The CR chondrite clan: Implications for early solar system processes

Abstract— In this paper, we review the mineralogy and chemistry of calcium‐aluminum‐rich inclusions (CAIs), chondrules, FeNi‐metal, and fine‐grained materials of the CR chondrite clan, including CR, CH, and the metal‐rich CB chondrites Queen Alexandra Range 94411, Hammadah al Hamra 237, Bencubbin, Gujba, and Weatherford. The members of the CR chondrite clan are among the most pristine early solar system materials, which largely escaped thermal processing in an asteroidal setting (Bencubbin, Weatherford, and Gujba may be exceptions) and provide important constraints on the solar nebula models. These constraints include (1) multiplicity of CAI formation; (2) formation of CAIs and chondrules in spatially separated nebular regions; (3) formation of CAIs in gaseous reservoir(s) having ¹⁶O‐rich isotopic compositions; chondrules appear to have formed in the presence of ¹⁶O‐poor nebular gas; (4) isolation of CAIs and chondrules from nebular gas at various ambient temperatures; (5) heterogeneous distribution of ²⁶Al in the solar nebula; and (6) absence of matrix material in the regions of CAI and chondrule formation.     

Parent body histories recorded in Rumuruti chondrite sulfides: Implications for the onset of oxidized,sulfur-rich core formation

Models of planetary core formation beginning with melting of Fe,Ni metal and troilite are not readily applicable to oxidized and sulfur-rich chondrites containing only trace quantities of metal. Cores formed in these bodies must be dominated by sulfides. Siderophile trace elements used to model metallic core formation could be used to model oxidized, sulfide-dominated core formation and identify related meteorites if their trace element systematics can be quantified. Insufficient information exists regarding the behavior of these core-forming elements among sulfides during metamorphism prior to anatexis. Major, minor, and trace element concentrations of sulfides are reported in this study for petrologic type 3–6 R chondrite materials. Sulfide-dominated core-forming components in such oxidized chondrites (ƒO₂ ≥ iron-wüstite) follow metamorphic evolutionary pathways that are distinct from reduced, metal-bearing counterparts. Most siderophile trace elements partition into pentlandite at approximately 10× chondritic abundances, but Pt, W, Mo, Ga, and Ge are depleted by 1–2 orders of magnitude relative to siderophile elements with similar volatilities. The distribution of siderophile elements is further altered during hydrothermal alteration as pyrrhotite oxidizes to form magnetite. Oxidized, sulfide-dominated core formation differs from metallic core formation models both physically and geochemically. Incongruent melting of pentlandite at 865°C generates melts capable of migrating along solid silicate grains, which can segregate to form a Ni,S-rich core at lower temperatures compared to reduced differentiated parent bodies and with distinct siderophile interelement proportions.     

26Mg excess in hibonites of the Rumuruti chondrite Hughes 030

Abstract— The Rumuruti chondrites (R chondrites) constitute a new, well-established, chondrite group different from carbonaceous, ordinary, and enstatite chondrites. Most samples of this group are gas-rich regolith breccias showing the typical light/dark structure and consist of abundant fragments of various parent body lithologies embedded in a fine-grained, olivine-rich matrix. Most R chondrites contain the typical components of primitive chondrites including chondrules, chondrule and mineral fragments, sulfides, and rare calcium-aluminum-rich inclusions (CAIs). In Hughes 030, an interesting CAI consisting of abundant hibonite and spinel was found. Mg isotopic analyses revealed excess ²⁶Mg in components of R chondrites for the first time. The hibonite grains with high Al/Mg values (∼1500 to 2600) show resolved ²⁶Mg excess. The slope of the correlation line yields an initial ²⁶Al/ ²⁷Al = (1.4 ± 0.3) × 10⁻⁶, which is ∼40 times lower than the initial value measured in CAIs from primitive meteorites. The inferred difference in ²⁶Al abundance implies a time difference of ∼4 million years for the closure of the Al-Mg system between CAIs from primitive chondrites and the Hughes 030 CAI. Based on mineralogy and the petrographic setting of the hibonite-rich CAI, it is suggested that 4 million years reflect the time interval between the formation of the CAI and the end of its secondary alteration. It is also suggested that most of this alteration may have occurred in the nebula (e.g. Zn- and Fe-incorporation in spinels). However, the CAI could not have survived in the nebula as a free floating object for a long period of time. Therefore, the possibility of storage in a precursor planetesimal for a few million years, resetting the magnesium-aluminum isotopic system, prior to impact brecciation, excavation, and accretion of the final R chondrite parent body cannot be ruled out.     

