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The glass phase in volcanic rocks presents a challenge to obtaining compositional data from visible and short‐wave‐infrared (VSWIR) and mid‐infrared (MIR) spectral data of remote surfaces due to its amorphous structure and variable composition. Nonetheless, glass is a common phase in volcanic materials because it forms via the rapid quench of magma and can constitute up to the entirety of a volcanic deposit. Use of partial least squares regression (PLS) to predict glass contents creates models that are insensitive to viewing geometry and sample conditions such as grain size and spectrally inactive compositional variables, enhancing the ability to detect glasses with remote sensing. PLS models are used here to predict crystallinity and oxide composition of samples from VSWIR and MIR spectral data using training spectra from natural volcanic rocks and geologically relevant synthetic samples. Three spectral resolutions of VSWIR and MIR spectra (1, 10, and 100 nm/band, and 1.9, 19, and 190 cm−1/band, respectively) were tested to assess the effects of collection configuration on different spectrometers. PLS models trained on 1 nm and 1.9 cm−1 data sets have the lowest uncertainties of glass modal abundance for VSWIR and MIR, respectively. MIR models predicting sample wt. % SiO2 and FeO, and VSWIR models of wt. % FeO provide accurate estimates (e.g., RMSE‐P of 3.4 wt. % FeO) at all spectral resolutions. Results are based on training data sets skewed to mafic compositions, which affects model accuracies. Plain Language Summary: Glass is difficult to identify and characterize from remote sensing observations of volcanic rocks. Differences in the number of spectral bands between laboratory and currently orbiting instruments further complicate interpretation of remote observations because glass features are subtle. A machine learning regression technique (partial least squares, or PLS) is used to predict the amount of glass and chemical composition using reflectance and emission spectral data of natural volcanic and synthetic glass samples. Results from three spectral resolutions were compared and affirm that spectral data can quantify phase abundance and composition in natural glassy samples such as volcanic materials. Key Points: PLS models can predict the modal glass abundance of volcanic samples with accuracies of 12%–15% from VSWIR and MIR spectraPLS models can predict bulk sample oxide abundances with accuracies of 1.5–6.9 and 0.9–4.2 wt. % from VSWIR and MIR spectra, respectivelyPLS model accuracy is minimally impacted by the number of available spectral bands [ABSTRACT FROM AUTHOR]

Wavelength‐dependent mid‐infrared (400–1,800 cm−1; 5.56–25 μm) optical constants (real and imaginary indices of refraction; n and k) are determined using reflectance spectra at a spectral sampling of 4 cm−1 for several silicate glasses of varying SiO2 wt% which include (a) basaltic volcanic glass from Kīlauea, Hawaii, (b) synthetic andesite, (c) two synthetic dacites, (d) obsidian volcanic glass from Mount Lassen, California, (e) synthetic rhyodacite, and (f) rhyolitic volcanic glass from Mexico. Because glasses are optically isotropic, no specific orientation was required for spectral measurements, and polished glass samples were measured at random orientations using micro‐FTIR spectrometer. Lorentz‐Lorenz dispersion theory and Fresnel reflectance model for high symmetry materials were used to model the optical constants by optimizing oscillator parameters in modeled spectra to match laboratory spectra. A genetic algorithm (GA) approach automatically finds the natural oscillators and their parameters for each spectrum, and then uses these parameters in a non‐linear least squares optimization routine. The study compared spectral parameters such as Christiansen feature, reststrahlen bands (RBs), and the peaks centered around 860–1,100 (peak 1) and 400–480 cm−1 (peak 2) of n and k to their respective SiO2 wt% of the glasses. CF, RBs, peak 1 of n and k, and peak 2 of n shift to higher wavenumbers with increased SiO2 wt%, whereas peak 2 of k shifts to lower wavenumbers with increased SiO2 wt%. Derived optical constants of these glasses will improve quantitative abundance mapping of volcanic materials on the surfaces of silicate targets in the Solar System. Plain Language Summary: The effective mapping of minerals and their abundances on a planet's surface from orbit is enabled by understanding the nature of the interaction of light of varying wavelengths with the planetary surface. The reflected/emitted light from the surface directly depends on the optical properties of the materials and their interaction with each other. In this study, we numerically modeled