Evidence for widespread hydrated minerals on asteroid (101955) Bennu

Origin of asteroid (101955) Bennu and its connection to the New Polana family

The asteroid (142) Polana is classified as a B-type asteroid located in the inner Main Belt. This asteroid is the parent of the New Polana family, which has been proposed to be the likely source of primitive near-Earth asteroids such as the B-type asteroid (101955) Bennu. To investigate the compositional correlation between Polana and Bennu at the 3 µm band and their aqueous alteration histories, we analyzed the spectra of Polana in the ~ 2.0-4.0 µm spectral range using the NASA Infrared Telescope Facility in Hawai'i. Our findings indicate that Polana does not exhibit discernable 3 µm hydrated mineral absorption (within 2σ), which is in contrast to asteroid Bennu. Bennu displayed a significant 3 µm absorption feature similar to CM- and CI-type carbonaceous chondrites. This suggests two possibilities: either Bennu did not originate from the New Polana family parented by asteroid Polana or the interior of Bennu's parent body was not homogenous, with diverse levels of aqueous alteration. Several explanations support the latter possibility, including heating due to shock waves and pressure, which could have caused the current dehydrated state of Bennu's parent body.

Aqueous breakdown of aspartate and glutamate to n-ω-amino acids on the parent bodies of carbonaceous chondrites and asteroid Ryugu

Amino acids in carbonaceous chondrites may have seeded the origin of life on Earth and possibly elsewhere. Recently, the return samples from a C-type asteroid Ryugu were found to contain amino acids with a similar distribution to Ivuna-type CI chondrites, suggesting the potential of amino acid abundances as molecular descriptors of parent body geochemistry. However, the chemical mechanisms responsible for the amino acid distributions remain to be elucidated particularly at low temperatures (<50°C). Here, we report that two representative proteinogenic amino acids, aspartic acid and glutamic acid, decompose to β-alanine and γ-aminobutyric acid, respectively, under simulated geoelectrochemical conditions at 25°C. This low-temperature conversion provides a plausible explanation for the enrichment of these two n-ω-amino acids compared to their precursors in heavily aqueously altered CI chondrites and Ryugu's return samples. The results suggest that these heavily aqueously altered samples originated from the water-rich mantle of their water/rock differentiated parent planetesimals where protein α-amino acids were decomposed.

Igneous processes in the small bodies of the Solar System I. Asteroids and comets

Igneous processes were quite widespread in the small bodies of the Solar System (SBSS) and were initially fueled by short-lived radioisotopes, the proto-Sun, impact heating, and differentiation heating. Once they finished, long-lived radioisotopes continued to warm the active bodies of the Earth, (possibly) Venus, and the cryovolcanism of Enceladus. The widespread presence of olivine and pyroxenes in planets and also in SBSS suggests that they were not necessarily the product of igneous processes and they might have been recycled from previous nebular processes or entrained in comets from interstellar space. The difference in temperature between the inner and the outer Solar System has clearly favored thermal annealing of the olivine close to the proto-Sun. Transport of olivine within the Solar System probably occurred also due to protostellar jets and winds but the entrainment in SBSS from interstellar space would overcome the requirement of initial turbulent regime in the protoplanetary nebula.

Formation and evolution of carbonaceous asteroid Ryugu: Direct evidence from returned samples

Samples of the carbonaceous asteroid Ryugu were brought to Earth by the Hayabusa2 spacecraft. We analyzed 17 Ryugu samples measuring 1 to 8 millimeters. Carbon dioxide-bearing water inclusions are present within a pyrrhotite crystal, indicating that Ryugu's parent asteroid formed in the outer Solar System. The samples contain low abundances of materials that formed at high temperatures, such as chondrules and calcium- and aluminum-rich inclusions. The samples are rich in phyllosilicates and carbonates, which formed through aqueous alteration reactions at low temperature, high pH, and water/rock ratios of <1 (by mass). Less altered fragments contain olivine, pyroxene, amorphous silicates, calcite, and phosphide. Numerical simulations, based on the mineralogical and physical properties of the samples, indicate that Ryugu's parent body formed ~2 million years after the beginning of Solar System formation.

A dehydrated space-weathered skin cloaking the hydrated interior of Ryugu

Without a protective atmosphere, space-exposed surfaces of airless Solar System bodies gradually experience an alteration in composition, structure and optical properties through a collective process called space weathering. The return of samples from near-Earth asteroid (162173) Ryugu by Hayabusa2 provides the first opportunity for laboratory study of space-weathering signatures on the most abundant type of inner solar system body: a C-type asteroid, composed of materials largely unchanged since the formation of the Solar System. Weathered Ryugu grains show areas of surface amorphization and partial melting of phyllosilicates, in which reduction from Fe³⁺ to Fe²⁺ and dehydration developed. Space weathering probably contributed to dehydration by dehydroxylation of Ryugu surface phyllosilicates that had already lost interlayer water molecules and to weakening of the 2.7 µm hydroxyl (-OH) band in reflectance spectra. For C-type asteroids in general, this indicates that a weak 2.7 µm band can signify space-weathering-induced surface dehydration, rather than bulk volatile loss.

Rare earth element identification and quantification in millimetre-sized Ryugu rock fragments from the Hayabusa2 space mission

Millimetre-sized primordial rock fragments originating from asteroid Ryugu were investigated using high energy X-ray fluorescence spectroscopy, providing 2D and 3D elemental distribution and quantitative composition information on the microscopic level. Samples were collected in two phases from two sites on asteroid Ryugu and safely returned to Earth by JAXA's asteroid explorer Hayabusa2, during which time the collected material was stored and maintained free from terrestrial influences, including exposure to Earth's atmosphere. Several grains of interest were identified and further characterised to obtain quantitative information on the rare earth element (REE) content within said grains, following a reference-based and computed-tomography-assisted fundamental parameters quantification approach. Several orders of magnitude REE enrichments compared to the mean CI chondrite composition were found within grains that could be identified as apatite phase. Small enrichment of LREE was found for dolomite grains and slight enrichment or depletion for the general matrices within the Ryugu rock fragments A0055 and C0076, respectively.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s40623-022-01705-3.

