Distribution by Scientific Domains

Selected Abstracts

ASTEROID , when spinning a star leads to a collision with reality

Graham Jackson
No abstract is available for this article. [source]

"Ultraviolet spring" and the ecological consequences of catastrophic impacts

Charles S. Cockell
Asteroid and comet impacts cause ozone depletion. For the first time, we have quantified the photobiological characteristics of these events and speculate on some of the associated ecological consequences. Following the clearing of stratospheric dust after "impact winter", levels of damaging UVB radiation (280,315 nm) could increase by at least 100%, resulting in an "ultraviolet spring". Many of the taxa stressed by the cold and dark conditions of impact are the same that would be stressed by large increases in UVB radiation. Furthermore, depletion of dissolved organic carbon (DOC) by impact-induced acid rain would increase UVB penetrability into freshwater systems. Although an increase in UVB radiation is an attractive hypothesis for exacerbating the demise of land animals at the Cretaceous-Tertiary (K/T) boundary, e.g. dinosaurs, our calculations suggest the impact into rare sulphate-rich target rock may have prevented an ultraviolet spring in this case. If the K/T impact event had occurred in any other region on Earth, the stress to the biosphere would probably have been considerably greater. [source]

Asteroid 3628 Bo,n,mcovß: Covered with angrite-like basalts?

Edward A. Cloutis
The clinopyroxene is Fe2+ -bearing (likely in the range Fs,10,20), with >90% of the Fe2+ being present in the M1 crystallographic site (spectral type A). The clinopyroxene:plagioclase feldspar ratio is between ,2 and 3 (,55,75% clinopyroxene, ,20,33% plagioclase feldspar). If olivine is present, the clinopyroxene:olivine ratio is >,3 (<20% olivine). The derived mineralogy of Bo,n,mcovß is most similar, but not identical, to the known angrite meteorites. The data suggest that Bo,n,mcovß formed by melting and differentiation of an oxidized chondritic precursor and probably represents an unsampled angrite-like body. [source]

MUSES-C target asteroid (25143) 1998 SF36: A reddened ordinary chondrite

Richard P. BINZEL
High signal-to-noise and relatively high-resolution (50 ┼) visible and near-infrared spectroscopic measurements obtained during this asteroid's favorable 2001 apparition reveal it to have a red-sloped S(IV)-type spectrum with strong 1 and 2 ,m absorption bands analogous to those measured for ordinary chondrite meteorites. This red slope, which is the primary spectral difference between (25143) 1998 SF36 and ordinary chondrite meteorites, is well modeled by the spectrum of 0.05% nanophase iron (npFe0) proposed as a weathering mechanism by Pieters et al. (2000). Asteroid 1998 SF36 appears to have a surface composition corresponding to that of ordinary chondrite meteorites and is most similar in spectral characteristics and modeled olivine/pyroxene content to the LL chondrite class. [source]

Vesta, Vestoids, and the howardite, eucrite, diogenite group: Relationships and the origin of spectral differences

T. H. Burbine
All of the measured small asteroids (except for 2579 Spartacus) have reflectance spectra consistent with surface compositions similar to eucrites and howardites and consistent with all being derived from Vesta. None of the observed asteroids have spectra similar to diogenites. We find no spectral distinction between the 15 objects tabulated as members of the Vesta dynamical family and 6 of the 7 sampled "non-family" members that reside just outside the semi-major axis (a), eccentricity (e), and inclination (i) region of the family. The spectral consistency and close orbital (a-e-i) match of these "non-family" objects to Vesta and the Vesta family imply that the true bounds of the family extend beyond the subjective cut-off for membership. Asteroid 2579 Spartacus has a spectrum consistent with a mixture of eucritic material and olivine. Spartacus could contain olivine-rich material from Vesta's mantle or may be unrelated to Vesta altogether. Laboratory measurements of the spectra of eucrites show that samples having nearly identical compositions can display a wide range of spectral slopes. Finer particle sizes lead to an increase in the slope, which is usually referred to as reddening. This range of spectral variation for the best-known meteoritic analogs to the Vestoids, regardless of whether they are actually related to each other, suggests that the extremely red spectral slopes for some Vestoids can be explained by very fine-grained eucritic material on their surfaces. [source]

