The OSIRIS-REx mission acquired spatially resolved (2-10 m spot sizes) visible-near infrared (VNIR) and thermal infrared (TIR) spectra across four candidate sampling sites on asteroid (101955) Bennu: Nightingale, Osprey, Sandpiper, and Kingfisher. To quantify heterogeneity across a small body (about 500 m radius) like Bennu, we explore remotely observed spectral data to draw conclusions about the mineralogical composition and key physical processes that drive surface variability. We derive diagnostic band parameters from the OSIRIS-REx Visible and Infrared Spectrometer and the OSIRIS-REx Thermal Emission Spectrometer datasets to quantify compositional and physical variability across sites and assess their mineralogical context. The VNIR spectra exhibit similar overall reflectance shapes but systematic differences in spectral slopes and the 2.74 micron OH absorption. TIR emissivity spectra reveal modest but statistically significant shifts in the Christiansen Feature, silicate stretching, and bending band positions, indicating differences in silicate composition, hydration state, and Mg/Fe relative abundance. Principal component analysis separates each site into distinct clusters in multivariate band-parameter space, whereas K-means clustering identifies intra-site spectral sub-populations. Welch's Analysis of Variance and Hotelling's tests confirm that band-parameter variations between sites are significant. These results reveal that Bennu's surface preserves measurable spectral heterogeneity at 2-10 m scales, with site-to-site variations in hydration indicators and silicate band positions. The spectral properties of Nightingale encompass the full range observed across all four sites, establishing a remote sensing baseline for contextualizing laboratory analyses of the returned sample within Bennu's broader composition diversity and alteration history.

Near-Earth rubble-pile asteroids Bennu and Ryugu are part of the carbonaceous taxonomic complex (C-complex), and samples returned from both bodies resemble the most aqueously altered carbonaceous chondrites. However, telescopic and spacecraft visible-near infrared (VIS-NIR) reflectance spectra of Ryugu exhibit a red (positive) spectral slope, whereas Bennu has a blue (negative) spectral slope characteristic of the rare B-type subclass of asteroids. The asteroid spectra also suggest different levels of hydration, with Ryugu dominated by OH and Bennu containing spectral evidence of more H2O. To understand what causes these differences, we acquired VIS-NIR reflectance data (similar to 0.3-5 mu m) from a variety of Bennu samples over spatial scales of 100 mu m to several millimeters. No single sample reproduces the average spectral properties of Bennu, but by evaluating samples of different petrology and physical states-groups of particles, isolated particles, and larger stones-we demonstrate that primary composition, and highly hydrated Mg-rich phosphate in particular, plays a strong role in controlling the spectral slope and average hydration absorption strength of Bennu materials. Bennu and Ryugu may be dominated by different lithologies originating from different regions of a common planetesimal, thus explaining their different spectral evolution. The spectral characteristics of B-type asteroids, particularly those with blue slopes at near-infrared wavelengths and broad hydration features at similar to 3 mu m, may indicate the presence of Mg phosphate and thus a history of complex fluid-rock interactions relevant to prebiotic chemistry.

The main belt asteroid (52246) Donaldjohanson (DJ) is a likely member of the Erigone asteroid family. This implies that DJ is a fragment of a larger parent body that was destroyed in a collision about 155 million years ago. We report observations taken during a flyby of DJ by the Lucy spacecraft. We found that DJ is composed of two heavily cratered lobes, connected by a smoother neck, with overall dimensions 8.8 kilometers (km) by 4.4 km by 3.1 km. The crater density is consistent with the Erigone family's age, except for craters <0.4 km, which have been preferentially erased. DJ rotates slowly in a tumbling state, likely owing to spin-down by radiative forces. Surface spectra show iron-bearing phyllosilicates, indicating moderate aqueous evolution on the parent body.

A key early discovery of the Mars Exploration Rover Opportunity on Meridiani Planum was the hematite spherules that are a ubiquitous component of the Burns formation sandstones at the rover's landing site (colloquially known as "blueberries"). The Meridiani spherules possess a suite of characteristics that are collectively very rare in terrestrial settings, including their gray color, a thermal spectral signature that indicates preferential exposure of the c crystal axis, a spherical shape that is evidently attributable to radially oriented crystallite growth, and high chemical and mineralogical purity. The origin of the Meridiani "blueberries" has remained a matter of considerable debate, but one leading hypothesis is that they formed through the decomposition of iron-rich sulfate minerals from the alunite group, specifically jarosite. To date, however, there has been no described terrestrial analog where the formation of hematite spherules is shown to be directly linked to jarosite decomposition. Here, we report the discovery of hematite spherules in Aztec Sandstone that possess many of the same characteristics as the martian "blueberries," albeit with substantially smaller size. The spherules occur primarily in narrow gray bands within mineralized fractures where the pore spaces are predominantly occupied by jarosite-alunite solid solutions (JASS). The spherules formed through partial decomposition and release of Fe3+ from adjacent JASS, supporting the possibility that analogous processes may have been responsible for the formation of hematite spherules during diagenesis of the sulfate-rich Burns sandstones on Mars. Continued study of the Aztec Sandstone spherules may provide new constraints on near-surface environmental conditions on early Mars.

