Asteroid 4 Vesta is thought to be a differentiated object with intact internal structure due to its achondritic surface. In this paper, we report K‐L band spectroscopic observations of Vesta. We have detected the presence of a 3‐μm absorption feature at about the 1% level on the surface of Vesta at longitudes between 155° and 195°. This result indicates that OH and/or H2O‐bearing minerals are present in this region of Vesta; the sources are plausibly fragments of carbonaceous chondrite impactors or solar wind implantation. The detection of 3‐μm absorption features from Vesta, regarded as the smallest terrestrial planet, may provide clues to the origin of volatile materials on terrestrial planets.

Chondritic meteorites (chondrites) contain evidence for the interaction of liquid water with the interiors of small bodies early in Solar System history. Here we review the processes, products and timings of the low-temperature aqueous alteration reactions in CR, CM, CI and ungrouped carbonaceous chondrites, the asteroids Ryugu and Bennu, and hydrated dark clasts in different types of meteorites. We first consider the nature of chondritic lithologies and the insights that they provide into alteration conditions, subdivided by the mineralogy and petrology of hydrated chondrites, the mineralogy of hydrated dark clasts, the effects of alteration on presolar grains, and the evolution of organic matter. We then describe the properties of the aqueous fluids and how they reacted with accreted material as revealed by physicochemical modelling and hydrothermal experiments, the analysis of fluid inclusions in aqueously formed minerals, and isotope tracers. Lastly, we outline the chronology of aqueous alteration reactions as determined using the ⁵³Mn-⁵³Cr and ¹²⁹I-¹²⁹Xe systems.<h4>Supplementary information</h4>The online version contains supplementary material available at 10.1007/s11214-024-01132-8. The principal products of the reaction of water with other accreted materials were phyllosilicate minerals whose abundance helps to classify such altered carbonaceous chondrites into petrologic types 1 and 2. Samples of these hydrated lithologies are available as the CR (Renazzo-like), CM (Mighei-like), and CI (Ivuna-like) meteorites, ungrouped carbonaceous chondrites, clasts that occur in a wide variety of meteorite groups, and grains returned from the Cb-type asteroid Ryugu and B-type asteroid Bennu. Hydrated lithologies include interplanetary dust particles and Antarctic micrometeorites (e.g., Bradley 2014 ), but they are not covered here. Next, this article explores the properties of the aqueous fluids, and how they reacted with the chondritic parent bodies (e.g., chemistry, temperature, water-rock (W/R) ratio, duration of water-rock interaction) as investigated by physicochemical modelling and hydrothermal experiments, by analyzing samples of water preserved as fluid inclusions, and through measurement of the oxygen, hydrogen, carbon, nitrogen and sulfur isotope compositions of bulk rocks and constituent minerals. This article finishes by describing the chronology of aqueous alteration as revealed by the 53 Mn- 53 Cr and 129 I- 129 Xe systems. As such, the 15 N-enrichments observed in metal-rich carbonaceous chondrite dark clasts are likely outer disk signatures from vertical mixing of small icy grains (van Kooten et al. 2024 ). Hydration of Clasts What Is the Protolith of These Clasts? Variations in In situ Analysis of Aqueously Formed Minerals The proportions of the original components of chondritic parent bodies (refractory inclusions, chondrules, Fe-Ni metal beads, sulfides, fine-grained matrix, and water ices) are highly variable between different chondrite groups. Heat released from the decay of short-lived 26 Al led to the melting of the water ice, resulting in the establishment of prolonged fluid alteration processes within asteroids commencing shortly after their accretion (Sect. 9 ). In water-rich chondrites, bulk δ D values range from − 300 ‰ in the ungrouped C1 chondrite Flensburg (Bischoff et al. 2021 ) to > + 600 ‰ in CRs and some C-ungrouped chondrites (see McCubbin and Barnes 2019 ; Piani et al. 2021 for compilations). In CM and CR chondrites, bulk H-isotope measurements reveal the presence of positive and distinct correlations between the δ D values and C/H ratios (Fig. S5A; Alexander et al. 2012 ). These correlations were interpreted to reflect mixing lines between a D-poor and C-free component (presumably the water from which the hydrated minerals formed) with D-rich organic matter. Hydrated amorphous silicates, that are ubiquitous in these two chondrites, were proposed as