 µm (Lebofsky 1978, 1980; Vilas & Gaffey 1989; Jones et al. 1990; Fornasier et al. 1999). The CO and CV chondrites were associated with K asteroids by Bell (1988), and the CR group was linked to C asteroids by Hiroi et al. (1996). Finally, the rare CH chondrite group was linked to C or M asteroids by Burbine et al. (2002). Despite this, most of these links are temptatively assigned so that any further progress in characterizing the asteroids that are sources of meteorites is particularly relevant. Probably, the most poorly known asteroid classes are those associated with undifferentiated carbon-rich asteroids that are linked with CCs (see Table 1). These objects are very primitive and are linked with highly porous and fragile CCs (Consolmagno & Britt 1998; Britt & Consolmagno 2000) that rarely survive atmospheric entry (Trigo-Rodr´ ıguez & Blum 2009). Not all CC meteorites have reflectance properties systematically studied, but recent comprehensive studies exist dealing with powders of the most abundant ones: CI, CM and CR (Cloutis et al. 2011a,b, 2012a,b,c,d,e,f). In this paper, we analyse CCs from Antarctica with the goal of identifying diagnostic absorption features in both common and rare CC groups. Using two different spectrometers, we have measured their reflective properties in a wide wavelength range: 0.2–25 µm. Some of the patterns and features found in our spectra have some promise to allow us to distinguish between different chondrite groups. Finally, as a byproduct of our current study, we have found a plausible link between the CH chondrites and the asteroid 21 Lutetia recently visited by the Rosetta spacecraft. 2 ANALYTICAL PROCEDURE AND DATA REDUCTION The meteorites studied here have been carefully selected from the NASA Antarctic collection. As noted above, the spectra presented here cover the full wavelength range from 0.2 to 30 µmbyusing two spectrometers. The first one is a Shimadzu UV3600 ultraviolet (UV)–visible (Vis)–near-infrared (NIR) spectrometer that covers the 0.2–2.6 µm window, while the second one is a smart orbit attenuated total reflectance (ATR) IR spectrometer that covers the 2.5–40 µm window. Both spectrometers allow us to infer the reflectivity of the meteorites in a very wide range, but with great detail in crucial spectral windows for comparison with the remote spectra of asteroids taken from ground or space-based telescopes. In reference with the meteorite samples, to obtain the UV–NIR spectra, we used mostly thin (∼30 µm), but also some thick sections (with a thickness of a few mm) were measured (e.g. Allende or Kainsaz). To avoid unwished contributions from the surroundings, all samples were having larger surface than the beam slot used by the spectrograph. On the other hand, to make sure that the thin sections were not too thin, and therefore were not allowing some of the incident light being transmitted through the section and be lost, we performed an analysis of some meteorites in thin and thick sections obtaining similar results. The Shimadzu UV3600 UV–Vis–NIR spectrometer allows the measurement of the transmission, absorbance and reflectance spectra of powder, solid or liquid samples. The standard stage for the spectrometer is an integrating sphere (ISR) with a working range 185–2600 nm. The spectrometer uses multiple lamps, detectors and diffraction gratings to work over a wide range of wavelengths. The light originates at one of two lamps, passes through a variable slit, is filtered to select the desired wavelength with a diffraction grating and is then split into two alternating but identical beams with a chopper. The sample beam interacts with the sample and is routed to one of the two or three detectors (depending on the sample stage). The reference beam interacts with the reference material and then goes to the same detector. The inside of the ISR is coated with a duraflect reflecting polymer. For calibration of the detector, a standard baseline was created using a conventional BaSO4substrate that provided an ∼100 per cent reflectance signal better than 1σall along the 