the mid‐infrared optical constants of silicate glasses using a genetic algorithm (GA) approach. The GA allows the model to automatically locate the natural harmonic oscillators and their parameters that are responsible for creating the spectral signals that can be detected by spectrometers on orbital spacecraft. Derived optical constants of the glasses could be used in radiative transfer models for quantitative mineral mapping applications. Glasses on planetary surfaces are common in volcanic and/or impact materials and the mid‐infrared spectral range has been widely used to map the surface mineralogy of planets including the Moon and Mars. Mapping the composition and abundance of these glasses therefore will help us to understand the thermal evolution of the planets themselves. Key Points: MIR optical constants (5.56–25 μm; 400–1,800 cm−1) and their oscillator parameters were modeled for seven silicate glasses of varying SiO2 wt%MicroFTIR reflectance spectra of the glasses studied were measured for the spectral region 5.56–25 μm (400–1,800 cm−1)Genetic algorithm was adopted to automatically find the natural oscillators and their parameters for the glasses studied [ABSTRACT FROM AUTHOR]

We used laboratory analysis to investigate the effect of mineral albedo, as defined at 750 nm, on the midinfrared emissivity spectra of silicates under lunar environment conditions. Optical darkening has long been recognized as an effect of space weathering on the visible‐to‐near‐infrared spectra of the Moon. However, space weathering has not been as thoroughly investigated in the mid‐infrared. Because mid‐infrared spectra are strongly influenced by the anisothermality of the lunar surface environment, it is likely that any darkening effects of space weathering would also change the thermal gradient in heavily space weathered lunar regolith. To isolate this variable, we added nanophase carbon to particulate samples of forsterite, augite, and anorthite to achieve a range of albedo samples and measured their midinfrared spectra under lunar environment conditions within the Planetary and Asteroid Regolith Spectroscopy Environment Chamber at Stony Brook University. We observe a shift in the Christiansen Feature maximum to longer wavelengths and decreasing spectral contrast with decreasing albedo. These shifts are well correlated with the observation of space weathering effects on the Diviner Lunar Radiometer Experiment compositional data, and point to the need for further investigation into the effects of space weathering on the midinfrared spectra of airless bodies. Plain Language Summary: Space weathering is a major process on the Moon that matures lunar soils over time. In this work, we try to isolate one of the effects of space weathering, darkening, to determine its effects on our interpretation of midinfrared emissivity spectra. We do so by adding nano‐scale carbon particles to three silicate minerals and examine how the spectral features change as the sample gets darker under both ambient and simulated lunar environment conditions. We observe a decrease in overall spectral contrast and a shift in the Christiansen Feature (CF) position to longer wavelengths with increasing carbon content. This correlation between the darkening sample and CF value is similarly observed in the remote sensing data from Diviner Lunar Radiometer Experiment and Kaguya Multiband Imager instruments, demonstrating that it is an important factor to consider to accurately interpret composition from mid‐infrared data of space weathered soils on airless bodies. Key Points: Space weathering affects midinfrared data from the Moon largely by altering regolith albedoAlbedo affects the position of the Christiansen feature (CF) of minerals in the midinfrared under ambient and simulated lunar environment conditionsCorrelation between albedo and CF is shown in remote sensing data and laboratory experiments [ABSTRACT FROM AUTHOR]

Bennu, the target of the OSIRIS‐REx mission, is an asteroid with compositions analogous to low petrologic type CI, CM, CR, and/or ungrouped carbonaceous chondrites. Asteroids like Bennu provide information about the building blocks of the early Solar System. Analysis of the mid‐infrared remote sensing data informs mineral quantification. We apply a phyllosilicate specific model, developed by Breitenfeld et al. (2021, https://doi.org/10.1029/2021je007035) that distinguishes between Mg and Fe serpentines, to Baseball Diamond 1 (BBD1), equatorial station 3 (EQ3), and touch‐and‐go OSIRIS‐REx thermal emission spectrometer data. The average total phyllosilicate predictions are 73 (BBD1) and 72 vol% (EQ3). We observe higher Fe‐cronstedtite and lower Mg‐rich serpentine content in the equatorial region of Bennu than