A primordial noble gas component discovered in the Ryugu asteroid and its implications.

Ryugu is the C-type asteroid from which material was brought to Earth by the Hayabusa2 mission. A number of individual grains and fine-grained samples analysed so far for noble gases have indicated that solar wind and planetary (known as P1) noble gases are present in Ryugu samples with concentrations higher than those observed in CIs, suggesting the former to be more primitive compared to the latter. Here we present results of analyses of three fine-grained samples from Ryugu, in one of which Xe concentration is an order of magnitude higher than determined so far in other samples from Ryugu. Isotopically, this Xe resembles P1, but with a much stronger isotopic fractionation relative to solar wind and significantly lower 36Ar/132Xe ratio than in P1. This previously unknown primordial noble gas component (here termed P7) provides clues to constrain how the solar composition was fractionated to form the planetary components.

Breunnerite grain and magnesium isotope chemistry reveal cation partitioning during aqueous alteration of asteroid Ryugu.

Returned samples from the carbonaceous asteroid (162173) Ryugu provide pristine information on the original aqueous alteration history of the Solar System. Secondary precipitates, such as carbonates and phyllosilicates, reveal elemental partitioning of the major component ions linked to the primordial brine composition of the asteroid. Here, we report on the elemental partitioning and Mg isotopic composition (25Mg/24Mg) of breunnerite [(Mg, Fe, Mn)CO3] from the Ryugu C0002 sample and the A0106 and C0107 aggregates by sequential leaching extraction of salts, exchangeable ions, carbonates, and silicates. Breunnerite was the sample most enriched in light Mg isotopes, and the 25Mg/24Mg value of the fluid had shifted lower by ~0.38‰ than the initial value (set to 0‰) before dolomite precipitation. As a simple model, the Mg2+ first precipitated in phyllosilicates, followed by dolomite precipitation, at which time ~76-87% of Mg2+ had been removed from the primordial brine. A minor amount of phyllosilicate precipitation continued after dolomite precipitation. The element composition profiles of the latest solution that interacted with the cation exchange pool of Ryugu were predominantly Na-rich. Na+ acts as a bulk electrolyte and contributes to the stabilization of the negative surface charge of phyllosilicates and organic matter on Ryugu.

Variations of organic functional chemistry in carbonaceous matter from the asteroid 162173 Ryugu.

Primordial carbon delivered to the early earth by asteroids and meteorites provided a diverse source of extraterrestrial organics from pre-existing simple organic compounds, complex solar-irradiated macromolecules, and macromolecules from extended hydrothermal processing. Surface regolith collected by the Hayabusa2 spacecraft from the carbon-rich asteroid 162173 Ryugu present a unique opportunity to untangle the sources and processing history of carbonaceous matter. Here we show carbonaceous grains in Ryugu can be classified into three main populations defined by spectral shape: Highly aromatic (HA), Alkyl-Aromatic (AA), and IOM-like (IL). These carbon populations may be related to primordial chemistry, since C and N isotopic compositions vary between the three groups. Diffuse carbon is occasionally dominated by molecular carbonate preferentially associated with coarse-grained phyllosilicate minerals. Compared to related carbonaceous meteorites, the greater diversity of organic functional chemistry in Ryugu indicate the pristine condition of these asteroid samples.

Primordial aqueous alteration recorded in water-soluble organic molecules from the carbonaceous asteroid (162173) Ryugu.

We report primordial aqueous alteration signatures in water-soluble organic molecules from the carbonaceous asteroid (162173) Ryugu by the Hayabusa2 spacecraft of JAXA. Newly identified low-molecular-weight hydroxy acids (HO-R-COOH) and dicarboxylic acids (HOOC-R-COOH), such as glycolic acid, lactic acid, glyceric acid, oxalic acid, and succinic acid, are predominant in samples from the two touchdown locations at Ryugu. The quantitative and qualitative profiles for the hydrophilic molecules between the two sampling locations shows similar trends within the order of ppb (parts per billion) to ppm (parts per million). A wide variety of structural isomers, including α- and ß-hydroxy acids, are observed among the hydrophilic molecules. We also identify pyruvic acid and dihydroxy and tricarboxylic acids, which are biochemically important intermediates relevant to molecular evolution, such as the primordial TCA (tricarboxylic acid) cycle. Here, we find evidence that the asteroid Ryugu samples underwent substantial aqueous alteration, as revealed by the presence of malonic acid during keto-enol tautomerism in the dicarboxylic acid profile. The comprehensive data suggest the presence of a series for water-soluble organic molecules in the regolith of Ryugu and evidence of signatures in coevolutionary aqueous alteration between water and organics in this carbonaceous asteroid.

Nonmagnetic framboid and associated iron nanoparticles with a space-weathered feature from asteroid Ryugu.