200–2000 nm range. Over that range, the BaSO4reflectivity is dropping off so the measurements here must be taken with caution and are not considered in this work. The sampled area during the measurements corresponds to a slot of 2 ×1cm 2that was larger than the common size of the analysed samples. A comparative view with these remote spectra is possible, but the major mineral absorption bands obtained by our Shimadzu are almost saturated; therefore, we cannot reliably extract band positions, so we restrict ourselves to identify the more clear features in UV–NIR. The received chips of each meteorite sample were ground using an agate mortar to obtain the IR spectra. Only a few minutes elapsed Downloaded from https://academic.oup.com/mnras/article-abstract/437/1/227/996705 by UNIVERSIDAD DE SEVILLA user on 21 March 2018 Reflectance spectra of carbonaceous chondrites 229 Table 1. Asteroid classes and temptative meteorite analogues as deduced from the observed UV-to-IR features. Only primitive taxonomic classes are included in this table. Reflectance classes are taken from Tholen (1989) and DeMeo et al. (2009). Some features from UV to NIR are taken from Burbine et al. (2002) and de Le´ on et al. (2012). Asteroid taxonomic class Example of asteroids belonging to the class Suggested chondrite group Possible meteorite in our sample Spectral properties: (slope; albedo) Main spectral features A 446 Aeternitas R3 PRE 95404 Red spectrum below 0.4 µm Absorption by olivine B 2 Pallas CM/CR SCO 06043 Blue in Vis; albedo usually >0.1 Weak UV feature C 10 Hygiea, 324 Bamberga CM/CI MAC 02606 Flat to reddish below 0.4 µm; albedo <0.1 Weak UV feature D 152 Atala, 624 Hektor, 773 Irmintraud, 944 Hidalgo CI QUE 99355 Reddish spectra Featureless and with very low albedo F 704 Interamnia CR – CM MET 01079 Flat to bluish below 0.4 µm; albedo <0.1 Vis wavelength only as DeMeo et al. 2009 found it diverging in IR. Very weak UV feature G 1 Ceres CM1 Murchison Flat past 0.4 µm; low albedo Defined only for UV–Vis region as its distinguishing feature is in the UV. Strong UV feature; usually strong 3µmOHband K 181 Eucharis, 221 Eos, 402 Chlo¨ e CO3/CV3 MET 01074 Shallow 1 µm feature and lack of 2 µm absorption Intermediate between C and S asteroids L 83 Beatrix CV3 Allende Flat longwards 0.75 µm, but strongly reddish below it Very strong UV P 46 Hestia, 65 Cybele, 76 Freia, 87 Sylvia, 153 Hilda CI/CM Orgueil Flat to slightly red; albedo <0.05 Very low albedo and featureless spectra between grinding the samples to powder and placing them in between a diamond detector of the smart orbit ATR IR spectrometer. The diamond ATR detector is ideal for the analysis of hard materials of very different nature, but particularly meteorites because it is inert and extremely strong to deal with tough chondritic materials (e.g. metals or refractory inclusions). At the same time, this diamond-based detector has a wide spectral range and good depth of penetration, which makes it a good choice for meteoritic samples. In this work, we focus on the overall description of the spectra and the identification of the main absorption lines in the wavelength range from 2 to 25 µm. Over 30 µm, we noticed a higher data scattering due to laboratory conditions, and therefore, we avoid a discussion here (Trigo-Rodr´ ıguez et al. 2012). In general, our ATR IR spectrometer provides high-resolution internal reflection spectra of meteorite powders following standard procedures. We remark that these results are similar in their general trends to that obtained by other techniques (e.g. IR microspectroscopy; Beck et al. 2010). 