Carbonaceous Chondrite Parent Bodies Research Articles

Aqueous alteration in icy planetesimals: The effect of outward transport of gaseous hydrogen

Parent bodies of carbonaceous chondrites that initially contained metallic iron potentially exert strong reduction power during aqueous alteration to generate molecular hydrogen in excess of hydrogen solubility in water-rich fluids. The surplus hydrogen escapes from the system, which is subsequently supplied to overlying regions in planetesimals. Based on this concept, we conducted chemical equilibrium modeling of the aqueous alteration and simulated gaseous H2 migration within the icy planetesimal that has a melted mantle and an icy shell during the early stages of radiogenic heating. In the chemical equilibrium modeling, we simulated the aqueous alteration of chondritic rocks at 0–350 °C and a water/rock mass ratio of 0.2–10 with initial CO2 contents of 0–10 mol% in the fluid. The results showed that the mineral assemblage and solution composition change with the temperature, water/rock mass ratio, and initial fluid composition. The reproduced mineral paragenesis and abundance well explain those of carbonaceous chondrites. Furthermore, it was revealed that the initial H2 fugacity of the system influences not only the stability of minerals and solution compositions, but also the preservation potential of organic molecules. Indeed, within these parameter spaces, the modeling results account for the organic/inorganic carbon-rich alterations reported for the Tagish Lake meteorite, Ceres, and Ryugu. Simulations of gaseous H2 migration in a planetesimal revealed that gaseous H2 in the deep interior can be transported to the interface with an icy shell even if the permeability is low. Moreover, it is highly possible that an H2-rich layer would have been widely formed just below the icy shell. Therefore, it is expected that H2-rich regions beneath the ice layer in planetesimals have substantial potential for the synthesis and preservation of organic molecules. These results imply that the alteration of carbonaceous chondrite parent bodies and C-complex asteroids is characterized by not only the type of parent bodies (e.g., formation age and distance from the Sun) but also the locations within their parent bodies.

Large Carbonaceous Chondrite Parent Bodies Favored by Abundance–Volatility Modeling: A Possible Chemical Signature of Pebble Accretion

Primitive meteorite groups such as the Vigarano, Mighei, and Karoonda carbonaceous chondrites have enigmatic patterns of elemental abundances, with moderately volatile elements—those that transition from vapor to condensate between ∼400 and ∼900 K—defining plateaus of subequal abundances despite a wide range in volatility. In detail, each group defines a plateau with distinctive nonmonotonic “chemical fingerprints” that have been attributed to combinations of mixing, vaporization/condensation, and fluid-mediated metasomatism—but the extent to which these processes can reproduce the observed variability has not been quantified. Starting with primitive Ivuna chondrite, a two-stage, two-component equilibrium condensation–vaporization model—with gravity implemented as Jeans escape—can explain large-scale plateaus in these chondrite groups, as well as more complex, nonmonotonic small-scale variations. For all three chondritic meteorite groups, models favor earlier high-temperature fractionation under low-gravity conditions followed by a low-temperature fractionation event that took place on a protoplanet at least as large as Ceres. The second fractionation event may represent the fractionation of incoming materials to the planetesimal during protracted pebble accretion. Models with only thermally driven volatile loss, gravity, and mixing can explain more than 80% of the observed compositional variability in these meteorite groups. In our five-parameter model, using only five randomly selected elements yields uselessly large ranges of planet sizes and temperatures, ranges that converge with increasing numbers of elements. These results suggest that even simple models are prone to generating inaccurate conclusions when constrained by too few observations, a fault likely held by more complex models as well.

Gamma-Ray-Induced Amino Acid Formation during Aqueous Alteration in Small Bodies: The Effects of Compositions of Starting Solutions.