Space weathering signatures in sulfide and silicate minerals from asteroid Itokawa

Transmission electron microscopy analyses of the polymineralic regolith particle RC-MD01-0025 show microstructural and microchemical characteristics indicative of space weathering on the surface of asteroid Itokawa. The depletion of sulfur and nickel was identified in space weathered rims on troilite and pentlandite minerals. This corresponds to the first report of nickel depletion in samples returned from asteroid Itokawa by the Hayabusa mission. Microstructurally, the sulfide minerals present crystalline rims and the olivine presents both crystalline and amorphous zones in the rim. These results suggest that sulfides might be more resistant to amorphization caused by solar wind irradiation. The space weathering features identified in the regolith particle analyzed here are likely formed via solar wind irradiation. Additionally, the differences in the space weathering features in olivine, pentlandite, and troilite suggest that silicates and sulfides respond differently to the same space weathering conditions in interplanetary space.

Near-zero cohesion and loose packing of Bennu's near subsurface revealed by spacecraft contact

When the OSIRIS-REx spacecraft pressed its sample collection mechanism into the surface of Bennu, it provided a direct test of the poorly understood near-subsurface physical properties of rubble-pile asteroids, which consist of rock fragments at rest in microgravity. Here, we find that the forces measured by the spacecraft are best modeled as a granular bed with near-zero cohesion that is half as dense as the bulk asteroid. The low gravity of a small rubble-pile asteroid such as Bennu effectively weakens its near subsurface by not compressing the upper layers, thereby minimizing the influence of interparticle cohesion on surface geology. The underdensity and weak near subsurface should be global properties of Bennu and not localized to the contact point.

Spacecraft sample collection and subsurface excavation of asteroid (101955) Bennu

Carbonaceous asteroids, such as (101955) Bennu, preserve material from the early Solar System, including volatile compounds and organic molecules. We report spacecraft imaging and spectral data collected during and after retrieval of a sample from Bennu’s surface. The sampling event mobilized rocks and dust into a debris plume, excavating a 9-m-long elliptical crater. This exposed material that is darker, spectrally redder, and more abundant in fine particulates than the original surface. The bulk density of the displaced subsurface material was 500–700 kg per cubic meter, about half that of the whole asteroid. Particulates that landed on instrument optics spectrally resemble aqueously altered carbonaceous meteorites. The spacecraft stored 250 ± 101 g of material, which will be delivered to Earth in 2023.

In Situ FIB-TEM-TOF-SIMS Combination Technique: Application in the Analysis of Ultra-Light and Trace Elements in Phyllosilicates

At present, a single technical method has difficulty in obtaining microscopic data of ultra-light elements, trace elements, and crystal structures in samples simultaneously. This work combined an in situ focused ion beam—transmission electron microscopy—time of flight secondary ion mass spectrometry (FTT) technique and analyzed the composition and crystal structure of four phyllosilicate samples. These materials were comprised of antigorite, clinochlore, and cookeite phases. An FIB sample preparation technique was found to provide a sample thickness suitable for TEM observations and a degree of surface roughness appropriate for TOF-SIMS analysis. In addition, the relative amounts and distributions of various elements could be obtained, as well as crystal structure data, such that the composition and crystal structure of each specimen were determined. The in situ FTT method demonstrated herein successfully combines the advantages of all three analytical techniques and offers unique advantages with regard to analyzing ultra-light and trace elements as well as the structural data of phyllosilicates.

Assessing the Sampleability of Bennu's Surface for the OSIRIS-REx Asteroid Sample Return Mission

NASA's first asteroid sample return mission, OSIRIS-REx, collected a sample from the surface of near-Earth asteroid Bennu in October 2020 and will deliver it to Earth in September 2023. Selecting a sample collection site on Bennu's surface was challenging due to the surprising lack of large ponded deposits of regolith particles exclusively fine enough ( ≤ 2 cm diameter) to be ingested by the spacecraft's Touch-and-Go Sample Acquisition Mechanism (TAGSAM). Here we describe the Sampleability Map of Bennu, which was constructed to aid in the selection of candidate sampling sites and to estimate the probability of collecting sufficient sample. "Sampleability" is a numeric score that expresses the compatibility of a given area's surface properties with the sampling mechanism. The algorithm that determines sampleability is a best fit functional form to an extensive suite of laboratory testing outcomes tracking the TAGSAM performance as a function of four observable properties of the target asteroid. The algorithm and testing were designed to measure and subsequently predict TAGSAM collection amounts as a function of the minimum particle size, maximum particle size, particle size frequency distribution, and the tilt of the TAGSAM head off the surface. The sampleability algorithm operated at two general scales, consistent with the resolution and coverage of data collected during the mission. The first scale was global and evaluated nearly the full surface. Due to Bennu's unexpected boulder coverage and lack of ponded regolith deposits, the global sampleability efforts relied heavily on additional strategies to find and characterize regions of interest based on quantifying and avoiding areas heavily covered by material too large to be collected. The second scale was site-specific and used higher-resolution data to predict collected mass at a given contact location. The rigorous sampleability assessments gave the mission confidence to select the best possible sample collection site and directly enabled successful collection of hundreds of grams of material.

Distinguishing the Origin of Asteroid (16) Psyche

The asteroid (16) Psyche may be the metal-rich remnant of a differentiated planetesimal, or it may be a highly reduced, metal-rich asteroidal material that never differentiated. The NASA Psyche mission aims to determine Psyche's provenance. Here we describe the possible solar system regions of origin for Psyche, prior to its likely implantation into the asteroid belt, the physical and chemical processes that can enrich metal in an asteroid, and possible meteoritic analogs. The spacecraft payload is designed to be able to discriminate among possible formation theories. The project will determine Psyche's origin and formation by measuring any strong remanent magnetic fields, which would imply it was the core of a differentiated body; the scale of metal to silicate mixing will be determined by both the neutron spectrometers and the filtered images; the degree of disruption between metal and rock may be determined by the correlation of gravity with composition; some mineralogy (e.g., modeled silicate/metal ratio, and inferred existence of low-calcium pyroxene or olivine, for example) will be detected using filtered images; and the nickel content of Psyche's metal phase will be measured using the GRNS.

Pebbles and sand on asteroid (162173) Ryugu: In situ observation and particles returned to Earth

The Hayabusa2 spacecraft investigated the C-type (carbonaceous) asteroid (162173) Ryugu. The mission performed two landing operations to collect samples of surface and subsurface material, the latter exposed by an artificial impact. We present images of the second touchdown site, finding that ejecta from the impact crater was present at the sample location. Surface pebbles at both landing sites show morphological variations ranging from rugged to smooth, similar to Ryugu's boulders, and shapes from quasi-spherical to flattened. The samples were returned to Earth on 6 December 2020. We describe the morphology of >5 grams of returned pebbles and sand. Their diverse color, shape, and structure are consistent with the observed materials of Ryugu; we conclude that they are a representative sample of the asteroid.