Chromium isotopic systematics of the Sutter's Mill carbonaceous chondrite: Implications for isotopic heterogeneities of the early solar system

Recent studies have shown that major meteorite groups possess their own characteristic ⁵⁴Cr values, demonstrating the utility of Cr isotopes for identifying genetic relationships between the planetary materials in conjunction with other classical tools, such as oxygen isotopes. In this study, we performed Cr isotope analyses for whole rocks and chemically separated phases of the new CM2 chondrite, Sutter's Mill (SM 43 and 51). The two whole rocks of Sutter's Mill show essentially identical ε⁵⁴Cr excesses (SM 43 = +0.95 ± 0.09ε, SM 51 = +0.88 ± 0.07ε), relative to the Earth. These values are the same within error with that of the CM2‐type Murchison (+0.89 ± 0.08ε), suggesting that parent bodies of Sutter's Mill and Murchison were formed from the same precursor materials in the solar nebula. Large ε⁵⁴Cr excess of up to 29.40ε is observed in the silicate phase of Sutter's Mill, while that of Murchison shows 15.74ε. Importantly, the leachate fractions of both Sutter's Mill and Murchison form a steep linear anticorrelation between ε⁵⁴Cr and ε⁵³Cr, cross‐cutting the positive correlation previously observed in carbonaceous chondrites. The fact that L4 acid leachate fraction contains higher ⁵⁴Cr excesses than that of L5 step designed to dissolve refractory minerals suggests that spinel is not a major ⁵⁴Cr carrier. We also note that L5 contains ⁵³Cr anomalies lower than the solar initial value, suggesting it carries a component of nucleosynthetic anomaly unrelated to the ⁵³Mn decay. We have identified five endmember components of nucleosynthetic origin among the early solar system materials.     

Pecora Escarpment 91002: A member of the new Rumuruti (R) chondrite group

Abstract— Pecora Escarpment (PCA)91002 is a light/dark-structured chondrite breccia related to Carlisle Lakes and Rumuruti; the meteorite contains ～10–20 vol% equilibrated (type ?5 and ?6) clasts within a clastic groundmass, much of which was metamorphosed to type-3.8 levels. The olivine compositional distribution forms a tight cluster that peaks at Fa_38–40; by contrast, low-Ca pyroxene compositions are highly variable. Opaque phases identified in PCA91002 and its paired specimen, PCA91241, include pyrrhotite, pentlandite, pyrite, chromite, ilmenite, metallic Cu and magnetite. The majority of the rock is of shock stage S3-S4; there are numerous sulfide-rich shock veins and 50-μm plagioclase melt pockets. Instrumental neutron activation analysis shows that, unlike Carlisle Lakes and ALH85151, PCA91002 exhibits no Ca enrichment or Au depletion; because PCA91002 is relatively unweathered, it seems probable that the Ca and Au fractionations in Carlisle Lakes and ALH85151 were caused by terrestrial alteration. The Rumuruti-like (formerly Carlisle-Lakes-like) chondrites now include eight separate meteorites. Their geochemical and petrographic similarities suggest that they constitute a distinct chondrite group characterized by unfractionated refractory lithophile abundances (0.95 ± 0.05x CI), high bulk Δ¹⁷O, a low chondrule/groundmass modal abundance ratio, mean chondrule diameters in the 400 ± 100 μm range, abundant NiO-bearing ferroan olivine, sodic plagioclase, titanian chromite, abundant pyrrhotite and pentlandite and negligible metallic Fe-Ni. We propose that this group be called R chondrites after Rumuruti, the only fall. The abundant NiO-bearing ferroan olivine grains, the occurrence of Cu-bearing sulfide, and the paucity of metallic Fe-Ni indicate that R chondrites are highly oxidized. It is unlikely that appreciable oxidation took place on the parent body because of the essential lack of plausible oxidizing agents (e.g., magnetite or hydrated silicates). Therefore, oxidation of R chondrite material must have occurred in the nebula. A few type-I porphyritic olivine chondrules containing olivine grains with cores of Fa_3–4 composition occur in PCA91002; these chondrules probably formed initially as metallic-Fe-Ni-bearing objects at high nebular temperatures. As temperatures decreased and more metallic Fe was oxidized, these chondrules accreted small amounts of oxidized material and were remelted. The ferroan compositions of the >5-μm olivine grains in the R chondrites reflect equilibration with fine-grained FeO-rich matrix material during parent body metatnorphism.     