Organic compounds, such as amino acids, are essential for the origin of life, and they may have been delivered to the prebiotic Earth from extra-terrestrial sources, such as carbonaceous chondrites. In the parent bodies of carbonaceous chondrites, the radioactive decays of short-lived radionuclides, such as 26Al, cause the melting of ice, and aqueous alteration occurs in the early stages of solar system formation. Many experimental studies have shown that complex organic matter, including amino acids and high-molecular-weight organic compounds, is produced by such hydrothermal processes. On the other hand, radiation, particularly gamma rays from radionuclides, can contribute to the formation of amino acids from simple molecules such as formaldehyde and ammonia. In this study, we investigated the details of gamma-ray-induced amino acid formation, focusing on the effects of different starting materials on aqueous solutions of formaldehyde, ammonia, methanol, and glycolaldehyde with various compositions, as well as hexamethylenetetramine. Alanine and glycine were the most abundantly formed amino acids after acid hydrolysis of gamma-ray-irradiated products. Amino acid formation increased with increasing gamma-ray irradiation doses. Lower amounts of ammonia relative to formaldehyde produced more amino acids. Glycolaldehyde significantly increased amino acid yields. Our results indicated that glycolaldehyde formation from formaldehyde enhanced by gamma rays is key for the subsequent production of amino acids.

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.

Analysis of layered boulders on asteroid (101955) Bennu and their implications for fluid flow on the parent body

AbstractThe observation of carbonate veins on asteroid Bennu supports the idea that large‐scale water flow may have occurred in carbonaceous asteroids in the early solar system. We identified and analyzed 11 boulders with layered structures on asteroid Bennu's surface using high‐resolution (centimeter‐scale) image and altimetry data obtained by the OSIRIS‐REx mission. The boulders' linear layer boundaries and parallel bedding follow the principle of original horizontality and suggest that they formed from sediment deposition by fluid flow on Bennu's parent body. We developed a simple model of the parent body (100‐km diameter with the density of CM chondrite material) and found that the water flow velocity had to be at least 21.1 cm s−1 to transport the largest clast observed embedded in a layered rock, which is 85 cm in average length. The flow velocity could have been as high as 26.5 cm s−1 if a larger clast observed on top of a layered rock was once embedded therein. Our results strongly support open‐system aqueous alteration on carbonaceous chondrite parent bodies.

Neodymium‐142 deficits and samarium neutron stratigraphy of C‐type asteroid (162173) Ryugu

AbstractWe report Nd and Sm isotopic compositions of four samples of Ryugu returned by the Hayabusa2 mission, including “A” (first touchdown) and “C” (second touchdown) samples, and several carbonaceous chondrites to evaluate potential genetic relationships between Ryugu and known chondrite groups and track the cosmic ray exposure history of Ryugu. We resolved Nd and Sm isotopic anomalies in small (<20 ng Nd and Sm) sample sizes via thermal ionization mass spectrometer using 1013 Ω amplifiers. Ryugu samples exhibit resolvable negative μ142Nd values consistent with carbonaceous chondrite values, suggesting that Ryugu is related to the parent bodies of carbonaceous chondrites. Ryugu's negative μ149Sm values are the result of exposure to galactic cosmic rays, as demonstrated by the correlation between 150Sm/152Sm and 149Sm/152Sm ratios that fall along the expected neutron capture correlation line. The neutron fluence calculated in the “A” samples (2.75 ± 1.94 × 1015 n cm−2) is slightly higher compared to the “C” samples (0.95 ± 2.04 × 1015 n cm−2), though overlapping within measurement uncertainty. The Sm results for Ryugu, at this level of precision, thus are consistent with a well‐mixed surface layer at least to the depths from which the “A” and “C” samples derive.