Deciphering Redox State for a Metal-Rich World

The Psyche mission's Oxidation-Reduction Working Group is focused on understanding, determining, and applying the redox state of (16) Psyche to understand the origin of a metal-rich world. The oxidation-reduction state of an asteroid, along with its temperature, parent body size, and composition, is a key parameter in determining the history of an asteroid. Determining the redox state from spacecraft data is most easily done by examining potential metal-oxide buffer pairs. The occurrence of Ni, Fe, C, Cr, P and Si, in that order, in the metal or sulfide phase of an asteroidal body indicates increasingly reduced conditions. Key observations by the Imager and Gamma-Ray and Neutron Spectrometer (GRNS) of Psyche can bracket the redox state using metal-oxide buffers. The presence of Fe,Ni metal can be confirmed by the ratios of Fe/O or Fe/Si and the concentration of Ni variability in metal across the asteroid can be determined by GRNS. The FeO concentration of silicates is complementary to the Ni concentration of metal and can be constrained using filters on the Imager. The presence of FeO in silicates from ground-based observations is one of the few measurements we already have of redox state, although available data permit a wide range of silicate compositions and mineralogies. The presence of C, P or Si concentrated in the metallic, Fe-rich portion of the asteroid, as measured by GRNS, or Ca-sulfide, determined by imaging, would indicate increasingly reducing conditions. Linkage to known types of meteorites, whether metal-rich chondrites, stony-irons or irons, expands the mineralogical, chemical and isotopic data not available from remote observations alone. Redox also controls both silicate and metal mineralogy, influencing differentiation, solidification, and subsolidus cooling, including the relative abundance of sulfur in the core and possible magnetic signatures. The redox state of Psyche, if a fully-differentiated metallic core, might constrain the location and timing of both the formation of Psyche and any oxidation it might have experienced.

Mid-infrared emissivity of partially dehydrated asteroid (162173) Ryugu shows strong signs of aqueous alteration

The near-Earth asteroid (162173) Ryugu, the target of Hayabusa2 space mission, was observed via both orbiter and the lander instruments. The infrared radiometer on the MASCOT lander (MARA) is the only instrument providing spectrally resolved mid-infrared (MIR) data, which is crucial for establishing a link between the asteroid material and meteorites found on Earth. Earlier studies revealed that the single boulder investigated by the lander belongs to the most common type found on Ryugu. Here we show the spectral variation of Ryugu's emissivity using the complete set of in-situ MIR data and compare it to those of various carbonaceous chondritic meteorites, revealing similarities to the most aqueously altered ones, as well as to asteroid (101955) Bennu. The results show that Ryugu experienced strong aqueous alteration prior to any dehydration.

Fine-regolith production on asteroids controlled by rock porosity

Spacecraft missions have observed regolith blankets of unconsolidated subcentimetre particles on stony asteroids^1-3. Telescopic data have suggested the presence of regolith blankets also on carbonaceous asteroids, including (101955) Bennu⁴ and (162173) Ryugu⁵. However, despite observations of processes that are capable of comminuting boulders into unconsolidated materials, such as meteoroid bombardment^6,7 and thermal cracking⁸, Bennu and Ryugu lack extensive areas covered in subcentimetre particles^7,9. Here we report an inverse correlation between the local abundance of subcentimetre particles and the porosity of rocks on Bennu. We interpret this finding to mean that accumulation of unconsolidated subcentimetre particles is frustrated where the rocks are highly porous, which appears to be most of the surface¹⁰. The highly porous rocks are compressed rather than fragmented by meteoroid impacts, consistent with laboratory experiments^11,12, and thermal cracking proceeds more slowly than in denser rocks. We infer that regolith blankets are uncommon on carbonaceous asteroids, which are the most numerous type of asteroid¹³. By contrast, these terrains should be common on stony asteroids, which have less porous rocks and are the second-most populous group by composition¹³. The higher porosity of carbonaceous asteroid materials may have aided in their compaction and cementation to form breccias, which dominate the carbonaceous chondrite meteorites¹⁴.

Spectrally blue hydrated parent body of asteroid (162173) Ryugu

Ryugu is a carbonaceous rubble-pile asteroid visited by the Hayabusa2 spacecraft. Small rubble pile asteroids record the thermal evolution of their much larger parent bodies. However, recent space weathering and/or solar heating create ambiguities between the uppermost layer observable by remote-sensing and the pristine material from the parent body. Hayabusa2 remote-sensing observations find that on the asteroid (162173) Ryugu both north and south pole regions preserve the material least processed by space weathering, which is spectrally blue carbonaceous chondritic material with a 0-3% deep 0.7-µm band absorption, indicative of Fe-bearing phyllosilicates. Here we report that spectrally blue Ryugu's parent body experienced intensive aqueous alteration and subsequent thermal metamorphism at 570-670 K (300-400 °C), suggesting that Ryugu's parent body was heated by radioactive decay of short-lived radionuclides possibly because of its early formation 2-2.5 Ma. The samples being brought to Earth by Hayabusa2 will give us our first insights into this epoch in solar system history.

Organic Matter and Associated Minerals on the Dwarf Planet Ceres

Ceres is the largest object in the main belt and it is also the most water-rich body in the inner solar system besides the Earth. The discoveries made by the Dawn Mission revealed that the composition of Ceres includes organic material, with a component of carbon globally present and also a high quantity of localized aliphatic organics in specific areas. The inferred mineralogy of Ceres indicates the long-term activity of a large body of liquid water that produced the alteration minerals discovered on its surface, including ammonia-bearing minerals. To explain the presence of ammonium in the phyllosilicates, Ceres must have accreted organic matter, ammonia, water and carbon present in the protoplanetary formation region. It is conceivable that Ceres may have also processed and transformed its own original organic matter that could have been modified by the pervasive hydrothermal alteration. The coexistence of phyllosilicates, magnetite, carbonates, salts, organics and a high carbon content point to rock–water alteration playing an important role in promoting widespread carbon occurrence.

