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Abstract-We reevaluate the systematics and geologic setting of terrestrial, lunar, Martian, and asteroidal "impactites" resulting from single or multiple impacts. For impactites derived from silicate rocks and sediments, we propose a unified and updated system of progressive shock metamorphism. "Shock-metamorphosed rocks" occur as lithic clasts or melt particles in proximal impactites at impact craters, and rarely in distal impactites. They represent a wide range of metamorphism, typically ranging from unshocked to shock melted. As the degree of shock metamorphism, at a given shock pressure, depends primarily on the mineralogical composition and the porosity of a rock or sediment sample, different shock classification systems are required for different types of planetary rocks and sediments. We define shock classification systems for eight rock and sediment classes which are assigned to three major groups of rocks and sediments (1) crystalline rocks with classes F, M, A, and U; (2) chondritic rocks (class C); and (3) sedimentary rocks and sediments with classes SR, SE, and RE. The abbreviations stand for felsic (F), mafic (M), anorthositic (A), ultramafic (U), sedimentary rocks (SR), unconsolidated sediments (SE), and regoliths (RE). In each class, the progressive stages of shock metamorphism are denominated S1 to Sx. These progressive shock stages are introduced as: S1-S7 for F, S1-S7 for M, S1-S6 for A, S1-S7 for U, S1-S7 for C, S1-S7 for SR, S1-S5 for SE, and S1-S6 for RE. S1 stands for "unshocked" and Sx (variable between S5 and S7) stands for "whole rock melting." We propose a sequence of symbols characterizing the degree of shock metamorphism of a sample, i.e

The outcome of the first stage of planetary formation, which is characterized by ballistic agglomeration of preplanetary dust grains due to Brownian motion in the free molecular flow regime of the solar nebula, is still somewhat speculative. We performed a microgravity experiment flown onboard the space shuttle in which we simulated, for the first time, the onset of free preplanetary dust accumulation and revealed the structures and growth rates of the first dust agglomerates in the young solar system. We find that a thermally aggregating swarm of dust particles evolves very rapidly and forms unexpected open-structured agglomerates. PACS numbers: 96.35.Cp, 61.43.Hv, 81.10.Mx It is now widely accepted that planets form from the nebula of gas and dust that comprises nascent solar systems. Inelastic, adhesive collisions between these dust particles eventually form kilometer-sized bodies, called planetesimals, which then collide under the influence of their mutual gravity to form planets [1][2][3][4][5][6]. After condensation of the micron-sized dust grains in the cooling gas, these initially collide with each other due to thermal (Brownian) motion, and, by adhesion due to van der Waals forces, form aggregates. The agglomeration rate of freshly condensed [7,8] preplanetary dust grains is determined by three factors: the collision cross section, the collision velocity, and the sticking probability of the dust particles, which are mutually interdependent. Laboratory experiments with micron-sized solid particles and dust agglomerates thereof have shown that, for moderate collision velocities y c # 1 m s 21 , the sticking probability is always unity [9][10][11]. The collision cross section and the collision velocity strongly depend on the morphology of the interacting preplanetary dust aggregates. Open-structured, fluffy particles generally have a larger cross section than compact grains, but couple also much better to the gas motion, so that relative velocities between fluffy agglomerates are suppressed. The gas-grain interaction is best described by the dust particles' response time to the gas motion, t f . In the free molecular flow regime, t f~m s a , where m and s a are the mass and the geometrical cross section (i.e., the projected area) of the dust aggregate. Aggregation models [1,12] for the Brownian motion-driven dust growth predict a scenario in which dust clusters of similar mass predominantly contribute to the agglomeration process. This leads to the evolution of a quasimonodisperse distribution of aggregate masses and to a relation between aggregate mass and size s ofwith an exponent ("fractal dimension") in the range of D f ഠ 1.8 2.1. For quasimonodisperse systems, the mean aggregate mass grows by a power law in time, m͑t͒~t z , and is related to the mass dependence of the collision cross section s c~m m and the collision velocity y c~m n of the aggregating particles through m 1 n z21 z (e.g., see the review in [13]). Here, z, m, and n are the respective exponents of the assumed power law functions for ...

Chondrules formed by the melting of dust aggregates in the solar protoplanetary disk and as such provide unique insights into how solid material was transported and mixed within the disk. Here Ti variations can be attributed to the addition of isotopically heterogeneous CAI-like material to enstatite and ordinary chondrite-like chondrule precursors. The new Ti isotopic data demonstrate that isotopic variations among carbonaceous chondrite chondrules do not require formation over a wide range of orbital distances, but can instead be fully accounted for by the incorporation of isotopically anomalous 'nuggets' into chondrule precursors. As such, these data obviate the need for disk-wide transport of chondrules prior to chondrite parent body accretion and are consistent with formation of chondrules from a given chondrite group in localized regions of the disk. Lastly, the ubiquitous presence of 50 Ti-enriched material in carbonaceous chondrites, and the lack of this material in the non-carbonaceous chondrites support the idea that these two meteorite groups derive from areas of the disk that remained isolated from each other through the formation of Jupiter.

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