Mineralogy and chemistry of Rumuruti: The first meteorite fall of the new R chondrite group

Abstract— The Rumuruti meteorite shower fell in Rumuruti, Kenya, on 1934 January 28 at 10:43 p.m. Rumuruti is an olivine-rich chondritic breccia with light-dark structure. Based on the coexistence of highly recrystallized fragments and unequilibrated components, Rumuruti is classified as a type 3–6 chondrite breccia. The most abundant phase of Rumuruti is olivine (mostly Fa_～39) with about 70 vol%. Feldspar (～14 vol%; mainly plagioclase), Ca-pyroxene (5 vol%), pyrrhotite (4.4 vol%), and pentlandite (3.6 vol%) are major constituents. All other phases have abundances below 1 vol%, including low-Ca pyroxene, chrome spinels, phosphates (chlorapatite and whitlockite), chalcopyrite, ilmenite, tridymite, Ni-rich and Ge-containing metals, kamacite, and various particles enriched in noble metals like Pt, Ir, arid Au. The chemical composition of Rumuruti is chondritic. The depletion in refractory elements (Sc, REE, etc.) and the comparatively high Mn, Na, and K contents are characteristic of ordinary chondrites and distinguish Rumuruti from carbonaceous chondrites. However, S, Se, and Zn contents in Rumuruti are significantly above the level expected for ordinary chondrites. The oxygen isotope composition of Rumuruti is high in δ¹⁷O (5.52 ‰) and δ¹⁸O (5.07 ‰). Previously, a small number of chondritic meteorites with strong similarities to Rumuruti were described. They were called Carlisle Lakes-type chondrites and they comprise: Carlisle Lakes, ALH85151, Y-75302, Y-793575, Y-82002, Acfer 217, PCA91002, and PCA91241, as well as clasts in the Weatherford chondrite. All these meteorites are finds from hot and cold deserts having experienced various degrees of weathering. With Rumuruti, the first meteorite fall has been recognized that preserves the primary mineralogical and chemical characteristics of a new group of meteorites. Comparing all chondrites, the characteristic features can be summarized as follows: (a) basically chondritic chemistry with ordinary chondrite element patterns of refractory and moderately volatile lithophiles but higher abundances of S, Se, and Zn; (b) high degree of oxidation (37–41 mol% Fa in olivine, only traces of Fe, Ni-metals, occurrence of chalcopyrite); (c) exceptionally high Δ¹⁷O values of about 2.7 for bulk samples; (d) high modal abundance of olivine (～70 vol%); (e) Ti-Fe³⁺?rich chromite (～5.5 wt% TiO₂); (f) occurrence of various noble metal-rich particles; (g) abundant chondritic breccias consisting of equilibrated clasts and unequilibrated lithologies. With Rumuruti, nine meteorite samples exist that are chemically and mineralogically very similar. These meteorites are attributed to at least eight different fall events. It is proposed in this paper to call this group R chondrites (rumurutiites) after the first and only fall among these meteorites. These meteorites have a close relationship to ordinary chondrites. However, they are more oxidized than any of the existing groups of ordinary chondrites. Small, but significant differences in chemical composition and in oxygen isotopes between R chondrites and ordinary chondrites exclude formation of R chondrites from ordinary chondrites by oxidation. This implies a separate, independent R chondrite parent body.     