The best temperature range to acquire reliable thermal infrared spectra from orbit

Solar System bodies undergo to daily and periodical variations of temperature that mainly depend on their closeness to the Sun. It is known that mineral expansion and contraction due to such variations modify the thermal infrared spectra acquired on solid surfaces. Therefore, it becomes crucial to know the best temperature range at which the acquisition itself should be carried out to get reliable information on the mineralogy of such bodies. Here we provide the thermal expansion of olivine between 20 and 298 K determined by X-ray diffraction. Our data reveal the non-linear behaviour of silicates that undergo to low temperatures, where volume variations appear positively correlated with temperatures. Subtle bond-length variations occurring at low temperatures are then expected to minimally affect vibrational absorption positions. We suggest that thermal infrared spectra of those Solar-System surfaces that are not exceeding 300 K provide reliable information about not only the silicate mineral identification but also on their chemical composition, regardless of the instantaneous temperature.

The Fe/S ratio of pyrrhotite group sulfides in chondrites: An indicator of oxidation and implications for return samples from asteroids Ryugu and Bennu

Determining compositional trends among individual minerals is key to understanding the thermodynamic conditions under which they formed and altered, and is also essential to maximizing the scientific value of small extraterrestrial samples, including returned samples and meteorites. Here we report the chemical compositions of Fe-sulfides, focusing on the pyrrhotite-group sulfides, which are ubiquitous in chondrites and are sensitive indicators of formation and alteration conditions in the protoplanetary disk and in small Solar System bodies. Our data show that while there are trends with the at.% Fe/S ratio of pyrrhotite with thermal and aqueous alteration in some meteorite groups, there is a universal trend between the Fe/S ratio and degree of oxidation. Relatively reducing conditions led to the formation of troilite during: (1) chondrule formation in the protoplanetary disk (i.e., pristine chondrites) and (2) parent body thermal alteration (i.e., LL4 to LL6, CR1, CM, and CY chondrites). Oxidizing and sulfidizing conditions led to the formation of Fe-depleted pyrrhotite with low Fe/S ratios during: (1) aqueous alteration (i.e., CM and CI chondrites), and (2) thermal alteration (i.e., CK and R chondrites). The presence of troilite in highly aqueously altered carbonaceous chondrites (e.g., CY, CR1, and some CM chondrites) indicates they were heated after aqueous alteration. The presence of troilite, Fe-depleted pyrrhotite, or pyrite in a chondrite can provide an estimate of the oxygen and sulfur fugacities at which it was formed or altered. The data reported here can be used to estimate the oxygen fugacity of formation and potentially the aqueous and/or thermal histories of sulfides in extraterrestrial samples, including those returned by the Hayabusa2 mission and due to be returned by the OSIRIS-REx mission in the near future.

The relationship between CM and CO chondrites: Insights from combined analyses of titanium, chromium, and oxygen isotopes in CM, CO, and ungrouped chondrites

A close relationship between CM and CO chondrites has been suggested by previous petrologic and isotopic studies, leading to the suggestion that they may originate from similar precursor materials or even a common parent body. In this study, we evaluate the genetic relationship between CM and CO chondrites using Ti, Cr, and O isotopes. We first provide additional constraints on the ranges of ε⁵⁰Ti and ε⁵⁴Cr values of bulk CM and CO chondrites by reporting the isotopic compositions of CM2 chondrites Murchison, Murray, and Aguas Zarcas and the CO3.8 chondrite Isna. We then report the ε⁵⁰Ti and ε⁵⁴Cr values for several ungrouped and anomalous carbonaceous chondrites that have been previously reported to exhibit similarities to the CM and/or CO chondrite groups, including Elephant Moraine (EET) 83226, EET 83355, Grosvenor Mountains (GRO) 95566, MacAlpine Hills (MAC) 87300, MAC 87301, MAC 88107, and Northwest Africa (NWA) 5958, and the O-isotope compositions of a subset of these samples. We additionally report the Ti, Cr, and O isotopic compositions of additional ungrouped chondrites LaPaz Ice Field (LAP) 04757, LAP 04773, Lewis Cliff (LEW) 85332, and Coolidge to assess their potential relationships with known carbonaceous and ordinary chondrite groups. LAP 04757 and LAP 04773 exhibit isotopic compositions indicating they are low-FeO ordinary chondrites. The isotopic compositions of Murchison, Murray, Aguas Zarcas, and Isna extend the compositional ranges defined by the CM and CO chondrites in ε⁵⁰Ti versus ε⁵⁴Cr space. The majority of the ungrouped carbonaceous chondrites with documented similarities to the CM and/or CO chondrites plot outside the CM and CO group fields in plots of ε⁵⁰Ti versus ε⁵⁴Cr, Δ¹⁷O versus ε⁵⁰Ti, and Δ¹⁷O versus ε⁵⁴Cr. Therefore, based on differences in their Ti, Cr, and O isotopic compositions, we conclude that the CM, CO, and ungrouped carbonaceous chondrites likely represent samples of multiple distinct parent bodies. We also infer that these parent bodies formed from precursor materials that shared similar isotopic compositions, which may indicate formation in regions of the protoplanetary disk that were in close proximity to each other.

Thermal metamorphism of CM chondrites: A dehydroxylation-based peak-temperature thermometer and implications for sample return from asteroids Ryugu and Bennu

The target bodies of C-complex asteroid sample return missions are carbonaceous chondrite-like near-Earth asteroids (NEAs), chosen for the abundance and scientific importance of their organic compounds and "hydrous" (including hydroxylated) minerals, such as serpentine-group phyllosilicates. Science objectives include returning samples of pristine carbonaceous regolith from asteroids for study of the nature, history, and distribution of its constituent minerals, organic material, and other volatiles. Heating after the natural aqueous alteration that formed the abundant phyllosilicates in CM and similar carbonaceous chondrites dehydroxylated them and altered or decomposed other volumetrically minor constituents (e.g., carbonates, sulfides, organic molecules; Tonui et al. 2003, 2014). We propose a peak-temperature thermometer based on dehydroxylation as measured by analytical totals from electron probe microanalysis (EPMA) of matrices in a number of heated and aqueously altered (but not further heated) CM chondrites. Some CM lithologies in Maribo and Sutter's Mill do not exhibit the matrix dehydroxylation expected for surface temperatures expected from insolation of meteoroids with their known orbital perihelia. This suggests that insolated-heated meteoroid surfaces were lost by ablation during passage through Earth's atmosphere, and that insolation-heated material is more likely to be encountered among returned asteroid regolith samples than in meteorites. More generally, several published lines of evidence suggest that episodic heating of some CM material, most likely by impacts, continued intermittently and locally up to billions of years after assembly and early heating of ancestral CM chondrite bodies. Mission spectroscopic measures of hydration can be used to estimate the extent of dehydroxylation, and the new dehydroxylation thermometer can be used directly to select fragments of returned samples most likely to contain less thermally altered inventories of primitive organic molecules.

