Helena C. Bates

Samples returned by NASA's OSIRIS‐REx mission from the carbonaceous asteroid (101955) Bennu hold clues about conditions in the early solar system and the formation of planetary bodies. Initial investigation of Bennu samples found that they are rich in phyllosilicates and other secondary minerals formed during aqueous alteration of a larger parent body. To better understand the phyllosilicate minerals and constrain the extent and settings of alteration, we used X‐ray powder diffraction (XRD) to characterize the mineralogy of homogenized aggregate (unsorted) samples and an angular particle from Bennu. We find that these samples consist of abundant (>80 vol%) phyllosilicates and few (≤2 vol%) precursor anhydrous silicates, in excellent agreement with remote observations of the global asteroid surface, and consistent with extensive aqueous alteration of Bennu's parent body. The XRD patterns and modal mineralogy of the Bennu samples are similar to those of CI carbonaceous chondrites and particles returned from the carbonaceous asteroid (162173) Ryugu by JAXA's Hayabusa2 mission, offering further support for the close genetic relationship among these materials suggested by other studies. Bennu's phyllosilicates are dominated by a mixture of trioctahedral Mg‐rich clay minerals that formed from alkaline fluids under high water to rock ratios and likely evolved in response to changes in temperature and/or fluid chemistry as alteration progressed. Based on the XRD characteristics of the phyllosilicates, the Bennu samples did not experience a peak temperature at or above ~300°C after aqueous alteration. Finally, we show that the interlayer space of the clay minerals potentially contains up to ~2 wt% water and does not host trapped organic species. The abundance of interlayer water in Bennu samples is notably higher than reported for Ryugu samples (<0.3 wt%), which we speculate is due to differences in either the timing at which the asteroids decoupled from the Main Belt or the sampling depths and/or mechanisms of the respective spacecraft.

Studying asteroids addresses a number of unknowns in planetary science. Investigating them makes use of asteroidal meteorites, remote observations and space-based missions. However, to fully investigate these objects, analysis of pristine asteroid material in Earth-based laboratories is necessary. This requires missions which travel to an asteroid and return samples to Earth. The Hayabusa, Hayabusa2 and OSIRIS-REx sample return missions have showcased the criticality of having asteroid material in hand. Returned samples exhibit fragile mineralogy which does not survive meteorite fall, and is too small to be identified via remote sensing. These missions paved the way for the deployment of future missions including sample return from the Martian moon Phobos, and visiting Psyche - the remanent core of a differentiated body. The study of asteroids, and particularly asteroid sample return, is necessary for the next generation of discoveries.

Aqueously altered carbonaceous astromaterials are dominated by secondary minerals, but a minor fraction of primary, anhydrous silicates and oxides escape alteration, offering insight into the original composition of asteroid parent bodies. We report the mineralogy, petrology, mineral chemistry, and oxygen isotopes of anhydrous minerals—50 olivine grains, 12 pyroxene grains, two spinel grains, and one hibonite grain—in aqueously altered particles returned from asteroid Bennu by the OSIRIS‐REx mission. These primordial grains are heterogeneously distributed, being more abundant (up to a few area percent for olivine) in less aqueously altered clasts. Olivine is typically forsteritic (Fo ₁₀₀ –Fo ₉₃ ), but we found three fayalitic examples (Fo ₈₀ –Fo ₇₅ ). Of the pyroxenes, seven grains are enstatite (En ₁₀₀ –En ₉₄ ); two are enstatite with more Fe (En ₈₃ and En ₈₇ ); one is Fe‐rich, low‐Ca clinopyroxene; one is Mg‐rich, aluminous low‐Ca pyroxene; and one is high‐Al, Ca‐rich clinopyroxene. Chromium is present in olivine grains at high abundances (up to 1.3 wt% Cr ₂ O ₃ ), indicating that the grains have not experienced significant thermal metamorphism. The olivines fall into two clusters: one with ¹⁶ O‐rich compositions (δ ¹⁸ O = −43 to −51‰) typical of refractory inclusions in carbonaceous chondrites and one with comparatively ¹⁶ O‐poor compositions (δ ¹⁸ O = −11 to +7.5‰) more typical of chondrules. Pyroxenes all fall in the ¹⁶ O‐poor cluster (δ ¹⁸ O = −2.6 to +13.5‰). For both olivine and pyroxene, the more Fe‐rich examples are associated with heavier oxygen isotope compositions. One isolated spinel grain has an oxygen isotope composition typical of Ca‐Al–rich inclusions, whereas a spinel–hibonite object has an unusual oxygen isotope composition consistent with fractionated unknown nuclear (FUN) compositions. Together, our data suggest that Bennu's parent body accreted a population of finer grained (≤10 μm) silicate and oxide grains that as a population are distinct from the anhydrous minerals in most chondrite groups but comparable to those in CI chondrites. The Bennu samples, as well as those from asteroid Ryugu and CI chondrites, appear to have accreted from a distinct reservoir that may have been in the outer protoplanetary disk.

