Visible‐IR and Raman microspectroscopic investigation of three Itokawa particles collected by Hayabusa: Mineralogy and degree of space weathering based on nondestructive analyses

Hayabusa‐returned samples offer a unique perspective for understanding the link between asteroids and cosmomaterials available in the laboratory, and provide insights on the early stages of surface space weathering. This study characterizes the mineralogy and the extent of space weathering of the three Itokawa particles RA‐QD02‐0163, RA‐QD02‐0174, and RA‐QD02‐0213 provided by JAXA to our consortium. We report here a series of results based on nondestructive analyses through visible‐near‐infrared reflectance and Raman spectroscopy. Results were obtained on the raw particles, both in their original containers and deposited on diamond windows. Identification of the minerals, characterization of their elemental compositions, and measurements of their relative abundances were led through Raman spectroscopy in punctual and automatic mode. Reflectance spectra in the visible and near‐IR wavelengths constrain the mineralogy of the grains and allow direct comparison with the surface of Itokawa. The spectra reflect the extent of space weathering experienced by the three particles. Particle RA‐QD02‐0163 consists of a heterogeneous mixture of minerals: olivine (Fo₇₆) dominates an assemblage with both Ca‐rich (En₅₀, Wo₅₀) and Ca‐poor (En₈₅) pyroxenes. The elemental compositions of the silicates are consistent with those previously reported for distinct Hayabusa particles. Particles RA‐QD‐0174 and RA‐QD02‐0213 are solely composed of olivine, whose chemical composition is similar to that observed in RA‐QD02‐0163. It has been previously shown that the S‐type asteroid 25143 Itokawa is a breccia of poorly equilibrated LL4 and highly equilibrated LL5 and LL6 materials. The three particles studied here can be related to the least metamorphosed lithology (LL4) based on the high forsterite content of the olivine. Neither carbonaceous matter nor hydrated minerals were detected through Raman on the three allocated particles. The NIR‐VIS reflectance (incidence = 45°, light collection at e = 0°) spectra of the three particles, in particular the 1 μm band, are consistent with the presence of both olivine and pyroxene detected via Raman. The spectra of particles RA‐QD02‐0163 and RA‐QD02‐0213 are also fully compatible with the ground‐based observations of asteroid (25143) Itokawa in terms of both spectral features and slope. By contrast, particle RA‐QD02‐0174 has a similar 1 μm band depth but higher (redder) spectral slope than the surface of Itokawa. This probably reveals a variable extent of space weathering among the regolith particles. RA‐QD02‐0174 may contain a higher amount of nanophase metallic iron and nanophase FeS. Such phases are products by space weathering induced by solar wind, previously detected on other Itokawa particles.     

Impacts of rapid sea‐level rise on mangrove deposit erosion: application of taraxerol and Rhizophora records

We investigated a well‐dated marine sediment core from the tropical SE Atlantic covering the last 25 kyr, applying taraxerol and Rhizophora pollen as organic geochemical and palynological proxies for mangrove, respectively. Taraxerol records are positively correlated with Rhizophora pollen records, showing an enhanced supply of mangrove materials into deep‐sea environments during the last deglaciation (Termination I). Sedimentation rates peaked during Meltwater Pulses 1A and 1B, which were associated with the maxima of taraxerol and Rhizophora pollen. This study supports the view that mangrove input was dominantly controlled by erosion of mangrove‐rich shelf sediments during the transgressions. Whether reworked materials were penecontemporaneous or from much older deposits formed during previous sea‐level cycles is discussed. Copyright © 2005 John Wiley & Sons, Ltd.     

High resolution TEM of chondritic carbonaceous matter: Metamorphic evolution and heterogeneity

Abstract– The insoluble carbonaceous matter from 12 chondrites (CI, CM, CO, CV, EH, and UOC), was characterized by high resolution transmission electron microscopy (HRTEM). Besides ubiquitous nanoglobules, the insoluble organic matter from petrologic type 1 and 2 chondrites and Semarkona (LL 3.0) is composed of a highly disordered polyaromatic component. No structural differences were observed between these IOMs, in agreement with the limited thermal metamorphism they all experienced. In chondrites of petrologic type >3.0, the evolution of the IOM is controlled by the extent of thermal metamorphism. The polyaromatic layers, shorter than 1 nm in petrologic type ≤3.0 chondrites, grow up to sizes between 5 and 10 nm in petrologic type >3.6 chondrites, contributing to the increase of the degree of structural order. In addition, we find rare, but ubiquitous onion‐like carbons, which may be the product of nanodiamond graphitization. The insoluble carbonaceous matter of the enstatite chondrite Sahara 97096 (EH 3) is different from the other meteorites studied here. It is more heterogeneous and displays a high abundance of graphitized particles. This may be the result of a mixture between (1) the disordered carbon located in the matrix, and (2) catalytic graphitized phases associated with metal, potentially originating from partial melting events. The structural and nanostructural evolution are similar in all IOMs. This suggests that the structure of the accreted precursors and the parent body conditions of their secondary thermal modifications (temperature, duration, and pressure) were similar. The limited degree of organization of the most metamorphosed IOMs compared with terrestrial rocks submitted to similar temperature suggests that the conditions are not favorable to graphitization processes, due to the chemical nature of the precursor or the lack of confinement pressure.     

