Meteorite Spectra Research Articles

Testing the Bus–DeMeo Asteroid Taxonomy Using Meteorite Spectra

The most widely used method to spectrally classify asteroids is the Bus–DeMeo taxonomy. To test how well the Bus–DeMeo taxonomy groups asteroids on the basis of their mineralogy, we have classified ∼1500 meteorite spectra using this Bus–DeMeo system. Some asteroid classes group together meteorites with similar compositions better than others. Howardite, eucrite, and diogenite spectra tend to be classified as V-types, while ordinary chondrite spectra tend to be classified as S-complex or Q-type bodies. The relatively featureless D- and X-types tend to be dominated by CM carbonaceous chondrites but with a substantial number of matches also with iron meteorites. The large proportion of CM chondrite matches for the D- and X-classes is most likely due to the large number of CM chondrite spectra and the rarity of spectra of more fragile carbonaceous chondrites in our data set. A number of relatively featureless asteroid classes like the C-, B-, L-, and Xc-types group meteorite types together with a wide variety of mineralogies and thermal histories. Visual albedos are vital for distinguishing between many of these assemblages. The Bus–DeMeo taxonomy does have trouble classifying olivine-dominated meteorites that do not have red-sloped spectra because this type of spectrum is rare among asteroids. For many asteroid classes, care must be used when making mineralogical interpretations based solely on spectral type.

Nuclear and Orbital Characterization of the Transition Object (4015) 107P/Wilson–Harrington

Comet 107P/Wilson–Harrington, cross-listed as asteroid 4015, is one of the original transition objects whose properties do not neatly fit into a cometary or asteroidal origin. Discovered in a period of apparently gas-dominated activity in 1949, it was subsequently lost and recovered as the inactive asteroid 1979 VA. We obtained new and reanalyzed archival observations of the object, compared to meteorites, and conducted new orbital integrations in order to understand the nature of this object and to understand where it falls on the asteroid–comet continuum. Wilson–Harrington’s reflectance spectrum is approximately neutral from visible to near-infrared wavelengths, but has a reflectance maximum near 0.8–0.9 μm. The object’s spectrum is well matched by laboratory spectra of carbonaceous chondrite meteorites like the CM Murchison or the C i Ivuna. The object’s phase curve slope is compatible with either an asteroidal or cometary origin, and its recent orbital history has no periods with high enough temperatures to have altered its surface. While it is possible that some unknown process has acted to change the surface from an originally cometary one, we instead prefer a fundamentally asteroidal origin for Wilson–Harrington, which can explain its surface and orbital properties. However, this would require a way to maintain significant (hyper)volatile supplies on the near-Earth objects beyond what is currently expected. Wilson–Harrington’s similar meteorite affinity and possible orbital link to sample return targets (162173) Ryugu and (101955) Bennu suggest that the returned samples from the Hayabusa-2 and OSIRIS-REx missions might hold the key to understanding this object.

Advances in Analysis of the Fe-Ni-Co Alloy and Iron-Bearing Minerals in Meteorites by Mössbauer Spectroscopy with a High Velocity Resolution

Meteorites are the space messengers bringing us the unique information about the Solar System formation and evolution as well as about the effects of various extreme space conditions on meteorites and their parent bodies. The main iron-bearing compounds in meteorites are Fe-Ni-Co alloy, olivine (Fe, Mg)2SiO4, orthopyroxene (Fe, Mg)SiO3, clinopyroxene (Ca, Fe, Mg)SiO3, troilite FeS, chromite FeCr2O4, hercynite FeAl2O4, ilmenite FeTiO3, daubréelite FeCr2S4, schreibersite (Fe, Ni)3P and some other compounds. Therefore, 57Fe Mössbauer spectroscopy was successfully applied for the analyses of various meteorites for about 60 years of experience. The development of Mössbauer spectrometers with a high velocity resolution, i.e., with a high discretization of the velocity reference signal up to 212, provides much better adjustment to resonance and significantly increases the spectra quality and analytical possibilities of Mössbauer spectroscopy. In fact, this permits us to decompose the complex Mössbauer spectra of meteorites using the larger number of spectral components related to reliable compounds in comparison with the results obtained using conventional Mössbauer spectrometers with discretization of the velocity reference signal up to 29. In the present review we consider the results and advances of various meteorites analyses by means of Mössbauer spectroscopy with a high velocity resolution.

