Current NASA funded Research

The main goal of the proposed study is to better understand the origin of dust particles in the outer Solar System through high-precision oxygen three-isotope analyses of crystalline silicate Wild 2 particles, and Bennu and Ryugu returned samples.

Task 1: We conduct coordinated petrology, mineralogy, and oxygen isotope investigation of anhydrous minerals using scanning electron microscope (SEM), electron microprobe analyzer (EPMA) and secondary ion mass spectrometer (SIMS). For comet samples, we will extract relatively large (≥ 10 µm) particles from ≥ cm sized large aerogel tracks as we have done for track T227, which allow us to obtain oxygen isotope ratios at higher precisions. Multiple ≤mm sized fragments of Ryugu and Bennu "aggregate" samples will be mounted and polished in epoxy disk for the analyses. Energy dispersive X-ray detectors on SEM will be used to obtain elemental maps of individual fragments in order to search small olivine and pyroxene grains (≤10 µm) at trace quantities (0.1-1%). From the results, we examine the ∆17O = δ17O -- 0.52 × δ18O versus Mg# = molar [Mg]/[Mg+Fe] relationships of ferromagnesian silicate samples from these returned samples, which will be compared to those of chondrules in primitive chondrites. 

Task 2: We will analyze rare aluminous silicate particle in Wild 2 for Al-Mg chronology using SIMS with improved analytical precisions. The results will address formation time of the cometary particles and further test if any Wild 2 particles formed within 2-3 Myr after the formation of Ca, Al-rich inclusions, the first solids in the solar system.

Task 3: Water ice was abundant in the outer protoplanetary disk that accreted to comet and icy planetesimal, though water ice was not collected from Wild 2 in Stardust mission. We analyze oxygen isotope ratios of magnetite and Ca-carbonate in less altered lithology of Ryugu and Bennu in order to estimate initial water accreted to the parent asteroids in outer disk regions.

The results of isotope ratio measurements would address the origin of solid particles as well as water ice in the outer solar system and how they were related to the dynamical evolution of our Solar system. The proposed work will "maximize the science derived from planetary sample-return missions" by the comprehensive studies of samples from three missions, Stardust, Hayabusa2, and OSIRIS-REx.

Chondrules in primitive meteorites formed by transient heating in the proto-planetary disk and postdated the oldest refractory inclusions (Ca, Al-rich inclusions; CAIs) by 2-5 million years (Ma). They were likely formed by mechanism that involve growth of planetary system, such as proto-Jupiter, planetary embryos, impacts between planetesimals. Oxygen three-isotope systematics among chondrules have indicated a variety of precursor solids with distinct oxygen isotopes and different environments of their formation. Under the assumption of homogeneous distribution of 26Al (half-life of 0.7 Ma), studies on the Al-Mg chronology of chondrules suggest that chondrule formation ages are systematically different among chondrite groups; chondrules in ordinary chondrites (OC) formation in the inner disk at 1.8-2.2 Ma, which are systematically older than those in carbonaceous chondrites (CC), typically 2.2-2.7 Ma for CO, CM, CV, and Acfer 094 chondrites, and >3-4 Ma after CAIs for CR, CH, CB (CR-clan) chondrites.

Systematic investigation of formation ages at higher precisions (0.1-0.2 Ma) are required to fully resolve detail systematics against their chemistry and oxygen isotope signatures. Nucleosynthetic anomalies in 54Cr and 50Ti show two distinct systematic changes between carbonaceous chondrites (CC) and other meteorites (NC), which is known as isotope dichotomy and may help us to understand mixing of solids in the history of protoplanetary disk . However, investigations 54Cr and 50Ti anomaly among individual chondrules, especially those coordinated with oxygen isotopes and Al-Mg chronology are scarce.

Task -1: We propose to conduct high time-resolution (<0.1 Ma) Al-Mg chronology of 15-20 chondrules in pristine CV, CM, CO,

CR chondrites, which are combined with detailed petrology, oxygen three-isotopes. Least metamorphosed and aqueously altered carbonaceous chondrites are carefully selected. Silicate Mg# (= [MgO]/ [MgO+FeO] mole%) and mass independent fractionation of oxygen isotope ratios would indicate dust density of local disk and abundance of 16O-poor water ice in chondrule precursor solids. For selected chondrules in CV and CR, we will extract chondrules after SIMS analyses and obtain nucleosynthetic anomalies of 54Cr and 50Ti by using TIMS and ICPMS, respectively. Furthermore, we produce Na-rich plagioclase standards in order to improve accuracy of Al-Mg isochron ages to be better than 0.1 Ma.

Task -2: W we examine detail oxygen three isotope systematics of Al-rich chondrules (ARC) in CV and CO chondrites. Some ARCs show internally heterogeneous oxygen isotopes with extreme 16O-rich isotope signatures, close to those in CAIs. Examination

of oxygen isotope zoning in each ARC would help understanding the isotope exchange between 16O-rich chondrule melt with surrounding 16O-poor ambient gas. We will determine Al-Mg ages of ARCs to understand possible 26Al heterogeneity and formation time.