a H-bearing component that could retain H acquired before the onset of CM parent-body alteration (Marrocchi et al. 2023b ). Origin of H-Isotope Variations of Organic Matter and Water Among Chondrite Groups The highly variable H-isotope signatures of hydrated minerals and organics among chondritic materials (Fig. 7 ) can be interpreted in two ways. They can either be the result of secondary processes occurring in asteroids and disturbing the initial isotope signatures, or they reflect the heterogeneous distributions and compositions of water ice and organics in the protoplanetary disk at the time of asteroid accretion. In asteroids, the accreted ice, once melted, percolates among the solid phases. The main process thought to have affected the H-isotope composition in H-rich chondrites is H-isotope exchange between D-rich organic components and D-poor aqueous fluids (e.g., Alexander et al. 2010 ). However, no evidence for H-isotope exchange between organic particles and water retained in the surrounding hydrated minerals was observed at the micrometer scale in the matrix (Remusat et al. 2010 ) nor H-isotope evolution in CM chondrites of increasing alteration degrees (Fig. 7 ). Temperatures might explain the differences in exchange efficiency for water and organics in laboratory experiments performed above 150 °C (Oba and Naraoka 2009 ; Kebukawa et al. 2021 ; Foustoukos et al. 2021 ). In CM and CO chondrites, bulk δ 15 N value decreases with increasing degree of alteration, likely due the progressive removal of an organic 15 N-rich component during alteration (Pearson et al. 2006 ). The presence of extreme δ 15 N variations in the hydrated clasts of the CB/CH chondrite Isheyevo with D-depletion compared to organic matter were proposed to derive from the heterogeneous accretion of NH 3 and HCN-bearing ices during the cooling of the protoplanetary disk (van Kooten et al. 2017a ). Such ice reservoirs do not appear to have been sampled by other water-rich CCs.

The first bodies to form in the Solar System acquired their materials from stars, the presolar molecular cloud and the protoplanetary disk. Asteroids that have not undergone planetary differentiation retain evidence of these primary accreted materials. However, geologic processes such as hydrothermal alteration can dramatically change their bulk mineralogy, isotopic compositions and chemistry. Here we analyse the elemental and isotopic compositions of samples from asteroid Bennu to uncover the sources and types of material accreted by its parent body. We show that some primary accreted materials escaped the extensive aqueous alteration that occurred on the parent asteroid, including presolar grains from ancient stars, organic matter from the outer Solar System or molecular cloud, refractory solids that formed close to the Sun, and dust enriched in neutron-rich Ti isotopes. We find Bennu to be richer in isotopically anomalous organic matter, anhydrous silicates, and light isotopes of K and Zn than its closest compositional counterparts, asteroid Ryugu and Ivuna-type (CI) carbonaceous chondrite meteorites. We propose that the parent bodies of Bennu, Ryugu and CI chondrites formed from a common but spatially and/or temporally heterogeneous reservoir of materials in the outer protoplanetary disk. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/ . Abstract The first bodies to form in the Solar System acquired their materials from stars, the presolar molecular cloud and the protoplanetary disk. Asteroids that have not undergone planetary differentiation retain evidence of these primary accreted materials. It is a rubble pile, consisting of reaccumulated fragments of a much larger parent body (≥100 km) that was collisionally disrupted in the main asteroid belt 3 . Unlike meteorites, the pristine Bennu samples returned by OSIRIS-REx have not been subjected to heating from entry through Earth’s atmosphere and have experienced minimal or no interaction with the ambient atmosphere and biosphere. These qualities make them ideal for probing the nature and formation of early planetesimals, particularly their volatile and organic contents. This alteration was likely caused when water, carbon dioxide, ammonia 8 and other ices accreted by the parent body melted due to heat generated from the decay of short-lived radioactive nuclides and impact events. Detailed study of the returned samples is required to understand the diversity of materials accreted by the parent asteroid, the chemical and isotopic reservoirs in the protoplanetary disk where it formed, and the extent to which it was hydrothermally altered. We investigated the bulk elemental