A machine learning classification of meteorite spectra applied to understanding asteroids

Understanding the distribution of matter within our Solar System requires a robust methodology for evaluating the composition of small objects in the asteroid belt. Existing asteroid taxonomies have variously been based on spectral features relating to mineralogy and on classification of asteroid spectra alone. This project tests a fundamentally different approach, using machine learning algorithms to classify asteroids based on spectroscopic characteristics of existing meteorite classes. After evaluating four classification techniques built on labeled meteorite spectral data, logistic regression (LR) was determined to provide the most accurate results that distinguish eight robust groups of meteorite classes to which asteroid spectra can then be matched. The groups are rooted in mineralogical composition and directly relate meteorites to potential host bodies. A standalone LR algorithm classifies unknown asteroid spectra uniquely as one of eight specific group, allowing the distribution of compositions in the asteroid belt to be evaluated.

Grain Size Effects on Visible and Near-infrared (0.35–2.5 μm) Laboratory Spectra of Ordinary Chondrite and HED Meteorites

Remote spectral characterization of near-Earth asteroids (NEAs) relies on laboratory spectral calibration to constrain their surface composition, including mineral chemistry and relative mineral abundances. Often these calibrations are based on fine meteorite powders that are representative of regolith observed on large NEAs such as (433) Eros. However, spacecraft observations of smaller NEAs such as (25143) Itokawa, (101955) Bennu, and (162173) Ryugu show surfaces devoid of a thick layer of regolith and instead find variegated landscapes with millimeter-sized particles to meter-scale boulders. Here we present the results of a laboratory study to understand the effects of grain size on the spectral properties of meteorites and how this can impact ground-based characterization of NEAs. Our study focuses on ordinary chondrites (H, L, LL) and HED meteorites, as they comprise ∼90% of all meteorites that fall on Earth. Compared to ordinary chondrites, the spectral band parameters of HED meteorites are less affected by changing grain size. Among the ordinary chondrites, LL chondrites are least affected, but the spectral band parameters and mineral chemistries and abundances for H and L chondrites are most affected by changing grain size. Grain size does not seem to have any significant effect on the taxonomic classification of our meteorite spectra. We also used the Hapke model to investigate trends in single-scattering albedo as a function of grain size and present equations to recover the grain size from a spectrum.

Compositional indication of E- and M-type asteroids by VIS-NIR reflectance spectra of meteorites

Context. E-type asteroids have been linked to aubrites, while M-type asteroids have been linked to enstatite chondrites (ECs) and iron meteorites (IMs). However, as ECs and IMs generally lack absorption characteristics, distinguishing their parent bodies by spectroscopy generally poses a challenge. Aims. We aim to develop a method to distinguish two kinds of M-type asteroids, the parent bodies of ECs and IMs, and to infer their composition. Methods. We measured the visible to near-infrared (VIS-NIR) reflectance spectra of aubrite, ECs, and IMs. Then we analyzed and compared their spectral parameters, such as the reflectance at 0.55 µm (R0.55), absorption bands, and spectral slopes. We also compared the geometric albedo and spectral slopes of a total of 13 E-type and 14 M-type asteroids. Furthermore, combining the collected radar albedo and density data of M-type asteroids, we discuss their potential composition at different depths. Results. We find that for most meteorites, with the exception of very weak absorption in an aubrite and an EH7 chondrite, ECs and IMs do not show any absorption characteristics. Aubrite shows extremely high reflectance and a negative near-infrared slope (NIRS) and ECs show relatively low reflectance and moderately positive NIRS, while IMs show relatively moderate reflectance and the steepest positive NIRS. Two diagrams plotting with R0.55 and NIRS calculated in the 1.1–1.2 µm and 1.1–1.4 µm bands were subsequently shown to perform optimally at distinguishing aubrite, ECs, and IMs. In addition, M-type asteroids have a wide range of NIRS and diverse radar albedo and densities, whereby 16 Psyche shows high NIRS, radar albedo, and density, while 21 Lutetia is dominated by low values for these parameters. Conclusions. We demonstrate that NIRS is correlated with metal content and increases with metal content. In particular, the NIRS calculated in the 1.1–1.4 µm band is a potentially useful parameter for inferring the surface metal content of E- and M-type asteroids. Based on our results, we suggest that the featureless M-type asteroids ought to be divided into two subtypes: Mm- (e.g., 16 Psyche) and Me-type (e.g., 21 Lutetia) in the aim of characterizing the sources of IMs and ECs, respectively.