average. Mid‐infrared spectral variability may be explained by sorting effects through mass movement. Plain Language Summary: Bennu is an asteroid made of primitive materials that provide information about the early Solar System. Using infrared emission spectra and machine learning modeling, we estimate the amount and composition of phyllosilicate minerals across the surface of Bennu. Our results suggest that Bennu is primarily composed of phyllosilicate minerals as expected from previous studies. Mg‐ and Fe‐rich serpentine abundances are mapped across the asteroid Bennu. Near the equator, our model predicts higher cronstedtite (Fe‐rich serpentine) and lower Mg‐rich serpentine compared to the average composition of the asteroid. The spectral and compositional variability on Bennu can be explained by aqueous processing and sorting effects. Key Points: We estimate Bennu's phyllosilicate content is primarily Mg‐rich serpentine with smaller amounts of cronstedtite and saponiteWe predict higher cronstedtite and lower Mg‐rich serpentine abundances in the equatorial region and vice versa for the middle latitudesWe observe thermal emission spectrometer spectral variability between the equatorial and middle latitude regions of Bennu that may be due to sorting effects [ABSTRACT FROM AUTHOR]

Clay minerals detected with orbital and in situ instruments in ancient Martian terrains constrain Mars' climate and aqueous alteration history. Early in its history, Mars experienced an atmospheric redox change and iron‐bearing clay minerals may preserve the effects of that transition. Ferrous smectites, the thermodynamically predicted product of chemical weathering of basalts under anoxic conditions, may have undergone oxidation by exposure to chemical oxidants in the atmosphere or regolith, or by direct photooxidation at the surface. To assess these potential oxidation pathways, ferrous trioctahedral smectites of varying initial iron content were synthesized and subjected to oxidation by ultraviolet (UV) irradiation. Experimental UV irradiation under an anoxic atmosphere equivalent to approximately 7 years of flux on the Martian surface caused partial oxidation of smectite‐bound Fe (Fe3+/ΣFe = 16–18%) and octahedral sheet contraction. Metal‐OH vibrational bands in visible/near infrared (VNIR) reflectance spectra of oxidized smectites changed in band depth and asymmetry with higher iron content. X‐ray diffraction patterns of UV irradiated samples indicate the formation of a mixed di‐ and trioctahedral smectite or a secondary nontronite phase, possibly on the surfaces of higher iron content smectites. These experiments suggest that UV irradiation is able to oxidize structurally bound iron in smectites without the presence of other chemical oxidants. Photooxidation may have influenced the mineralogy, both syndepositionally and postdepositionally, of Martian alteration assemblages formed near the surface and this process needs not be limited to one part of their formation history. Plain Language Summary: Martian mineral assemblages observed with orbiters and rover instruments allow us to understand the planet's past climate and aqueous activity. Hydrated minerals that contain Fe(II) are of particular interest as they would have been affected by the early atmosphere becoming more oxidizing overtime, changing the structural iron into Fe(III). In this study, we synthesized Fe(II) smectites with varying iron content and irradiated them with an ultraviolet (UV) light source to test whether they could be photooxidized. Iron in the smectite minerals was incompletely oxidized by UV radiation after being exposed for the equivalent of approximately 7 years of exposure on the early Mars surface. The smectites with higher iron content showed more changes with photooxidation in their visible/near infrared spectra as well as in their X‐ray diffraction patterns, two datasets collected by instruments on currently active Martian missions. These results suggest that UV photooxidation is a plausible contributor to the mineralogy and redox state of clay mineral assemblages observed at the Martian surface. Key Points: Ultraviolet (UV) irradiation of initially ferrous smectites of varying iron content partially oxidized structural iron (Fe3+/ΣFe = 16–18%)Smectites with greater iron had more observable changes in their visible/near infrared spectra regardless of the similar extent of oxidationUV photooxidation is proposed as a viable oxidation pathway for surficial Martian smectites [ABSTRACT FROM AUTHOR]

