Our Solar System formed approximately 4.6 billion years ago from the collapse of a dense core inside an interstellar molecular cloud. The subsequent formation of solid bodies took place rapidly. The period of &<10 million years over which planetesimals were assembled can be investigated through the study of meteorites. Although some planetesimals differentiated and formed metallic cores like the larger terrestrial planets, the parent bodies of undifferentiated chondritic meteorites experienced comparatively mild thermal metamorphism that was insufficient to separate metal from silicate. There is debate about the nature of the heat source as well as the structure and cooling history of the parent bodies. Here we report a study of 244Pu fission-track and 40Ar-39Ar thermochronologies of unshocked H chondrites, which are presumed to have a common, single, parent body. We show that, after fast accretion, an internal heating source (most probably 26Al decay) resulted in a layered parent body that cooled relatively undisturbed: rocks in the outer shells reached lower maximum metamorphic temperatures and cooled faster than the more recrystallized and chemically equilibrated rocks from the centre, which needed approximately 160 Myr to reach 390K.

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The Cassini-Huygens Cosmic Dust Analyzer (CDA) is intended to provide direct observations of dust grains with masses between 10 −19 and 10 −9 kg in interplanetary space and in the jovian and saturnian systems, to investigate their physical, chemical and dynamical properties as functions of the distances to the Sun, to Jupiter and to Saturn and its satellites and rings, to study their interaction with the saturnian rings, satellites and magnetosphere. Chemical composition of interplanetary meteoroids will be compared with asteroidal and cometary dust, as well as with Saturn dust, ejecta

Abstract-Radiochronometry of L chondritic meteorites yields a rough age estimate for a major collision in the asteroid belt about 500 Myr ago. Fossil meteorites from Sweden indicate a highly increased influx of extraterrestrial matter in the Middle Ordovician ~480 Myr ago. An association with the L-chondrite parent body event was suggested, but a definite link is precluded by the lack of more precise radiometric ages. Suggested ages range between 450 ± 30 Myr and 520 ± 60 Myr, and can neither convincingly prove a single breakup event, nor constrain the delivery times of meteorites from the asteroid belt to Earth. Here we report the discovery of multiple 40 Ar- 39 Ar isochrons in shocked L chondrites, particularly the regolith breccia Ghubara, that allow the separation of radiogenic argon from multiple excess argon components. This approach, applied to several L chondrites, yields an improved age value that indicates a single asteroid breakup event at 470 ± 6 Myr, fully consistent with a refined age estimate of the Middle Ordovician meteorite shower at 467.3 ± 1.6 Myr (according to A Geologic Time Scale 2004). Our results link these fossil meteorites directly to the L-chondrite asteroid destruction, rapidly transferred from the asteroid belt. The increased terrestrial meteorite influx most likely involved larger projectiles that contributed to an increase in the terrestrial cratering rate, which implies severe environmental stress.

C hondritic meteorites, in particular the CM-type carbonaceous chondrites, make up a unique subset of primitive meteorites, which are of particular interest in the context of origins of life because of their relatively high carbon content and because most of this carbon is present as organic matter. This material is a diverse mixture of compounds that in particular includes carboxylic acids, dicarboxylic acids, hydroxy acids, sulfonic acids, phosphonic acids, and amino acids in the form of monoamino alkanoic acids and monoamino alkandioic acids (1). Among these classes of compounds, a very small fraction of the meteoritic isomers is believed to support prebiotic evolutionary processes. We analyzed hydrolyzed hot-water extracts of a fresh sample of the Murchison CM-chondrite by GC-MS with a chiral column. With an efficient derivatization method and an improved detection technique we focused on compounds with more than two functional groups. MethodsA 1.19-g sample (MPI 320͞14) was taken from the interior of the Murchison meteorite by using a stone crusher. The sample provided fresh fracture surfaces. The sample was powdered for 2 ϫ 2 min at 600 rpm by a planetary micro mill (Pulverisette 7, Fritsch, Idar-Oberstein, Germany) in a positive-pressure ''class 100'' clean room. A 5-mg aliquot of the sample was imaged by a raster electron microscope (CS44, CamScan, Cranberry Township, PA), showing a homogeneous particle size distribution in the low micrometer range. A cold-water (water for organic trace analysis, Fluka) extraction was performed with a 130-mg aliquot that was subjected to diamino acid analysis. In parallel, 347 mg of the powdered sample was extracted with 700 l of water for 20 h at 100°C. After centrifugation (Eppendorf safe-lock tubes in Biopur-quality), the liquid phase was split. One hundred microliters was taken for direct derivatization by using the procedure described ref.2, leading to N,NЈ-diethoxycarbonyl diamino acid ethyl ester (ECEE) derivatives. Another aliquot of 100 l was hydrolyzed in 6 M HCl (hydrochloric acid for amino acid analysis, ampoule, Fluka) at 110°C for 24 h (3). After evaporation of the 6 M HCl, the residue was dissolved in 0.1 M HCl and derivatized by the above protocol. The ECEE derivatives obtained in this way were subjected to enantioselective GC-MS (Varian Chrompack Chirasil-L-Val capillary column; 12 m ϫ 0.25-mm inner diameter, 0.12-m film thickness, 250°C inlet temperature, pulsed splitless injection, 1.5 ml⅐min Ϫ1 constant f low of He carrier gas). The oven temperature program applied for the solvent trapping mode started at 50°C with 10°C⅐min Ϫ1 up to 90°C and then with 2°C⅐min Ϫ1 to 110°C, then was 10°C⅐min Ϫ1 to 180°C, where it was held constant for 39 min. The Agilent 6890͞5973 GC-MSD system was used. The identities of the diamino acid peaks obtained via enantioselective GC-MS were verified by comparing the retention times and the mass spectra with literature data (4) and external standards purchased from Fluka. A serpentine sample was taken as a blank (5). Results a...