Synthetic MgSiO3 glasses were irradiated at room temperature by 300 keV electrons in a transmission electron microscope. One of the samples had been previously irradiated by 50 keV He+ ions. Electron irradiation induces the nucleation and growth of randomly oriented nanometer-sized crystallites. The crystallites first consist of MgO and subsequently of forsterite (Mg2SiO4). Both are seen to form within an amorphous SiO2 matrix. The rate of crystallisation of the samples has been monitored by conventional TEM imaging and electron diffraction. The sample that had been pre-irradiated with He+ ions is found to transform faster than the as-quenched glass. The crystallization of metastable MgSiO3 glasses is explained by ionising radiation-induced elemental diffusion which allows the reorganization of matter into a more favourable thermodynamic state. These results show that ionizing radiation interactions could account for crystal formation as observed in IR spectroscopy in some young stellar environments.

The laboratory analyses of cosmic dust analogues--that in the context of this paper include interstellar, circumstellar as well as cometary dust--have a critical role in the study of circumstellar and cometary dust. The morphological, structural and chemical characterisation of these analogues are critical for comparisons of their IR and UV spectra with those obtained by astronomical observations, as well as for modelling purposes. Besides, the results from these laboratory studies are important to the success of space missions to comets when testing and calibrating the payload instruments. The interpretations of returned scientific data would benefit from the comparison with data recorded by the instruments in a laboratory setting for different classes of previously characterised analogues. We produced various types of condensed samples: (1) Mg,Fe-silicates, (olivine; pyroxene), (2) carbon-rich dust and (3) mixed carbon-silicate dust. The samples were prepared using different techniques, viz. (1) laser bombardment of solid targets in an Ar and O2 atmosphere, (2) arc discharge in an Ar and H2 atmosphere, and (3) grinding powders of natural minerals. We simulated various post-condensation processes, such as thermal annealing, UV irradiation, ion bombardment and exposure to atomic hydrogen. These processes produced compound samples of a wide range of physico-chemical properties. To identify their textures, morphologies, grain compositions and crystallographic properties we used electron microscopy and far-ultraviolet to far-infrared (millimiter range) spectroscopy.

We discuss eight trecento (fourteenth century) paintings containing depictions of astronomical events to reveal the revolutionary advances made in both astronomy and naturalistic painting in early Renaissance Italy, noting that an artistic interest in naturalism predisposed these pioneering painters to make their scientific observations. In turn, the convincing representations of their observations of astronomical phenomena in works of art rendered their paintings more believable, convincing. Padua was already a renowned center for mathematics and nascent astronomy (which was separating from astrology) when Enrico Scrovegni commissioned the famous Florentine artist Giotto di Bondone to decorate his lavish family chapel (circa 1301-1303). Giotto painted a flaming comet in lieu of the traditional Star of Bethlehem in the Adoration of the Magi scene. Moreover, he painted a historical apparition that he recently had observed with a great accuracy even by modern standards. Halley's Comet of 1301 (Olson, 1979). While we do not know the identity of the artist's theological advisor, we discuss the possibility that Pietro d'Abano, the Paduan medical doctor and "astronomer" who wrote on comets, might have been influential. We also compare Giotto's blazing comet with two others painted by the artist's shop in San Francesco at Assisi (before 1316) and account for the differences. In addition, we discuss Giotto's pupil, Taddeo Gaddi, reputed to have been partially blinded by a solar eclipse, whose calamity may find expression in his frescoes in Santa Croce, Florence (1328-30; 1338?). Giotto also influenced the Sienese painter Pietro Lorenzetti, two of whose Passion cycle frescoes at Assisi (1316-20) contain dazzling meteor showers that reveal the artist's observed astronomical phenomena, such as the "radiant" effect of meteor showers, first recorded by Alexander von Humboldt in 1799 and only accepted in the nineteenth century. Lorenzetti also painted sporadic, independent meteors, which do not emanate from the radiant. It is also significant that these artists observed differences between comets and meteors, facts that were not absolutely established until the eighteenth century. In addition we demonstrate that artistic and scientific visual acuity were part of the burgeoning empiricism of the fourteenth century, which eventually yielded modern observational astronomy.

Three samples of Darwin Glass, an impact glass found in Tasmania, Australia at the edge of the Australasian tektite strewn field were dated using the 40Ar/39Ar single-grain laser fusion technique, yielding isochron ages of 796-815 ka with an overall weighted mean of 816 +/- 7 ka. These data are statistically indistinguishable from those recently reported for the Australasian tektites from Southeast Asia and Australia (761-816 ka; with a mean weighted age of 803 +/- 3 ka). However, considering the compositional and textural differences and the disparity from the presumed impact crater area for Australasian tektites, Darwin Glass is more likely to have resulted from a distinct impact during the same period of time.

A shock experiment has been devised to produce large shear in a single crystal sample of olivine. The recovered sample exhibits macroscopic shear faults resembling shock veins in ordinary chondrites. Examination with transmission electron microscopy reveals a high density of dislocations in the bulk olivine. The shear faults appear as thin veins containing small grains of olivine and pockets of glass. The microstructure and composition of the material in the veins point to fractional crystallization of a melt. An order of magnitude calculation is consistent with the idea that the veins were produced by shear melting. These results support the view that shock veins in meteorites are the result of shear heating rather than of pressure heterogeneities.

We have identified four comets which have produced low velocity Earth-crossing dust streams within the past century: 7P/Pons-Winnecke, 26P/Grigg-Skjellerup, 73P/Schwassmann-Wachmann 3, and 103P/Hartley 2. These comets have the rare characteristics of low eccentricity, low inclination orbits with nodes very close to 1 AU. Dust from these comets is directly injected into Earth-crossing orbits by radiation pressure, unlike the great majority of interplanetary dust particles collected in the stratosphere which spend millennia in space prior to Earth-encounter. Complete dust streams from these comets form within a few decades, and appreciable amounts of dust are accreted by the Earth each year regardless of the positions of the parent comets. Dust from these comets could be collected in the stratosphere and identified by its short space exposure age, as indicated by low abundances of implanted solar wind noble gases and/or lack of solar flare tracks. Dust from Grigg-Skjellerup probably has the highest concentration at Earth orbit. We estimate that the proportion of dust from this comet reaches at least several percent of the background interplanetary dust flux in the >40 micrometer size range during April 23-24 of 2003.