and isotopic composition of Bennu aggregate material—loose, unsorted particles <0.5 cm—and the in situ isotopic compositions of individual components, including presolar grains, organic matter and anhydrous silicates. Comparing the composition of Bennu samples with those of carbonaceous chondrites (CCs) and samples of asteroid (162173) Ryugu returned by the Japan Aerospace Exploration Agency’s Hayabusa2 mission 9 , 10 places the accretion history and chemical evolution of Bennu’s parent body in the broader context of other primitive astromaterials. Our data reinforce existing dynamical and geologic evidence for common histories of the parent bodies of Bennu and Ryugu 3 , 41 . The bulk solar elemental abundances in samples from both asteroids affirms their primitive nature (Extended Data Fig. 1 ). Their shared mineral inventories 2 , 12 , 41 indicate that both underwent hydrothermal alteration by alkaline, salt-rich water, before catastrophic disruption and subsequent reaccumulation into rubble-pile asteroids 3 , 41 . Two isotopically distinct reservoirs in the Solar System are well resolved, representing non-carbonaceous and carbonaceous astromaterials 24 , 44 . The neutron-rich Ti isotope signatures measured here suggest that the reservoir(s) sourcing the parent bodies of Bennu, Ryugu and CIs were distinct from those of all other chondritic meteorites. Moreover, the overlapping ranges of O isotopes in Bennu and Ryugu samples 9 , 26 (Extended Data Fig. 3 ) implies a common primordial source or exposure to similar physicochemical environments during early Solar System evolution. Bennu’s parent asteroid could have accreted in a reservoir located close to the water snowline that was seeded with sunwards-drifting ice, refractory solids and dust 47 . However, the CIs likely derive from parent bodies that accreted at distances >5 au (refs. 41 , 49 ). Our analyses of aggregate samples indicate that Bennu’s parent body experienced substantial aqueous alteration but preserved enough pre-accretion components from diverse stellar, interstellar and Solar System sources to provide insight into its early formation environment. There are genetic similarities in the main rock-forming elements between Bennu, Ryugu and CI materials, but also distinctions. In particular, the analysed Bennu samples contain more anhydrous silicates and isotopically anomalous organic matter than samples of the hydrated Ryugu lithology 29 , 30 , 41 and Orgueil 39 . The difference with Bennu samples is believed to be related to the additional oxygen present in the meteorites as a result of formation of ferrihydrite and sulfates through interaction with Earth’s atmosphere, as these phases have not been observed in either the Ryugu or Bennu samples, plus the abundant interlayer water present in CIs 9 . While the low yield has the potential to induce unwanted isotopic effects, the high temperatures associated with the laser-assisted fluorination should minimize any isotopic fractionation effects.

Abstract Large main-belt asteroids (diameter D ≳ 120 km) exhibit a surface composition gradient as a function of heliocentric distance, ranging from anhydrous bodies to those rich in hydrated and, possibly, ammoniated materials. Their primordial nature holds key clues to the evolution of the solar system. It has been suggested that volatile-rich bodies formed in the outer solar system and were implanted into the main belt. Alternatively, volatiles may have been delivered via inward-drifting icy pebbles in the protosolar disk. Here, we examine whether in situ formed rocky embryos can acquire volatiles through pebble accretion as the snow line migrated inward. With the turbulence strength of the disk, radial pebble flux, and the dimensionless stopping time of pebbles scaled with the Keplerian frequency (i.e., Stokes number (St)) as parameters, we calculate the growth of large asteroids. The results are then compared with mass and compositional constraints based on asteroid observations. We find that a moderate pebble flux (≲18 M ⊕ Myr −1 ) is required to enable volatile delivery while preventing the largest asteroids from becoming more massive than Ceres. Water accretion is feasible with St ∼ 10 −3 (∼1 mm). However, only the largest asteroids ( D ≳ 200 km) can accumulate sufficient ammonia under such conditions. For most asteroids with D ≃ 100–200 km, ammonia ice accretion requires St ∼ 10 −4 (∼100 μ m). Such small particle sizes may pose both theoretical and observational challenges. Thus, we propose that the intermediate-sized, potentially ammonia-bearing asteroids serve as a record of the solar system’s dynamic evolution.