3 RESULTS In our study, we have included samples of relatively well-known and common CC groups (e.g. CMs, COs and CVs) that can be directly compared with much rare specimens (e.g. those belonging to the CH, CK, CR and R groups) and also to a few ungrouped CCs. Table 2 compiles all analysed meteorites. We have found evidence for a continuum in the reflectivity of chondrite groups, from very low reflectance groups to high-reflectance ones represented by CHs and CBs. It is remarkable that the last four cited groups have distinctive reflectance properties, particularly higher reflectivity than those found in other chondrite groups. Our measurements in PCA 91467 thin section indicate that this CH chondrite is rich in 10–200 µ-sized metal grains, also characteristic of the CH group of CCs that might raise somehow the albedo (Campbell & Humayun 2004). MAC 02675 is a CB that exhibits few chondrules embedded in the Fe–Ni alloy structure (see e.g. Weisberg, McCoy & Krot 2006). The main mineralogical properties, like e.g. the endmember composition of olivine and pyroxene, and a list of secondary minerals that can play a role in the spectral features observed in the spectra are compiled in Table 3. 3.1 CCs spectra from UV to NIR The reflectance spectra of different groups of CCs reflect a large compositional diversity. Spectral diversity is seen among the presumed CC parent asteroids, reflecting the diversity in mineral components seen in the meteorites (DeMeo et al. 2009; de Le´ on et al. 2012). Most data in the literature cover spectral regions 0.4–1.7 µm (for instance, the Small Main-Belt Asteroid Spectroscopic Survey, Small Solar System Objects Spectroscopic Survey and Small Main- Belt Asteroid Spectroscopic Survey IR extension surveys), although Downloaded from https://academic.oup.com/mnras/article-abstract/437/1/227/996705 by UNIVERSIDAD DE SEVILLA user on 21 March 2018 230 J. M. Trigo-Rodr´ ıguez et al. Table 2. Selection of CCs analysed in this work. The main grain size measured for the grained meteorite powders is 100±50 µm. Meteorite name Chondrite group/petrologic subtype Weathering grade Year of find (F, for year of fall) Spectra obtained in this study ALHA 77003 CO 3.6 Ae 1977 UV–NIR ALH 83108 CO 3.5 A 1983 UV–NIR +IR ALH 84028 CV 3 Ae 1984 UV–NIR ALH 85002 CK 4 A 1985 IR Allende CV 3 A F 1969 UV–NIR +IR Cold Bokkeveld CM 2 A F 1838 IR EET 92159 CR 2 B/C 1992 UV–NIR GRA 95229 CR 2 A 1995 UV–NIR Kainsaz CO 3.2 A F 1937 UV–NIR +IR LAP 02342 CR 2 A/B 2002 UV–NIR Leoville CV 3 A/B 1961 IR LEW 87148 CM 2 Ae 1987 UV–NIR +IR MAC 02606 CM 2 A 2002 UV–NIR +IR MAC 02675 CBb B 2002 IR MET 01070 CM 1 Be 2001 UV–NIR +IR MET 01074 CV 3 B 2001 UV–NIR MIL 07689 CM 1 C 2007 UV–NIR +IR Murchison CM 2 A F 1969 UV–NIR +IR Murray CM 2 A F 1950 IR Orgueil CI 1 B F 1864 IR PCA 91467 CH 3 B/C 1991 UV–NIR +IR PRE 95404 R3 A 1995 UV–NIR +IR QUE 97990 CM 2 Be 1997 UV–NIR +IR QUE 99038 CM 2 A/B 1999 UV–NIR +IR QUE 99355 CM 2 B 1999 UV–NIR +IR SCO 06043 CM 1 ∼B 2006 UV–NIR +IR Tagish Lake C 2 Ungrouped A F 2000 UV–NIR Table 3. Main mineralogical properties of the analysed meteorites (the full table is available online as Supporting Information). Abbreviations: Fa – fayalite, Fo – forsterite, Fs – ferrosilite, CAIs – Ca–Al rich inclusions, AOAs – Ameboid Olivine Aggregates and n.a. – non-available. Meteorite name Chondrite group/petrologic subtype Olivine Fa content range (average) Pyroxene Fs content range (average) Notes and secondaryminerals Reference ALHA 77003 CO 3.6 4–48 (22) 2–25 (14) Pyroxene contains ∼1 per cent CaO, spinel grains Met. Bull. #76 ALH 83108 CO 3.5 0.9–38 1–17 Pyroxene compositions range from pure MgSiO3 to Wo5Fs16 Met. Bull. #76 ALH 84028 CV 3 0–50 2 Large CAIs with gehlenitic melilite, spinel and Ti-rich fassaitic pyroxene Met. Bull. #76 the Reflectance Experiment Laboratory comprehensive data base1 and current commonly used asteroid instruments typically reach 2.5 µm. Our data extend this window to wavelengths as short as 0.2 µm and to 2.6 µm as a long-wavelength limit. This is particularly interesting as absolute reflectivity measurements in those windows are rare. We have found e.g. that below 0.4 µm, the samples’ reflectivity tends to converge into a 5–10 per cent range. Longwards of 2.0 µm, the reflectivity diversifies among 15–35 per cent, and this seems to be a direct consequence of the non-ideal reflectivity of our BaSO4standard for those wavelengths. Consequently, we decided to cut the plotted data at 2 µm. 1Brown Univ. Keck/NASA RELAB Spectra Catalog (http://www.planetary. brown.edu/relab/). Polished sections of the selected meteorites were measured using the UV–Vis–NIR spectrometer. The reflectance spectra were obtained at least two times to get an average value for the reflectance in each wavelength, and no significant differences were found by rotating the sections. The slot size was chosen to almost cover the size of the section, usually about 1cm2, just following the same procedure explained in previous studies (Trigo-Rodr´ ıguez et al. 2011, 2012). However, we are dealing with polished samples, so we would like to know what are the main differences with the reflectance spectra obtained from grained powders of some of these meteorites by Cloutis et al. (2011a,b, 2012a,b,c,d,e,f). We choose to separate the spectra by chondrite groups to test the homogeneity of reflectance properties for meteorites presumably coming from the same parent body. It has been suggested, from the major element differences among the chondrite groups, that each one comes Downloaded from https://academic.oup.com/mnras/article-abstract/437/1/227/996705 by UNIVERSIDAD DE SEVILLA user on 21 March 2018 Reflectance spectra of carbonaceous chondrites 231 Figure 1. (a) UV–NIR spectra of CM and CK CCs. (b) UV–NIR spectra of CR and R CCs. (c) UV–NIR spectra of CO and CV CCs (d) UV–NIR spectra of all studied CC groups. Most of them have reflectivity between 5 and 15 per cent along the full spectral window, but CH and CB chondrites. from a different formation location in the protoplanetary disc and is probably accreted in a different parent body (Wasson 1985). In any case, as meteorites arriving to Earth have presumably experienced quite complex and peculiar evolutionary histories, important compositional and mineralogical properties among the different specimens can exist. The later statement is reinforced by the fact that recovered meteorites are small samples of their parent asteroids, probably representative of local processes in their evolutionary paths. Quite distinctive reflectance spectra were obtained for the different chondrite groups, and to make easy an overall discussion, we have separated the spectra in different graphs (see Fig. 1). Reflectance spectra of CM and CK chondrites are plotted in Fig. 1(a). Spectra of CR and R chondrites are plotted in Fig. 1(b). Finally, CO Downloaded from https://academic.oup.com/mnras/article-abstract/437/1/227/996705 by UNIVERSIDAD DE SEVILLA user on 21 March 2018 232 J. M. Trigo-Rodr´ ıguez et al. and CV are plotted in Fig. 1(c). To have a more general view, we have compiled in Fig. 1(d), the reflectivity of all samples including the CH and CB analysed specimens. We will discuss the main features in Section 4.1. 3.2 Carbonaceous chondrites spectra from NIR to far-IR Significant differences in the reflectance spectra of CCs (differing spectral slopes, presence or absence of absorption bands, overall reflectance levels and so on) suggest compositional diversity. Sometimes the IR spectra are plotted as a function of emissivity, but we decided to use the direct transformation provided by our smart orbit ATR spectrometer to plot them directly as relative reflectance (Figs 4–7). The spectral window goes from 2.5 to 45.5 µmwitha resolution of about 0.1 µm. The grinding procedure of the meteorite samples was always similar, but depending on the toughness and specific mineralogy of each meteorite sample, we noticed that the resulting averaged size of grains was slightly different. Performing some direct measurements of the size of the grains, we concluded that it was about 100 ±50 µm. The granulometric studies that we made for example for Allende indicate that a significant population of the grained materials produce powders in the range of a few hundred µm that might have direct implications in the inferred positions of the Christiansen feature and the Reststrahlen bands in each chondrite group (see e.g. Mustard & Hays 1997). Depending on the matrix abundance and degree of alteration of mafic silicates forming chondrules, we observed that the grinding was more efficient for altered