Mid‐infrared (MIR) spectroscopy has been used with great success to quantitatively determine the mineralogy of geologic samples. It has been employed in a variety of contexts from determining bulk composition of powdered samples to spectroscopic imaging of rock thin sections via micro‐Fourier transform infrared (micro‐FTIR) imaging spectroscopy. Recent advances allow for IR measurements at the nanoscale. Near field nanoscale infrared imaging and spectroscopy with a broadband source (nano‐FTIR) enable understanding of the spatial relationships between compositionally distinct materials within a sample. This will be of particular use when analyzing returned samples from Bennu and Ryugu, which are thought to be compositionally like CI or CM1/2 carbonaceous chondrites. Returned samples will likely contain olivine/pyroxene chondrules that have been transformed into hydrous phyllosilicates, sulfides, carbonates, and other alteration phases. The use of near‐field infrared techniques to probe the boundaries between once pristine chondrules and alteration phases at the nanoscale is a novel approach to furthering our understanding of the compositional evolution of carbonaceous asteroids and the processes that drive their evolution. Here we report the results of nano‐FTIR spectroscopy and imaging measurements performed on the carbonaceous chondrite Allan Hills (ALH) 83100 (CM1/2). We show with nanoscale resolution that spatially resolved Fe‐Mg variations exist within the phyllosilicates around a chondrule rim. We also present effects of crystal orientation on the nano‐FTIR spectra to account for the spectral differences between the meteorite and mineral spectra. Plain Language Summary: National Aeronautics and Space Administration's OSIRIS‐REx mission will return a sample of near‐Earth asteroid Bennu to Earth in September 2023. Based on remote sensing analyses of the asteroid, it is expected that the sample will be similar to CI or CM chondrite meteorites. Key remote sensing analyses of Bennu included thermal infrared emission spectroscopy. In this work, we conduct infrared spectral analyses of Allan Hills (ALH) 83100, a CM1/2 chondrite that shares spectral similarities to Bennu. We use synchrotron near‐field infrared nano‐infrared (nano‐IR) measurements to analyze the sample at scales of about 30 nm/pixel. This allows us to capture the spectra of very fine‐grained components in meteorites that contribute to their overall bulk spectra measured from remote sensing platforms. While nano‐IR spectra do not directly translate to remote‐sensing infrared measurements, identification of mineral and organic components in analog extraterrestrial materials provides key constraints on models of Bennu's composition from orbital infrared spectroscopy. We document the chemical alteration of a chondrule and show strong alteration gradients in the sample over length scales smaller than 10 microns. The variability in Fe and Mg in the phyllosilicate‐rich matrix of the meteorite over small spatial scales may indicate aqueous alteration in the ALH 83100 parent body as well as in the solar nebula. Key Points: Nano‐Fourier transform infrared (nano‐FTIR) spectroscopy provides extremely fine‐scale (∼20 nm) spatially resolved mineralogic compositional informationPhyllosilicate spectra in Allan Hills (ALH) 83100 show Fe‐Mg variations around a chondrule rimEffects of crystallographic orientation on the nano‐FTIR spectra are shown [ABSTRACT FROM AUTHOR]

Planetary surfaces can be complex mixtures of coarse and fine particles that exhibit linear and nonlinear mixing behaviors at mid‐infrared (MIR) wavelengths. Machine learning multivariate analysis can estimate modal mineralogy of mixtures and is favorable because it does not assume linear mixing across wavelengths. We used partial least squares (PLS) and least absolute shrinkage and selection operator (lasso), two types of machine learning, to build MIR spectral models to determine the surface mineralogy of the asteroid (101955) Bennu using OSIRIS‐REx Thermal Emission Spectrometer (OTES) data. We find that PLS models outperform lasso models. The cross‐validated root‐mean‐square error of our final PLS models (consisting of 317 unique spectra of samples derived from 13 analog mineral samples and eight meteorites) range from ∼4 to 13 vol% depending on the mineral group. PLS predictions in vol% of Bennu's average global composition are 78% phyllosilicate, 9% olivine, 11% carbonates, and 6% magnetite. Pyroxene is not predicted for the global average spectrum, though it has been detected in small amounts on Bennu. These mineral abundances confirm previous findings that the composition of Bennu is consistent with CI/CM chondrites with high degrees of aqueous alteration. The predicted mineralogy of two previously identified OTES spectral types vary minimally from the global average. In agreement with previous work, we interpret OTES spectral differences as primarily caused by relative abundances of fine particulates