Alteration processes in the CV chondrite parent body based on analysis of NWA 2086 meteorite

Analysis of the NWA 2086 CV3 chondrite showed a matrix/chondrule ratio of 52%, similar to Bali, Mokoia, and Grosanaja. Nearly twice as many chondrule fragments as intact ones demonstrate that an early fragmentation phase occurred prior to final accretion. After this event, no substantial mechanical change or redeposition is evident. Rims with double‐layered structures were identified around some chondrules, which, in at least one case, is attributed to an accretionary origin. The rim's outer parts with a diffuse appearance were formed by in situ chemical alteration. During this later process, Mg content decreased, Fe content increased, and olivine composition was homogenized, producing a rim composition close to that of the matrix. This alteration occasionally happened along fractures and at confined locations, and was probably produced by fluid interactions. Iron oxides are the best candidate for a small grain‐sized alteration product; however, technical limitations in the available equipment did not allow exact phase identification. These results suggest that NWA 2086 came from a location (possible more deeply buried) in the CV parent body than Mokoia or Bali, and suffered less impact effects—although there is no evidence of sustained thermal alteration. This meteorite may represent a sample of the CV parent asteroid interior and provide a useful basis for comparison with other CV meteorites in the future.     

Mass distribution in the solar system

The present-day observed mass distribution in the solar system including the Sun is shown to be compatible with the idea of the splitting of a number of ring-shaped rotating clouds of particles in the equatorial plane of a single contracting nebula. The formation of such a nebula is discussed and it is inferred that during the course of contraction this nebula has remained a sphere of uniform density spinning with the Keplerian velocity of its surface layer. The mass of a planet is taken as the portion of this spherical solar nebula gained at the time of splitting by its gaseous ring of dimensions satisfying Roche and accretional limits.     

Accretion of differentiated achondritic and aqueously altered chondritic materials in the early solar system—Significance of an igneous fragment in the CM chondrite NWA 12651

One approach to decipher the dynamics of material transport and planetary accretion in the early solar system is to investigate xenolithic fragments in meteorites. In this work, we examined an igneous fragment from the NWA 12651 meteorite—the first igneous fragment found in any CM chondrite—by analyzing its mineralogy, rare earth elements (REEs), and O‐isotopes. The study shows that the exsolution lamellae of the igneous fragment consist of Fe‐rich and Ca‐rich pyroxene. Thus, the fragment was part of a progressive crystallization in a closed system, such as in a depleted magma reservoir or mantle. In this environment, the pyroxene co‐crystallized with plagioclase, resulting in a negative Eu anomaly and enrichment of the heavy REEs compared to the light REEs. The O‐isotopes of the fragment are more ¹⁶O‐enriched than the mafic minerals in the matrix or in other bulk CM chondrites; therefore, the fragment was formed in a different region than the NWA 12651 parent body. The iron meteorites Tucson and Deep Springs, the pallasite Milton, and the CB chondrites have similar O‐isotopes as the igneous fragment. However, no direct connection can be drawn and it is questionable if the fragment shares a same parent body with one of these meteorites. The close formation region to the CB chondrites may suggest a formation of the fragment in the carbonaceous chondrite region. Thus, a wide transport through the nebula of the early solar system may not have been necessary to move the fragment to the CM chondrite formation region.     

Accretional and alterational differences in a carbonaceous chondrite parent body: Evidence from the NWA 5491 CV3 meteorite

The NWA 5491 CV3 meteorite is a CVoxA subtype, and composed of two substantially different units (titled “upper” and “lower” units) in the cm size range with original accreted material and also subsequent alteration produced features. Based on the large chondrules in the “upper” unit and the small chondrules plus CAIs in the “lower” unit, they possibly accreted material from different parts of the solar nebula and/or at different times, whereas substantial changes happened in the nebula's composition. Differences are observed in the level of early fragmentation too, which was stronger in the upper units. During later alteration oxidizing fluids possibly circulated only in the upper unit, mechanical fragmentation and resorption were also stronger there. In the last phase of the geological history these two rock units came into physical contact, but impact‐driven shock effects were not observed. The characteristics of this meteorite provide evidence that the same parent body might accrete substantially different material and also the later processes could differ spatially in the parent body.     