Xenoliths of carbonaceous chondrites (CC) in meteoritic breccias can provide samples of primitive solar system materials that are not represented by individual meteorites and thus expand our knowledge of chemical and isotopic reservoirs in the early solar system and early geologic processes on CC parent bodies. The Almahata Sitta (AhS) polymict ureilite contains one such xenolith, referred to here as AhS 202. Hamilton et al. (2020a) discovered that, unlike any other known CC, the AhS 202 xenolith contains abundant (similar to 12-14 vol%) amphibole, a hydrous mineral that characteristically forms in greenschist to amphibolite facies metamorphism and requires a significantly larger parent body than typically inferred (<= 100 km diameter) for CC meteorite bodies. Building on that initial work, we report additional analyses of the mineralogy and petrology, and new analyses of the chemical composition, oxygen and chromium isotope compositions, and physical properties of this xenolith that further constrain its petrogenesis and provenance. Our results show that the AhS 202 precursor was chondritic and experienced aqueous alteration similar to many low petrologic type CC meteorites at temperatures of similar to 30-100 degrees C and fluid pressures of P-H2O < 0.1 kbar, leading to formation of serpentines, magnetite, and chlorite. However, unlike any known CC meteorite, AhS 202 was heated further under water-saturated conditions similar to prograde metamorphism of terrestrial serpentinites, leading to formation of chemically pure diopside, secondary olivine, and tremolite amphibole. Peak metamorphic conditions determined from thermodynamic modeling, constrained by olivine-magnetite oxygen isotope thermometry, were similar to 380-430 degrees C and similar to 0.5-2.25 kbar. Based on our measured density of 2.27 g/cc for AhS 202, these conditions imply parent body sizes of 600-1875 km diameter, confirming the previous estimate (640-1800 km) of Hamilton et al. (2020a). The fluid-assisted metamorphic conditions experienced by AhS 202 cannot be represented in current classification systems of meteorite petrologic type, which recognize only anhydrous metamorphism; we discuss an alternative approach to the classification of such materials. Oxygen and chromium isotope compositions show an affinity between AhS 202 and CR chondrites and/or CR-related achondrites, suggesting derivation from a common reservoir. However, petrology, refractory element composition, and extremely low carbon content indicate that it did not form on the same parent body as known CR chondrites or CR-related achondrites. The existence of this sample, in combination with several even higher-pressure clasts observed in CR chondrites (Kimura et al., 2013; Hiyagon et al., 2016), suggests that this reservoir contained multiple large planetesimals.

Mars Sample Return (MSR) has been the highest flagship mission priority in the last two Planetary Decadal Surveys of the National Academies of Science, Engineering, and Medicine (hereafter, “the National Academies”) and was the highest priority flagship for Mars in the Decadal Survey that preceded them. This inspirational and challenging campaign, like the Apollo program’s returned lunar samples, will potentially revolutionize our understanding of Mars and help inform how other planets are explored. MSR’s technological advances will keep the NASA and European Space Agency at the forefront of planetary exploration, and data on returned samples will fill knowledge gaps for future human exploration. Investigations of the ancient rocks collected in and around Jezero crater, as well as samples of the regolith and atmosphere, will be fundamentally different in scope, depth, and certainty from what is achievable with spaceborne observations. Returned Mars samples can address critical science issues including the discovery and characterization of ancient extraterrestrial life, prebiotic organic chemistry, the history of habitable planetary environments, planetary geological, geochemical, and geophysical evolution, orbital dynamics of bodies in the early Solar System, and the formation and evolution of atmospheres.