Abundant extraterrestrial purine nucleobases in the Murchison meteorite: Implications for a unified mechanism for purine synthesis in carbonaceous chondrite parent bodies

Extraterrestrial nucleobases are of key interest owing to their implications for the chemical evolution of primordial molecules in the solar system and their potential contributions to prebiotic chemistry on the early Earth. Recent advances in analytical techniques capable of detecting femto-mole-order nucleobases have allowed us to identify all five exogenous nucleobases—uracil, cytosine, thymine, adenine, and guanine—in carbonaceous chondrites and to quantify uracil obtained from the carbonaceous asteroid (162173) Ryugu. In this study, we optimized the analytical method using hot-water extraction followed by 6 M hydrochloric acid (HCl) extraction from a sample of the CM2 Murchison meteorite to reassess the abundances and distributions of extraterrestrial nucleobases. The target analyses performed using high-performance liquid chromatography paired with electrospray-ionization, high-resolution mass spectrometry revealed that purine nucleobases were substantially more enriched in the subsequent 6 M HCl extract (951 ± 104 parts per billion, ppb) than in the hot-water extract (199 ± 3 ppb). The most abundant nucleobase was guanine (649 ± 103 ppb in total). The 6 M HCl extract not only contained canonical nucleobases but also included rare nucleobase analogs from the purine family, such as 8-substituted purines. Unlike purines, we preferentially detected pyrimidine nucleobases in the hot-water extract and the acid hydrolysate (185 ± 17 ppb and 297 ± 5 ppb, respectively) rather than in the 6 M HCl extract (51 ± 4 ppb). The disparate distributions of purine and pyrimidine bases in the Murchison meteorite suggests that purines are more robustly incorporated than pyrimidines into meteoritic matrices and/or insoluble macromolecular organic matter. We propose a unified formation mechanism for purine nucleobases—which involves the synthesis of 5-aminoimidazole derivatives from hydrogen cyanide and its related molecules—that can account for the molecular distributions of the extraterrestrial purine nucleobases found in the Murchison meteorite.

RAMAN-GEOTHERMOMETER FOR CARBONACEOUS CHONDRITES

Samples of the Murchison meteorite (carbonaceous chondrite, type CM2) were kept isothermally in a specially designed device at temperatures of 200, 500 and 800°C. After the samples cooled down in an inert helium atmosphere, Raman scattering spectra were taken. An increase in the intensity of the G- and D-lines of graphite was detected depending on the degree of heating. It is shown that using such a characteristic parameter of these lines as the area ratio, SD/SG, it is possible to define a geothermometer to determine the maximum temperature of thermal metamorphism of the parent bodies of carbonaceous chondrites. A comparison with the known data for carbonaceous chondrite Allende (CM3), which has experienced a significant thermal metamorphism, is carried out.

Fossil micrometeorites from Monte dei Corvi: Searching for dust from the Veritas asteroid family and the utility of micrometeorites as a palaeoclimate proxy