Asteroids, meteorites, impacts and their consequences (AMICO 2000) N÷rdlingen im Ries, Germany 2000 May 16,20

Article first published online: 4 FEB 2010

EURONEAR: Data mining of asteroids and Near Earth Asteroids

O. Vaduvescu
Abstract Besides new observations, mining old photographic plates and CCD image archives represents an opportunity to recover and secure newly discovered asteroids, also to improve the orbits of Near Earth Asteroids (NEAs), Potentially Hazardous Asteroids (PHAs) and Virtual Impactors (VIs). These are the main research aims of the EURONEAR network. As stated by the IAU, the vast collection of image archives stored worldwide is still insufficiently explored, and could be mined for known NEAs and other asteroids appearing occasionally in their fields. This data mining could be eased using a server to search and classify findings based on the asteroid class and the discovery date as "precoveries" or "recoveries". We built PRECOVERY, a public facility which uses the Virtual Observatory SkyBoT webservice of IMCCE to search for all known Solar System objects in a given observation. To datamine an entire archive, PRECOVERY requires the observing log in a standard format and outputs a database listing the sorted encounters of NEAs, PHAs, numbered and un-numbered asteroids classified as precoveries or recoveries based on the daily updated IAU MPC database. As a first application, we considered an archive including about 13 000 photographic plates exposed between 1930 and 2005 at the Astronomical Observatory in Bucharest, Romania. Firstly, we updated the database, homogenizing dates and pointings to a common format using the JD dating system and J2000 epoch. All the asteroids observed in planned mode were recovered, proving the accuracy of PRECOVERY. Despite the large field of the plates imaging mostly 2.27░ Î 2.27░ fields, no NEA or PHA could be encountered occasionally in the archive due to the small aperture of the 0.38m refractor insufficiently to detect objects fainter than V , 15. PRECOVERY can be applied to other archives, being intended as a public facility offered to the community by the EURONEAR project. This is the first of a series of papers aimed to improve orbits of PHAs and NEAs using precovered data derived from archives of images to be data mined in collaboration with students and amateurs. In the next paper we will search the CFHT Legacy Survey, while data mining of other archives is planned for the near future (ę 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Mucocutaneous Splendore-Hoeppli phenomenon

Mahmoud R. Hussein
Splendore-Hoeppli phenomenon (asteroid bodies) is the in vivo formation of intensely eosinophilic material (radiate, star-like, asteroid or club-shaped configurations) around microorganisms (fungi, bacteria and parasites) or biologically inert substances. This study presents a literature review concerning Splendore-Hoeppli reaction in the mucocutaneous diseases. It examines the histopathological features, nature and differential diagnosis of this reaction. It also discusses the mucocutaneous infections and the non-infective diseases associated with it. Available studies indicate that several mucocutaneous infections can generate Splendore-Hoeppli reaction. The fungal infections include sporotrichosis, pityrosporum folliculitis, zygomycosis, candidiasis, aspergillosis and blastomycosis. The bacterial infections include botryomycosis, nocardiosis and actinomycosis. The parasitic conditions include orbital pythiosis, strongyloidiasis, schistosomiasis and cutaneous larva migrans. In addition, Splendore-Hoeppli reaction may be seen with non-infective pathology such as hypereosinophilic syndrome and allergic conjunctival granulomas. The Splendore-Hoeppli reaction material comprises antigen-antibody complex, tissue debris and fibrin. Although the exact nature of this reaction is unknown, it is thought to be a localized immunological response to an antigen-antibody precipitate related to fungi, parasites, bacteria or inert materials. The characteristic formation of the peribacterial or perifungal Splendore-Hoeppli reaction probably prevents phagocytosis and intracellular killing of the insulting agent leading to chronicity of infection. To conclude, Splendore-Hoeppli reaction is a tell tale of a spectrum of infections and reactive conditions. The molecular pathways involved in the development of this reaction are open for future investigations. [source]