From the results, we will determine the total range of Al-Mg ages for each chondrite and evaluate any correlation between ages, chemical and isotope variabilities. We address following questions in order to better understand the mechanism of chondrule formation and the evolution of protoplanetary disk.

(1) Distribution of chondrule formation ages: Short time intervals (≤0.1 Ma) or continuous formation (>0.5 Ma), which may relate to different chondrule formation mechanisms.

(2) Younger ages for FeO-rich chondrules than FeO-poor chondrules in CC may imply increase of disk density and decrees of disk temperatures with time.

(3) Evidence for radial transport of chondrules, such as older chondrules in CCs show NC-like isotope signatures.

The proposed research is to explore the evolution of solids in the protoplanetary disk through the chemical and isotopic properties of extraterrestrial materials that represent primitive solids in the early Solar System. Therefore, it is relevant to the scope of the Emerging Worlds Program.

Refractory inclusions, i.e., calcium-aluminum-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs), are the oldest solids of our solar system. They were mostly formed by condensation of a nebular gas of near solar composition close to the protosun (refractory inclusion factory), recording the isotopic features (i.e., 26Al and 17O) of the initial gas during/after the collapse of the molecular cloud. Hereafter, they were efficiently distributed across the protoplanetary disk by turbulent diffusion or disk wind and acted as basic building blocks of solar system bodies or partially recycled by chondrules, another major product of the protoplanetary disk's high-temperature events.

The proposal focuses on anorthite (An)-rich CAIs (task 1) and AOAs (task 2) in primitive CV (Kaba), CO (DOM 08006), CR (GRA 95229 etc.), and Acfer 094 chondrites. We will conduct comprehensive studies for the two types of objects, including their mineralogy, major and trace elements, oxygen isotopes, and Al-Mg systematics, using in situ techniques like SEM, EPMA, LA-ICP-MS, and SIMS. Their SIMS Al-Mg systematics will be primarily determined from their anorthite (>5 µm domains), utilizing a 3 µm primary ion beam generated by the radiofrequency plasma ion source and three electronic multipliers (3EM) for simultaneously detecting Mg three isotopes. Furthermore, ~10 large AOAs will be drilled out from their polished thick sections (guided by micro-CT images) for high-precision MC-ICP-MS bulk Mg isotope measurements. The dataset will be used systematically to address three questions. 

Q1: The formation timescale (initial condensation and later reprocessing in their factory) of refractory inclusions, in astrophysical speaking, the duration of the hottest (>1300 K) environments in the protoplanetary disk. This will be achieved by obtaining high-precision Al-Mg chronology of An-rich CAIs and AOAs based primarily on SIMS analyses on anorthite (27Al/24Mg >300). BSE, CL of anorthite, major elements, oxygen isotopes, and δ25Mg of olivine will be used to exclude those objects that experienced post-aggregation remelting. The possibility of heterogeneity distribution of 26Al will be discussed too. 

Q2: Determine the solar system's initial 26Mg/24Mg ratio [µ26Mg*0 (ss)], a critical parameter for the 26Al--26Mg chronometer. When the initial 26Al/27Al [(26Al/27Al)0] of AOAs is unknown, they were assumed to be the same as CAIs and used to construct a bulk CAI-AOA isochron. The µ26Mg*0 (ss) was determined to be -16 ppm, arguing for a large-scale spatial heterogeneity of 26Al. However, it is modeled to be -35 ppm based on close-system Mg isotope evolution with canonical (26Al/27Al)0 and the Al/Mg ratio of CI chondrites. This was confirmed by recent Mg isotope analyses of refractory forsterites in carbonaceous chondrite chondrules, supporting the homogeneous distribution of 26Al. We will, for the first time, determine the 26Al/27Al)0 of AOAs using SIMS, and then combine it with their bulk Al/Mg ratios and Mg isotope ratios determined by MC-ICP-MS to further constrain the µ26Mg*0 (ss). The result will argue or dispute the bulk CAI-AOA isochron and provide an important constraint to the homo-/heterogeneity distribution of 26Al in the early solar system.

Q3: Elucidate the genetic relationships between refractory inclusions and chondrules. PI's recent study on Al-rich chondrules identified abundant relict anorthite in addition to spinel and olivine, suggesting that An-rich CAIs and AOAs are possibly the most abundant refractory objects in chondrule-forming regions. The comprehensive dataset of the two types of objects will be used to confirm their relationships and discuss why lower-temperature condensates were preferentially recycled by chondrule-forming events.   

The three questions are fundamental to the chemical and dynamic evolution of the early solar system. Thus, the proposed works are relevant to the scope of the Emerging Worlds Program.