Review of meteorite irradiation tests to support next C-type asteroid missions

ABSTRACT Effect of space weathering of airless asteroids could be better understood by artificial irradiation tests on meteorites in laboratories. This work surveys the infrared and Raman analysis based interpretation of simulated charged particle irradiation tests in order to better understand near-future observational possibilities of asteroid visiting missions and also to support the planning of next missions and directions of detector improvement. Recent works properly targeted different meteors and meteor relevant minerals, evaluating bulk meteorite spectra, during the irradiation tests. He+, (Ne+, Kr+), and Ar+ ions were used with fluxes characteristic for inner planetary system solar wind, considering 1–10 million yr exposure durations. Although main meteorite minerals were irradiated and analysed, one missing aspect is that only bulk analysis have been done, not minerals separately in their original embedded context. Some Earth based mineral references were also analysed; however, they might not necessarily behave similar to the same type of reference minerals and irradiation effect is poorly known for feldspar, troilite, and magnetite. Darkening should be also further analysed for separate minerals too, together with the record of peak shape and position changes. Infrared ATR analysis might still provide such data in the future using the recently emerged technology, as well as Raman analysis – however for flyby missions’ infrared is the useful method while Raman can be used only at in situ missions. The overview including the tables to support the identification of specific missing information related gaps in our current knowledge and directions for future research.

Mineralogical analysis of 14 PHAs from ViNOS data

ABSTRACT Potentially hazardous asteroids (PHAs) are objects that can have close encounters with Earth and are significantly large to cause significant regional damage in the event of impact. The mitigation strategy requires to understand their physical properties. We aim to investigate the physical nature of PHAs, using data obtained within the framework of the Visible NEOs Observations Survey (ViNOS). We analysed and characterized a sample comprised of 14 of these objects. To conduct this study, we obtained visible spectra of 14 PHAs in the 0.5–0.9-μm region using the 2.5 m Nordic Optical Telescope, located at the El Roque De Los Muchachos Observatory in La Palma (Spain). The resulting spectra were combined with their corresponding near-infrared counterparts, available in the literature from the Small Main-Belt Asteroid Spectroscopic Survey (SMASS). We performed a taxonomical classification, computed several diagnostic spectral parameters (slopes, band centres, and band area ratios), and provide completely new mineralogical information for 10 of these objects. We also compared the data with laboratory spectra of meteorites from the RELAB data base. Among the studied sample of PHAs, approximately 90 per cent of the objects (13 out of 14) were classified as silicaceous (S-types and subclasses). Only one object, 489 486, was classified as carbonaceous. Five of the studied PHAs did not have previous taxonomic classifications. The comparisons of the silicaceous PHAs with meteoritic spectra yielded, in all cases, ordinary chondrites as the best match for meteoritic analogues. The computed mineralogy of all of our targets is also consistent with this results.