Carbonates in CM chondrites: Complex formational histories and comparison to carbonates in CI chondrites

Simone De LEUW
Two different carbonate minerals (calcite/aragonite and dolomite) together constitute 1.4,2.8 vol% of CM chondrites. In contrast, CI chondrites contain four different carbonate minerals: calcite/aragonite, dolomite, breunnerite, and siderite. CI chondrites have abundant dolomite, a mineral that seems to be absent in the most aqueously altered CM chondrites. In this study, carbonates in seven CM chondrites (Y-791198, LaPaz Icefield 04796, Cold Bokkeveld, Nogoya, Queen Alexandra Range 93005, Allan Hills 83100, and Meteorite Hills 01070) were studied petrographically and by electron microprobe. The results indicate that carbonate formation in CM chondrites differs from that in CI chondrites and is more complex than previously recognized. Our studies of CM chondrites indicate that (1) carbonates formed on the parent asteroid in an aqueous environment that gradually changed in composition, (2) at some stage, Ca and Mg activities in the environment were high enough to form metastable dolomite, and (3) dolomites disappeared in the most aqueously altered CM chondrites. [source]

The Fountain Hills unique CB chondrite: Insights into thermal processes on the CB parent body

This meteorite is closely related to the CBa class. Mineral compositions and O-isotopic ratios are indistinguishable from other members of this group. However, many features of Fountain Hills are distinct from the other CB chondrites. Fountain Hills contains 23 volume percent metal, significantly lower than other members of this class. In addition, Fountain Hills contains porphyritic chondrules, which are extremely rare in other CBa chondrites. Fountain Hills does not appear to have experienced the extensive shock seen in other CB chondrites. The chondrule textures and lack of fine-grained matrix suggests that Fountain Hills formed in a dust-poor region of the early solar system by melting of solid precursors. Refractory siderophiles and lithophile elements are present in near-CI abundances (within a factor of two, related to the enhancement of metal). Moderately volatile and highly volatile elements are significantly depleted in Fountain Hills. The abundances of refractory siderophile trace elements in metal grains are consistent with condensation from a gas that is reduced relative to solar composition and at relatively high pressures (10,3bars). Fountain Hills experienced significant thermal metamorphism on its parent asteroid. Combining results from the chemical gradients in an isolated spinel grain with olivine-spinel geothermometry suggests a peak temperature of metamorphism between 535 ░C and 878 ░C, similar to type-4 ordinary chondrites. [source]

Evidence for K-rich terranes on Vesta from impact spherules

Some of them are complex breccias that contain impact glasses whose compositions mirror that of their source regions. Some K-rich impact glasses (up to 2 wt% K2O) suggest that in addition to basalts and ultramafic cumulates, K-rich rocks are exposed on Vesta's surface. One K-rich glass (up to 6 wt% K2O), with a felsic composition, provides the first evidence of highly differentiated K-rich rocks on a large asteroid. They can be compared to the rare lunar granites and suggest that magmas generated in a large asteroid are more diverse than previously thought. [source]

Physical properties of meteorites,Applications in space missions to asteroids

However, more detailed observations indicate that differences exist in composition between asteroids and meteorites resulting in difficulties when searching for meteorite-asteroid match. We show that among other physical parameters the magnetic susceptibility of an asteroid can be determined remotely from the magnetic induction by solar wind using an orbiting spacecraft or directly using the AC coil on the lander, or it can be measured in samples returned to the laboratory. The shape corrected value of the true magnetic susceptibility of an asteroid can be compared to those of meteorites in the existing database, allowing closer match between asteroids and meteorites. The database of physical properties contains over 700 samples and was recently enlarged with measurements of meteorites in European museums using mobile laboratory facility. [source]

Geochemistry and origin of metal, olivine clasts, and matrix in the Dong Ujimqin Qi mesosiderite