rather than major compositional variations. Plain Language Summary: The OTES instrument onboard the OSIRIS‐REx spacecraft collects infrared emission spectra that can, in principle, be used to determine the mineralogy of Bennu, the target asteroid of the OSIRIS‐REx mission. However, predicting mineral abundances on remote planetary bodies from infrared spectra is particularly complex when there are fine particles (<∼100 μm) on the surface. To circumvent this problem, we created a training set of mineral mixture spectra acquired under asteroid (vacuum) conditions and used machine learning to create models for mineral abundance predictions on asteroids like Bennu. Our results support previous findings that Bennu has a composition consistent with carbonaceous chondrites, the most primitive meteorites. Key Points: Machine learning models were constructed to predict phyllosilicate, olivine, carbonate, pyroxene, and magnetite abundances using mid‐infrared spectraMineral abundance predictions of Bennu indicate the composition is consistent with CI/CM chondrites with high degrees of aqueous alterationThe predicted mineralogy of two previously identified OSIRIS‐REx Thermal Emission Spectrometer spectral types vary minimally from the global average [ABSTRACT FROM AUTHOR]

The Didim meteorite contains multiple lithologies and clasts of different petrologic types in a single stone. A mixture of H5 clasts in an unequilibrated H3 host was previously observed in Didim, according to the initial characterization reported in the Meteoritical Bulletin Database, providing an opportunity to investigate molecular composition that contains varying degree of equilibrium with varying mineralogy. We have taken a "from large scale to small scale" approach to spectroscopically investigate the chemical content of Didim. Centimeter‐scale biconical reflectance spectra show that Didim contains abundant olivine, pyroxene, and other optically active minerals, evident from a strong Band I near 0.93 µm and a weak Band II near 1.75 µm. Micrometer‐scale Raman spectroscopic investigations reveal the presence of carbonaceous material in addition to forsteritic olivine, pyroxene (augite and enstatite), feldspars, and opaque phases such as chromite and hematite. 3‐D Raman tomographic imaging shows that the carbonaceous material near chondrules extends underneath a large olivine grain, going further down toward the interior, indicating that the observed carbonaceous matter is likely indigenous. Nano‐scale infrared measurements reveal that the observed chemical materials in Didim contain spectral, and therefore, molecular, variations at the ~20 nm spatial scale. These chemical variations are normally not accessible via conventional infrared techniques, and indicate the presence of different cations in the molecular composition of observed minerals. By taking the "large scale to small scale" approach, we show that these compositional variations can be captured and investigated nondestructively in meteorites to understand formation/evolution of chemical components in the parent body. [ABSTRACT FROM AUTHOR]

Mid‐infrared spectroscopic analysis of the Moon and other airless bodies requires a full accounting of spectral variation due to the unique thermal environment in airless body regoliths and the substantial differences between spectra acquired under airless body conditions and those measured in an ambient environment on Earth. Because there exists a thermal gradient within the upper hundreds of microns of lunar regolith, the data acquired by the Diviner Lunar Radiometer Experiment are not isothermal with wavelength. While this complication has been previously identified, its effect on other known variables that contribute to spectral variation, such as particle size and porosity, have yet to be well characterized in the laboratory. Here we examine the effect of particle size on mid‐infrared spectra of silicates common to the Moon measured within a simulated lunar environment chamber. Under simulated lunar conditions, decreasing particle size is shown to enhance the spectral contrast of the Reststrahlen bands and transparency features, as well as shift the location of the Christiansen feature to longer wavelengths. This study shows that these variations are detectable at Diviner spectral resolution and emphasizes the need for simulated environment laboratory data sets, as well as hyperspectral mid‐infrared instruments on future missions to airless bodies. Plain Language Summary: When trying to determine the composition of a planetary surface, it is important to have a basis for comparison. Currently, infrared data acquired from missions to airless bodies, like the Moon and asteroids, are mostly compared to data measured under