Organic matter in meteorites reveals clues about early Solar System chemistry and the origin of molecules important to life, but terrestrial exposure complicates interpretation. Samples returned from the B-type asteroid Bennu by the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer mission enabled us to study pristine carbonaceous astromaterial without uncontrolled exposure to Earth’s biosphere. Here we show that Bennu samples are volatile rich, with more carbon, nitrogen and ammonia than samples from asteroid Ryugu and most meteorites. Nitrogen-15 isotopic enrichments indicate that ammonia and other N-containing soluble molecules formed in a cold molecular cloud or the outer protoplanetary disk. We detected amino acids (including 14 of the 20 used in terrestrial biology), amines, formaldehyde, carboxylic acids, polycyclic aromatic hydrocarbons and N-heterocycles (including all five nucleobases found in DNA and RNA), along with 10,000 N-bearing chemical species. All chiral non-protein amino acids were racemic or nearly so, implying that terrestrial life’s left-handed chirality may not be due to bias in prebiotic molecules delivered by impacts. The relative abundances of amino acids and other soluble organics suggest formation and alteration by low-temperature reactions, possibly in NH3-rich fluids. Bennu’s parent asteroid developed in or accreted ices from a reservoir in the outer Solar System where ammonia ice was stable. Rocks and dust from the asteroid Bennu contain some of the molecular building blocks of life on Earth, such as amino acids and nucleobases. They also carry ammonia that formed billions of years ago in cold, distant regions of our Solar System.

It is now possible to bring back samples from planetary bodies of the Solar System other than the moon. This research method enables a direct link between astronomical observations and meteorite analyses, which were previously disconnected. The Hayabusa, Hayabusa2, and OSIRIS REx sample return missions have provided detailed information on the composition of S-, C-, and B-type asteroids, respectively, and the processes by which they were formed. This paper reviews the results of these three asteroid sample return missions, and also introduces the next Martian moon sample return mission MMX. Although sample returns are currently achieved only from near-Earth objects in the Solar System, it is hoped that in the future it will be possible to collect samples from outer Solar System objects and even from small objects flying from outside the Solar System.

The Lucy Thermal Emission Spectrometer (L’TES) instrument acquired hyperspectral thermal infrared (TIR) observations of the Earth's Moon during Lucy's 2022 Earth gravity assist. L’TES covers the spectral range of 100–1,750 cm −1 (100–5.8 μm) at a spectral sampling of 8.64 cm −1 (Christensen et al., 2023, https://doi.org/10.1007/s11214‐023‐01029‐y ). The field of view (FOV) is 7.3‐mrad, giving a spatial resolution on the Moon of 1,650 km. Seventeen high‐quality spectra of the warm disk were acquired of Oceanus Procellarum that provide the first well‐calibrated TIR observations of the Moon with high spectral resolution. The lunar surface emissivity was determined by modeling the surface radiance using two different methods that gave nearly identical results. The L’TES spectra have Christiansen feature (CF) maxima at 1,226 cm −1 (8.15 μm), a spectral band depth of ∼0.04, and a downward slope at wavenumbers >1,200 cm −1 that is characteristic of <100 μm particles. Comparison with Diviner 3‐point spectral data (Greenhagen et al., 2010, https://doi.org/10.1126/science.1192196 ) shows excellent agreement in the CF location and band shape. The L’TES spectra closely match several lunar soil laboratory spectra (Donaldson‐Hanna et al., 2017, https://doi.org/10.1016/j.icarus.2016.05.034 ), providing excellent ground truth for the L’TES observations, validating the L’TES data processing, and demonstrating that high‐spatial and spectral resolution TIR data would provide a powerful tool for remote compositional mapping. The L’TES nightside observations accurately derived surface temperatures at 110 K, even when the Moon only filled 10% of the FOV, confirming that L’TES will accurately determine the cold Trojan asteroid temperatures.

Evaporation or freezing of water-rich fluids with dilute concentrations of dissolved salts can produce brines, as observed in closed basins on Earth1 and detected by remote sensing on icy bodies in the outer Solar System2,3. The mineralogical evolution of these brines is well understood in regard to terrestrial environments4, but poorly constrained for extraterrestrial systems owing to a lack of direct sampling. Here we report the occurrence of salt minerals in samples of the asteroid (101955) Bennu returned by the OSIRIS-REx mission5. These include sodium-bearing phosphates and sodium-rich carbonates, sulfates, chlorides and fluorides formed during evaporation of a late-stage brine that existed early in the history of Bennu's parent body. Discovery of diverse salts would not be possible without mission sample return and careful curation and storage, because these decompose with prolonged exposure to Earth's atmosphere. Similar brines probably still occur in the interior of icy bodies Ceres and Enceladus, as indicated by spectra or measurement of sodium carbonate on the surface or in plumes2,3.

