An ice crystal halo display that contains several previously unknown halo phenomena was observed in Northern Chile. Analysis of computer simulations of the halos demonstrates that most of the new halo arcs in the display can be explained by the presence of airborne and preferentially oriented crystals of cubic ice. These observations therefore provide evidence of the existence of the cubic phase of ice in the Earth's atmosphere.

No abstract

Abstract-Recently, samples of allochthonous melt rocks from the ∼10 km and ≤1.9 Ga Paasselkä impact structure, SE Finland, were obtained. In this study, we present a first detailed petrographic and geochemical description of clast-rich Paasselkä impact melt rocks. Shock metamorphic features comprise shocked feldspar grains, intensely shocked and toasted quartz, marginally molten and recrystallized clasts thought to have been diaplectic quartz glass, largely fresh and recrystallized feldspar glasses, decomposed biotite flakes, recrystallized fluidal silica glass (originally probably lechatelierite) in partially molten sandstone clasts, all set into a glassy to cryptocrystalline melt matrix. The degree of shock metamorphism of clasts suggests initial whole-rock melting at peak shock pressures of ≥35 GPa and post-shock temperatures of up to ∼1500 °C. Glass components vary in geochemical composition corresponding to the mixed character of the crystalline target rock (i.e., representing different monomineralic and mixed-mineral melts). Feldspar glasses and the fresh glassy to cryptocrystalline melt matrix indicate that the Paasselkä melt rocks are not intensely altered. The geochemical composition of the Paasselkä impact melt rocks is roughly consistent with the compositions of melt rocks from a number of impact structures located within the crystalline basement of the Baltic Shield.

When an object enters the atmosphere it may be detected as a meteor. A bright meteor, called a fireball, may be a sign of a meteorite fall. Instrumentally observed meteorite falls provide unique opportunities to recover and analyse unweathered planetary samples supplemented with the knowledge on the Solar System orbit they had. To recover a meteorite from a fireball event, it is essential that recovery teams can be directed to a well-defined search area. Until recently, simulations showing the realistic mapping of a strewn field was difficult, in particular due to the large number of unknowns not directly retrieved from the fireball observations. These unknowns include the number of fragments and their aerodynamic properties, for which the masses of the fragments need to be assumed in a traditional approach. Here we describe a new Monte Carlo model, which has already successfully assisted in several meteorite recoveries. The model is the first of its kind as it provides an adequate representation of the processes occurring during the luminous trajectory coupled together with the dark flight. In particular, the model comprises a novel approach to fragmentation modelling that leads to a realistic fragment mass distribution on the ground. We present strewn field simulations for the well-documented Košice and Neuschwanstein meteorite falls, which demonstrate good matches to the observations. We foresee that our model can be used to revise the flux of extra-terrestrial matter onto the Earth, as it provides a possibility of estimating the terminal mass of meteorite fragments reaching the ground.