ambient terrestrial conditions, and the difference in measurement environment complicates analysis. In this work, we measure minerals of varying particle size in the laboratory under a simulated lunar environment to understand how this variable affects the data, and whether we can detect the variations with the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment. Acquiring infrared data under simulated lunar environment conditions will improve our interpretation of data not only from the Moon but also from other airless planetary bodies such as Mercury and asteroids. Key Points: Particle size is an important variable affecting the position of the Christiansen feature under a simulated lunar environmentParticle size variation is within the detectable limits of the Diviner Lunar Radiometer ExperimentFuture mid‐infrared missions will benefit from a simulated lunar environment spectral library [ABSTRACT FROM AUTHOR]

McLaughlin crater is a 92‐km diameter complex crater that formed on Mars ~4 billion years ago. The resulting basin was the site of a large (~3,000 km2), deep (~500 m), voluminous (~1,500 km3) Martian lake circa 3.8 Ga. While there is strong evidence that hundreds of lakes have existed on Mars at some point during the same time period, the geology of McLaughlin crater is extraordinary for a number of reasons. Detailed spectral analyses show that the deep‐water sediments include detrital inputs of olivine and pyroxene, but the lake‐floor sediments include lithologies with abundant Fe‐rich, Mg‐bearing smectite, serpentine‐rich deposits, and ferrihydrite. For astrobiologists, this site provides a treasure trove of high‐priority targets. Serpentinization reactions are thought to have played a key role in abiogenesis on Earth, and within McLaughlin crater, deposits of subterranean and probably sublacustrine serpentinites are well preserved. In addition, delta sequences are well exposed throughout the east side of the basin; such deposits are endorsed by some as the highest priority targets for preservation of organics on Mars. Yet deep‐water turbidites, which might have flowed through hydrothermal environments, may be the most intriguing aspect of this geology. Such rapid sedimentation could have sequestered and preserved any potential organic materials for future exploration by a rover. Plain Language Summary: Though Mars is cold, dry, and inhospitable today, it contains evidence for many dry lake beds—relics of an ancient climate that was at least episodically warmer and wetter. Considering that lakes can be an excellent environments for life on Earth, a common thread of Mars research has been to characterize the environmental conditions under which Martian lakes formed in order to better understand their implications for past habitability of the red planet. The goal of this work is to characterize the geology of McLaughlin crater, where a lake existed on Mars over 3.8 billion years ago. McLaughlin lake was vast, deep (~ 500 m), and probably long lived. Many characteristics of McLaughlin lake are unlike those seen in any other ancient lake basins on Mars. This lake contained a range of sedimentary environments including delta deposits, shallow‐water fan deposits, and deep‐water, fine‐grained materials. It is the only known place on Mars where fine‐grained, subaqueous "landslides" called turbidites are identified, and these are important because we know that such deposits can be sites of excellent preservation of organic matter on Earth (due to rapid burial). Key Points: McLaughlin crater contained a deep, voluminous lake >3.8 Ga with features unlike any other known Martian paleolakesSedimentary units include delta deposits, turbidites, and deep‐water sediments; and hydrothermal deposits include serpentinitesGeochemical gradients occurred in a long‐lived lake that was in sustained communication with groundwater and subsurface environments [ABSTRACT FROM AUTHOR]

Field portable instrumentation, such as in situ geochemical analyzers or broader field of view instruments like multispectral imagers or other imaging capabilities, has the potential to dramatically increase the science return of a planetary surface exploration mission. However, more work is needed to determine how emerging portable technologies should be designed and implemented into evolving mission architectures. This work summarizes the efforts of the RIS4E (Remote, In Situ and Synchrotron Studies for Science and Exploration) SSERVI (Solar System Exploration Research Virtual Institute) team in investigating how field portable instruments should be including into planning for future exploration EVAs (extravehicular activities). EVA crews of geologists and astronauts tested a variety of portable and handheld technologies