chondrites. CC groups exhibit different components and distinctive abundance ratios, so initially we expected significant differences among the studied groups. Chondrules are the most abundant ingredients, but they vary in average size and proportions, like also occurs for the Ca–Al-rich inclusions (CAIs) and other refractory oxides (Rubin 1989; Scott & Krott 2005; Jones 2012). Metal grain abundances are also highly variable from being almost absent to ubiquitous depending on the chondrite group, and their presence inside the chondrules or in the matrix has direct implications for the reflectance. For instance, the abundance of metal grains is probably the reason for the uniformly red-sloped spectra of CRs. Further complicating the interpretation, some water rich groups (e.g. the CMs) exhibit secondary minerals formed by precipitation from a fluid (Trigo-Rodr´ ıguez, Rubin & Wasson 2006; Rubin et al. 2007). To exemplify the complexity and heterogeneity of aqueous alteration, we know that the mobilization of soluble elements by water was a local process in CMs (Trigo-Rodr´ ıguez et al. 2006). It has been suggested that the availability of heat and/or water could have been depth dependent in the parent asteroid (Rubin et al. 2007). Consequently, the reflectance spectra of CM chondrites exhibit different degrees of aqueous alteration and the extent of aqueous products is highly variable. Each chondrite group exhibits distinctive absorption bands that support the notion that secondary variations in mineralogy influence the meteorite reflectance spectra. On the other hand, among meteorite members of each group exist important differences in water adsorption. Most of that water was probably incorporated in the parent asteroids, but it is also known that carbonaceous chondrites incorporate terrestrial water into their mineral structure after their falls (Fuchs, Olsen & Jensen 1973). By using the ATR technique, it is possible to identify the main absorption hydroxyl bands corresponding to clay minerals and also several functional groups of organic compounds present in the C-rich matrix of CCs (Table 4). Then, it seems that this technique has an important potential to identify the presence of such Table 4. Main absorption bands in IR found for the analysed chondrites (the full table is available online as Supporting Information). Relative band depth can be estimated from the maximum reflectance. Band abbreviations are listed in Table 5. Meteorite Chondrite Maximum name group reflectance Main identified absorption bands λ(µm) Reflectance in band minimum Band assignment ALH 77003 CO 3.6 91.3 (at 8.3 µm) 3.8 10.5 11.6 12.1 17.3 21.2 88.6 65.3 45.9 51.9 64.4 40.3 AS/CA/CR/HI/CO/HYD ALH 83108 CO 3.5 92.6 (at 8.3 µm) 3.1 6.1 9.4 10.5 11.6 12.1 17.1 21.1 89.2 90.6 86.5 65.7 46 50.7 64.6 41 OH/HCC/NC/AS/CR/HI/CO/HYD ALH 85002 CK 4 90.1 (at 8.1 µm) 4.31 10.4 11.5 12.1 17.1 20.9 68.7 55.6 58.5 67.1 59.7 46.1 CO2/AS/CR/HI/CO/HYD Allende CV 3 87.2 (at 8.8 µm) 4.3 9.4 10.4 11.6 12.1 17.2 21.2 83.5 80.4 53.4 28.5 31.7 53.4 25.3 CO2/NC/AS/CR/HI/CO/HYD Downloaded from https://academic.oup.com/mnras/article-abstract/437/1/227/996705 by UNIVERSIDAD DE SEVILLA user on 21 March 2018 Reflectance spectra of carbonaceous chondrites 233 Figure 2. (a) IR spectra of CM CCs from 2.5 to 25 µm. Note the affinity with Orgueil CI chondrite, which is most distinguishable for showing deeper absorption bands than CM members. (b) (available online as Supporting Information) The 2.5–8 µm window to show the 3-µm-band characteristic of the water bounded in phyllosilicates and other features described in Table 4 (available online as Supporting Information). For example we marked the location of the prominent OH absorption band, the two emission peaks located around 4.25 µm from atmospheric CO2, and the absorption bands of CC, CH2and CH3(for Fig. 2b see the online version for colour). components in CCs. The following graphs exemplify how the different groups can be identified obtaining such ATR IR spectra. In Fig. 2 are plotted 10 CM chondrites together with Murchison