Presolar stardust in highly pristine CM chondrites Asuka 12169 and Asuka 12236

We report a NanoSIMS search for presolar grains in the CM chondrites Asuka (A) 12169 and A12236. We found 90 presolar O-rich grains and 25 SiC grains in A12169, giving matrix-normalized abundances of 275 (+55/-50, 1σ) ppm or, excluding an unusually large grain, 236 (+37/-34) ppm for O-rich grains and 62 (+15/-12) ppm for SiC grains. For A12236, 18 presolar silicates and 6 SiCs indicate abundances of 58 (+18/-12) and 20 (+12/-8) ppm, respectively. The SiC abundances are in the typical range of primitive chondrites. The abundance of presolar O-rich grains in A12169 is essentially identical to that in CO3.0 Dominion Range 08006, higher than in any other chondrites, while in A12236, it is higher than found in other CMs. These abundances provide further strong support that A12169 and A12236 are the least-altered CMs as indicated by petrographic investigations. The similar abundances, isotopic distributions, silicate/oxide ratios, and grain sizes of the presolar O-rich grains found here to those of presolar grains in highly primitive CO, CR, and ungrouped carbonaceous chondrites (CCs) indicate that the CM parent body(ies) accreted a similar population of presolar oxides and silicates in their matrices to those accreted by the parent bodies of the other CC groups. The lower abundances and larger grain sizes seen in some other CMs are thus most likely a result of parent-body alteration and not heterogeneity in nebular precursors. Presolar silicates are unlikely to be present in high abundances in returned samples from asteroids Ryugu and Bennu since remote-sensing data indicate that they have experienced substantial aqueous alteration.

Meteoritic Evidence for a Ceres-sized Water-rich Carbonaceous Chondrite Parent Asteroid

Carbonaceous chondrite meteorites record the earliest stages of Solar System geo-logical activities and provide insight into their parent bodies' histories. Some carbonaceous chondrites are volumetrically dominated by hydrated minerals, providing evidence for low temperature and pressure aqueous alteration¹. Others are dominated by anhydrous minerals and textures that indicate high temperature metamorphism in the absence of aqueous fluids¹. Evidence of hydrous metamorphism at intermediate pressures and temperatures in carbonaceous chondrite parent bodies has been virtually absent. Here we show that an ungrouped, aqueously altered carbonaceous chondrite fragment (numbered 202) from the Almahata Sitta (AhS) meteorite contains an assemblage of minerals, including amphibole, that reflect fluid-assisted metamorphism at intermediate temperatures and pressures on the parent asteroid. Amphiboles are rare in carbonaceous chondrites, having only been identified previously as a trace component in Allende (CV3_oxA) chondrules². Formation of these minerals requires prolonged metamorphism in a large (~640-1800 km diameter), unknown asteroid. Because Allende and AhS 202 represent different asteroidal parent bodies, intermediate conditions may have been more widespread in the early Solar System than recognized from known carbonaceous chondrite meteorites, which are likely a biased sampling.

Prokaryotic and Fungal Characterization of the Facilities Used to Assemble, Test, and Launch the OSIRIS-REx Spacecraft

To characterize the ATLO (Assembly, Test, and Launch Operations) environment of the OSIRIS-REx spacecraft, we analyzed 17 aluminum witness foils and two blanks for bacterial, archaeal, fungal, and arthropod DNA. Under NASA’s Planetary Protection guidelines, OSIRIS-REx is a Category II outbound, Category V unrestricted sample return mission. As a result, it has no bioburden restrictions. However, the mission does have strict organic contamination requirements to achieve its primary objective of returning pristine carbonaceous asteroid regolith to Earth. Its target, near-Earth asteroid (101955) Bennu, is likely to contain organic compounds that are biologically available. Therefore, it is useful to understand what organisms were present during ATLO as part of the larger contamination knowledge effort—even though it is unlikely that any of the organisms will survive the multi-year deep space journey. Even though these samples of opportunity were not collected or preserved for DNA analysis, we successfully amplified bacterial and archaeal DNA (16S rRNA gene) from 16 of the 17 witness foils containing as few as 7 ± 3 cells per sample. Fungal DNA (ITS1) was detected in 12 of the 17 witness foils. Despite observing arthropods in some of the ATLO facilities, arthropod DNA (COI gene) was not detected. We observed 1,009 bacterial and archaeal sOTUs (sub-operational taxonomic units, 100% unique) and 167 fungal sOTUs across all of our samples (25–84 sOTUs per sample). The most abundant bacterial sOTU belonged to the genus Bacillus. This sOTU was present in blanks and may represent contamination during sample handling or storage. The sample collected from inside the fairing just prior to launch contained several unique bacterial and fungal sOTUs that describe previously uncharacterized potential for contamination during the final phase of ATLO. Additionally, fungal richness (number of sOTUs) negatively correlates with the number of carbon-bearing particles detected on samples. The total number of fungal sequences positively correlates with total amino acid concentration. These results demonstrate that it is possible to use samples of opportunity to characterize the microbiology of low-biomass environments while also revealing the limitations imposed by sample collection and preservation methods not specifically designed with biology in mind.

Motion of dust ejected from the surface of asteroid (101955) Bennu

From Jan. 6, 2019 to Feb. 18, 2019, OSIRIS-REx observed asteroid (101955) Bennu ejecting 11 plumes of dust, of which part is escaping and another part is re-captured by the asteroid. The relative magnitudes of the typical forces acting on the emitted dust are quite different from the environments of the planets and other minor planets in the solar system. Here we show that ejected dust grains from the surface of Bennu can be caught in the gravitational field of Bennu. To this end, we calculated numerically the trajectories of dust grains of various sizes, from the 0.1μm to the ten millimeter range. The shape and the fate of an emitted cloud of particles depend on the size of the grains: smaller grains form a more narrowly confined dust trail while trails formed by larger grains disperse more rapidly. Four different fates are possible for ejected dust. All grains with radius less than 1.0μm, directly re-impact on Bennu or they escape directly. In contrast, a fraction of grains with a radius larger than 10.0 μm will impact or escape only after performing a number of non-Keplerian revolutions around Bennu. Our findings show how dust grains may populate the vicinity of Bennu and other active asteroids and that they can reach interplanetary space and other celestial bodies, implying that organic matter can be transported from carbonaceous asteroids to other celestial bodies, including Earth.

