Hydrogen recombination lines from the Gum nebula

Hydrogen recombination lines in the H156 α and H139 α transitions have been detected at four widely separated positions in the Gum nebula. This confirms that the radio continuum emission seen in parts of the nebula is predominantly bremsstrahlung rather than synchrotron emission.
The derived electron temperatures and emission measures are in the range 4200 to 8500 K and 220 to 470 pc cm⁻⁶ respectively. This is consistent with the presence of a low-density, photoionized plasma. The linewidth observed at the position away from the edge of the nebula is significantly larger than those near the edge of the nebula. Together with the negative line velocity observed at this position, this suggests there is systematic expansion of the near side of the nebula.     

Iron in silicate glasses of granitic composition: a Mössbauer spectroscopic study

⁵⁷Fe Mössbauer spectra of natural glasses (pumices and obsidians) and of synthetic glasses of granitic composition have been analyzed. — Ferric iron is found in tetrahedral coordination if enough M⁺-cations are available to balance the charge of both M⁺Fe³⁺O₂ and M⁺AlO₂ complexes. In other compositions the ratio of tetrahedrally to octahedrally coordinated Fe³⁺ depends on the ratio of mono-to divalent cations. — Ferrous iron occurs in two distinctly different octahedral sites. The existence of these sites can be attributed to different anionic units adjacent to Fe²⁺. The degree of polymerization of these units is reflected in the quadrupole splitting. The anionic units adjacent to Fe²⁺ are depolymerized for increasing mean Z/r ² of the network modifiers, which do not stabilize M³⁺ in the tetrahedra by local charge balance. — Increasing pressure diminishes the geometric differences between these types of ferrous iron-oxygen-octahedra, which gives rise to a more even distribution of Fe²⁺ among these sites and thereby to an ordering in the network of melts.     

A study of lunar impact crater size-distributions

Discrepancies in published crater frequency data prompted this study of lunar crater distributions. Effects modifying production size distributions of impact craters such as surface lava flows, blanketing by ejecta, superposition, infilling, and abrasion of craters, mass wasting, and the contribution of secondary and volcanic craters are discussed. The resulting criteria have been applied in the determination of the size distributions of unmodified impact crater populations in selected lunar regions of different ages. The measured cumulative crater frequencies are used to obtain a general calibration size distribution curve by a normalization procedure. It is found that the lunar impact crater size distribution is largely constant in the size range 0.3 km ?D ? 20 km for regions with formation ages between ≈ 3 × 10⁹ yr and ? 4 × 10⁹ yr. A polynomial of 4th degree, valid in the size range 0.8 km ?D ? 20 km, and a polynomial of 7th degree, valid in the size range 0.3 km ?D ? ? 20 km, have been approximated to the logarithm of the cumulative crater frequencyN as a function of the logarithm of crater diameterD. The resulting relationship can be expressed asN ～D ^α(D) where α is a function depending onD. This relationship allows the comparison of crater frequencies in different size ranges. Exponential relationships with constant α, commonly used in the literature, are shown to inadequately approximate the lunar impact crater size distribution. Deviations of measured size distributions from the calibration distribution are strongly suggestive of the existence of processes having modified the primary impact crater population.     

Reference Values Following ISO Guidelines for Frequently Requested Rock Reference Materials

We present new reference values for nineteen USGS, GSJ and GIT‐IWG rock reference materials that belong to the most accessed samples of the GeoReM database. The determination of the reference values and their uncertainties at the 95% confidence level follows as closely as possible ISO guidelines and the Certification Protocol of the International Association of Geoanalysts. We used analytical data obtained by the state‐of‐the‐art techniques published mainly in the last 20 years and available in GeoReM. The data are grouped into four categories of different levels of metrological confidence, starting with isotope dilution mass spectrometry as a primary method. Data quality was checked by careful investigation of analytical procedures and by the application of the Horwitz function. As a result, we assign a new and more reliable set of reference values and respective uncertainties for major, minor and a large group of trace elements of the nineteen investigated rock reference materials.