Ping Kong
According to silicate textures and metal composition, this meteorite is classified as a member of subgroup IB. Instrumental neutron activation analyses (INAA) of metals show that the matrix metal has lower concentrations of Os, Ir, Re, and Pt, but higher concentrations of Ni and Au than the 7.5 cm metal nodule present in the meteorite. We attribute these compositional differences to fractional crystallization of molten metal. Studies of olivine clasts show that FeO contents are uniform in individual olivine crystals but are variable for different olivine clasts. Although concentrations of rare earth elements (REEs) change within olivine clasts, they all exhibit a vee-shaped pattern relative to CI chondrites. The relatively high concentrations of REEs in olivine and the shape of REE patterns require a liquid high in REEs and especially in light REEs. As such a liquid was absent from the region where basaltic and gabbroic clasts formed, mesosiderite olivine must have formed in a part of the differentiated asteroid that is different from the location where other mesosiderite silicate clasts formed. [source]

Tsunami generation and propagation from the Mj°lnir asteroid impact

S. Glimsdal
The geological structure resulting from the impact is today known as the Mj°lnir crater. The present work attempts to model the generation and the propagation of the tsunami from the Mj°lnir impact. A multi-material hydrocode SOVA is used to model the impact and the early stages of tsunami generation, while models based on shallow-water theories are used to study the subsequent wave propagation in the paleo-Barents Sea. We apply several wave models of varying computational complexity. This includes both three-dimensional and radially symmetric weakly dispersive and nonlinear Boussinesq equations, as well as equations based on nonlinear ray theory. These tsunami models require a reconstruction of the bathymetry of the paleo-Barents Sea. The Mj°lnir tsunami is characteristic of large bolides impacting in shallow sea; in this case the asteroid was about 1.6 km in diameter and the water depth was around 400 m. Contrary to earthquake- and slide-generated tsunamis, this tsunami featured crucial dispersive and nonlinear effects: a few minutes after the impact, the ocean surface was formed into an undular bore, which developed further into a train of solitary waves. Our simulations indicate wave amplitudes above 200 m, and during shoaling the waves break far from the coastlines in rather deep water. The tsunami induced strong bottom currents, in the range of 30,90 km/h, which presumably caused a strong reworking of bottom sediments with dramatic consequences for the marine environment. [source]

Spectral properties of angrites

T. H. Burbine
Almost all angrites (e.g., D'Orbigny, Lewis Cliff [LEW] 86010, and Sahara 99555) are composed predominately of anorthite, Al-Ti diopside-hedenbergite, and Ca-rich olivine, except for the type specimen, Angra dos Reis, which is composed almost entirely of Al-Ti diopside-hedenbergite. D'Orbigny, LEW 86010, and Sahara 99555 also have spectral properties very different from Angra dos Reis. These newly measured angrites all have broad absorption features centered near 1 ,m with very weak to absent absorption bands at ,2 ,m, which is characteristic of some clinopyroxenes. The spectrum of Angra dos Reis has the characteristic 1 and 2 ,m features due to pyroxene. One asteroid, 3819 Robinson, has similar spectral properties to the newly measured angrites in the visible wavelength region, but does not appear to spectrally match these angrites in the near-infrared. [source]

Fabric analysis of Allende matrix using EBSD

Lauren E. Watt
Electron backscatter diffraction (EBSD) has allowed fabrics in these fine-grained materials to be visualized in detail for the first time. Our data reveal that Allende, a CV3 chondrite, possesses a uniform, planar, short-axis alignment fabric that is pervasive on a broad scale and is probably the result of deformational shortening related to impact or gravitational compaction. Interference between this matrix fabric and the larger, more rigid components, such as dark inclusions (DIs) and calcium-aluminium-rich inclusions (CAIs), has lead to the development of locally oriented and intensified matrix fabrics. In addition, DIs possess fabrics that are conformable with the broader matrix fabric. These results suggest that DIs were in situ prior to the deformational shortening event responsible for these fabrics, thus providing an argument against dark inclusions being fragments from another lithified part of the asteroid (Kojima and Tomeoka 1996; Fruland et al. 1978). Moreover, both DIs and Allende matrix are highly porous (,25%) (Corrigan et al. 1997). Mobilizing a highly porous DI during impact-induced brecciation without imposing a fabric and incorporating it into a highly porous matrix without significantly compacting these materials is improbable. We favor a model that involves Allende DIs, CAIs, and matrix accreting together and experiencing the same deformation events. [source]