Widespread carbon-bearing materials on near-Earth asteroid (101955) Bennu

Asteroid (101955) Bennu is a dark asteroid on an Earth-crossing orbit that is thought to have assembled from the fragments of an ancient collision. We use spatially resolved visible and near-infrared spectra of Bennu to investigate its surface properties and composition. In addition to a hydrated phyllosilicate band, we detect a ubiquitous 3.4-micrometer absorption feature, which we attribute to a mix of organic and carbonate materials. The shape and depth of this absorption feature vary across Bennu's surface, spanning the range seen among similar main-belt asteroids. The distribution of the absorption feature does not correlate with temperature, reflectance, spectral slope, or hydrated minerals, although some of those characteristics correlate with each other. The deepest 3.4-micrometer absorptions occur on individual boulders. The variations may be due to differences in abundance, recent exposure, or space weathering.

Bright carbonate veins on asteroid (101955) Bennu: Implications for aqueous alteration history

The composition of asteroids and their connection to meteorites provide insight into geologic processes that occurred in the early Solar System. We present spectra of the Nightingale crater region on near-Earth asteroid Bennu with a distinct infrared absorption around 3.4 micrometers. Corresponding images of boulders show centimeters-thick, roughly meter-long bright veins. We interpret the veins as being composed of carbonates, similar to those found in aqueously altered carbonaceous chondrite meteorites. If the veins on Bennu are carbonates, fluid flow and hydrothermal deposition on Bennu's parent body would have occurred on kilometer scales for thousands to millions of years. This suggests large-scale, open-system hydrothermal alteration of carbonaceous asteroids in the early Solar System.

Variations in color and reflectance on the surface of asteroid (101955) Bennu

Visible-wavelength color and reflectance provide information about the geologic history of planetary surfaces. Here we present multispectral images (0.44 to 0.89 micrometers) of near-Earth asteroid (101955) Bennu. The surface has variable colors overlain on a moderately blue global terrain. Two primary boulder types are distinguishable by their reflectance and texture. Space weathering of Bennu surface materials does not simply progress from red to blue (or vice versa). Instead, freshly exposed, redder surfaces initially brighten in the near-ultraviolet region (i.e., become bluer at shorter wavelengths), then brighten in the visible to near-infrared region, leading to Bennu's moderately blue average color. Craters indicate that the time scale of these color changes is ~10⁵ years. We attribute the reflectance and color variation to a combination of primordial heterogeneity and varying exposure ages.

Asteroid (101955) Bennu's weak boulders and thermally anomalous equator

Thermal inertia and surface roughness are proxies for the physical characteristics of planetary surfaces. Global maps of these two properties distinguish the boulder population on near-Earth asteroid (NEA) (101955) Bennu into two types that differ in strength, and both have lower thermal inertia than expected for boulders and meteorites. Neither has strongly temperature-dependent thermal properties. The weaker boulder type probably would not survive atmospheric entry and thus may not be represented in the meteorite collection. The maps also show a high-thermal inertia band at Bennu's equator, which might be explained by processes such as compaction or strength sorting during mass movement, but these explanations are not wholly consistent with other data. Our findings imply that other C-complex NEAs likely have boulders similar to those on Bennu rather than finer-particulate regoliths. A tentative correlation between albedo and thermal inertia of C-complex NEAs may be due to relative abundances of boulder types.

Introduction to the Special Issue: Exploration of the Activity of Asteroid (101955) Bennu

Near-Earth asteroid (101955) Bennu is an active asteroid experiencing mass loss. The activity manifests itself in the form of ejection events emitting up to hundreds of millimeter- to centimeter-scale particles. The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer spacecraft monitored particle activity for a 10-month period that included Bennu's perihelion and aphelion. Novel and classical methods were utilized to detect the particles and characterize their orbital and physical properties. Roughly 30% of the observed particle mass escaped to heliocentric orbit. A majority of particles fell back onto the surface of Bennu after ejection, with the longest-lived particle surviving for 6 days on a temporary orbit. Particle ejection events appear to preferentially take place in the afternoon and evening and from low latitudes, although they can occur at any time or latitude. The reaccumulation of material is biased toward low latitudes resulting in the possible in-fill of craters and growth of Bennu's equatorial bulge. Of the potential mechanisms behind this activity that were investigated in focused studies, meteoroid impacts, thermal fracturing, and ricochet-but not water ice sublimation-were found to be consistent with observations. While phyllosilicate dehydration was not investigated with a focused study, it remains a possible mechanism. These mechanisms are not unique to Bennu, suggesting that many near-Earth asteroids may exhibit activity that has gone undetected thus far. Spacecraft missions with wide-field imagers are encouraged to further characterize this phenomenon.

Initial Orbit Determination and Event Reconstruction From Estimation of Particle Trajectories About (101955) Bennu

The OSIRIS-REx mission has observed multiple instances of particles being ejected from the surface of near-Earth asteroid (101955) Bennu. The ability to quickly identify the particle trajectories and origins is necessary following a particle ejection event. Using proven initial orbit determination techniques, we can rapidly estimate particle trajectories and ejection locations. We present current results pertaining to the identification of particle tracks, an evaluation of the estimated orbits and the excess velocity necessary to induce the particle ejection from the surface, and the uncertainty quantification of the ejection location. We estimate energies per particle ranging from 0.03 to 11.03 mJ for the largest analyzed events and velocities ranging from 5 to 90 cm/s, though we exclude the highest-velocity particles in this technique. We estimate ejection times for eight events and constrain six of the analyzed ejection events to have occurred between about 16:30 and 19:00 local solar time, with the largest events occurring between 16:30 and 18:05.