We searched late Miocene sedimentary rocks in an attempt to recover fossil micrometeorites derived from the Veritas asteroid family. This study was motivated by the previous identification of a pronounced 3He peak (4-5x above background) within marine sediments with ages between ∼8.5–6.9 Ma ago (Montanari et al., 2017. GSA Bulletin, 129:1357–1376). We processed 118.9 kg of sediment from the Monte dei Corvi beach section (Italy), the global type-section for the Tortonian epoch (11.6–7.2 Ma). Samples were collected both before and within the 3He peak. Although a small number of iron-rich (I-type) fossil micrometeorites were recovered from each horizon studied (Ntotal = 20), there is no clear difference between the pre- and intra- 3He peak samples. All micrometeorites are compositionally similar, and three out of five horizons yielded similar abundances and particle sizes. Micrometeorites extracted from sediments at the base of the 3He peak were exclusively small (ø <75 µm), while micrometeorites extracted from sediments near the highest 3He values were relatively large (ø <270 µm). The recovered fossil micrometeorites are interpreted as samples of the background dust flux derived from metal-bearing chondritic asteroids. The presence of a 3He signature combined with the absence of fossil micrometeorites or extraterrestrial spinels (Boschi et al., 2019, Spec. Pap. Geol. Soc. Am. 542:383–391) unambiguously related to the Veritas event suggests that the Veritas family is composed of highly friable materials that rarely survive on the sea floor to become preserved in the geological record. Our data supports the existing hypothesis that the Veritas asteroid family is an aqueously altered carbonaceous chondrite parent body, one that contains minimal native metal grains or refractory Cr-spinels. The low yield of fossil micrometeorites at Monte dei Corvi is attributed to loss of particles by dissolution whilst they resided on the sea floor but also due to high sedimentation rates leading to dilution of the extraterrestrial dust flux at this site. As with other fossil micrometeorite collections (e.g. Cretaceous chalk [Suttle and Genge, EPSL, 476:132–142]) the I-type spherules have been altered since deposition. In most particles, both magnetite and wüstite remain intact but have been affected by solid state geochemical exchange, characterised by partial leaching of Ni, Co and Cr and implantation of Mn, Mg, Si and Al. In some particles Mn concentrations reach up to 16.6 wt%. Conversely, in some micrometeorites wüstite has been partially dissolved, or even replaced by calcite or ankerite. Finally, we observe evidence for wüstite recrystallisation, forming a second generation of magnetite. This process is suggested to occur by oxidation during residence on the seafloor and has implications for the use of fossil I-type micrometeorites as a potential proxy for probing Earth’s upper atmospheric composition (oxidative capacity) in the geological past. However, solutions to the limitations of post-depositional recrystallisation are suggested. Fossil I-type spherules remain a potential tool for palaeo-climatic studies.

Extended time scales of carbonaceous chondrite aqueous alteration evidenced by a xenolith in LaPaz Icefield 02239 (CM2)

AbstractLaPaz Icefield (LAP) 02239 is a mildly aqueously altered CM2 carbonaceous chondrite that hosts a xenolith from a primitive chondritic parent body. The xenolith contains chondrules and calcium‐ and aluminum‐rich inclusions (CAIs) in a very fine‐grained matrix. The chondrules are comparable in mineralogy and oxygen isotopic composition with those in the CMs, and its CAIs are also mineralogically similar to the CM population apart for being unusually small and abundant. The presence of serpentine demonstrates that the xenolith has been aqueously altered, and its phyllosilicate‐rich matrix has a comparable oxygen isotopic composition to the matrices of CM meteorites. The xenolith's chondrules lack fine‐grained rims, whereas the xenolith itself has a fine‐grained rim that is petrographically and chemically comparable with the rims on coarse grained objects in LAP 02239 and other CM meteorites. These properties show that the xenolith's parent body was formed from similar materials to the CM parent body(ies). Following its lithification by aqueous alteration, a piece of the xenolith's parent body was impact‐ejected, acquired a fine‐grained rim while free‐floating in the protoplanetary disc, then was accreted along with rimmed chondrules and other materials to make the LAP 02239 parent body. Subsequent aqueous processing of the LAP 02239 parent body altered the fine‐grained rims on the xenolith, chondrules, and CAIs. The xenolith shows that the timespan of geological evolution of carbonaceous chondrite parent bodies was sufficiently long for some of them to have been aqueously altered before others had formed.

Origin of Isotopic Diversity among Carbonaceous Chondrites

Carbonaceous chondrites are some of the most primitive meteorites and derive from planetesimals that formed a few million years after the beginning of the solar system. Here, using new and previously published Cr, Ti, and Te isotopic data, we show that carbonaceous chondrites exhibit correlated isotopic variations that can be accounted for by mixing among three major constituents having distinct isotopic compositions, namely refractory inclusions, chondrules, and CI chondrite-like matrix. The abundances of refractory inclusions and chondrules are coupled and systematically decrease with increasing amount of matrix. We propose that these correlated abundance variations reflect trapping of chondrule precursors, including refractory inclusions, in a pressure maximum in the disk, which is likely related to the water ice line and the ultimate formation location of Jupiter. The variable abundance of refractory inclusions/chondrules relative to matrix is the result of their distinct aerodynamical properties resulting in differential delivery rates and their preferential incorporation into chondrite parent bodies during the streaming instability, consistent with the early formation of matrix-poor and the later accretion of matrix-rich carbonaceous chondrites. Our results suggest that chondrules formed locally from isotopically heterogeneous dust aggregates, which themselves derive from a wide area of the disk, implying that dust enrichment in a pressure trap was an important step to facilitate the accretion of carbonaceous chondrite parent bodies or, more generally, planetesimals in the outer solar system.