at both the December 1974 lava flow, Kilauea Volcano, Hawai'i, and Kilbourne Hole, New Mexico, both of which are planetary analog sites. The timeline data gathered during instrument deployment were then mapped onto EVA timelines used in large‐scale NASA planetary surface exploration analog missions. Results and recommendations for future instrument hardware and software development are discussed, as is the operational framework necessary for incorporating in situ analytical capabilities into future planetary surface exploration. Key Points: Advancements in field portable technologies have the capability to increase the science return from crewed planetary surface missionsSimulated extravehicular activities were conducted in planetary analog sites to investigate the utility of portable instrumentsMore integrated testing is needed to evaluate varying modes of integrating field portable instruments beyond a handheld capacity [ABSTRACT FROM AUTHOR]

Introduction: Raman spectroscopy is highly sensitive to molecular functional groups including carbon-rich structures such as aromatic moieties in meteorites. Raman spectroscopic investigation of meteoritic carbon is particularly useful as thermal metamorphic grade/history of carbonaceous chondrites can be roughly estimated through the spectral properties of polyaromatic structures in meteorites. For instance, mathematical expressions that allow estimating the peak metamorphic temperature of chondrites (mostly high-temperature chondrites) using the Raman spectral parameters of the first-order carbon peaks were previously reported [1, 2]. Recently, an expression based on the full-width half-maxima of the G band was reported and it seems to be particularly appropriate for lowtemperature carbonaceous chondrites [3]. In this work, we are collecting hyperspectral Raman images and spectra for various low temperature carbonaceous chondrites such as CM2 and C2 ungrouped (C2-ung) chondrites in an effort to estimate and compare the peak thermal metamorphic temperatures experienced by these meteorites. Samples: Our sample set for this investigation contains CM2 chondrites (Aguas Zarcas, Jbilet Winselwan, Murchison, Asuka (A) 12437, Asuka 12236, Asuka 12408, Asuka 12248, Yamato 980039, Northwest Africa 13711) and C2-ung (Tagish Lake, Tarda, and Queen Alexandra Range (QUE) 99038) chondrites. All meteorites were prepared in the form of polished sections with large sample surface areas for measurements. Technique: Raman spectroscopic experiments were conducted using a commercial confocal Raman microspectroscopy system (WiTec alpha300R) equipped with a 600 g/mm grating, 532 nm Nd:YAG laser, and a 50× objective (NA = 0.8). The spectrograph was calibrated using a silicon wafer substrate prior to measurements. Individual Raman spectra were acquired with a 1-s integration time for 60 accumulations within 800-2000 cm-1, while twodimensional maps were acquired with 0.1 s integration time. The power density of the laser beam was ~0.2-0.5 mW/µm2 on the sample surface. The collected spectra were processed using the commercial ProjectPLUSv4 (WiTec GmbH) software package. Artificial fluorescence background was removed from the spectra, which were then fitted with Lorentzian-Breit-Wigner-Fano (LBWF) spectral model to extract the D and G band parameters of aromatic hydrocarbons in the meteorites (e.g., Ferrari and Robertson, 2000; Chan et al. 2020). This method has previously used to successfully obtain the PMT values for different meteorites [e.g., 6]. Results: Raman spectra of all meteorites present well-developed D and G carbon bands at ~1370 and 1581 cm-1. These carbon bands are due to disordered sp3 and graphitic-like sp2 carbon bonds, respectively [7, 8]. The fullwidth half-maxima (FWHM) values for the G band of Aguas Zarcas, Jbilet Winselwan, A 12236, Tarda, Tagish Lake, QUE 99038 were found to be, respectively, 97 cm-1, 93 cm-1, 84 cm-1, 94 cm-1, 90 cm-1, and 60 cm-1. Using the expression by [3], we found that these values roughly correspond to ~71 °C, 79 °C, 101 °C, 77 °C, 86 °C, and 195 °C. For Jbilet Winselwan, we note that we did not include the shock-heated parts into consideration for this work, which would yield much higher temperatures. Based on these results, A 12236 seems to be slightly more heated compared to other currently studied CM2 chondrites. Tarda and Tagish Lake appears to be compatible with being type 2; however, QUE 99038 presents a much higher peak metamorphic temperature. The rest of the meteorites are currently being measured as well, and their results will be presented at the meeting. Acknowledgements: We thank NIPR and NASA-JSC for providing some of the meteorite samples used in this study. MY acknowledges support from TUBITAK (grants #119N207 and #120Y115). [ABSTRACT FROM AUTHOR]