Sample studies and SELENE (Kaguya) observations of purest anorthosite (PAN) in the primordial lunar crust for future sample return mission

Purest anorthosite (PAN), which consists mostly of plagioclase (≥98%), provides key information for understanding the deep crust of the Moon, because PAN observed at central peaks of large impact craters was uplifted from a deep-seated layer. For future sample return of PAN, we examined the visible and near-infrared spectra of lunar meteorites and the mineralogical and petrological studies for Apollo FAN 60015. Sample analyses for lunar meteorites and Apollo FAN 60015 showed the existence of PAN in lunar samples. However, PAN clasts in lunar meteorites were so small that not enough sampled material could be secured for multiple analyses, such as determining their crystallization ages. The lunar meteorites were also heavily brecciated by multiple impacts on the surface. The brecciation and recrystallization on the surface may have disturbed the original information (i.e., age, texture, etc). Therefore, sample return of PAN rocks that have recently fallen from the central peaks of the large craters is required for analysis to determine the parent magma composition and timing of PAN formation. We investigated the PAN distributions in Jackson crater on the lunar farside and found that PAN rocks are widely distributed over the central peak and parts of the crater wall, using SELENE (Kaguya) observational data to locate where PAN would best be collected from the lunar surface. Based on the slope that a rover can manage, we recommend two areas appropriate for collecting samples of PAN rocks that have separated from the central peak.

Probing the surface environment of large T-type asteroids

Context. The thermal and radiative environments encountered by asteroids have shaped their surface features. Recent observations have focused on asteroids in the main belt and showed indications for ices and organics in the interiors of the asteroids that were likely significant on prebiotic Earth. They stand out in reflectance spectra as darker, redder colours than most colocated asteroids. Aims. We probe the surface environment of large (>80km in diameter) T-type asteroids. This taxonomic type is relatively ill-constrained as an independent group. We discuss their place of origin based on our probing. Methods. We performed spectroscopic observations of two T-type asteroids, (96) Aegle and (570) Kythera, over the L band (2.8–4.0 µm) using the Subaru telescope. The spectra of other T-type asteroids are available in the literature, as are survey datasets. Based on this, we strove to find commonalities and global trends in this group. We also used the archival polarimetric data of the asteroids and meteorite spectra from laboratory experiments to constrain their surface texture and composition. Results. Our targets exhibit red L-band continuum slopes, (0.30±0.04) µm−1 for (96) Aegle and (0.31 ± 0.03) µm−1 for (570) Kythera, that are similar to those of (1) Ceres and 67P/Churyumov-Gerasimenko, and have an OH-absorption feature with band centres <2.8 µm. For (96) Aegle, we find an indication of a shallow N–H band near 3.1 µm and a C–H band of organic materials over 3.4–3.6 µm, whereas no diagnostic bands of water ice and other volatiles exceeding the noise of the data were seen for both asteroids. The large T-type asteroids except for (596) Scheila display spectral shapes similar to those of to our targets. About 50% of large T-type asteroids contain an absorption band near 0.6−0.65 µm that is likely associated with hydrated minerals. For T-type asteroids (except for Jupiter Trojans) of all sizes, we found a weak correlation that the smaller the diameter and the closer to the Sun, the redder the visible (0.5−0.8 µm) slope. Conclusions. The 2.9 µm band depths of large T-type asteroids suggest that they might have experienced aqueous alteration comparable to Ch-type asteroids, but that it was more intense than for most of the main-belt asteroids. The polarimetric phase curve of the T-type asteroids is well described by a particular surface structure. The 0.5−4.0 µm reflectance spectra of large T-type asteroids appear most similar to those of CI chondrites with grain sizes of ~25−35 µm. Taken as a whole, we propose that large T-type asteroids may have been dislodged roughly around 10 au in the early Solar System.