Tracking and dating incipient melting of a new grouplet of primitive achondrites

Primitive achondrites, representing ‘transitional’ samples between chondrites and differentiated achondrites, provide a great opportunity to decode the thermal histories of planetesimals in the early solar system. In this work, we report a comprehensive study of petrography, mineralogy, O-Cr isotopes and 53Mn-53Cr systematics of four ungrouped primitive achondrites, Northwest Africa (NWA) 12869, NWA 3250, NWA 11112, and Tafassasset. High resolution X-ray tomographic microscopy (HR-XTM) observations yield the 3D spatial distributions of metal, sulfide, and plagioclase, providing the direct petrologic evidence for incipient melting. In NWA 12869, NWA 3250, and NWA 11112, only FeNi-FeS eutectic melting occurs, with small fractions of metal and sulfide melt and amalgamate sporadically. As for Tafassasset, in addition to common metal and sulfide melt, plagioclase (±pyroxene) extensively melts and interconnects to form networks, with the generation of basaltic melts. The 53Mn-53Cr systematics in NWA 12869 [initial 53Mn/55Mn of (3.24 ± 0.32) × 10−6] and Tafassasset [initial 53Mn/55Mn of (3.48 ± 0.20) × 10−6] are determined with mineral separates and bulk samples. When anchored to the D’Orbigny angrite, NWA 12869 has a 53Mn-53Cr age of 4563.4 ± 0.6 Ma, while Tafassasset has an age of 4563.8 ± 0.4 Ma. The Mn-Cr ages of NWA 12869 and Tafassasset record the time of incipient melting within their parent bodies.Considering that NWA 12869, NWA 3250, and NWA 11112 exhibit the same olivine-dominated (47–69 vol%) mineral assemblages with recrystallized textures and a great similarity in mineral chemistry (olivine: Fa35–38; pyroxene: Fs27–30Wo3.0–3.5; plagioclase: An47–50; chromite: Cr#: 74–76; Mg#: 14.0–14.6), oxygen fugacity (IW-2 to IW-1), as well as 54Cr nucleosynthetic anomalies (ε54Cr: 1.01 ± 0.09 to 1.44 ± 0.18) and oxygen isotopes (Δ17O: −1.72 to −1.78 ± 0.05‰), we first classify them into a new grouplet of primitive achondrites, different from Tafassasset. This grouplet does not share a parent body with CR chondrites, but rather derives from incipient melting of a carbonaceous chondrite parent body which formed in a nebular reservoir physically close to that sampled by CR chondrites. More chondrite, differentiated achondrite, and/or iron samples associated with this new grouplet would provide further constraints on the rate and onset time of accretion, as well as the interior structure of the planetesimal in the outer solar system.

Meteorites and the RNA World: Synthesis of Nucleobases in Carbonaceous Planetesimals and the Role of Initial Volatile Content