Ureilite petrogenesis: A limited role for smelting during anatexis and catastrophic disruption

Paul H. Warren
Several mass-balance problems arise from this hypothesis. Smelting inevitably consumes a large proportion of any plausible initial carbon while generating significant proportions of Fe metal and copious proportions of CO gas. The most serious problem concerns the yield of CO gas. If equilibrium smelting produced the ureilites' entire 21 mol% range in olivine-core mg, the proportion of gas within the asteroidal mantle (assuming plausibly low pressure <,80 bar) should have reached ,85 vol%. Based on the remarkably stepwise cooling history inferred from ureilite texture and mineralogy, a runaway, CO-leaky process that can loosely be termed smelting appears to have occurred, probably triggered by a major impact. The runaway scenario appears likely because, by Le ChÔtelier's principle, CO leakage would tend to accelerate the smelting process. Also, the copious volumes of gas produced by smelting would have led to explosive, mass-leaky eruptions into the vacuum surrounding the asteroid. Loss of mass would mean diminution of interior pressure, which would induce further smelting, leading to further loss of mass (basalt), and so on. Such a disruptive runaway process may have engendered the ureilites' distinctive reduced olivine rims. But the only smelting, according to this scenario, was a short-lived disequilibrium process that reduced only the olivine rims, not the cores; and the ureilites were cooling, not melting, during the abortive "smelting" episode. [source]

Ibitira: A basaltic achondrite from a distinct parent asteroid and implications for the Dawn mission

The mean Fe/Mn ratio of pyroxenes in Ibitira with <10 mole% wollastonite component is 36.4 ▒ 0.4; this value is well resolved from those of similar pyroxenes in five basaltic eucrites studied for comparison, which range from 31.2 to 32.2. Data for the latter five eucrites completely overlap. Ibitira pyroxenes have lower Fe/Mg than the basaltic eucrite pyroxenes; thus, the higher Fe/Mn ratio does not reflect a simple difference in oxidation state. Ibitira also has an oxygen isotopic composition, alkali element contents, and a Ti/Hf ratio that distinguish it from basaltic eucrites. These differences support derivation from a distinct parent asteroid. Thus, Ibitira is the first recognized representative of the fifth known asteroidal basaltic crust, the others being the HED, mesosiderite, angrite, and NWA 011 parent asteroids. 4 Vesta is generally assumed to be the HED parent asteroid. The Dawn mission will orbit 4 Vesta and will perform detailed mapping and mineralogical, compositional, and geophysical studies of the asteroid. Ibitira is only subtly different from eucritic basalts. A challenge for the Dawn mission will be to distinguish different basalt types on the surface and to attempt to determine whether 4 Vesta is indeed the HED parent asteroid. [source]

Northwest Africa 011: A "eucritic" basalt from a non-eucrite parent body

Christine Floss
This meteorite bears many similarities to the eucrites it was initially identified with, although oxygen isotopic compositions rule out a genetic relationship. Like many eucrites, NWA 011 crystallized from a source with approximately chondritic proportions of REE, although a slightly LREE-enriched bulk composition with a small positive Eu anomaly, as well as highly fractionated Fe/Mg ratios and depleted Sc abundances (Korotchantseva et al. 2003), suggest that the NWA 011 source experienced some pyroxene and/or olivine fractionation. Thermal metamorphism resulted in homogenization of REE abundances within grains, but NWA 011 did not experience the intergrain REE redistribution seen in some highly metamorphosed eucrites. Despite a similarity in oxygen isotopic compositions, NWA 011 does not represent a basaltic partial melt from the acapulcoite/lodranite parent body. The material from which NWA 011 originated may have been like some CH or CB chondrites, members of the CR chondrite clan, which are all related through oxygen isotopic compositions. The NWA 011 parent body is probably of asteroidal origin, possibly the basaltic asteroid 1459 Magnya. [source]