Connecting asteroids and meteorites with visible and near-infrared spectroscopy

In this work we identify spectral similarities between asteroids and meteorites. Using spectral features such as absorption bands and spectral curvature, we identify spectral matches between 500 asteroid spectra and over 1,000 samples of RELAB meteorite spectra over visible plus near-infrared wavelengths (0.45–2.5μm). We reproduce and confirm many major and previously known meteorite–asteroid connections and find possible new, more rare or less-established connections. Well-established connections include: ordinary chondrites with S-complex asteroids; pristine CM carbonaceous chondrites with Ch-type asteroids and heated CMs with C-type asteroids ; HED meteorites with V-types; enstatite chondrites with Xc-type asteroids; CV meteorites with K-type asteroids; Brachinites, Pallasites and R chondrites with olivine-dominated A-type asteroids.In addition to the link between ordinary chondrite meteorites with S-complex asteroids, we find a trend from Q, Sq, S, Sr to Sv correlates with LL to H, with Q-types matching predominately to L and LL ordinary chondrites, and Sr and Sv matching predominantly with L and H ordinary chondrites. We find ordinary chondrite samples that match to the X-complex. These are measurements of slabs and many are labeled as dark or black (shocked) ordinary chondrites. We find carbonaceous chondrite samples having spectral slopes large enough to match D-type asteroid spectra.We find in many cases the asteroid type to meteorite type links are not unique, for classes with and without distinct spectral features. While there are examples of dominant matches between an asteroid class and meteorite class that are well established, there are less common but still spectrally compatible matches between many asteroid types and meteorite types. This result emphasizes the diversity of asteroid and meteorite compositions and highlights the degeneracy of classification by spectral features alone requiring additional measurements to firmly establish asteroid–meteorite links. Recent and upcoming spacecraft missions will shed light on the compositions of many of the asteroid classes, particularly those without diagnostic features, (C-, B-, X-, and D-types), with measurements of C-type Ceres, C-type Ryugu, B-type Bennu, M-type Psyche, and C-, P-, and D-types as part of the Lucy mission.

Linear Modeling of Spectra of Fine Particulate Materials: Implications for Compositional Analyses of Primitive Asteroids

AbstractIn this study, we applied a sum to one constraint weighted least squares (STO WLS) model to thermal infrared (TIR) spectra of a suite of primitive asteroid analogs spectrally and volumetrically dominated by fine particulates (<38 μm). Since coarse particulate emissivity spectra combine linearly, deriving compositions from these are fairly straightforward. Across the TIR it is not as straightforward for fine particulate emissivity spectra due to the nonlinear behavior that arises from volumetric scattering between particles when the particle size becomes comparable to the wavelength of light. Using a WLS model, mixed spectra may be deconvolved into areal percentages of each end member which we assume corresponds to their volume percentages. We used a spectral library of pure mineral spectra to model TIR spectra of a suite of physical mixtures and meteorites obtained under ambient (Earth‐like) and simulated asteroid environment (SAE) conditions collected for the OSIRIS‐REx team (Donaldson Hanna et al., 2021, https://doi.org/10.1029/2020JE006624). The STO WLS model underestimated the modal abundances of the dominant mineral phases by ∼25.0 ± 0.1% on average over the full spectral range and ∼22.0 ± 0.4% over a limited spectral range in all of the ambient and SAE physical mixture and meteorite spectra. Minor phases present in the mixtures and meteorites (abundances ≤5%) were typically not modeled. Also, phases not present in the mixtures were commonly selected (∼2 phases on average with an average abundance of ∼25.2 ± 1.9%) to get the best mathematical fit.

Can M\u04e7ssbauer methods of classifying ordinary chondrites help to identify non-representative samples of these meteorites?

Mössbauer spectra of nonweathered ordinary chondrites consist of four main mineral phases: olivines, pyroxenes, metallic phase and troilite. These minerals represent more than 95% of the whole mass of an ordinary chondrite. Distribution of these mineral phases in micro-scale is not homogeneous. Nevertheless, preparation of representative sample of ordinary chondrite for Mössbauer measurements is possible. To do that a part of 1 g nonweathered material, selected from inside of meteorite without any specific intention is needed. The Warsaw group has been working on investigation of meteorites for 25 years and has analysed about 150 Mössbauer spectra of various meteorites. Among them we found 15 spectra, which could be suspected of being non-representative. These spectra were obtained from Baszkówka, Amber, Bjurböle, Krasnoi-Ugol and Chelyabinsk meteorites. The analysis of how the samples of meteorites were selected for investigation, has shown that the non-representativeness of samples may be due to: intentional choice of sample, preparation of sample from a too small part of material or the use of non-credible source of meteoritic samples. For confirmation of these assumptions, we used a new method of classification of ordinary chondrites – the 4M method. It turned out that this method is a very useful tool for investigation of non-representative samples of equilibrated ordinary chondrites.