that, due to its availability, is usually considered as a proxy. Aqueous alteration seems to be directly associated with meteorite reflectance. Note for example that the less reflective LEW 87148, QUE 97990 and QUE 99355 are also exhibiting the deepest water absorption bands, suggesting that reflectivity is also affected by aqueous processing. As an example of the applicability of these spectra to characterize asteroid surfaces, we note that the 3 µm OH band was identified in the IR spectrum of the near-Earth object (NEO) 1996 FG3 that is the backup target of Marco Polo-R mission (see e.g. fig. 1 in Rivkin et al. 2013) and that it has also been identified in other NEO called: 1992 UY4. It is also seen in most of the C-complex objects in the main asteroid belt (Jones et al. 1990, Rivkin et al. 2002, 2003). In Fig. 3, we plotted four CO chondrites, two CV chondrites and again CI Orgueil for general comparison. The differences in the location of the main absorption bands compared with Orgueil (but also with the CMs) are evident in this figure and a consequence of different bulk mineralogy (see e.g. Cloutis et al. 2011a, 2012b,d). In Fig. 4, we compiled the rest of spectra belonging to CB, CH, CK and R chondrites. The R3 PRE 95404 and the CK4 ALH 85002 have absorption bands in similar locations than CO and CV chondrites. Surprisingly, we found a spectral similitude (peaks in similar locations) between a preliminarily classified CM QUE 99038 with the reflectance spectra of CVs. We suggested Alan Rubin that QUE 99038 was misclassified or having an anomalous composition, so he reported to the Meteoritical Society, a reclassification as CV3 chondrite (Rubin, private communication). This assignation is already established in the Meteoritical Bulletin online data base. The affinity with CV3 is also suggested by Fig. 5 where we can directly compare QUE 99038 reflectance spectrum with the spectrum of CV3 chondrite Allende, the CK4 ALH 85002 and the R3 PRE 95404. In particular, QUE 99038 features and bands are much closer to those exhibited by the R3 chondrite, even in the position of the olivine peak that occurs at about 12.05 µm, instead of 12.12 µmfor CV3 Allende. We also show CM2 Murchison to exemplify that this method allows identifying misclassified chondrites. Cloutis et al. (2012f) remarked that QUE 99038 has a reflectance spectrum similar to the CO3 chondrite ALH 77003 in the 1µm window, but clearly differing in the 2 µm region in which QUE 99038 exhibits a dominant olivine absorption band. 4 DISCUSSION 4.1 General reflectance patterns for CC groups In next subsections, we will discuss the main reflectance patterns observed in primitive CCs. Each spectral window exhibits distinctive spectral features that can be used to characterize the nature of a primitive asteroid, the main goal after all. 4.1.1 General patterns seen in the UV-to-NIR spectral window CCs exhibit a wide diversity in reflectance that is a direct consequence of their original and current mineralogy. The similarities in UV-to-NIR spectra that many groups share complicate efforts at interpretations that use only that spectral range. For example, the CM and CK chondrite groups exhibit absolute reflectivity values between 5 and 10 per cent (see Fig. 1a). The main characteristics of the CM chondrite spectra are modestly blue-, flat or red-sloped profiles, the presence of a ubiquitous absorption band in the 700 nm region and a broader region of absorption between ∼900 and 1200 nm attributable to olivine and phyllosilicates (Cloutis et al. 2011b). Our results also confirm that the respective depths of the previously cited absorption bands vary largely so they should be a consequence of different degree of aqueous alteration or, in other words, of the presence of serpentine-rich silicates in their mineral structure (Cloutis et al. 2011b). Table 3 compiles the main mineralogical characteristics of the analysed meteorites. CK chondrites exhibit similar spectra to CM but higher overall reflectance probably due to less pervasive aqueous alteration. CKs are in general