Extraterrestrial minerals on the surface of airless Solar System bodies undergo gradual alteration processes known as space weathering over long periods of time. The signatures of space weathering help us understand the phenomena occurring in the Solar System. However, meteorites rarely retain the signatures, making it impossible to study the space weathering processes precisely. Here, we examine samples retrieved from the asteroid Ryugu by the Hayabusa2 spacecraft and discover the presence of nonmagnetic framboids through electron holography measurements that can visualize magnetic flux. Magnetite particles, which normally provide a record of the nebular magnetic field, have lost their magnetic properties by reduction via a high-velocity (>5 km s-1) impact of a micrometeoroid with a diameter ranging from 2 to 20 µm after destruction of the parent body of Ryugu. Around these particles, thousands of metallic-iron nanoparticles with a vortex magnetic domain structure, which could have recorded a magnetic field in the impact event, are found. Through measuring the remanent magnetization of the iron nanoparticles, future studies are expected to elucidate the nature of the nebular/interplanetary magnetic fields after the termination of aqueous alteration in an asteroid.

Microstructural and chemical features of impact melts on Ryugu particle surfaces: Records of interplanetary dust hit on asteroid Ryugu.

The Hayabusa2 spacecraft delivered samples of the carbonaceous asteroid Ryugu to Earth. Some of the sample particles show evidence of micrometeoroid impacts, which occurred on the asteroid surface. Among those, particles A0067 and A0094 have flat surfaces on which a large number of microcraters and impact melt splashes are observed. Two impact melt splashes and one microcrater were analyzed to unveil the nature of the objects that impacted the asteroid surface. The melt splashes consist mainly of Mg-Fe-rich glassy silicates and Fe-Ni sulfides. The microcrater trapped an impact melt consisting mainly of Mg-Fe-rich glassy silicate, Fe-Ni sulfides, and minor silica-rich glass. These impact melts show a single compositional trend indicating mixing of Ryugu surface materials and impactors having chondritic chemical compositions. The relict impactor in one of the melt splashes shows mineralogical similarity with anhydrous chondritic interplanetary dust particles having a probable cometary origin. The chondritic micrometeoroids probably impacted the Ryugu surface during its residence in a near-Earth orbit.

Reassigning CI chondrite parent bodies based on reflectance spectroscopy of samples from carbonaceous asteroid Ryugu and meteorites.

The carbonaceous asteroid Ryugu has been explored by the Hayabusa2 spacecraft to elucidate the actual nature of hydrous asteroids. Laboratory analyses revealed that the samples from Ryugu are comparable to unheated CI carbonaceous chondrites; however, reflectance spectra of Ryugu samples and CIs do not coincide. Here, we demonstrate that Ryugu sample spectra are reproduced by heating Orgueil CI chondrite at 300°C under reducing conditions, which caused dehydration of terrestrial weathering products and reduction of iron in phyllosilicates. Terrestrial weathering of CIs accounts for the spectral differences between Ryugu sample and CIs, which is more severe than space weathering that likely explains those between asteroid Ryugu and the collected samples. Previous assignments of CI chondrite parent bodies, i.e., chemically most primitive objects in the solar system, are based on the spectra of CI chondrites. This study indicates that actual spectra of CI parent bodies are much darker and flatter at ultraviolet to visible wavelengths than the spectra of CI chondrites.

Polycyclic aromatic hydrocarbons in samples of Ryugu formed in the interstellar medium.

Polycyclic aromatic hydrocarbons (PAHs) contain ≲20% of the carbon in the interstellar medium. They are potentially produced in circumstellar environments (at temperatures ≳1000 kelvin), by reactions within cold (~10 kelvin) interstellar clouds, or by processing of carbon-rich dust grains. We report isotopic properties of PAHs extracted from samples of the asteroid Ryugu and the meteorite Murchison. The doubly-13C substituted compositions (Δ2×13C values) of the PAHs naphthalene, fluoranthene, and pyrene are 9 to 51‰ higher than values expected for a stochastic distribution of isotopes. The Δ2×13C values are higher than expected if the PAHs formed in a circumstellar environment, but consistent with formation in the interstellar medium. By contrast, the PAHs phenanthrene and anthracene in Ryugu samples have Δ2×13C values consistent with formation by higher-temperature reactions.

Soluble organic matter Molecular atlas of Ryugu reveals cold hydrothermalism on C-type asteroid parent body.

The sample from the near-Earth carbonaceous asteroid (162173) Ryugu is analyzed in the context of carbonaceous meteorites soluble organic matter. The analysis of soluble molecules of samples collected by the Hayabusa2 spacecraft shines light on an extremely high molecular diversity on the C-type asteroid. Sequential solvent extracts of increasing polarity of Ryugu samples are analyzed using mass spectrometry with complementary ionization methods and structural information confirmed by nuclear magnetic resonance spectroscopy. Here we show a continuum in the molecular size and polarity, and no organomagnesium molecules are detected, reflecting a low temperature and water-rich environment on the parent body approving earlier mineralogical and chemical data. High abundance of sulfidic and nitrogen rich compounds as well as high abundance of ammonium ions confirm the water processing. Polycyclic aromatic hydrocarbons are also detected in a structural continuum of carbon saturations and oxidations, implying multiple origins of the observed organic complexity, thus involving generic processes such as earlier carbonization and serpentinization with successive low temperature aqueous alteration.

Chemical evolution of primordial salts and organic sulfur molecules in the asteroid 162173 Ryugu.