Discovery of Earth's quasi-satellite

Martin Connors
The term quasi-satellite is used since these large orbits are not completely closed, but rather perturbed portions of the asteroid's orbit around the Sun. Due to its extremely Earth-like orbit, this asteroid is influenced by Earth's gravity to remain within 0.1 AU of the Earth for approximately 10 years (1997 to 2006). Prior to this, it had been on a horseshoe orbit closely following Earth's orbit for several hundred years. It will re-enter such an orbit, and make one final libration of 123 years, after which it will have a close interaction with the Earth and transition to a circulating orbit. Chaotic effects limit our ability to determine the origin or fate of this object. [source]

First discovery of stishovite in an iron meteorite

Dan Holtstam
The mineral occurs intimately mixed with amorphous silica, forming tabular grains up to ,3 mm wide, with a hexagonal outline. It was identified using X-ray diffraction and Raman microspectroscopy. The unit-cell parameters of stishovite are a = 4.165(3) ┼ and c = 2.661(6) ┼, and its chemical composition is nearly pure SiO2. Raman spectra show relatively sharp bands at 231 and 754 cm,1 and a broad band with an asymmetric shape and a maximum around 500 cm,1. The rare grains are found within troilite nodules together with chromite, daubreelite, and schreibersite. From their composition and morphology, and by comparisons with silica inclusions in, e.g., the Gibeon IVA iron, we conclude that these rare grains represent pseudomorphs after tridymite. The presence of stishovite in Muonionalusta is suggested to reflect shock metamorphic conditions in the IVA parent asteroid during a cosmic impact event. [source]

The clay mineralogy of sediments related to the marine Mj°lnir impact crater

Henning DYPVIK
It was formed about 142 ▒ 2.6 Myr ago by the impact of a 1,2 km asteroid into the shallow shelf clays of the Hekkingen Formation and the underlying Triassic to Jurassic sedimentary strata. A core recovered from the central high within the crater contains slump and avalanche deposits from the collapse of the transient crater and central high. These beds are overlain by gravity flow conglomerates, with laminated shales and marls on top. Here, impact and post-impact deposits in this core are studied with focus on clay mineralogy obtained from XRD decomposition and simulation analysis methods. The clay-sized fractions are dominated by kaolinite, illite, mixed-layered clay minerals and quartz. Detailed analyses showed rather similar composition throughout the core, but some noticeable differences were detected, including varying crystal size of kaolinite and different types of illites and illite/smectite. These minerals may have been formed by diagenetic changes in the more porous/fractured beds in the crater compared to time-equivalent beds outside the crater rim. Long-term post-impact changes in clay mineralogy are assumed to have been minor, due to the shallow burial depth and minor thermal influence from impact-heated target rocks. Instead, the clay mineral assemblages, especially the abundance of chlorite, reflect the impact and post-impact reworking of older material. Previously, an ejecta layer (the Sindre Bed) was recognized in a nearby well outside the crater, represented by an increase in smectite-rich clay minerals, genetically equivalent to the smectite occurring in proximal ejecta deposits of the Chicxulub crater. Such alteration products from impact glasses were not detected in this study, indicating that little, if any, impact glass was deposited within the upper part of the crater fill. Crater-fill deposits inherited their mineral composition from Triassic and Jurassic sediments underlying the impact site. [source]