Heating of carbonaceous materials: Insights into the effects of thermal metamorphism on spectral properties of carbonaceous chondrites and asteroids

AbstractCarbonaceous chondrites (CCs) are important materials for understanding the early evolution of the solar system and delivery of organic material to the early Earth. Spectral analysis of CCs can establish the relationship between them and their possible parent asteroids, which helps to determine the surface composition of the asteroid. In this paper, the 0.3–26 μm reflectance spectra of a series of coals ranging from lignite to anthracite (Earth analogs of organic matter contained in CCs), a coal heated to various durations and temperatures, and reflectance spectra of CM2 meteorites were analyzed in conjunction with compositional information to derive spectral–compositional relationships. All types of coals have strong aromatic absorptions (3.28 and 5–6.5 μm) and aliphatic “triplet” absorptions (3.38, 3.41, and 3.48 μm). In contrast, CM2 meteorites have obvious aliphatic absorptions and lack aromatic absorptions. The reason is the weak absorption coefficients of aromatic materials and the overlap with strong OH/H2O absorption. Absorptions in the coal spectra are strongly related to elemental H/C ratio. When the H/C ratio is >0.55, the absorption intensity of an aliphatic increases linearly with the increase of H/C. For heated coal, increasing heating time above 1 h at 450 °C causes the disappearance of the aliphatic “triplet” absorptions. Similarly, heating Murchison meteorite to 400 °C for 1 week causes all the organic absorptions to disappear. This implies that in remote sensing detections, only asteroids (e.g., with CM and CI carbonaceous chondrites compositions) that experienced low thermal metamorphism (<400 °C) are suitable as potential targets for detecting organic compounds using features in the 3–4 µm region.

A Spectral Investigation of Aqueously and Thermally Altered CM, CM‐An, and CY Chondrites Under Simulated Asteroid Conditions for Comparison With OSIRIS‐REx and Hayabusa2 Observations

AbstractVolatile‐rich asteroids are crucial to understanding the transport of water and organics to the terrestrial planet forming region in the early Solar System. Observations of two such asteroids by Hayabusa2 and Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS‐REx) suggest a relationship between these bodies and CI, CM, and CY chondrites. To confirm this, meteorite spectra need to be collected under appropriate conditions for comparison with asteroid observations. We report midinfrared (MIR) emissivity spectra (5.5–50 μm) obtained under ambient and simulated asteroid environment conditions and near‐infrared (NIR) reflectance spectra (2–5 μm) of CM and CY chondrite fine‐particulate (<35 μm) powders for which bulk mineralogy was determined using X‐ray diffraction. Reflectance spectra show a 3 μm feature associated with ‐OH/H2O that shifts from shorter (∼2.72 μm) to longer (∼2.90 μm) wavelengths and develops a rounder shape and reduced band area with increasing thermal metamorphism. In the MIR, the transparency feature (TF) and features in the Si‐O bending region (>15 μm) can be used to infer the relative degree of aqueous alteration, and to resolve the effects of aqueous and thermal alteration, when combined with NIR spectral parameters. The MIR spectra of metamorphosed CY chondrites are distinct from CM chondrite spectra, including a plateau around the Christiansen feature (∼8.00–12.50 μm) and features at longer wavelengths in the Si‐O bending region (e.g., ∼25.50 μm compared to ∼24.30 μm in the CM spectra). We additionally report potential implications of the spectra and parameters determined in this study for the results from Hayabusa2 and OSIRIS‐REx.