NASA’s Lucy spacecraft flew by the main-belt asteroid (152830) Dinkinesh on 2023 November 1, providing a test of its instruments and systems prior to its encounters with the Jupiter Trojans and enabling an opportunity for scientific investigation of this asteroid. Analysis of disk-integrated radiance spectra of Dinkinesh collected by the Lucy Thermal Emission Spectrometer (L’TES) instrument during the close approach reveals a thermal inertia for Dinkinesh of 91 ± 24 J m −2 K −1 s −1/2 and a surface roughness of 35° ± 7° rms slope. These values for the thermal inertia and surface roughness are comparable to values derived for other small S-type asteroids such as (65803) Didymos. The Dinkinesh flyby also provided the opportunity to develop new techniques for extracting data when the target body does not fill the field of view of the L’TES instrument, which proved challenging for predecessors of this instrument such as OTES on OSIRIS-REx. The grain size of the regolith of Dinkinesh, estimated to be r = 1 . 2 − 0.6 + 0.9 mm, is below expected trends with size but is comparable to that of similarly sized asteroids that are either binaries or may have undergone rotational fission in the past. These findings imply that fine-grained materials are being preferentially retained on the primaries of multiple systems, either by cohesive forces or by redeposition after impact events on the secondaries.

The OSIRIS‐REx mission returned a sample of regolith from the carbonaceous asteroid Bennu in September 2023. We present preliminary in situ investigations of the petrology and petrography of selected particles ranging in size from 0.5 to 3 mm. Using a combination of optical and electron beam techniques, we investigate whole specimens and polished sections belonging to morphologically and visually distinct categories of particles. We find that morphological differences in the particles are reflective of petrographic and petrologic differences, leading to the conclusion that we have at least two distinct major lithologies in the bulk sample. Our findings support predictions from remote sensing, suggesting that the morphological differences observed in the boulder population of Bennu correspond to petrologic differences. Our data provide insight into the geologic activity on Bennu's parent body and the petrographic framework needed to contextualize the detailed analyses of this pristine asteroidal material.

. IntroductionWe present visible to near infrared (VNIR) and thermal infrared (TIR) spectral data for asteroid (101955) Bennu collected by the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) [1, 2] and the OSIRIS-REx Thermal Emission Spectrometer (OTES) [3]. The data discussed here were collected during the 12:30 pm Equatorial Station of the Detailed Survey mission phase and Reconnaissance A (varying local times). Constraints applied to the selection of data are described by [4-6].2. OVIRS resultsEarly results [7] revealed that an unambiguous “3-µm” band is present, consistent with the presence of hydrated (phyllo-)silicates. The specific position of this band in OVIRS spectra, 2.74 µm ± 0.01, is consistent with the positions observed in low petrologic subtype CM2 meteorites [8]. The global distribution of this feature is described by [9].Since the acquisition of global mapping data at ~20-30 m/spot, we have identified a complex of features in the 3.2–3.6 µm region that we attribute to the presence of C-bearing compounds (organics and carbonate minerals) [4, 5]. Absorption band positions, widths, and relative strengths appear to be associated with a variable mixture of organics and multiple carbonate minerals. The varying shape and depth of a 3.4-µm absorption feature across Bennu’s surface spans the range seen among disk-averaged spectra of main-belt carbonaceous asteroids. Bennu’s distribution of carbon-bearing materials does not correlate with the distribution of hydrated minerals, surface brightness, or geologic features. Carbonate features identified in this spectral region are interpreted as having a variety of cation compositions. The organic features are consistent with aromatic and aliphatic C-H bonds like those of insoluble organic matter in meteorites and other primitive objects [10, 11]. The deepest 3.4-micron absorptions occur on individual boulders, and surface variation may be attributable to differences in abundance, fresh exposure by processes such as thermal fracturing, or differences in space weathering. There is no definitive spectral evidence of either organics or carbonates outside of the 3.2–3.6 μm region.Several weak absorption bands also have been observed [4]. These are consistent with phyllosilicates (e.g., the 1.4-µm region), Fe-bearing phases (e.g., 1.05-µm region), and magnetite (0.55 µm).3. OTES resultsOTES spectra acquired during the Preliminary Survey mission phase are broadly consistent with carbonaceous chondrites (CCs) in the CI/CM groups [7]. Potential evidence of magnetite is present in features at 555 and 346 cm–1 [1] and is consistent with aqueous alteration. Detailed Survey measurements at ~40 m/spot exhibit spectral variability, primarily in the shape of the silicate stretching feature and the depth of the silicate bending feature. These variations can be described by two endmember spectral types, T1 and T2, which appear to primarily represent differences in the amount of fine particulate (