Simulated asteroid materials based on carbonaceous chondrite mineralogies

A set of high‐fidelity simulated asteroid materials, or simulants, was developed based on the mineralogy of carbonaceous chondrite meteorites. Three varieties of simulant were developed based on CI1 chondrites (typified by Orgueil), CM2 chondrites (typified by Murchison), and CR2/3 chondrites (multiple samples). The simulants were designed to replicate the mineralogy and physical properties of the corresponding meteorites and anticipated asteroid surface materials as closely as is reasonably possible for bulk amounts. The simulants can be made in different physical forms ranging from larger cobbles to fine‐grained regolith. We analyzed simulant prototypes using scanning electron microscopy, X‐ray fluorescence, reflectance spectroscopy at ambient conditions and in vacuum, thermal emission spectroscopy in a simulated asteroid environment chamber, and combined thermogravimetry and evolved gas analysis. Most measured properties compare favorably to the reference meteorites and therefore to predicted volatile‐rich asteroid surface materials, including boulders, cobbles, and fine‐grained soils. However, there were also discrepancies, and mistakes were made in the original mineral formulations that will be updated in the future. The asteroid simulants are available to the community from the nonprofit Exolith Lab at UCF, and the mineral recipes are freely published for other groups to reproduce and modify as they see fit.     

Palynological evidence for climatic and oceanic variability off NW Africa during the late Holocene

Pollen and organic-walled dinoflagellate cyst assemblages from core GeoB 9503-5 retrieved from the mud-belt ( 50 m water depth) off the Senegal River mouth have been analyzed to reconstruct short-term palaeoceanographic and palaeoenvironmental changes in subtropical NW Africa during the time interval from ca. 4200 to 1200 cal yr BP. Our study emphasizes significant coeval changes in continental and oceanic environments in and off Senegal and shows that initial dry conditions were followed by a strong and rapid increase in humidity between ca. 2900 and 2500 cal yr BP. After ca. 2500 cal yr BP, the environment slowly became drier again as indicated by slight increases in Sahelian savannah and desert elements in the pollen record. Around ca. 2200 cal yr BP, this relatively dry period ended with periodic pulses of high terrigenous contributions and strong fluctuations in fern spore and river plume dinoflagellate cyst percentages as well as in the fluxes of pollen, dinoflagellate cysts, fresh-water algae and plant cuticles, suggesting “episodic flash flood” events of the Senegal River. The driest phase developed after about 2100 cal yr BP.     

RETRIEVING ATMOSPHERIC TEMPERATURE FROM NOAA-15 ATOVS MEASUREMENTS

This paper describes briefly the sounding capabilities of TOVS/ATOVS onboard the NOAApolar-orbiting meteorological satellites,followed by a more detailed review of the retrievalschemes.The ICI physical retrieval scheme with some adaptations is implemented in ourexperiment.The analyses of the Chinese regional NWP model are utilized to create a rolling libraryof initial guess field.Retrieval results validated against both NWP analyses and radiosondesindicate good agreement between ICI retrievals and conventional observations.Preliminary resultfrom the PC-ATOVS Windows display system of NSMC will also be shown.     

RETRIEVING ATMOSPHERIC TEMPERATURE FROM NOAA-15 ATOVS MEASUREMENTS*

This paper describes briefly the sounding capabilities of TOVS/ATOVS onboard the NOAA polar-orbiting meteorological satellites,followed by a more detailed review of the retrieval schemes.The ICI physical retrieval scheme with some adaptations is implemented in our experiment.The analyses of the Chinese regional NWP model are utilized to create a rolling library of initial guess field.Retrieval results validated against both NWP analyses and radiosondes indicate good agreement between ICI retrievals and conventional observations.Preliminary result from the PC-ATOVS Windows display system of NSMC will also be shown.     

Hydrous mineralogy of CM and CI chondrites from infrared spectroscopy and their relationship with low albedo asteroids