A cornucopia of presolar and early solar system materials at the micrometer size range in primitive chondrite matrix

Abstract— We have used a variety of complementary microanalytical techniques to constrain the mineralogy, trace‐element distributions, and oxygen‐isotopic compositions in a 50 × 50 μm area of Acfer 094 matrix. The results reveal the exceptional mineralogical and compositional heterogeneity of this material at the sub‐μm level. We observe μm‐scale and sub‐μm grains with elemental associations suggesting feldspar, metal with widely varying Ni contents, and a Cr‐Fe alloy (in addition to forsterite, pyroxene, sulfide, ferrihydrite, and amorphous groundmass previously described). A new class of μm‐scale CAI (μCAI) is also observed, which show sub‐μm compositional zoning, and a range of oxygen isotopic compositions. Unlike the larger CAIs in Acfer 094, which are uniformly ¹⁶O‐enriched, two of the three μCAIs we analyzed are isotopically normal. We also observed a Li‐rich hotspot that detailed analysis by ToF‐SIMS suggests may be a LiCr‐oxide grain. Within the resolution of the NanoSIMS, this grain has isotopically normal Li. Finally, in our 50 × 50 μm area, we positively identified a presolar grain that is the most ¹⁸O‐rich silicate found so far in meteorites. The grain may originate from an asymptotic giant branch (AGB) star, or more likely, a supernova. In line with previous TEM studies (Greshake 1997), we find no evidence for clastic material (e.g., fragmental chondrules) in the matrix of Acfer 094: although the matrix is volatile‐depleted, this depletion does not appear to result from dilution of a primordial starting material with (depleted) chondrule fragments. Assuming that matrix experienced the depletion event, our data on the detailed mineralogy of Acfer 094 are currently equivocal in constraining the nature of that event. We observe carrier phases for several elements consistent with conditions approaching equilibrium condensation; however, the presence of an amorphous groundmass is suggestive of more rapid cooling.     

Evolution of the angrite parent body: Implications of metamorphic coronas in NWA 3164

Northwest Africa 3164 is a coarse‐grained angrite that shows reaction coronas, a unique character among achondrites. Olivine (Fo₅₇; 1.2 wt% CaO), fassaitic clinopyroxene, anorthite, and spinel account for 46–47, 28–29, 8–13, and 4–8 vol%, respectively; kamacite is an accessory phase. The spinel grains in contact with clinopyroxene are bounded by discontinuous 20 μm thick coronas of anorthite and olivine, indicating the reaction Cpx + Spl → Ol + An (R1). In addition, irregular coronas of clinopyroxene and spinel developed around the primary anorthite in contact with primary olivine, during the reaction Ol + An → Cpx + Spl (R2). R2 also generated clinopyroxene and spinel films between the secondary olivine and anorthite coronas produced during R1, implying that R1 preceded R2. Both are metamorphic reactions that developed in the solid state. Finally, the coronas are cross cut by μm‐thick veinlets due to a late shock. A mass‐balance study shows that R2 is almost the reverse of R1. The P–T metamorphic evolution of the rock, modeled by calculating a P–T isochemical diagram, indicates an equilibrium T of 940 ± 120 °C at P < 0.9 GPa for the initial assemblage, followed by an increase of T up to approximately 1000–1200 °C during reaction R1 and a subsequent cooling during R2. Several causes are envisaged to account for this metamorphic evolution. Contact metamorphism due to a hot magmatic intrusion in the angrite parent body is favored, as similar metamorphic coronas are well known in metamorphic terrestrial rocks. In addition to differentiation and magmatism, there is now evidence for metamorphism in the angrite parent body, which would have been a large asteroid or a planetary‐sized body.     