On 24 September 2023, the NASA OSIRIS-REx mission dropped a capsule to Earth containing approximately 120 g of pristine carbonaceous regolith from Bennu. We describe the delivery and initial allocation of this asteroid sample and introduce its bulk physical, chemical, and mineralogical properties from early analyses. The regolith is very dark overall, with higher-reflectance inclusions and particles interspersed. Particle sizes range from sub-micron dust to a stone about 3.5 cm long. Millimeter-scale and larger stones typically have hummocky or angular morphologies. A subset of the stones appears mottled by brighter material that occurs as veins and crusts. Hummocky stones have the lowest densities and mottled stones have the highest. Remote sensing of the surface of Bennu detected hydrated phyllosilicates, magnetite, organic compounds, carbonates, and scarce anhydrous silicates, all of which the sample confirms. We also find sulfides, presolar grains, and, less expectedly, Na-rich phosphates, as well as other trace phases. The sample composition and mineralogy indicate substantial aqueous alteration and resemble those of Ryugu and the most chemically primitive, low-petrologic-type carbonaceous chondrites. Nevertheless, we find distinct hydrogen, nitrogen, and oxygen isotopic compositions, and some of the material we analyzed is enriched in fluid-mobile elements. Our findings underscore the value of sample return, especially for low-density material that may not readily survive atmospheric entry, and lay the groundwork for more comprehensive analyses.

The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission recently returned a sample of rocks and dust collected from asteroid Bennu. We analyzed the highest-resolution thermal data obtained by the OSIRIS-REx Thermal Emission Spectrometer (OTES) to gain insight into the thermal and physical properties of the sampling site, including rocks that may have been sampled, and the immediately surrounding Hokioi Crater. After correcting the pointing of the OTES data sets, we find that OTES fortuitously observed two dark rocks moments before they were contacted by the spacecraft. We derived thermal inertias of 100–150 (±50) J m −2 K −1 s −1/2 for these two rocks—exceptionally low even compared with other previously analyzed dark rocks on Bennu (180–250 J m −2 K −1 s −1/2 ). Our simulations indicate that monolayer coatings of sand- to pebble-sized particles, as observed on one of these rocks, could significantly reduce the apparent thermal inertia and largely mask the properties of the substrate. However, the other low-thermal-inertia rock that was contacted is not obviously covered in particles. Moreover, this rock appears to have been partially crushed, and thus potentially sampled, by the spacecraft. We conclude that this rock may be highly fractured and that it should be sought in the returned sample to better understand its origin in Bennu’s parent body and the relationship between its thermal and physical properties.