39Ar- 40Ar ages of eucrites and thermal history of asteroid 4 Vesta

Donald D. Bogard
Past studies have shown that after most eucrites formed, they underwent metamorphism in temperatures up to ,800░C. Much later, many were brecciated and heated by large impacts into the parent body surface. The less common basaltic, unbrecciated eucrites also formed near the surface but, presumably, escaped later brecciation, while the cumulate eucrites formed at depths where metamorphism may have persisted for a considerable period. To further understand the complex HED parent body thermal history, we determined new 39Ar- 40Ar ages for 9 eucrites classified as basaltic but unbrecciated, 6 eucrites classified as cumulate, and several basaltic-brecciated eucrites. Precise Ar-Ar ages of 2 cumulate eucrites (Moama and EET 87520) and 4 unbrecciated eucrites give a tight cluster at 4.48 ▒ 0.02 Gyr (not including any uncertainties in the flux monitor age). Ar-Ar ages of 6 additional unbrecciated eucrites are consistent with this age within their relatively larger age uncertainties. By contrast, available literature data on Pb-Pb isochron ages of 4 cumulate eucrites and 1 unbrecciated eucrite vary over 4.4,4.515 Gyr, and 147Sm- 143Nd isochron ages of 4 cumulate and 3 unbrecciated eucrites vary over 4.41,4.55 Gyr. Similar Ar-Ar ages for cumulate and unbrecciated eucrites imply that cumulate eucrites do not have a younger formation age than basaltic eucrites, as was previously proposed. We suggest that these cumulate and unbrecciated eucrites resided at a depth where parent body temperatures were sufficiently high to cause the K-Ar and some other chronometers to remain as open diffusion systems. From the strong clustering of Ar-Ar ages at ,4.48 Gyr, we propose that these meteorites were excavated from depth in a single large impact event ,4.48 Gyr ago, which quickly cooled the samples and started the K-Ar chronometer. A large (,460 km) crater postulated to exist on Vesta may be the source of these eucrites and of many smaller asteroids thought to be spectrally or physically associated with Vesta. Some Pb-Pb and Sm-Nd ages of cumulate and unbrecciated eucrites are consistent with the Ar-Ar age of 4.48 Gyr, and the few older Pb-Pb and Sm-Nd ages may reflect an isotopic closure before the large cratering event. One cumulate eucrite gives an Ar-Ar age of 4.25 Gyr; 3 additional cumulate eucrites give Ar-Ar ages of 3.4,3.7 Gyr; and 2 unbrecciated eucrites give Ar-Ar ages of ,3.55 Gyr. We attribute these younger ages to a later impact heating. Furthermore, the Ar-Ar impact-reset ages of several brecciated eucrites and eucritic clasts in howardites fall within the range of 3.5,4.1 Gyr. Among these, Piplia Kalan, the first eucrite to show evidence for extinct 26Al, was strongly impact heated ,3.5 Gyr ago. When these data are combined with eucrite Ar-Ar ages in the literature, they confirm that several large impact heating events occurred on Vesta between ,4.1,3.4 Gyr ago. The onset of major impact heating may have occurred at similar times for both Vesta and the moon, but impact heating appears to have persisted for a somewhat later time on Vesta. [source]

Importance of the accretion process in asteroid thermal evolution: 6 Hebe as an example

Amitabha Ghosh
Previous simulations of asteroid heat transfer have assumed that accretion was instantaneous. For the first time, we present a thermal model that assumes a realistic (incremental) accretion scenario and takes into account the heat budget produced by decay of 26Al during the accretion process. By modeling 6 Hebe (assumed to be the H chondrite parent body), we show that, in contrast to results from instantaneous accretion models, an asteroid may reach its peak temperature during accretion, the time at which different depth zones within the asteroid attain peak metamorphic temperatures may increase from the center to the surface, and the volume of high-grade material in the interior may be significantly less than that of unmetamorphosed material surrounding the metamorphic core. We show that different times of initiation and duration of accretion produce a spectrum of evolutionary possibilities, and thereby, highlight the importance of the accretion process in shaping an asteroid's thermal history. Incremental accretion models provide a means of linking theoretical models of accretion to measurable quantities (peak temperatures, cooling rates, radioisotope closure times) in meteorites that were determined by their thermal histories. [source]

Clay mineral-organic matter relationships in the early solar system

Victoria K. Pearson
These organic-rich meteorites provide a valuable and tangible record of the chemical steps taken towards the origin of life in the early solar system. Chondritic organic matter is present in the inorganic meteorite matrix which, in the CM and CI chondrites, contains evidence of alteration by liquid water on the parent asteroid. An unanswered and fundamental question is to what extent did the organic matter and inorganic products of aqueous alteration interact or display interdependence? We have used an organic labelling technique to reveal that the meteoritic organic matter is strongly associated with clay minerals. This association suggests that clay minerals may have had an important trapping and possibly catalytic role in chemical evolution in the early solar system prior to the origin of life on the early Earth. [source]