Samples from the carbonaceous asteroid (162173) Ryugu provide information on the chemical evolution of organic molecules in the early solar system. Here we show the element partitioning of the major component ions by sequential extractions of salts, carbonates, and phyllosilicate-bearing fractions to reveal primordial brine composition of the primitive asteroid. Sodium is the dominant electrolyte of the salt fraction extract. Anions and NH4+ are more abundant in the salt fraction than in the carbonate and phyllosilicate fractions, with molar concentrations in the order SO42- > Cl- > S2O32- > NO3- > NH4+. The salt fraction extracts contain anionic soluble sulfur-bearing species such as Sn-polythionic acids (n < 6), Cn-alkylsulfonates, alkylthiosulfonates, hydroxyalkylsulfonates, and hydroxyalkylthiosulfonates (n < 7). The sulfur-bearing soluble compounds may have driven the molecular evolution of prebiotic organic material transforming simple organic molecules into hydrophilic, amphiphilic, and refractory S allotropes.

Visualization of nanoscale magnetic domain states in the asteroid Ryugu.

In the samples collected from the asteroid Ryugu, magnetite displays natural remanent magnetization due to nebular magnetic field, whereas contemporaneously grown iron sulfide does not display stable remanent magnetization. To clarify this counterintuitive feature, we observed their nanoscale magnetic domain structures using electron holography and found that framboidal magnetites have an external magnetic field of 300 A m-1, similar to the bulk value, and its magnetic stability was enhanced by interactions with neighboring magnetites, permitting a disk magnetic field to be recorded. Micrometer-sized pyrrhotite showed a multidomain magnetic structure that was unable to retain natural remanent magnetization over a long time due to short relaxation time of magnetic-domain-wall movement, whereas submicron-sized sulfides formed a nonmagnetic phase. These results show that both magnetite and sulfide could have formed simultaneously during the aqueous alteration in the parent body of the asteroid Ryugu.

Uracil in the carbonaceous asteroid (162173) Ryugu.

The pristine sample from the near-Earth carbonaceous asteroid (162173) Ryugu collected by the Hayabusa2 spacecraft enabled us to analyze the pristine extraterrestrial material without uncontrolled exposure to the Earth's atmosphere and biosphere. The initial analysis team for the soluble organic matter reported the detection of wide variety of organic molecules including racemic amino acids in the Ryugu samples. Here we report the detection of uracil, one of the four nucleobases in ribonucleic acid, in aqueous extracts from Ryugu samples. In addition, nicotinic acid (niacin, a B3 vitamer), its derivatives, and imidazoles were detected in search for nitrogen heterocyclic molecules. The observed difference in the concentration of uracil between A0106 and C0107 may be related to the possible differences in the degree of alteration induced by energetic particles such as ultraviolet photons and cosmic rays. The present study strongly suggests that such molecules of prebiotic interest commonly formed in carbonaceous asteroids including Ryugu and were delivered to the early Earth.

Insights into the formation and evolution of extraterrestrial amino acids from the asteroid Ryugu.

All life on Earth contains amino acids and carbonaceous chondrite meteorites have been suggested as their source at the origin of life on Earth. While many meteoritic amino acids are considered indigenous, deciphering the extent of terrestrial contamination remains an issue. The Ryugu asteroid fragments (JAXA Hayabusa2 mission), represent the most uncontaminated primitive extraterrestrial material available. Here, the concentrations of amino acids from two particles from different touchdown sites (TD1 and TD2) are reported. The concentrations show that N,N-dimethylglycine (DMG) is the most abundant amino acid in the TD1 particle, but below detection limit in the other. The TD1 particle mineral components indicate it experienced more aqueous alteration. Furthermore, the relationships between the amino acids and the geochemistry suggest that DMG formed on the Ryugu progenitor body during aqueous alteration. The findings highlight the importance of aqueous chemistry for defining the ultimate concentrations of amino acids in primitive extraterrestrial samples.

Chondrule-like objects and Ca-Al-rich inclusions in Ryugu may potentially be the oldest Solar System materials.

Chondrule-like objects and Ca-Al-rich inclusions (CAIs) are discovered in the retuned samples from asteroid Ryugu. Here we report results of oxygen isotope, mineralogical, and compositional analysis of the chondrule-like objects and CAIs. Three chondrule-like objects dominated by Mg-rich olivine are 16O-rich and -poor with Δ17O (=δ17O - 0.52 × Î´18O) values of ~ -23‰ and ~ -3‰, resembling what has been proposed as early generations of chondrules. The 16O-rich objects are likely to be melted amoeboid olivine aggregates that escaped from incorporation into 16O-poor chondrule precursor dust. Two CAIs composed of refractory minerals are 16O-rich with Δ17O of ~ -23‰ and possibly as old as the oldest CAIs. The discovered objects (<30 µm) are as small as those from comets, suggesting radial transport favoring smaller objects from the inner solar nebula to the formation location of the Ryugu original parent body, which is farther from the Sun and scarce in chondrules. The transported objects may have been mostly destroyed during aqueous alteration in the Ryugu parent body.

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

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.