Prebiotic molecules, fundamental building blocks for the origin of life, have been found in carbonaceous chondrites. The exogenous delivery of these organic molecules onto the Hadean Earth could have sparked the polymerization of the first RNA molecules in Darwinian ponds during wet-dry cycles. Here, we investigate the formation of the RNA and DNA nucleobases adenine, uracil, cytosine, guanine, and thymine inside parent body planetesimals of carbonaceous chondrites. An up-to-date thermochemical equilibrium model coupled with a 1D thermodynamic planetesimal model is used to calculate the nucleobase concentrations. Different from previous studies, we assume the initial volatile concentrations more appropriate for the formation zone of carbonaceous chondrite parent bodies. This represents more accurately cosmochemical findings that these bodies have formed inside the inner, ∼2–5 au, warm region of the solar system. Due to these improvements, our model represents the concentrations of adenine and guanine measured in carbonaceous chondrites. Our model did not reproduce per se the measurements of uracil, cytosine, and thymine in these meteorites. This can be explained by transformation reactions between nucleobases and the potential decomposition of thymine. The synthesis of prebiotic organic matter in carbonaceous asteroids could be well explained by a combination of (i) radiogenic heating, (ii) aqueous chemistry involving a few key processes at a specific range of radii inside planetesimals where water can exist in the liquid phase, and (iii) a reduced initial volatile content (H2, CO, , and CH2O) of the protoplanetary disk material in the parent body region compared to the outer region of comets.

Geoelectrochemistry-driven alteration of amino acids to derivative organics in carbonaceous chondrite parent bodies

A long-standing question regarding carbonaceous chondrites (CCs) is how the CCs’ organics were sourced and converted before and after the accretion of their parent bodies. Growing evidence shows that amino acid abundances in CCs decrease with an elongated aqueous alteration. However, the underlying chemical processes are unclear. If CCs’ parent bodies were water-rock differentiated, pH and redox gradients can drive electrochemical reactions by using H2 as an electron source. Here, we simulate such redox conditions and demonstrate that α-amino acids are electrochemically altered to monoamines and α-hydroxy acids on FeS and NiS catalysts at 25 °C. This conversion is consistent with their enrichment compared to amino acid analogs in heavily altered CCs. Our results thus suggest that H2 can be an important driver for organic evolution in water-rock differentiated CC parent bodies as well as the Solar System icy bodies that might possess similar pH and redox gradients.

Synthesis of Amino Acids from Aldehydes and Ammonia: Implications for Organic Reactions in Carbonaceous Chondrite Parent Bodies

Synthesis of Amino Acids from Aldehydes and Ammonia: Implications for Organic Reactions in Carbonaceous Chondrite Parent Bodies

Possible Ribose Synthesis in Carbonaceous Planetesimals.

The origin of life might be sparked by the polymerization of the first RNA molecules in Darwinian ponds during wet-dry cycles. The key life-building block ribose was found in carbonaceous chondrites. Its exogenous delivery onto the Hadean Earth could be a crucial step toward the emergence of the RNA world. Here, we investigate the formation of ribose through a simplified version of the formose reaction inside carbonaceous chondrite parent bodies. Following up on our previous studies regarding nucleobases with the same coupled physico-chemical model, we calculate the abundance of ribose within planetesimals of different sizes and heating histories. We perform laboratory experiments using catalysts present in carbonaceous chondrites to infer the yield of ribose among all pentoses (5Cs) forming during the formose reaction. These laboratory yields are used to tune our theoretical model that can only predict the total abundance of 5Cs. We found that the calculated abundances of ribose were similar to the ones measured in carbonaceous chondrites. We discuss the possibilities of chemical decomposition and preservation of ribose and derived constraints on time and location in planetesimals. In conclusion, the aqueous formose reaction might produce most of the ribose in carbonaceous chondrites. Together with our previous studies on nucleobases, we found that life-building blocks of the RNA world could be synthesized inside parent bodies and later delivered onto the early Earth.