Clearwater East impact structure: A re-interpretation of the projectile type using new platinum-group element data from meteorites

Iain McDonald
This is at odds with recent chromium isotope analyses that suggest ordinary chondrite-type material is present. The present study reviews and reinterprets the available PGE data in the light of new PGE data from meteorites and concludes that the PGE ratios in the impact melt are most consistent with ordinary (possibly type-L) chondrite source material, not carbonaceous chondrites. Therefore the structure was most probably formed by the impact of an asteroid composed of material similar to ordinary chondrites. [source]

MUSES-C target asteroid (25143) 1998 SF36: A reddened ordinary chondrite

Richard P. BINZEL
High signal-to-noise and relatively high-resolution (50 ┼) visible and near-infrared spectroscopic measurements obtained during this asteroid's favorable 2001 apparition reveal it to have a red-sloped S(IV)-type spectrum with strong 1 and 2 ,m absorption bands analogous to those measured for ordinary chondrite meteorites. This red slope, which is the primary spectral difference between (25143) 1998 SF36 and ordinary chondrite meteorites, is well modeled by the spectrum of 0.05% nanophase iron (npFe0) proposed as a weathering mechanism by Pieters et al. (2000). Asteroid 1998 SF36 appears to have a surface composition corresponding to that of ordinary chondrite meteorites and is most similar in spectral characteristics and modeled olivine/pyroxene content to the LL chondrite class. [source]

Formation of mesosiderites by fragmentation and reaccretion of a large differentiated asteroid

Edward R. D. SCOTT
To test whether impacts can excavate core iron and mix it with crustal material, we used a low-resolution, smoothed-particle hydrodynamics computer simulation. For 50,300 km diameter differentiated targets, we found that significant proportions of scrambled core material (and hence potential mesosiderite metal material) could be generated. For near-catastrophic impacts that reduce the target to 80% of its original diameter and about half of its original mass, the proportion of scrambled core material would be about 5 vol%, equivalent to ,10 vol% of mesosiderite-like material. The paucity of olivine in mesosiderites and the lack of metal-poor or troilite-rich meteorites from the mesosiderite body probably reflect biased sampling. Mesosiderites may be olivine-poor because mantle material was preferentially excluded from the metal-rich regions of the reaccreted body. Molten metal globules probably crystallized around small, cool fragments of crust hindering migration of metal to the core. If mantle fragments were much hotter and larger than crustal fragments, little metal would have crystallized around the mantle fragments allowing olivine and molten metal to separate gravitationally. The rapid cooling rates of mesosiderites above 850 ░C can be attributed to local thermal equilibration between hot and cold ejecta. Very slow cooling below 400 ░C probably reflects the large size of the body and the excellent thermal insulation provided by the reaccreted debris. We infer that our model is more plausible than an earlier model that invoked an impact at ,1 km/s to mix projectile metal with target silicates. If large impacts cannot effectively strip mantles from asteroidal cores, as we infer, we should expect few large eroded asteroids to have surfaces composed purely of mantle or core material. This may help to explain why relatively few olivine-rich (A-type) and metal-rich asteroids (M-type) are known. Some S-type asteroids may be scrambled differentiated bodies. [source]

Stability limits for the quasi-satellite orbit

S. Mikkola
ABSTRACT An asteroid moving around the Sun having approximately the same mean motion and mean longitude as a planet, but a different eccentricity, circles the planet like a retrograde satellite even when the distance is large enough so that it is not a bound satellite. If the orbits are coplanar, then the motion is stable in the secular approximation. When the orbits are inclined enough, an asteroid can be trapped into such a quasi-satellite (QS) motion for a finite period of time. The conditions under which this can occur are discussed, improved criteria for the recognition of this type of motion are developed, and numerical examples from real QS objects are provided. [source]

A solar system focus

Peter Bond
Solar system exploration has been grabbing the headlines over the past few months, with the first triple spacecraft observations of Jupiter followed by the first landing on an asteroid. Peter Bond reports. [source]