Hydrogen abundance estimation and distribution on (101955) Bennu

Asteroids were likely a major source of volatiles and water to early Earth. Quantifying the hydration of asteroids is necessary to constrain models of the formation and evolution of the Solar System and the origin of Life on Earth. The OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) mission showed that near-Earth asteroid (101955) Bennu contains widespread, abundant hydrated phyllosilicates, indicated by a ubiquitous absorption at ~ 2.7 μm. The objective of this work is to quantify the hydration—that is, the hydrogen content—of phyllosilicates on Bennu's surface and investigate how this hydration varies spatially. We analyse spectral parameters (normalized optical path length, NOPL; effective single-scattering albedo, ESPAT; and Gaussian modeling) computed from the hydrated phyllosilicate absorption band of spatially resolved visible–near-infrared spectra acquired by OVIRS (the OSIRIS-REx Visible and InfraRed Spectrometer). We also computed the same spectral parameters using laboratory-measured spectra of meteorites including CMs, CIs, and the ungrouped C2 Tagish Lake. We estimate the mean hydrogen content of water and hydroxyl groups in hydrated phyllosilicates on Bennu's surface to be 0.71 ± 0.16 wt%. This value is consistent with the hydration range of some aqueously altered meteorites (CMs, C2 Tagish Lake), but not the most aqueously altered group (CIs). The sample collection site of the OSIRIS-REx mission has slightly higher hydrogen content than average. Spatial variations in hydrogen content on Bennu's surface are linked to geomorphology, and may have been partially inherited from its parent body.

Visible-infrared spectroscopy of ungrouped and rare meteorites brings further constraints on meteorite-asteroid connections

The composition of asteroids gives crucial insights into the formation and evolution of the Solar System. Although spectral surveys and spacecraft missions provide information on small bodies, many important analyses can only be performed in terrestrial laboratories. Meteorites represent our main source of samples of extraterrestrial material. Determining the source asteroids of these meteorites is crucial to interpret their analyses in the broader context of the inner Solar System. For now, the total number of parent bodies represented in our meteorites collection is estimated to about 150 parent bodies, of which 50 parent bodies represented by the poorly studied ungrouped chondrites. Linking ungrouped meteorites to their parent bodies is thus crucial to significantly increase our knowledge of asteroids. To this end, the petrography of 25 ungrouped chondrites and rare meteorite groups was studied, allowing grouping into 6 petrographic groups based on texture, mineralogy, and aqueous and thermal parent body processing. Then, we acquired visible-near-infrared (VIS-NIR) reflectance spectroscopy data of those 25 meteorites, in order to compare them to ground-based telescopic observations of asteroids. The reflectance spectra of meteorites were obtained on powdered samples, as usually done for such studies, but also on raw samples and polished sections. With asteroids surfaces being more complex than fine-grained regolith (e.g., asteroid (101955) Bennu), in particular near-Earth asteroids, the use of raw samples is a necessary addition for investigating parent bodies. Our results showed that sample preparation influences the shape of the spectra, and thus asteroid spectral matching, especially for carbonaceous chondrites. Overall, the petrographic groups defined initially coincide with reflectance spectral groups, with only few exceptions. The meteorite spectra were then compared with reference end-member spectra of asteroids taxonomy. We matched the 25 studied meteorites to asteroids types, using a qualitative match of the shape of the spectra, as well as a quantitative comparison of spectral parameters (bands positions, bands depths and slopes at 1 and 2 μm). We define links between some of the studied ungrouped chondrites and asteroid types that had no meteorite connection proposed before, such as some very primitive type 3.00 ungrouped chondrites to B-type or Cg-type asteroids. We also matched metamorphosed ungrouped carbonaceous chondrites to S-complex asteroids, suggesting that this complex is not only composed of ordinary chondrites or primitive achondrites, as previously established, but may also host carbonaceous chondrites. Conversely, some ungrouped chondrites could not be matched to any known asteroid type, showing that those are potential samples from yet unidentified asteroid types.