Thermophysical analyses of planetary bodies such as the Moon, Mars, and numerous asteroids have allowed for remote estimates of regolith physical properties, such as particle size and packing density, as well as the relative spatial abundance of boulders. Here we define “regolith” as a particulate assemblage where most particles are comparable to or smaller than the length scale of the diurnal skin depth (the e-folding depth of the diurnal thermal wave). Until recently, regolith and boulders were believed to be thermally quite distinct; regolith on the Moon, Mars, and most asteroids was usually known or suspected to be fine (i.e., ranging from fine dust to sand), meaning that it was known or assumed to have a thermal inertia much lower than that of boulders and bedrock. Upon the arrival of NASA OSIRIS-REx at asteroid Bennu and JAXA Hayabusa2 at asteroid Ryugu and the subsequent thermophysical analyses of the respective asteroid surfaces, this preconceived notion of thermophysically distinct regolith and boulders/bedrock was found to be flawed (DellaGiustina and Emery et al., 2019; Sugita et al., 2019). Boulders cover the vast majority of both asteroids’ surfaces, yet the thermal inertia values determined for these boulder-rich surfaces fall within a range that was previously believed to exclusively represent coarse, sand-to-pebble–sized regolith. Recent work has been devoted to the analysis of the boulders’ thermal inertia and the physical interpretation thereof (Grott et al., 2019; Rozitis et al., in revision); the general conclusion so far is that the boulders have very low thermal conductivity and density owing to the presence of numerous pores and fractures, more so than most (or perhaps all) carbonaceous chondrite meteorites in Earth’s collections. As such, these boulders are likely to be structurally distinct from all known meteorite specimens. Although fine-particulate regolith is rare on the surface of Bennu, it is present in some locations. It is still of great interest to probe the physical properties of this regolith by means of thermal analysis so as to make predictions about the properties of the samples that will be returned by OSIRIS-REx and to learn about the evolution of the asteroid surface and the mechanisms by which regolith is produced and lost or destroyed. There remains the enigmatic question: although we suspect that the boulders on Bennu are distinct from the meteorite collection, will the returned samples of particulate material share these distinct properties? At what scales can we define the physical thermally relevant properties of boulders and regolith particles on Bennu? In other words, if the boulders are indeed highly fractured and porous, are the regolith particles also fractured and porous or are their dimensions below the relevant length scales? It is very challenging to estimate the physical properties of the regolith on Bennu using thermal data, even in regions where it appears to be abundant, due to the coarseness of the regolith. Commonly used planetary thermophysical models rely on the assumption that the material on the surface, be it regolith or rock, can be approximated as a continuous, non-discretized material with physical properties that are either constant with depth or are allowed to vary with depth in some well-defined way (e.g. an exponential density increase, or up to a few layers of physically distinct material, such as regolith on bedrock or dust coatings). This assumption of material continuity is valid when regolith particles are smaller, or perhaps even much smaller, than the diurnal skin depth of the thermal wave. However, high-resolution images of the Nightingale Crater on Bennu, which is the OSIRIS-REx mission’s primary sample collection site, revealed a particle size frequency distribution (SFD) that crosses this threshold; i.e., there are particles present that are smaller than, comparable to, and larger than the diurnal skin depth (~1­–5 cm) present within a single, meters-wide observation footprint. The thermophysical behavior of such a regolith configuration has never been comprehensively studied and likely cannot be properly approximated with standard 1D thermal modeling methods. We will present preliminary results using a 3D regolith model where we render hundreds of regolith particles and rocks, approximated as spheres, in a finite element mesh framework. The model is heated diurnally with a solar source to study the thermal response of such skin depth–crossing SFDs under Bennu surface-like conditions. The SFD of the particles is informed by particle size counts in Nightingale Crater from high-resolution visible images. With the SFD partially constrained, we are able to focus our efforts on exploring the material property parameter space, namely to estimate the thermal conductivity and density of individual regolith particles. Given the coarseness of the regolith on Bennu, we find that the model is more sensitive than one might expect to the thermal conductivity of the individual regolith particles owing to the effects of particle non-isothermality (Ryan et al., 2020). Although this present modeling work is focused on analyzing thermal emission data of Bennu obtained by the OSIRIS-REx Thermal Emission Spectrometer (OTES, Christensen et al., 2018), we aim to expand our efforts to study the more general thermal behavior of coarse regoliths and regoliths with wide SFDs under a range of solar heating conditions. Acknowledgements This material is based upon work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program. We are grateful to the entire OSIRIS-REx Team for making the encounter with Bennu possible. References: Christensen, P.R. et al. (2018). The OSIRIS-REx Thermal Emission Spectrometer (OTES) Instrument. Space Science Reviews 214, 87. DellaGiustina, D.N., Emery, J.P., et al. (2019) Properties of rubble-pile asteroid (101955) Bennu from OSIRIS-REx imaging and thermal analysis, Nature Astronomy, DOI:10.1038/s41550-019-0731-1. Grott, M., Knollenberg, J., et al. (2019) Low thermal conductivity boulder with high porosity identified on C-type asteroid (162173) Ryugu, Nature Astronomy, DOI:10.1038/s41550-019-0832-x. Rozitis, B., Ryan, A.J., Emery, J.P., et al. (in revision) Asteroid (101955) Bennu’s Weak Boulders and Thermally Anomalous Equator, Science Advances, submitted March 2020. Ryan, A., Pino Muñoz, D., Bernacki, M., Delbo, M. (2020) Full-Field Modeling of Heat Transfer in Asteroid Regolith: Radiative Thermal Conductivity of Polydisperse Particulates, JGR:Planets. DOI:10.1029/2019JE006100. Sugita, S., Honda, R., et al. (2019) The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes, Science. DOI:10.1126/science.aaw0422