Distant Formation and Differentiation of Outer Main Belt Asteroids and Carbonaceous Chondrite Parent Bodies

AbstractVolatile compositions of asteroids provide information on the Solar System history and the origins of Earth's volatiles. Visible to near‐infrared observations at wavelengths of &lt;2.5 µm have suggested a genetic link between outer main belt asteroids located at 2.5–4 au and carbonaceous chondrite meteorites (CCs) that show isotopic similarities to volatile elements on Earth. However, recent longer wavelength data for large outer main belt asteroids show 3.1 μm absorption features of ammoniated phyllosilicates that are absent in CCs and cannot easily form from materials stable at those present distances. Here, by combining data collected by the AKARI space telescope and hydrological, geochemical, and spectral models of water‐rock reactions, we show that the surface materials of asteroids having 3.1 μm absorption features and CCs can originate from different regions of a single, water‐rock‐differentiated parent body. Ammoniated phyllosilicates form within the water‐rich mantles of the differentiated bodies containing NH3 and CO2 under high water‐rock ratios (&gt;4) and low temperatures (&lt;70°C). CCs can originate from the rock‐dominated cores, that are likely to be preferentially sampled as meteorites by disruption and transport processes. Our results suggest that multiple large main belt asteroids formed beyond the NH3 and CO2 snow lines (currently &gt;10 au) and could be transported to their current locations. Earth's high hydrogen to carbon ratio may be explained by accretion of these water‐rich progenitors.

Limits on the contribution of early endogenous radiolysis to oxidation in carbonaceous chondrites’ parent bodies

Context.Carbonaceous chondrites have undergone alteration in their parent bodies and display oxidized secondary phases, including sulfates in CI and CM chondrites. The cause of the formation of these sulfates is yet to be determined.Aims.This study investigates the potential of endogenous radiolysis of water (i.e., radiolysis caused by radionuclides present in the rock) on the parent bodies of carbonaceous chondrites. Radiolysis may have contributed to the enhanced degree of oxidation of CI and CM chondrites, and we also examined CV chondrites as a case with no measured sulfates.Methods.We quantified the oxidants produced by radiolysis and how much of the sulfur content could be oxidized to form sulfates by this method. The amount of oxidants was calculated using a radiolytic production model developed and used for Earth and planetary applications that takes into account relevant physical parameters (water-to-rock ratio, grain density) and composition (amount of radionuclides, sulfur content).Results.For CM and CI parent bodies, even using a very favorable set of assumptions, only slightly more than 1% of the available sulfur can be oxidized into sulfates by this process, significantly below the amount of sulfates observed in these chondrites.Conclusions.Endogenous radiolysis is unlikely to have significantly contributed to the abundance of sulfate in CI and CM meteorites. The hypothesis of oxidation of sulfur by large quantities of O2accreted with primitive ice, on the other hand, is quantitatively supported by measurements of O2in comet 67P/Churyumov-Gerasimenko.

Assessment of Cr Isotopic Heterogeneities of Volatile-rich Asteroids Based on Multiple Planet Formation Models

Abstract Describing the comprehensive evolutionary scenario for asteroids is key to explaining the various physical processes of the solar system. Bulk-scale carbonaceous chondrites (CCs) possibly record the primordial information associated with the formation processes of their parent bodies. In this study, we tried to estimate the relative formation region of volatile-rich asteroids by utilizing the nucleosynthetic Cr isotopic variation (54Cr/52Cr) in bulk-scale CCs. Numerical calculations were conducted to track the temporal evolution of isotopically different (solar and presolar) dust and 54Cr/52Cr values for mixed materials with disk radius. First, we found that isotopic heterogeneities in CC formation regions were preserved with a weak turbulence setting that increased the timescales of the advection and diffusion in the disk. Second, we assessed the effects of gaps formed by giant planets. Finally, the distance from the injected supernovae and Cr isotopic compositions of the presolar grains were investigated in terms of the estimated formation region of CCs. In our results, a plausible formation region of four types of CCs could be obtained with the supernova from approximately 2 pc and typical Cr isotopic compositions of presolar grains. Among the parent bodies of CCs (i.e., volatile-rich asteroids), B-type asteroids formed in the outermost region, which is inconsistent with the present population, showing that D-type asteroids are generally located beyond most of the C-complex asteroids. Both the initial and present orbits of asteroids might be explained by the scatter attributed to the inward-outward migration of Jupiter and Saturn.

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 (CV3oxA) 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.