Dawn found Ceres's surface to be a mixture of water, ice, and hydrated minerals such as carbonates and clay. Gravity data suggest Ceres to be partially Ceres is a dwarf planet in the main asteroid belt between the orbits of Mars and Jupiter. It was the first object identified in the asteroid belt, discovered on 1 January 1801 by Giuseppe Piazzi at Palermo Astronomical Observatory in Sicily, and announced as a new planet. Ceres was later classified as an asteroid, and then more recently as the only confirmed dwarf planet within the asteroid belt, Ceres is a dwarf planet in the main asteroid belt between the orbits of Mars and Jupiter. It was the first object identified in the asteroid belt, discovered on 1 January 1801 by Giuseppe Piazzi at Palermo Astronomical Observatory in Sicily, and announced as a new planet. Ceres was later classified as an asteroid, and then more recently as the only confirmed dwarf planet within the asteroid belt, and the largest without a moon. It is also the only recognized dwarf planet in the solar system whose orbit lies within that of Neptune. In the minor planet numbering system, its designation is 1 Ceres or (1) Ceres. Ceres's diameter is about a quarter that of the Moon. Its small size means that even at its brightest it is too dim to be seen by the naked eye, except under extremely dark skies. Its apparent magnitude ranges from 6.7 to 9.3, peaking at opposition (when it is closest to Earth) once every 15- to 16-month synodic period. As a result, its surface features are barely visible even with the most powerful telescopes, and little was known about it until the robotic NASA spacecraft Dawn approached Ceres for its orbital mission in 2015. Dawn found Ceres's surface to be a mixture of water, ice, and hydrated minerals such as carbonates and clay. Gravity data suggest Ceres to be partially differentiated into a muddy (ice–rock) mantle/core and a less dense, but stronger crust that is at most thirty percent ice by volume. Although Ceres is a dwarf planet in the main asteroid belt between the orbits of Mars and Jupiter. It was the first object identified in the asteroid belt, discovered on 1 January 1801 by Giuseppe Piazzi at Palermo Astronomical Observatory in Sicily, and announced as a new planet. Ceres was later classified as an asteroid, and then more recently as the only confirmed dwarf planet within the asteroid belt, and the largest without a moon. It is also the only recognized dwarf planet in the solar system whose orbit lies within that of Neptune. In the minor planet numbering system, its designation is 1 Ceres or (1) Ceres. Ceres's diameter is about a quarter that of the Moon. Its small size means that even at its brightest it is too dim to be seen by the naked eye, except under extremely dark skies. Its apparent magnitude ranges from 6.7 to 9.3, peaking at opposition (when it is closest to Earth) once every 15- to 16-month synodic period. As a result, its surface features are barely visible even with the most powerful telescopes, and little was known about it until the robotic NASA spacecraft Dawn approached Ceres for its orbital mission in 2015. Dawn found Ceres's surface to be a mixture of water, ice, and hydrated minerals such as carbonates and clay. Gravity data suggest Ceres to be partially differentiated into a muddy (ice–rock) mantle/core and a less dense, but stronger crust that is at most thirty percent ice by volume. Although Ceres likely has an internal ocean of liquid water, brines still flow through the outer mantle and reach the surface, allowing cryovolcanoes such as Ahuna Mons to form roughly every fifty million years. This makes Ceres the closest known cryovolcanically active body to the Sun. Ceres has an extremely tenuous and transient atmosphere of water vapour, vented from localised sources on its surface. Ceres follows an orbit between Mars and Jupiter, near the middle of the asteroid belt, with an orbital period of 4.6 Earth years. Compared to other planets and dwarf planets, Ceres's orbit is moderately tilted relative to that of Earth; its inclination (i) is 10.6°, compared to 7° for Mercury and 17° for Pluto. It is also slightly elongated, with an eccentricity (e) = 0.08, compared to 0.09 for Mars. Ceres is the only widely accepted dwarf planet with an orbital period less than that of Neptune. Ceres is not part of an asteroid family, probably due to its large proportion of ice, as smaller bodies with the same composition would have sublimated to nothing over the age Ceres is the largest asteroid in the main asteroid belt. It has been classified as a C‑type or carbonaceous asteroid and, due to the presence of clay minerals, as a G-type asteroid. It has a similar, but not identical, composition to that of carbonaceous chondrite meteorites. It is an oblate spheroid, with an equatorial diameter 8% larger than its polar diameter. Measurements from the Dawn spacecraft found a mean diameter of 939.4 km (583.7 mi) and a mass of 9.38×1020 kg. This gives Ceres a density of 2.16 g/cm3, suggesting that a quarter of its mass is water ice. Ceres makes up 40% of the estimated (2394±5)×1018 kg mass of the asteroid belt, and it has 3+1⁄2 times the mass of the next asteroid, Vesta, but it has only 1⁄78 the mass of the Moon, and its surface gravity is 1⁄35 that of Earth (1⁄6 of the Moon's). It is close to being in hydrostatic equilibrium, but some deviations from an equilibrium shape have yet to be explained. Modelling has suggested Ceres's rocky material is partially differentiated, and that it may possess a small core, but the data is also consistent with a mantle of hydrated silicates and no core. Because Dawn lacked a magnetometer, it is not known if Ceres has a magnetic field; it is believed not to. Ceres's internal differentiation may be related to its lack of a natural satellite, as satellites of main belt asteroids are mostly believed to form from collisional disruption, creating an undifferentiated, rubble pile structure.

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.