IR spectroscopy is one of the few techniques that can directly probe water molecules in rocks. This method has been used to characterize the mineralogy of hydrated/hydrous carbonaceous chondrites, and to link known meteorite families with spectroscopic observations of low albedo asteroids. In this paper, we present measurements of the infrared transmission spectra of matrix chunks from 3 CI and 9 CM chondrites. Spectra were measured at ambient conditions and then at different temperatures along a dehydration path toward high-T (∼300 °C) under primary vacuum. At ambient conditions, the 3-μm spectral range is always dominated by adsorbed atmospheric water molecules. Upon moderate (∼100 °C) and high (∼300 °C) heating under low pressure (P < 10⁻⁴ mbar), adsorbed water and then phyllosilicates interlayer water are removed, revealing a residual absorption band around 3 μm. This band is a characteristic IR feature of the phyllosilicate phases which dominate the mineralogical assemblage of hydrated carbonaceous chondrites. Among the CM chondrites, the high-T spectra reveal a strong variability that appears correlated with the alteration classification scheme of Rubin et al. (2007) and Howard et al. (2009a). The 3-μm band continuously evolves from a broad feature peaking at 3550-3600 cm⁻¹ for the weakly altered CMs (Murchison-type) to a sharp asymmetric peak at ∼3675 cm⁻¹ for the more extensively altered samples (Cold Bokkeveld-type). We attribute this spectral evolution to variations in the chemistry of the phyllosilicate phases from Fe-rich to Mg-rich. On the other hand, the 10-μm spectral region shows a single broad peak which does not compare with known terrestrial serpentine spectra, probably due to high structural disorder of the chondrite phyllosilicate phases. The present work clearly shows that previously published reflectance spectra of chondrites are biased by the presence of adsorbed terrestrial water molecules. Laboratory data collected under dry conditions are needed to reinterpret the chondrite-asteroid connection from the comparison of their 3-μm absorption features.     

The parent bodies of CR chondrites and their secondary history

Renazzo-type (CR) chondrites are a relatively rare group of carbonaceous chondrites with the vast majority having escaped thermal alteration. This means that CRs are composed of relatively unprocessed material, depending on the extent of aqueous alteration they have experienced. Hydration in CRs ranges from incipient alteration of matrix glass, up to nearly complete replacement of the rock by hydration products. The extent of secondary processes is often difficult to assess in these meteorites, due to their heterogeneity and diversity of alteration products. Yet, this is crucial in order to understand the extent of geological processing that occurred on the primary parent body. Additionally, the parent asteroids of CRs remain a mystery, mainly because terrestrial oxyhydroxide signatures dominate the reflectance spectra of CRs. In this work, we have conducted optical and IR reflectance and transmission spectra of 25 CR chondrites in order to (i) better evaluate the extent of aqueous alteration that occurred on the CR parent body, and (ii) find possible parent body candidates. Terrestrial oxyhydroxides were removed from 12 samples, as these tend to interfere with the optical-IR spectra of CRs. Our results suggest, among other, that (i) aqueous alteration in most of our CRs was limited to the matrix and (ii) most CRs may stem from a continuum of X-to-C complex asteroids, depending on their extent of aqueous alteration. More specifically, the endmembers being Xk/Xn types and Cgh/Ch types. This has strong implication in regard to what we can expect from the Psyche mission.     

The iron record of asteroidal processes in carbonaceous chondrites

The valence of iron has been used in terrestrial studies to trace the hydrolysis of primary silicate rocks. Here, we use a similar approach to characterize the secondary processes, namely thermal metamorphism and aqueous alteration, that have affected carbonaceous chondrites. X‐ray absorption near‐edge structure spectroscopy at the Fe‐K‐edge was performed on a series of 36 CM, 9 CR, 10 CV, and 2 CI chondrites. While previous studies have focused on the relative distribution of Fe⁰ with respect to oxidized iron (Fe_ox = Fe²⁺ + Fe³⁺) or the iron distribution in some specific phases (e.g., Urey–Craig diagram; Urey and Craig 1953), our measurements enable us to assess the fractions of iron in each of its three oxidation states: Fe⁰, Fe²⁺, and Fe³⁺. Among the four carbonaceous chondrites groups studied, a correlation between the iron oxidation index (IOI = [2(Fe²⁺) + 3(Fe³⁺)]/[Fe_TOT]) and the hydrogen content is observed. However, within the CM group, for which a progressive alteration sequence has been defined, a conversion of Fe³⁺ to Fe²⁺ is observed with increasing degree of aqueous alteration. This reduction of iron can be explained by an evolution in the mineralogy of the secondary phases. In the case of the few CM chondrites that experienced some thermal metamorphism, in addition to aqueous alteration, a redox memory of the aqueous alteration is present: a significant fraction of Fe³⁺ is present, together with Fe²⁺ and sometimes Fe⁰. From our data set, the CR chondrites show a wider range of IOI from 1.5 to 2.5. In all considered CR chondrites, the three oxidation states of iron coexist. Even in the least‐altered CR chondrites, the fraction of Fe³⁺ can be high (30% for MET 00426). This observation confirms that oxidized iron has been integrated during formation of fine‐grained amorphous material in the matrix (Le Guillou and Brearley 2014; Le Guillou et al. 2015; Hopp and Vollmer 2018). Last, the IOI of CV chondrites does not reflect the reduced/oxidized classification based on metal and magnetite proportions, but is strongly correlated with petrographic types. The valence of iron in CV chondrites therefore appears to be most closely related to thermal history, rather than aqueous alteration, even if these processes can occur together (Krot et al. 2004; Brearley and Krot 2013).