The angular momenta of solar system bodies: Implications for asteroid strengths

The angular momentum H is plotted versus mass M for the planets and for all asteroids with known rotation rates and shapes, primarily taken from D. C. McAdoo and J. A. Burns [Icarus18, 285–293 (1973)]. An asteroid's angular momentum is derived from its rotation rate as determined by the period of its lightcurve, its shape as indicated by the lightcurve amplitude, and where possible its size as given by polarimetry or radiometry. The asteroid is assumed to be rotating about its axis of maximum moment of inertia. As previously found by F. F. Fish [Icarus7, 251–256 (1967]) and W. K. Hartmann and S. M. Larson [Icarus7, 257–260 (1967)], H is approximately proportional to $\text{M}{}^{\mathrm{<mtext>5</mtext><mtext>3</mtext>}}$, which shows that the asteroids and most planets spin with nearly the same rate. The very smallest asteroids on the plot deviate from the above reaction, usually containing excess angular momentum. This suggests that collisions have transferred substantial angular momentum to the smallest asteroids, perhaps causing their internal stress states to be substantially modified by centrifugal effects.The forces produced by gravitation are then compared to centrifugal effects for a rotating, triaxial ellipsoid of density 3 g cm^?3. For all asteroids with known properties the gravitational attraction is shown to be larger than the centrifugal acceleration of a particle on the surface: thus the observed asteroid regoliths are gravitationally bound. Poisson's equation for the gravitational potential is investigated and it is shown by mathematical and physical arguments that any arbitrarily shaped ellipsoid with the attractive surface force boundary condition found above will have only attractive internal forces. Thus the internal stress states in asteroids are always compressive so that asteroids could be internally fractured without losing their integrity.     

Reflectance spectrophotometry (∼0.5–1.0 μm) of outer-belt asteroids: Implications for primitive,organic solar system material

Narrowband spectrophotometric observations of outer-belt asteroids (semi-major axis greater than 3.2 AU) have been used to investigate the surface compositions of the D- and P-class asteroids in an effort to learn about their origins and formation conditions. Spectra of 19 outer-belt asteroids and 2 main-belt D-class asteroids were obtained using two charge-coupled device spectrographs. No mineralogical absorption features were evident. The spectra can be divided into four groups based upon four discrete slopes among the spectra. The slope increases (reddens) with increasing heliocentric distance. All of the outer-belt asteroids are locked into orbits controlled by Jupiter's gravitational attraction. The distinct slope changes suggest that these asteroids are the remnants of a gradation in composition of planetesimals in the outer Solar System, which were selectively retained in location when other material was ejected from the Solar System. The spectrophotometry of Iapetus and infrared photometry of quiescent comets of other researchers suggest that this compositional gradation could extend through the orbits of Uranus and Neptune. Two possible explanations for this compositional gradation based upon changes in organic polymer materials are discussed.     

Modal abundances of CAIs: Implications for bulk chondrite element abundances and fractionations

Abstract— Modal abundances of Ca,Al‐rich inclusions (CAIs) are poorly known and reported data scatter across large ranges. CAIs are Poisson distributed, and if only small areas (<1000 mm²) are studied, the data are probably not representative of the true CAI modal abundances, explaining their reported large scatter in a single chondrite group. We combine reported CAI modal abundances and our own set, and present a complete list of CAI modal abundances in carbonaceous chondrites. This includes (in area%): CV: 2.98, CM: 1.21, Acfer 094: 1.12, CO: 0.99, CK/CV (Ningqiang and Dar al Gani [DaG] 055): 0.77, CK: 0.2, CR: 0.12 and CB: 0.1. CAIs are Poisson distributed and if only small areas are studied, the data are probably not representative of the true CAI modal abundances, Carbonaceous chondrites have excess bulk Al concentrations when compared to the CI‐chondritic value. We find a correlation between this excess and CAI modal abundances and conclude that the excess Al was delivered by CAIs. The excess Al is only a minor fraction (usually ?10 rel%, but 25 rel% in case of CVs) of the bulk chondrite Al and cannot have contributed much ²⁶Al to heat the chondrite parent body. Ordinary, enstatite, R and K chondrites have an Al deficit relative to CI chondrites and only very low CAI modal abundances, if any are present at all. Carbonaceous chondrites also had an initial Al deficit if the contribution of Al delivered by CAIs is subtracted. Therefore all chondrites probably lost a refractory rich high‐T component. Only minor amounts of CAIs are present in the matrix or have been present in the chondrule precursor aggregates. Most CAI size distributions contain more than one size population, indicating that CAIs from within a single meteorite group had different origins.     