Carbonaceous chondrite meteorites, which include the sulfide-rich “Yamato-type” chondrites (CYs), have undergone a complex history of aqueous and thermal alteration and offer crucial insights into early outer solar system conditions. In this study, we evaluate thermal infrared (TIR) reflectance spectra of three CY chondrites. We observe a broad spectral plateau near 10 μ m, a spectral signature that has been observed in remote observations of some primitive, low-albedo asteroids, including Jupiter Trojans. We compare our data to CY emissivity spectra, spectra of Fe-sulfide and olivine mixtures, and remote Jupiter Trojan observations and establish the plateau and low albedo are a result of a high content of fine-particulate Fe-sulfide of these meteorites. We therefore suggest that D- and P-type asteroids, like Jupiter’s Trojan asteroids, could have a high abundance of Fe sulfide on their surfaces as a potential result of aqueous alteration followed by dehydration, shedding light on the processes shaping the outer solar system.

Thermal inertia is used to infer physical properties of asteroid surfaces. The carbonaceous asteroid Bennu has low thermal inertia suggestive of a surface covered in sub-centimeter rock fragments. However, spacecraft observations revealed that Bennu is instead blanketed by boulders of differing physical properties, with the most abundant population displaying very low thermal inertia compared to carbonaceous chondritic meteorites. Here we show that morphologically distinct particles in samples returned from Bennu also possess distinct thermal and physical properties, consistent with their genetic connection to the boulders. Angular particles have higher thermal inertia, greater hardness, and fewer but longer cracks that lead to more efficient splitting, relative to the hummocky particles. A hummocky particle exhibits low thermal inertia at sub-millimeter scales due to fine pores. Tortuous crack networks in hummocky particles further reduce thermal inertia while resisting disaggregation. Samples from Ryugu, a carbonaceous asteroid with similarly low thermal inertia, have cracks like those in Bennu’s hummocky particles yet have bulk densities that indicate lower porosity. These observations imply that the low thermal inertia of both asteroids is driven by cracks in rocks resulting from geological processes within the parent body or, more recently, micrometeoroid impacts and thermal fatigue.

Samples of asteroid (101955) Bennu delivered by the OSIRIS-REx mission offer the opportunity to study pristine planetary materials unchanged by exposure to the terrestrial environment. Here we use a combination of X-ray diffraction and various electron microscopy techniques to explore the detailed mineralogy of Bennu samples and determine the alteration history of the planetesimal protolith from which they originated. The samples consist largely of hydrated sheet-silicate minerals, namely nanoscale serpentine and saponite of varied grain size, which are decorated with micro- to nanoscale Fe-sulfides, magnetite and carbonates. We observe sheet silicates parallel and normal to sulfide surfaces and as inclusions in sulfides; sulfur-rich veins transecting the sheet-silicate matrix; zoned carbonates and phosphates and sulfide and magnetite grains exhibiting embayment. The mineralogical evidence indicates alteration of accreted minerals by a fluid that evolved with time, leading to etching, dissolution and reprecipitation. Sulfide compositions indicate alteration at ~25 °C, similar to conditions inferred for asteroid (162173) Ryugu and Ivuna-type (CI) chondrite meteorites. The fluid probably evolved from neutral to alkaline, culminating with the precipitation of highly soluble salts. We conclude that Bennu’s protolith comprised mainly nanometre to micrometre silicates, with fewer chondrules and calcium–aluminium-rich inclusions than those of most chondrite groups.