Meteoroid Impacts as a Source of Bennu's Particle Ejection Events

Asteroid (101955) Bennu, a near-Earth object with a primitive carbonaceous chondrite-like composition, was observed by the Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft to undergo multiple particle ejection events near perihelion between December 2018 and February 2019. The three largest events observed during this period, which all occurred 3.5 to 6 hr after local noon, placed numerous particles <10 cm on temporary orbits around Bennu. Here we examine whether these events could have been produced by sporadic meteoroid impacts using the National Aeronautics and Space Administration's (NASA) Meteoroid Engineering Model 3.0. Most projectiles that impact Bennu come from nearly isotropic or Jupiter-family comets and have evolved toward the Sun by Poynting-Robertson drag. We find that 7,000-J impacts on Bennu occur with a biweekly cadence near perihelion, with a preference to strike in the late afternoon (~6 pm local time). This timing matches observations. Crater scaling laws also indicate that these impact energies can reproduce the sizes and masses of the largest observed particles, provided the surface has the cohesive properties of weak, porous materials. Bennu's ejection events could be caused by the same kinds of meteoroid impacts that created the Moon's asymmetric debris cloud observed by the Lunar Atmosphere and Dust Environment Explorer (LADEE). Our findings also suggest that fewer ejection events should take place as Bennu moves further away from the Sun, a result that can be tested with future observations.

Thermal Fatigue as a Driving Mechanism for Activity on Asteroid Bennu

Many boulders on (101955) Bennu, a near-Earth rubble pile asteroid, show signs of in situ disaggregation and exfoliation, indicating that thermal fatigue plays an important role in its landscape evolution. Observations of particle ejections from its surface also show it to be an active asteroid, though the driving mechanism of these events is yet to be determined. Exfoliation has been shown to mobilize disaggregated particles in terrestrial environments, suggesting that it may be capable of ejecting material from Bennu's surface. We investigate the nature of thermal fatigue on the asteroid, and the efficacy of fatigue-driven exfoliation as a mechanism for generating asteroid activity, by performing finite element modeling of stress fields induced in boulders from diurnal cycling. We develop a model to predict the spacing of exfoliation fractures and the number and speed of particles that may be ejected during exfoliation events. We find that crack spacing ranges from ~1 mm to 10 cm and disaggregated particles have ejection speeds up to ~2 m/s. Exfoliation events are most likely to occur in the late afternoon. These predictions are consistent with observed ejection events at Bennu and indicate that thermal fatigue is a viable mechanism for driving asteroid activity. Crack propagation rates and ejection speeds are greatest at perihelion when the diurnal temperature variation is largest, suggesting that events should be more energetic and more frequent when closer to the Sun. Annual thermal stresses that arise in large boulders may influence the spacing of exfoliation cracks or frequency of ejection events.

In situ evidence of thermally induced rock breakdown widespread on Bennu's surface

Rock breakdown due to diurnal thermal cycling has been hypothesized to drive boulder degradation and regolith production on airless bodies. Numerous studies have invoked its importance in driving landscape evolution, yet morphological features produced by thermal fracture processes have never been definitively observed on an airless body, or any surface where other weathering mechanisms may be ruled out. The Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) mission provides an opportunity to search for evidence of thermal breakdown and assess its significance on asteroid surfaces. Here we show boulder morphologies observed on Bennu that are consistent with terrestrial observations and models of fatigue-driven exfoliation and demonstrate how crack propagation via thermal stress can lead to their development. The rate and expression of this process will vary with asteroid composition and location, influencing how different bodies evolve and their apparent relative surface ages from space weathering and cratering records.

In their study, the authors discuss the potential of thermal weathering on airless bodies. As a case study, they use boulder and fracture morphologies on asteroid Bennu.

Collisional formation of top-shaped asteroids and implications for the origins of Ryugu and Bennu

Asteroid shapes and hydration levels can serve as tracers of their history and origin. For instance, the asteroids (162173) Ryugu and (101955) Bennu have an oblate spheroidal shape with a pronounced equator, but contain different surface hydration levels. Here we show, through numerical simulations of large asteroid disruptions, that oblate spheroids, some of which have a pronounced equator defining a spinning top shape, can form directly through gravitational reaccumulation. We further show that rubble piles formed in a single disruption can have similar porosities but variable degrees of hydration. The direct formation of top shapes from single disruption alone can explain the relatively old crater-retention ages of the equatorial features of Ryugu and Bennu. Two separate parent-body disruptions are not necessarily required to explain their different hydration levels.

Concerns of Organic Contamination for Sample Return Space Missions

Analysis of organic matter has been one of the major motivations behind solar system exploration missions. It addresses questions related to the organic inventory of our solar system and its implication for the origin of life on Earth. Sample return missions aim at returning scientifically valuable samples from target celestial bodies to Earth. By analysing the samples with the use of state-of-the-art analytical techniques in laboratories here on Earth, researchers can address extremely complicated aspects of extra-terrestrial organic matter. This level of detailed sample characterisation provides the range and depth in organic analysis that are restricted in spacecraft-based exploration missions, due to the limitations of the on-board in-situ instrumentation capabilities. So far, there are four completed and in-process sample return missions with an explicit mandate to collect organic matter: Stardust and OSIRIS-REx missions of NASA, and Hayabusa and Hayabusa2 missions of JAXA. Regardless of the target body, all sample return missions dedicate to minimise terrestrial organic contamination of the returned samples, by applying various degrees or strategies of organic contamination mitigation methods. Despite the dedicated efforts in the design and execution of contamination control, it is impossible to completely eliminate sources of organic contamination. This paper aims at providing an overview of the successes and lessons learned with regards to the identification of indigenous organic matter of the returned samples vs terrestrial contamination.

Advances in Cosmochemistry Enabled by Antarctic Meteorites

At present, meteorites collected in Antarctica dominate the total number of the world's known meteorites. We focus here on the scientific advances in cosmochemistry and planetary science that have been enabled by access to, and investigations of, these Antarctic meteorites. A meteorite recovered during one of the earliest field seasons of systematic searches, Elephant Moraine (EET) A79001, was identified as having originated on Mars based on the composition of gases released from shock melt pockets in this rock. Subsequently, the first lunar meteorite, Allan Hills (ALH) 81005, was also recovered from the Antarctic. Since then, many more meteorites belonging to these two classes of planetary meteorites, as well as other previously rare or unknown classes of meteorites (particularly primitive chondrites and achondrites), have been recovered from Antarctica. Studies of these samples are providing unique insights into the origin and evolution of the Solar System and planetary bodies.