Orbital resonances in the solar nebula: Implications for planetary accretion

The influence of gas drag and gravitational perturbations by a planetary embryo on the orbit of a planetesimal in the solar nebula was examined. Non-Keplerian rotation of the gas causes secular decay of the orbit. If the planetesimal's orbit is exterior to the perturber's, resonant perturbations oppose this drag and can cause it to be trapped in a stable orbit at a commensurability of order j/(j + 1), where j is an integer. Numerical and analytical demonstrations show that resonant trapping occurs for wide ranges of perturbing mass, planetesimal size, and j. Induced eccentricities are large, causing overlap of orbits for bodies in different resonances with j > 2. Collisions between planetesimals in different resonances, or between resonant and nonresonant bodies, result in their disruption. Fragments smaller than a critical size can pass through resonances under the influence of drag and be accreted by the embryo. This effect speeds accretion and tends to prevent dynamical isolation of planetary embryos, making gas-rich scenarios for planetary formation more plausible.     

The origin of the solar system: Implications for transneptunian planets and the nature of the long-period comets

If the solar system origin is considered within the framework of the author's hypothesis on the binary stars formation as a result of rotational-exchange break-up of the rotating protostar, then difficulties involved in the usual nebular hypotheses are automatically removed (unclear aspects of the possibility of formation of the gas disc proper, the problems of the angular momentum including slow rotation of the Sun and coplanarity of the planetary orbits, of differences in planetary masses and composition, the need, for the disc remnants to be swept out, the long time of planetary formation as compared with the possible lifetime of a turbulized disc etc.).The major stages of division and evolution of the Jupiter-Sun system are described. Similarities between the massive rotating proto-Jupiter (PJ) and the classical protoplanetary discs are pointed out. The process of planetoid condensation inside PJ is discussed. The most probable site of the condensation is the region of the first Lagrangian point. The planetoids condensed were lost by PJ as a result of its fast mass decrease. A gas dynamic consideration of the motion of planetoids in PJ yields 1000–3000 yr as a time scale for the PJ's mass loss. The number of the moonlike bodies lost (the remaining Galilean satellites fixing their lower mass limit) could reach 10⁴.Evolution of such interacting bodies results in the formation beyond Neptune of a cloud (up to 10³) of moonlike (and more massive) planets.The excess concentration of the long-period comets aphelia in this area implies their genetic relation to the planets. A concept of a joint planeto-cometary cloud is introduced. A concrete hydrodynamic mechanism of ice ejection from planets into space, viz. the formation of cumulative (Monroe) jets, is pointed out.A program of further investigations is outlined and recommendations given for an experimental check on the implications of the new cosmogonic concepts.     

First recorded find of ordinary chondrite material in the Kaidun meteorite

For the first time, ordinary chondrite material—the most common type among the present-day fall meteorite—has been found in the unique Kaidun breccia. The discovered object is a large unequilibrated olivine-pyroxene porphyritic chondrule, with peripheral and central zones of different structures, suggesting different crystallization regimes. In chemical composition, the chondrule corresponds to unequilibrated ordinary chondrites of petrological type 3; it is enriched in lithophile elements and depleted in siderophiles, indicating formation by melting of the parent material, which preceded or was accompanied by metal-silicate fractionating. The chondrule material was subjected to aqueous alteration that formed smectite and calcite in the cavities and veins of its central part. The anomalous oxygen isotopic compositions of the chondrule are evidence of an oxygen reservoir different from known types of meteorites, including the ordinary-chondrite chondrules. Thus, the unique breccia Kaidun contains ordinary chondrite material along with carbonaceous and enstatite chondrite material, products of early nebular processes, and highly differentiated planetary-type material.__________Translated from Astronomicheskii Vestnik, Vol. 39, No. 2, 2005, pp. 169–176.Original Russian Text Copyright © 2005 by Ivanova, Kononkova, Ivanov.     

