Water-rich C-type asteroids as early solar system carbonate factories

Micrometeorites represent a major potential source of volatiles for the early Earth, although often overlooked due to their small sizes and the effects of atmospheric entry. In this study we explore an unusual ~2000 μm, fine-grained unmelted micrometeorite TAM19B-7 derived from a water-rich C-type asteroid. Previous analysis revealed a unique O-isotope composition and intensely aqueously altered geological history. We investigated its carbon isotopic composition using the NanoSIMS and characterized the carbon-bearing carriers using Raman and Near-Infrared spectroscopy. We found that TAM19B-7 has a 13C enriched bulk composition (δ13C = +3 ± 8 ‰), including a domain with 13C depletion (δ13C = −27.1 ‰). Furthermore, a few micro-scale domains show 13C enrichments (δ13C from +12.9 ‰ to +32.7 ‰) suggesting much of the particle’s carbon content was reprocessed into fine-grained carbonates, likely calcite. The heavy bulk C-isotope composition of TAM19B-7 indicates either open system gas loss during aqueous alteration or carbonate formation from isotopically heavy soluble organics. Carbonates have been detected on small body surfaces, including across dwarf planet Ceres, and on the C-type asteroids Bennu and Ryugu. The preservation of both carbonates with 13C enrichments and organic carbon with 13C depletion in TAM19B-7, despite having been flash heated to high temperatures (<1000 °C), demonstrates the importance of cosmic dust as a volatile reservoir

Isotopic and Elemental Compositions of Stardust and Protosolar Dust Grains in Primitive Meteorites

This dissertation presents the results and implications of the isotopic and elemental analyses of presolar silicate grains from the primitive chondrites, Acfer 094, SAH 97096, and ALHA77307. Oxygen-anomalous, C-anomalous, and N-anomalous grains were identified by O, C, and N isotopic imaging, respectively, using the NanoSIMS 50. Subsequently, the elemental compositions of the grains carrying the anomalous isotopic signatures were acquired in the PHI 700 Auger Nanoprobe. Some silicate grains with unique O isotopic compositions were measured for Si and Fe isotopes. The isotopic analyses indicate that a majority of the silicate and oxide grains are 17O-rich with solar to sub-solar 18O/16O ratios and come from less than 2.2Mï Red Giant or Asymptotic Giant Branch stars. The second most abundant fraction of grains show large 18O excesses and come from core collapse supernovae. The next most abundant fraction of grains comes from high metallicity AGB stars of approximately solar mass. A minor fraction of the grains exhibit large excesses in 16O and formed in core collapse supernova ejecta. Grains with extreme 17O excesses are the latest addition to the presolar grain inventory. These grains may come from binary star systems where one star goes nova. Numerous presolar SiC and N-anomalous carbonaceous grains were identified in the matrix of ALHA77307. The SiC grains are predominantly mainstream grains and may have condensed in 1-3Mï AGB stars. The carbonaceous grains may have formed by ionmolecule reactions in the protosolar nebula or interstellar medium. A few carbonaceous grains exhibit 13C-rich compositions; grains with such compositions are rare, which implies that either the fractionation effects that produce C anomalies in opposite directions cancel them out or secondary processing destroyed grains with C anomalies. The elemental compositions of the silicate grains are predominantly nonstoichiometric: 61%), with some grains exhibiting olivine- or pyroxene-like compositions. About 25% of the silicate grains contain Ca and/or Al. Furthermore, most of the presolar silicates are Fe-rich with Fe contents reaching up to about 45 at.% in contrast to equilibrium condensation models, which predict Mg-rich phases such as forsterite and enstatite to form. Although secondary alteration processes have probably modified the compositions of some presolar silicate grains in Acfer 094, the source of the Fe enrichments in most silicate grains is arguably primary. This work has led to the identification of presolar Si-oxide and Mg-oxide grains that had not been observed before. The formation of SiO2 grains requires non-equilibrium conditions in the outflows of stars. The abundance of SiC and silicate grains in ALHA77307 is high indicating its pristine characteristics, similar to the CR chondrites QUE 99177 and MET 00426. Although, the number of stardust grains identified in the enstatite chondrite SAH 97096 is low, their identification indicates that such grains were not all destroyed during the high temperature phase experienced by enstatite chondrites. Finally, silicate grain abundances are much higher than oxide grain abundances in all the three meteorites

Uncertainties in physical properties of Itokawa-like asteroids widen constraints on their formation time

Abstract One of the outstanding questions in planetary science is to determine how the fundamental mechanical and physical properties of materials determine the thermal evolution of asteroids, and which properties have the greatest influence. We investigate the effects of uncertainty in the material properties of asteroid parent bodies on the ability of thermal evolution models to constrain the sizes and formation times of ordinary chondrite parent asteroids. A simple model is formulated for the thermal evolution of the parent body of asteroid 25143 Itokawa. The effects of the uncertainties in the values specified for specific heat capacity, thermal diffusivity, and aluminum abundance are determined. The uncertainties in specific heat capacity and aluminum abundance, or heat production more generally, are found to both have significant and approximately equal effects on these results, substantially widening the range of possible formation times of Itokawa’s parent body. We show that Itokawa’s parent body could have formed between 1.6 and 2.5 million years after the origin of calcium–aluminum inclusions with a radius larger than 19 km, and it could have formed as early as 1.4 millions years, as late as 3.5 million years, or with a radius as small at 17 km if more lenient definitions of uncertainty in aluminum abundance are considered. These results stress the importance of precise data required of the material properties of a suite of LL type 4-6 ordinary chondrite meteorites to place better constraints on the thermal history of Itokawa’s parent body. Graphical Abstrac

Protracted Timescales for Nebular Processing of First-formed Solids in the Solar System

The calcium–aluminum-rich inclusions (CAIs) from chondritic meteorites are the first solids formed in the solar system. Rim formation around CAIs marks a time period in early solar system history when CAIs existed as free-floating objects and had not yet been incorporated into their chondritic parent bodies. The chronological data on these rims are limited. As seen in the limited number of analyzed inclusions, the rims formed nearly contemporaneously (i.e., <300,000 yr after CAI formation) with the host CAIs. Here we present the relative ages of rims around two type B CAIs from NWA 8323 CV3 (oxidized) carbonaceous chondrite using the ^26 Al– ^26 Mg chronometer. Our data indicate that these rims formed ∼2–3 Ma after their host CAIs, most likely as a result of thermal processing in the solar nebula at that time. Our results imply that these CAIs remained as free-floating objects in the solar nebula for this duration. The formation of these rims coincides with the time interval during which the majority of chondrules formed, suggesting that some rims may have formed in transient heating events similar to those that produced most chondrules in the solar nebula. The results reported here additionally bolster recent evidence suggesting that chondritic materials accreted to form chondrite parent bodies later than the early-formed planetary embryos, and after the primary heat source, most likely ^26 Al, had mostly decayed away