The Meteorite Hills dense collection area in the Transantarctic Mountains has yielded 1130 meteorites over several ANSMET field seasons. Twenty‐three CM carbonaceous chondrites were recovered as part of the 2000–2001 and 2001–2002 field seasons. Many of these CMs have unique or rare features, but most are small (χ values between 3.7 and 3.9, but there are a few exceptions such as MET 00432 (4.85), MET 01076 and 77 (4.06 and 4.63, respectively), and MET 01073 (3.21). Fifteen of the samples exhibit intermediate to high levels of aqueous alteration with phyllosilicate fractions (PSF) of 0.88–0.93. A trio of samples exhibit even higher levels of alteration with PSFs of 0.96–0.98. Find locations and cosmic ray exposure (CRE) ages of these two groups are similar and the latter very short at 0.1–0.2 Ma, raising the possibility that they are all part of the same heterogeneous fall. Since the three heavily altered samples are rare and have distinctive mineralogy relative to other MET CMs, they should be preserved regardless of whether they are from one large fall or two separate falls. Two samples (MET 01076 and MET 01077) contain a much greater fraction of olivine and pyroxene, have longer CRE ages, and most likely are heated CM chondrites. Three samples are unpaired and have unique characteristics: MET 00432 has a high magnetite fraction and other mineralogical and chemical properties comparable to C2 ungrouped chondrites such as Tagish Lake and Tarda, while MET 001087 (PSF = 0.77) and MET 00633 (PSF = 0.76) are less aqueously altered than the other meteorites, with the former in particular showing a significant tochilinite peak in its XRD pattern. Although MET 00633 could arguably be part of the larger pairing group of samples given its similar find location, we recommend keeping it unpaired given its distinct mineralogy.

Evaporation or freezing of water-rich fluids with dilute concentrations of dissolved salts can produce brines, as observed in closed basins on Earth ¹ and detected by remote sensing on icy bodies in the outer Solar System ^2,3 . The mineralogical evolution of these brines is well understood in regard to terrestrial environments ⁴ , 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 mission ⁵ . 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 plumes ^2,3 .

Reckling Peak (RKP) 17085 is a newly classified Antarctic CM chondrite that preserves a complex alteration history characterized by mild aqueous alteration (CM2.7), overprinted by a short‐lived thermal metamorphic event (heating stage III [<750°C]), and affected by low‐grade terrestrial weathering. This meteorite contains abundant Fe‐rich bands within the fine‐grained matrix, composed of micron‐scale Fe‐oxyhydroxide minerals. They are interpreted as “alteration fronts” arising due to the dissolution and transport of Fe (typically <500 μm) before being abruptly deposited. This alteration texture is relatively rare among hydrated carbonaceous chondrites, with only five reported instances to date (Murchison, Murray, Allan Hills 81002, Miller Range 07687, and Northwest Africa 5958). Evidence from RKP 17085 suggests that early aqueous alteration operated as multiple geochemically isolated microenvironments, which moved outwards from local point sources within the matrix. Low permeability fine‐grained rims on chondrules appear to have acted as barriers to fluid flow, controlling the migration of fluid across the parent body. Furthermore, the higher porosity regions within the altered fine‐grained matrix represent either void space generated by the dehydration of hydrated minerals during post‐hydration metamorphism and/or sites of ice accretion (water‐ice or C‐bearing ices) preserved within a mildly altered primitive matrix.

On September 24, 2023, NASA's OSIRIS‐REx mission dropped a capsule to Earth containing ~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 submicron dust to a stone ~3.5 cm long. Millimeter‐scale and larger stones typically have hummocky or angular morphologies. Some stones appear 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 Bennu's surface 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, Mg,Na‐rich phosphates, as well as other trace phases. The sample's 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.