Mid-infrared reflectance spectroscopy of carbonaceous chondrites and Calcium–Aluminum-rich inclusions

We obtained mid-infrared reflectance spectra of powdered size fractions (0–25 ​μm, 25–63 ​μm, 63–125 ​μm and 125–250 ​μm) of 5 carbonaceous chondrites HaH 280 (CK4), Allende (CV3), NWA 10574 (CM2) and Murchison (CM2), and C2 (ungrouped) Tagish Lake. In addition, in-situ micro-FTIR reflectance spectra of CAIs in Allende, Ornans and Vigarano have been made.These spectra are part of a database for the comparison with remote sensing data of asteroids and planetary bodies, but also for laboratory studies of meteorites.Carbonaceous chondrites HaH 280 (CK4), Allende (CV3) and NWA 10574 (CM2) show mid-IR spectra with Reststrahlen bands (RB) dominated by forsterite and pyroxene features. The transparency features (TF) are at 12.1 ​μm–12.9 ​μm and the Christiansen features (CF) at 8.5 ​μm–9.3 ​μm.Murchison (CM2) and ungrouped Tagish Lake (C2) show mainly phyllosilicate features in their spectra. The CF is between 8.6 ​μm and 9.0 ​μm.Micro-FTIR spectra of CAIs from Allende, Vigarano, and Ornans have spinel features between 14.1 ​μm and 14.3 ​μm. In addition, melilite was identified in the Vigarano and Ornans samples with characteristic features at 11.7 ​μm and 12.4 ​μm.Nepheline and sodalite features have been identified around 14 ​μm in the Allende CAI as alteration phases resulting from thermal metamorphism.A band shift in spinel with increasing Fe content from thermal metamorphism could be spectral indicator for parent body metamorphism.A comparison with other bulk samples from the same meteorites/type shows high similarity.Comparison of the finest size fractions modelled in emissivity with remote sensing data of asteroids shows some similarity between a linear mixture of Allende and Murchison and the spectra of asteroid 253 Mathilde. However, the main 10 ​μm plateau is clearly shifted by 0.5 ​μm between asteroid and meteorite spectra. Also, the general band shapes are different. Differences between asteroidal environment (e.g. fairy castle structure) and laboratory make direct comparisons difficult.

Illuminating the dark side of the asteroid population: Visible near-infrared (0.7–2.45 μm) surface mineralogy modeling of D-type asteroids using Shkuratov theory

D-type asteroids are a prime example of the many dark, low-albedo asteroids which do not reflect sufficient light to reveal detectable mineral absorptions. While D-type asteroids are relatively rare in the inner solar system and the main asteroid belt, they are dominant among the Jovian Trojans. In this study, we have applied Shkuratov radiative transfer modeling to laboratory spectra of meteorites for which mineral abundances have been measured using X-ray diffraction (XRD) and Rietveld refinement. The general agreement of radiative transfer and XRD estimates of mineral abundances demonstrates the applicability of the radiative transfer approach to featureless, low-albedo spectra. Shkuratov modeling was then applied to new spectral observations of D-type asteroids, along with numerous previously published spectra. The surface mineral abundances of 81 D-type objects, including NASA's Lucy Mission target (21900) Orus, were modeled using assemblages that are plausible based on meteorite analogues. Modeling results reveal D-types are composed of: low-iron olivine; magnesium saponite-dominant phyllosilicates; opaques such as pyrrhotite and tholin; as well as traces of water-ice and other constituents. Subtle compositional differences in model mineralogies exist between Trojan and non-Trojan D-types as well as between L4 and L5 Trojans suggesting differing formational as well as evolutional conditions have affected these bodies.

Role of spectral resolution for infrared asteroid compositional analysis using meteorite spectra

ABSTRACT In this work the potential mineral identification of meteorites is analysed for the mid-infrared range, to evaluate observational possibilities for future missions targeting small body surfaces. Three carbonaceous and three ordinary chondrite meteorites are examined by a diffuse reflection (DRIFT) instrument, and the presence of principal minerals is confirmed by a powder diffraction method as well. The possibilities and constraints of mineral identifications in the mid-infrared are simulated by artificially degrading the spectral resolution. Our research shows that for the definite identification of principal mineral bands, a spectral resolution ≤10 cm−1 (≤0.15 μm) is needed. At 20–100 cm−1 (0.3–1.5 μm) resolution the identification of these minerals is uncertain, and with a resolution >100 cm−1, it is almost impossible.