Hydrogen fluence in Genesis collectors: Implications for acceleration of solar wind and for solar metallicity

NASA's Genesis mission was flown to capture samples of the solar wind and return them to the Earth for measurement. The purpose of the mission was to determine the chemical and isotopic composition of the Sun with significantly better precision than known before. Abundance data are now available for noble gases, magnesium, sodium, calcium, potassium, aluminum, chromium, iron, and other elements. Here, we report abundance data for hydrogen in four solar wind regimes collected by the Genesis mission (bulk solar wind, interstream low‐energy wind, coronal hole high‐energy wind, and coronal mass ejections). The mission was not designed to collect hydrogen, and in order to measure it, we had to overcome a variety of technical problems, as described herein. The relative hydrogen fluences among the four regimes should be accurate to better than ±5–6%, and the absolute fluences should be accurate to ±10%. We use the data to investigate elemental fractionations due to the first ionization potential during acceleration of the solar wind. We also use our data, combined with regime data for neon and argon, to estimate the solar neon and argon abundances, elements that cannot be measured spectroscopically in the solar photosphere.     

Dust production from the hypervelocity impact disruption of the Murchison hydrous CM2 meteorite: Implications for the disruption of hydrous asteroids and the production of interplanetary dust

More than half of the C-type asteroids, the dominant type of asteroid in the outer half of the main-belt, show evidence of hydration in their reflectance spectra. In order to understand the collisional evolution of asteroids and the production of interplanetary dust and to model the infrared signature of small particles in the Solar System it is important to characterize the dust production from primary impact disruption events, and compare the disruption of hydrous and anhydrous targets. We performed a hypervelocity impact disruption experiment on an ∼30 g target of the Murchison CM2 hydrated carbonaceous chondrite meteorite, and compared the results with our previous disruption experiments on anhydrous meteorites including Allende, a CV3 carbonaceous chondrite, and nine ordinary chondrites. Murchison is significantly more friable than the ordinary chondrites or Allende. Nonetheless, on a plot of mass of the largest fragment versus specific impact energy, the Murchison disruption plots within the field of the anhydrous meteorites points, suggesting that Murchison is at least as resistant to impact disruption as the anhydrous meteorites, which require about twice the energy for disruption as terrestrial anhydrous basalt targets. We determined the mass-frequency distribution of the debris from the Murchison disruption over a nine order-of-magnitude mass range, from ∼10⁻⁹ g to the mass of the largest fragment produced in the disruption. The cumulative mass-frequency distribution from the Murchison disruption is fit by three power-law segments. For masses >10⁻² g the slope is only slightly steeper than that of the corresponding segment from the disruption of most anhydrous meteorites. Over the range from ∼10⁻⁶ to 10⁻² g the slope is significantly steeper than that for the anhydrous meteorites. For masses <10⁻⁶ g the slopes of both the Murchison and the anhydrous meteorites are almost flat. Thus the Murchison disruption significantly over-produced small fragments (10⁻⁶-10⁻³ g) compared to anhydrous meteorite targets. If the Murchison results are representative of hydrous asteroids, the hydrous asteroids may dominate over anhydrous asteroids in the production of interplanetary dust >100 μm in size, the size of micrometeorites recovered from the polar ices, while both types of asteroids might produce comparable amounts of ∼10 μm interplanetary dust. This would explain the puzzle that polar micrometeorites (>100 μm in size) are similar to hydrous meteorites, while the majority of the ∼10 μm interplanetary dust particles are anhydrous.