This proposal is a collaborative effort to characterize the petrologic and whole rock chemical diversity of enstatite chondrites (ECs) and re-evaluate their classification, chondrule origins, thermal histories and parent bodies. Meteorites are grouped (classified) into solar system materials with similar petrologic and geochemical characteristics, suggesting formation by similar processes and potentially common parent bodies. Petrologic and bulk chemical data will be collected to re-evaluate enstatite chondrite classification and assess the number of enstatite chondrite parent bodies, their thermal histories and the metamorphic trends they record, providing a better understand of this chondrite group and their parent bodies. The proposed work also includes oxygen isotopic measurements to better understand enstatite chondrite chondrules and their formation, in relation to chondrules in other chondrite groups and Al-Mg isotopic measurements to test for evidence of 26Al during chondrule formation and estimate the relative timing of EC chondrule formation. The oxygen isotope data will be used to test for chondrule recycling, as recorded in all other chondrite groups, mixing in of different solar system materials and multiple populations, potentially generations, of chondrules within the unequilibrated enstatite chondrites. Enstatite chondrites are a remarkable group of meteorites interpreted to have formed in the inner solar system, inside of the nebular snow line. They are the most reduced solar system materials known, with unusual reduced mineralogies unlike any other primitive meteorites. They include a range of material from type 3 to 6, impact melt rocks and are the only chondrites directly linked to a group of achondrites (i.e., the aubrites). They have stable isotopic compositions similar to Earth-Moon, suggesting they are important for understanding terrestrial planet formation and/or are possibly related to formation of the moon. Data from MESSENGER suggest that the surface of Mercury is reduced, potentially having enstatite meteorite-like mineral assemblages. Documenting the diversity of enstatite chondrites and understanding the solar system histories they record are essential for understanding the evolution of the terrestrial planets.

The work outlined in this proposal includes four inter-related tasks targeted at documenting the range of petrologic and geochemical properties of enstatite chondrites to evaluate their classification, origins, thermal histories, parent bodies, chondrule populations and potentially chondrule relative ages. The results will provide a comprehensive understanding of the enstatite chondrites, their parent bodies, their role in the evolution of the inner solar system and the primary evolution of their chondrules compared to chondrites in other chondrite groups. The goals will be obtained through a comprehensive petrologic and whole rock chemical study of a large suite of ECs and an oxygen and 26Al-26Mg isotopic study of EC chondrules.

We will enhance the science return from the Hayabusa2 mission by using returned samples from asteroid (162173) Ryugu, in conjunction with previously unstudied CI-like xenoliths in polymict ureilites, CI meteorites, and other possibly CI-related meteorites such as Tagish Lake and Tarda, to increase knowledge of the primordial (petrologic type 3) mineralogical, chemical, and oxygen isotopic properties of CI-like materials in the solar system, i.e., the protoliths of known CI.

Initial analyses of Ryugu particles returned by JAXA's Hayabusa2 mission showed mineralogical, chemical, and isotopic similarities to the CI carbonaceous chondrites. CI meteorites are of paramount importance because their bulk chemical compositions closely match the solar photosphere in all but the most volatile elements, and thus are thought to represent the starting composition of the solar system. However, CI materials are rare and so our knowledge of their properties is limited. In particular, all CI meteorites are petrologic type 1 - i.e., they have been extensively altered by aqueous fluids and their primary mineralogy, textures, and oxygen isotope compositions have been obscured. The direct return of samples from Ryugu provides a new source of CI-like materials, which are potentially less aqueously altered than CI themselves and so could yield new information about their primary properties.

In fact, studies of Ryugu samples have shown that they contain substantial abundances of anhydrous silicates that may be remnants (petrologic type 2) of primordial CI3 materials. This discovery motivates us to study Ryugu samples in the context of other type 2 CI-like samples, in particular a population of potentially Ryugu-like materials that occur as xenoliths in polymict ureilites. These xenoliths were implanted into ureilitic regolith in the inner solar system ~4.5 Ga ago, and so could provide direct information about the dynamical history of Ryugu's progenitor planetesimal.

We will determine the mineralogy, textures, chemical compositions, and oxygen isotope compositions of a previously unstudied Ryugu sample, along with a targeted selection of C2 xenoliths from polymict ureilites. We will also study selected areas of several CIs, Tagish Lake (C2), and Tarda (C2), for comparison. The resulting dataset will be used to address the following objectives:

1. To identify relatively unaltered (C2) areas in Ryugu and other CI-like samples and use them to constrain the primordial mineralogical and textural characteristics of CI-like materials in the solar system;

2. To determine oxygen isotope compositions of anhydrous silicates and Ca-Al-rich phases in relatively unaltered (C2) areas of Ryugu and other CI-like samples and use them to constrain the oxygen isotope properties of primordial anhydrous components of CI-like materials in the solar system;

3. To determine oxygen isotope compositions of magnetite and carbonates in the least altered areas of Ryugu and other CI-like samples and use them to constrain the primary oxygen isotope properties of aqueous fluids on CI-like planetesimals;

4. To use the least altered areas of Ryugu and other CI-like samples to explore the diversity of primordial CI-like materials in the early solar system.

This work is relevant to LARS because it uses laboratory-based analyses of returned samples from asteroid Ryugu to enhance the science return from the Hayabusa2 mission. It also includes analyses of meteorites that are essential to the objectives of the work on Ryugu, and thus is responsive to the recommendation of the 2022 Decadal Survey that returned samples and meteorites should be studied together to understand the initial conditions of the solar system.