The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) mission to carbonaceous asteroid (101955) Bennu performed detailed mapping with a suite of instruments to characterize the composition and geology of its surface. Here we use data from the OSIRIS-REx Thermal Emission Spectrometer (OTES) instrument to investigate the relationship of OTES-derived spectral indices to other derived data products from OTES, the OSIRIS-REx Camera Suite (OCAMS), and the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) at global and local scales. We quantitatively confirm that high values of the OTES silicate stretching slope (from similar to 10 to similar to 12 mu m) in midday spectra that are indicative of thin and/or patchy dust cover are strongly associated with low thermal inertia (high porosity), low albedo boulders on Bennu. These high porosity boulders have brecciated textures with embedded clasts that likely originated on Bennu's parent body or during its disruption. The high porosity of these boulders is a key factor in the local production of the dust or its entrapment, as some large, brecciated boulders with a lower porosity have little evidence of dust. A second OTES spectral parameter, the silicate bending band depth near 22.7 mu m applied to early evening spectra, is not correlated to thermal inertia, but is weakly to strongly correlated to albedo, OVIRS-derived 1.05 mu m and 2.74 mu m band depths, OVIRS-derived hydrogen abundance, and modeled nanophase troilite abundance. In several regions on Bennu there is a strong spatial relationship between these parameters, whereby areas with shallower silicate bending bands also have shallower 1.05 mu m and 2.74 mu m bands and lower albedo with higher nanophase troilite abundances. These correlations, combined with analysis of the silicate bending band in laboratory experiments of space weathered and mildly heated carbonaceous chondrites, suggests that decreased silicate bending band depths signify decomposition of phyllosilicates, likely Fe-bearing, due to space weathering or mild heating (<600(degrees)C) via solar radiation during Bennu's time in near-Earth space. There is a strong association of larger silicate bending band depths in areas dominated by small rocks and unresolved material and in areas with small (<= 25 m) craters identified as the spectrally reddest on Bennu, suggesting that this material has been more recently exposed due to impact and/or mass wasting processes. The shallowest silicate bending depths are associated with larger rocks and boulders that appear to have the longest surface exposure history, although there is band depth variation among them suggesting either initial composition variation that resulted in different responses to space weathering or heating, or varied exposure history of individual boulders themselves. We predict that any grains returned from Bennu with a significant surface exposure history will be characterized by shallower 22.7 mu m, 1.05 mu m and 2.7 mu m band depths and increased sulfide (troilite) abundance, as well as textural and chemical evidence for phyllosilicate dehydration.

Asteroids with diameters less than about 5 km have complex histories because they are small enough for radiative torques (that is, YORP, short for the Yarkovsky-O'Keefe-Radzievskii-Paddack effect)1 to be a notable factor in their evolution2. (152830) Dinkinesh is a small asteroid orbiting the Sun near the inner edge of the main asteroid belt with a heliocentric semimajor axis of 2.19 AU; its S-type spectrum3,4 is typical of bodies in this part of the main belt5. Here we report observations by the Lucy spacecraft6,7 as it passed within 431 km of Dinkinesh. Lucy revealed Dinkinesh, which has an effective diameter of only 720 m, to be unexpectedly complex. Of particular note is the presence of a prominent longitudinal trough overlain by a substantial equatorial ridge and the discovery of the first confirmed contact binary satellite, now named (152830) Dinkinesh I Selam. Selam consists of two near-equal-sized lobes with diameters of 210 m and 230 m. It orbits Dinkinesh at a distance of 3.1 km with an orbital period of about 52.7 h and is tidally locked. The dynamical state, angular momentum and geomorphologic observations of the system lead us to infer that the ridge and trough of Dinkinesh are probably the result of mass failure resulting from spin-up by YORP followed by the partial reaccretion of the shed material. Selam probably accreted from material shed by this event.