Near-infrared observations of active asteroid (3200) Phaethon reveal no evidence for hydration

Asteroid (3200) Phaethon is an active near-Earth asteroid and the parent body of the Geminid Meteor Shower. Because of its small perihelion distance, Phaethon’s surface reaches temperatures sufficient to destabilize hydrated materials. We conducted rotationally resolved spectroscopic observations of this asteroid, mostly covering the northern hemisphere and the equatorial region, beyond 2.5-µm to search for evidence of hydration on its surface. Here we show that the observed part of Phaethon does not exhibit the 3-µm hydrated mineral absorption (within 2σ). These observations suggest that Phaethon’s modern activity is not due to volatile sublimation or devolatilization of phyllosilicates on its surface. It is possible that the observed part of Phaethon was originally hydrated and has since lost volatiles from its surface via dehydration, supporting its connection to the Pallas family, or it was formed from anhydrous material.

The surface of active asteroid (3200) Phaethon, parent body of the Geminid meteor shower, reaches temperatures sufficient to destabilize hydrated materials. Here, the authors show that the northern hemisphere and the equatorial region of this asteroid reveal no evidence of hydration in the near-infrared spectra.

Episodes of particle ejection from the surface of the active asteroid (101955) Bennu

Active asteroids are those that show evidence of ongoing mass loss. We report repeated instances of particle ejection from the surface of (101955) Bennu, demonstrating that it is an active asteroid. The ejection events were imaged by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) spacecraft. For the three largest observed events, we estimated the ejected particle velocities and sizes, event times, source regions, and energies. We also determined the trajectories and photometric properties of several gravitationally bound particles that orbited temporarily in the Bennu environment. We consider multiple hypotheses for the mechanisms that lead to particle ejection for the largest events, including rotational disruption, electrostatic lofting, ice sublimation, phyllosilicate dehydration, meteoroid impacts, thermal stress fracturing, and secondary impacts.

Particle Ejection Contributions to the Rotational Acceleration and Orbit Evolution of Asteroid (101955) Bennu

This paper explores the implications of the observed Bennu particle ejection events for that asteroid's spin rate and orbit evolution, which could complicate interpretation of the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) and Yarkovsky effects on this body's spin rate and orbital evolution. Based on current estimates of particle ejection rates, we find that the overall contribution to Bennu's spin and orbital drift is small or negligible as compared to the Yarkovsky and YORP effects. However, if there is a large unseen component of smaller mass ejections or a strong directionality in the ejection events, it could constitute a significant contribution that could mask the overall YORP effect. This means that the YORP effect may be stronger than currently assumed. The analysis is generalized so that the particle ejection effect can be assessed for other bodies that may be subject to similar mass loss events. Further, our model can be modified to address different potential mechanisms of particle ejection, which are a topic of ongoing study.

A Mineralogical Context for the Organic Matter in the Paris Meteorite Determined by A Multi-Technique Analysis

This study is a multi-technique investigation of the Paris carbonaceous chondrite directly applied on two selected 500 × 500 µm² areas of a millimetric fragment, without any chemical extraction. By mapping the partial hydration of the amorphous silicate phase dominating the meteorite sample matrix, infrared spectroscopy gave an interesting glimpse into the way the fluid may have circulated into the sample and partially altered it. The TOF-SIMS in-situ analysis allowed the studying and mapping of the wide diversity of chemical moieties composing the meteorite organic content. The results of the combined techniques show that at the micron scale, the organic matter was always spatially associated with the fine-grained and partially-hydrated amorphous silicates and to the presence of iron in different chemical states. These systematic associations, illustrated in previous studies of other carbonaceous chondrites, were further supported by the identification by TOF-SIMS of cyanide and/or cyanate salts that could be direct remnants of precursor ices that accreted with dust during the parent body formation, and by the detection of different metal-containing large organic ions. Finally, the results obtained emphasized the importance of studying the specific interactions taking place between organic and mineral phases in the chondrite matrix, in order to investigate their role in the evolution story of primitive organic matter in meteorite parent bodies.

The unexpected surface of asteroid (101955) Bennu

NASA'S Origins, Spectral Interpretation, Resource Identification and Security-Regolith Explorer (OSIRIS-REx) spacecraft recently arrived at the near-Earth asteroid (101955) Bennu, a primitive body that represents the objects that may have brought prebiotic molecules and volatiles such as water to Earth1. Bennu is a low-albedo B-type asteroid2 that has been linked to organic-rich hydrated carbonaceous chondrites3. Such meteorites are altered by ejection from their parent body and contaminated by atmospheric entry and terrestrial microbes. Therefore, the primary mission objective is to return a sample of Bennu to Earth that is pristine-that is, not affected by these processes4. The OSIRIS-REx spacecraft carries a sophisticated suite of instruments to characterize Bennu's global properties, support the selection of a sampling site and document that site at a sub-centimetre scale5-11. Here we consider early OSIRIS-REx observations of Bennu to understand how the asteroid's properties compare to pre-encounter expectations and to assess the prospects for sample return. The bulk composition of Bennu appears to be hydrated and volatile-rich, as expected. However, in contrast to pre-encounter modelling of Bennu's thermal inertia12 and radar polarization ratios13-which indicated a generally smooth surface covered by centimetre-scale particles-resolved imaging reveals an unexpected surficial diversity. The albedo, texture, particle size and roughness are beyond the spacecraft design specifications. On the basis of our pre-encounter knowledge, we developed a sampling strategy to target 50-metre-diameter patches of loose regolith with grain sizes smaller than two centimetres4. We observe only a small number of apparently hazard-free regions, of the order of 5 to 20 metres in extent, the sampling of which poses a substantial challenge to mission success.

Shape of (101955) Bennu indicative of a rubble pile with internal stiffness

The shapes of asteroids reflect interplay between their interior properties and the processes responsible for their formation and evolution as they journey through the Solar System. Prior to the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) mission, Earth-based radar imaging gave an overview of (101955) Bennu's shape. Here, we construct a high-resolution shape model from OSIRIS-REx images. We find that Bennu's top-like shape, considerable macroporosity, and prominent surface boulders suggest that it is a rubble pile. High-standing, north-south ridges that extend from pole to pole, many long grooves, and surface mass wasting indicate some low levels of internal friction and/or cohesion. Our shape model indicates that, similar to other top-shaped asteroids, Bennu formed by reaccumulation and underwent past periods of fast spin leading to its current shape. Today, Bennu might follow a different evolutionary pathway, with interior stiffness permitting surface cracking and mass wasting.