darker than pure olivine and exhibit its 1.05 µm absorption band, also having blue-sloped spectra (Cloutis et al. 2012e). Fig. 1(b) shows the easily distinguishable red-sloped spectra of the CR and R groups. In general, chondrite groups unaffected by aqueous alteration (e.g. CO and CV) are dominated by the features of silicates. They are the most abundant minerals forming the components of chondrites, so they exhibit diagnostic features distinguishable in the Vis Downloaded from https://academic.oup.com/mnras/article-abstract/437/1/227/996705 by UNIVERSIDAD DE SEVILLA user on 21 March 2018 234 J. M. Trigo-Rodr´ ıguez et al. Figure 3. IR spectra of CO and CV chondrite groups in the 2.5–25 µm window. Note the different location of the main absorption bands compared to Orgueil CI chondrite. Figure 4. IR spectra of other CC groups in the 2.5–25 µm window. Note the extreme reflectivity found for CB chondrite MAC 02675, not so surprising due to this being about 60 volume per cent metal (Weisberg, Prinz & Nehru 1988). Figure 5. IR spectra of the currently classified CM chondrite QUE 99038 in the 8–25 µm window, compared with the spectra of the CV3 Allende, the R3 PRE 95404, the CK: ALH 85002 and the CM2 Murchison. and NIR regions. For example, crystal field absorptions appear in the spectra due to the presence of transition metal ions (often Fe2+) in the mafic minerals like e.g. olivine and pyroxene. In this sense, olivine is characterized by a typical absorption feature at 1 µm, while pyroxene shows two absorption bands at 1 and 2 µm. On the other hand, the 1.2 µm band is obvious in Fig. 1(c) and corresponds to olivine (see Table 3). Fig. 1(c) also shows that most of the analysed meteorites except Allende (that it is a fall) exhibit decreasing slopes below 500 nm that are probably a consequence of iron oxyhydroxides produced by terrestrial alteration. In fact, Buchwald & Clarke (1989) identified the primary Fe-bearing terrestrial weathering products in Antarctic chondrites are akagan´ eite (β-FeOOH), Downloaded from https://academic.oup.com/mnras/article-abstract/437/1/227/996705 by UNIVERSIDAD DE SEVILLA user on 21 March 2018 Reflectance spectra of carbonaceous chondrites 235 goethite (α-FeOOH), lepidocricite (γ-FeOOH) and maghemite (γ-Fe2O3). It is particularly remarkable that the narrow absorption seen in ALHA 77003 and Kainsaz, which we think could be associated with maghemite and magnetite, is due to its deep extinction below 400 nm (see e.g. Tang et al. 2003). In fact, this deep absorption feature around 300 nm is also visible in Fig. 1(a) for CK4 ALH 85002 and CK4/5 PCA 82500 as well as in the altered CM2 chondrites: MAC 02606 and SCO 06043. Also, the spectral profiles of CR2 and R3 in Fig. 1(b) are indicative of the abundant presence of iron oxyhydroxides as previously shown (Cloutis et al. 2012a). In general, an asteroid reflectance spectrum affected by aqueous alteration may be distinguished by several features (Dotto et al. 2005). Phyllosilicates show absorption bands centred around 0.7 µm, attributed to Fe2+→Fe3+charge transfer in iron oxides (Cloutis et al. 2011a,b). Pervasive water alteration changes the bulk mineralogy transforming Fe–Ni grains into magnetite and troilite. Magnetite for example has distinctive absorption bands in 0.48 and 1µm, respectively, (Cloutis & Gaffey 1994; Cloutis et al. 2011b; Yang & Jewitt 2010). Fornasier et al. (1999) also found a relationship between the albedo of the objects and the extent of aqueous alteration that can be explained with the progressive leaching of iron from silicates as the alteration proceeds. In fact, Vilas (1994) remarked that leached iron, the most opaque phase in the visible range associated with aqueous alteration, is enveloped in magnetite and iron sulphide grains, so much less material would be available to absorb the incoming sunlight and this would naturally increase the albedo. Yang & Jewitt (2010) also identified the magnetite absorption band as a distinctive feature in B-type asteroids. The highest reflectance in our data sample is achieved by