Introduction: Establishing links between the plethora of meteorite samples in collections on Earth and the parent body asteroids they originated from is a valuable outcome of past (NEAR-Shoemaker, Hayabusa, Dawn), present (Hayabusa2, OSIRIS-REx) and future (Lucy, Psyche) space missions [1]. Sample return missions (Hayabusa, Hayabusa2, OSIRIS-REx) provide much needed context for the study of relevant extraterrestrial samples, and by utilizing these specimens of asteroids in the laboratory, we can supply detailed chemical, mineralogical and petrological information for reference libraries of materials relevant to asteroid missions [2,3]. Laboratory-based spectral analysis of well-characterized meteorite samples can be applied to more quantitatively analyze asteroid remote sensing data in conjunction with returned samples. Previous work has evaluated the spectral features of ordinary chondrite material [2], and revealed the individual effects of temperature and grain size on the visible and near-infrared (VNIR) spectra of silicate [4,5] and meteorite [6] powders. In this work, we examine the combined effects of physical variables (temperature, particle size) and chemical variables (petrologic type, metal fraction) on VNIR spectra of ordinary chondrite meteorite powders. Sample Selection: Six ordinary chondrite falls spanning the full range of equilibrated petrologic types were sourced from the American Museum of Natural History (AMNH) meteorite collection: Soko-Banja (LL4), Mangwendi (LL6), Mount Tazerzait (L5), Suizhou (L6), Jilin (H5), and Zhovtnevyi (H6). Methods: Bulk chondrite samples (~2-6 cm3 volumes) were first scanned using AMNH's computed tomography scanner, with resolutions ranging from 6-11 microns/voxel, to characterize their 3D structure and determine the abundance of optically opaque (metal and sulfide) phases. These samples were subsequently mapped for the X-ray intensities of ten major and minor elements at 4 microns/pixel resolution, over a sufficient area (~1 cm2) to characterize mineralogy in a 2D slice [7]. Element maps were linearly combined to determine the mineralogy of each individual pixel in every map, enabling quantitative determination of the relative mineral abundances in each meteorite [8]. Samples were then powdered and sieved to four different size fractions (25-63 µm, 63-90 µm, 90-125 µm, 125-250 µm) to capture the spectral diversity associated with asteroid regoliths dominated by various grain sizes. Each individual size fraction for each meteorite was characterized via X-ray diffraction (XRD) to assess the distribution of metal grains across the size fractions, in case of preferential size sorting of metal grains during sieving. VNIR spectra of the ordinary chondrite material were then measured under simulated asteroid surface conditions (defined as ~10-6 mbar, -100°C chamber temperature, and low intensity illumination) at a series of temperatures chosen to mimic near-Earth asteroid surfaces. These reflectance spectra were collected in increments of 10°C, over the range 10°C to 100°C. Results: The VNIR spectra show minimal variation in both major absorption bands across the simulated near-Earth asteroid temperature regime. Spectral changes due to particle size are consistent across samples, with the smallest and largest grain sizes having the highest reflectance. Previous spectral investigations of ordinary chondrites [3] removed the metal fraction from the powdered meteorite sample. Notably, we retained the metal fraction in our powders instead of analyzing the silicate fraction only. In our measurements, we observe distinct offsets in spectral features when compared to analyses of purely silicate fractions. XRD analysis shows that the largest size fraction of nearly every sample contains relatively more metal, likely due to the retention of metal nuggets in the largest size fraction during sieving. The more petrologically pristine samples (e.g., LL4) from each ordinary chondrite group display relatively shallower band depths than their more petrologically altered counterparts (e.g., LL6). The band depths shift to higher wavelengths as temperature, grain size, and petrologic type increase. Spectral studies of meteorites combined with detailed petrologic analysis of the samples greatly enhance interpretation of current and future planetary remote sensing data sets. Importantly, understanding the spectral contribution of the metal fraction will aid in investigations of metal-rich mission targets such as asteroid 16 Psyche. This work continues an effort to develop a comprehensive spectral library of materials in conditions relevant to airless bodies and contemporaneous asteroid missions. [ABSTRACT FROM AUTHOR]