OVIRS [1, 2] acquired visible to near-infrared spectra of asteroid Bennu’s surface showing an asymmetric absorption band centered at 2.74 ± 0.01 μm [3], attributed to the presence of hydrated phyllosilicates. This feature is widespread across Bennu’s surface. Such an absorption band has been detected in some carbonaceous chondrite meteorites [4, 5].In this study, we report the results from two distinct methods to estimate the hydration of Bennu’s surface. We calculated the normalized optical path length (NOPL) as well as the effective single particle absorption thickness (ESPAT) [6, 7, 8] on Bennu’s hydration band and on the selected meteorite spectra. For both methods, we compare meteorite results with their H2O/OH– H content, to estimate a H2O/OH– H content of Bennu’s average surface. Carbonaceous chondrite meteorite H2O/OH– H contents are derived from laboratory studies [9, 10]. Bennu spectra. Analysed spectra were acquired by OVIRS during Equatorial Station 3 (EQ3) of the Detailed Survey mission phase, on May 9, 2019, at 12:30 pm local solar time [11]. The reflectance spectra have been calibrated and photometrically corrected to an incidence angle of 0°, emission angle of 30°, and phase angle of 30°, using a McEwen photometrical model [12].Meteorite spectra. We used three sets of meteorite absolute reflectance spectra, from [4,5], [8], and [13]. For each set, powdered meteorite sample spectra were measured under vacuum (asteroid-like conditions).We selected over 40 meteorites for which bulk H values have been independently measured [9, 10]. In the case of Orgueil, Bells, and Tagish Lake, several samples were analysed and several H contents were ultimately derived [9, 14], all of which were used. Normalized Optical Path Length (NOPL). The NOPL parameter was calculated as described in [6, 7, 8] on each meteorite spectrum, each individual Bennu reflectance spectrum, and the global average spectrum of Bennu. A linear continuum was fitted from 2.67 to 3.3 μm. The wavelength, at which the NOPL parameter is calculated, is the mean band minimum position for the EQ3 data set at 2.73 μm. Methods used to locate the band minimum are described in [3].Effective Single Particle Absorption Thickness (ESPAT). The ESPAT parameter was calculated following the method of [6, 7, 8]. Absolute reflectance spectra of meteorites and Bennu’s surface were first converted into single-scattering albedo spectra [6, 7, 8]. A linear continuum was then fitted from 2.67 to 3.3 μm and the ESPAT parameter is calculated at 2.73 μm as well. Our analyses do not include the organic absorption bands, present longwards of ~3.3 μm [11, 15]. Thus, we compare NOPL and ESPAT results with the hydrogen content of H2O/OH– groups in hydrated phyllosilicates only, measured for the selected meteorites [9, 10]. Figure 1 shows the NOPL parameter variations across Bennu’s surface using EQ3 spectra.Figure 1: Map of NOPL values computed at 2.73 μm for each EQ3 spectrum of Bennu.We find a linear correlation (Figure 2) between the NOPL parameter calculated at 2.73 μm on meteorite spectra and the meteorite H2O/OH– H content.Using this linear correlation, for the NOPL calculated on Bennu’s EQ3 average spectrum, we estimate a H2O/OH– H content for Bennu’s average surface of 0.54 ± 0.11 wt.%. Figure 2: Linear correlation between NOPL calculated at 2.73 μm and H2O/OH– H content of the seven selected meteorites (in colored points), and for average Bennu (blue circle).As with the NOPL parameter, we also find a linear correlation between the ESPAT parameter calculated at 2.73 μm on meteorite spectra and the meteorite H2O/OH– H content. We therefore estimate a H2O/OH– H content for Bennu’s average surface of 0.49 ± 0.13 wt.%, using Bennu’s EQ3 mean ESPAT value and the latter correlation. Discussion and ConclusionThe H2O/OH– H content for Bennu’s average surface obtained using NOPL parameters is consistent with the range obtained with the ESPAT parameter. Both methods are based on estimating global H content (in H2O/OH– groups of hydrated phyllosilicates) by analogy with meteorite data. The values of H2O/OH– H content of Bennu’s average surface we obtained are 0.54 ± 0.11 and 0.49 ± 0.13 wt.% using the NOPL parameter and the ESPAT parameter, respectively. From our results (Figure 2), Bennu’s average surface is most similar to heated CMs and Tagish Lake. Both estimated H2O/OH– H content ranges of Bennu’s average surface are more consistent with those of CM meteorites (0.46–1.36 wt%), Tagish Lake (0.50–0.69 wt.%), CR meteorites (0.30–1.20 wt.%), and CO meteorites (0.49–0.52 wt.%) [3, 9]. The gaussian modeling of the hydration band will complete those results. AcknowledgementsThis material is based on work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program. AP, MAB, FM, SF, PH and JDPD acknowledge funding support by CNES. INAF participation was supported by Italian Space Agency grant agreement n. 2017-37-H.0. We are grateful to the entire OSIRIS-REx Team for making the encounter with Bennu possible. References[1] Lauretta D. S. et al. (2017) Space Sci. Rev. 212, 925-984. [2] Reuter D. C. et al. (2018) Space Sci. Rev. 214, 54. [3] Hamilton V. E. et al. (2019) Nat. Astron. 3, 332. [4] Takir D. et al. (2013) Meteorit. Planet. Sci. 48, 1618–1637. [5] Takir D. et al. (2019) Icarus 333, 243–251. [6] Milliken R. E. et Mustard J. F. (2005) JGR, 110, E12001. [7] Milliken R. E. et al. (2007) JGR, 112, E08S07. [8] Garenne A. et al. (2016) Icarus, 264, 172-183. [9] Alexander C.M.O’D. et al. (2012) Science, 337, 721-723. 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