CCs belonging to the CR, R and CH groups. Fig. 1(b) shows the characteristic red sloped, and widely varying spectra of CR and R groups ranging from about 5 to 20 per cent maximum reflectance. The three studied CRs show distinctive magnetite absorption bands in 0.48 and 1 µm. These groups do not exhibit the absorption band in the 650 nm region characteristic of CMs attributable to Fe3+→Fe2+ charge transfers, as also noted by Cloutis et al. (2012a). This suggests that CMs are richer in Fe3+phyllosilicates and aqueous alteration was probably weaker in most CRs. Fig. 1(b) also shows weak absorption bands in the 900–1300 nm region, as also was found by Cloutis et al. (2012a). We initially interpret the largest reflectance of CR, R and CHs compared to other chondrite groups as probably due to their major abundances of metal grains in volume, but alternative explanations exist. For example, Gaffey (1986) showed that cut metal grains are redder than just crushed metal grains, so the red slope is probably due to the fact that the metal grains in our studied sections are cut; therefore, the red slope argument is probably not valid. On the other hand, Cloutis et al. (2012a) found that due to having low iron content in their silicates, even small amounts of terrestrial weathering products will cause a red slope. In other words, this non-linear relationship between metal content and reflectivity needs to be better explored, taking also into account the different availability of metal. For example, CR chondrites have most of the metal inside chondrules, while CH ones have them in the matrix. We expect that in nature chondrules are not perfectly cut by space weathering processes (unlike happens in thin or thick sections), and consequently, metal grains inside of chondrules will not contribute to the spectrum as much as they do in the matrix. It is also remarkable that CR, CH and CB chondrites are considered primitive meteorites forming a grouplet sharing similar characteristics and being particularly rich in metal (Weisberg et al. 2006). The metal content was estimated to be, respectively, 5–8, 20 and 60–80 in volume per cent for the above-mentioned chondrite groups (Scott & Krot 2005; Weisberg et al. 2006). We have found recently that aqueous alteration in CRs decreases the abundance of metal grains producing magnetite aggregates and sulphides (Trigo- Rodr´ ıguez et al. 2013). Consequently, as noted by Trigo-Rodr´ ıguez et al. (2013), aqueous alteration may be a natural pathway to decrease the reflectivity of the parent bodies of CR and CH chondrites as exemplified in Fig. 6. We have not included in the discussion here the CBs as they are probably formed by impacts, but we include in Fig. 1(d) the reflectivity of CBb chondrite MAC 02675, ranging from 30 per cent in the UV to about 47 per cent at 2 µm. CO and CV groups also can have high reflectance, and some COs like e.g. ALH 83108 and ALHA 77003 reflect more than 10 per cent of the light at wavelengths longwards of 500 nm. However, these Antarctic finds are showing pervasive, spectrally significant, terrestrial weathering due to the oxidation of metal and mafic silicates (Bland et al. 2006). This produces in both meteorites an increase in the UV region absorption, and the visible region absorption edge appears shifted to longer wavelengths (Cloutis et al. 2012e), as well as the appearance of an absorption feature near 0.9 µm due to Fe3+ charge transfers (Sherman 1985). Specimens from the CO and CV groups are plotted in Fig. 1(c) and mostly characterized by an overall flat spectrum until about 2000 nm and a reddish growing slope onwards. Both groups spectra also show weak absorption bands in Figure 6. A1mm 2window of three metal-rich CCs discussed here exhibiting increasing amounts of metal from left to right. Reflectance images taken with a Zeiss petrographic microscope. The specimens are (a) CR2 chondrite GRA 95229, (b) CR2 EET 92159 and (c) CH3 PCA 91467. Downloaded from https://academic.oup.com/mnras/article-abstract/437/1/227/996705 by UNIVERSIDAD DE SEVILLA user on 21 March 2018