Planetary Nebulae

This review summarizes spectroscopic and imaging results obtained on board the Infrared Space Observatory (ISO) on well-known planetary nebulae: the young nebula NGC 7027, and two fully evolved nearby nebulae, the Dumbbell and the Helix. This revised version was published online in August 2006 with corrections to the Cover Date.     

A practical approach to grain shape quantification

Grain shape is a key factor affecting the mechanical properties of granular materials. However, grain shape quantification techniques to distinguish one granular material from another have not reached a stage of development for inclusion in modeling the behavior of granular materials. Part of the problem is the equipment of choice for grain shape measurement is the scanning electron microscopes. This is a relatively expensive and complex device. In this paper, we investigate a practical approach using light microscopy to quantify grain shape and to identify the key shape parameters that can distinguish grains. A light microscope was found to produce grain images with sufficient quality for the purpose of observing the grain shape profile. Several grain shape parameters were determined for eight different sands. We found Circularity, Roundness, Compactness, Sphericity, Aspect Ratio and ModRatio to be the key shape parameters that differentiate these sand grains.     

Permeability development in vesiculating magmas: implications for fragmentation

 Fragmentation, or the "coming apart" of magma during a plinian eruption, remains one of the least understood processes in volcanology, although assumptions about the timing and mechanisms of fragmentation are key parameters in all existing eruption models. Despite evidence to the contrary, most models assume that fragmentation occurs at a critical vesicularity (volume percent vesicles) of 75–83%. We propose instead that the degree to which magma is fragmented is determined by factors controlling bubble coalescence: magma viscosity, temperature, bubble size distribution, bubble shapes, and time. Bubble coalescence in vesiculating magmas creates permeability which serves to connect the dispersed gas phase. When sufficiently developed, permeability allows subsequent exsolved and expanded gas to escape, thus preserving a sufficiently interconnected region of vesicular magma as a pumice clast, rather than fully fragmenting it to ash. For this reason pumice is likely to preserve information about (a) how permeability develops and (b) the critical permeability needed to insure clast preservation. We present measurements and calculations that constrain the conditions (vesicularity, bubble size distribution, time, pressure difference, viscosity) necessary for adequate permeability to develop. We suggest that magma fragments explosively to ash when and where, in a heterogeneously vesiculating magma, these conditions are not met. Both the development of permeability by bubble wall thinning and rupture and the loss of gas through a permeable network of bubbles require time, consistent with the observation that degree of fragmentation (i.e., amount of ash) increases with increasing eruption rate. Received: 5 July 1995 / Accepted: 27 December 1995     

Quantification of the effects of secondary matrix on the analysis of sandstone composition, and a petrographic-chemical technique for retrieving original framework grain modes of altered sandstones

Most studies of sandstone provenance involve modal analysis of framework grains using techniques that exclude the fine-grained breakdown products of labile mineral grains and rock fragments, usually termed secondary matrix or pseudomatrix. However, the data presented here demonstrate that, when the proportion of pseudomatrix in a sandstone exceeds 10%, standard petrographic analysis can lead to incorrect provenance interpretation. Petrographic schemes for provenance analysis such as QFL and QFR should not therefore be applied to sandstones containing more than 10% secondary matrix. Pseudomatrix is commonly abundant in sandstones, and this is therefore a problem for provenance analysis. The difficulty can be alleviated by the use of whole-rock chemistry in addition to petrographic analysis. Combination of chemical and point-count data permits the construction of normative compositions that approximate original framework grain compositions. Provenance analysis is also complicated in many cases by fundamental compositional alteration during weathering and transport. Many sandstones, particularly shallow marine deposits, have undergone vigorous reworking, which may destroy unstable mineral grains and rock fragments. In such cases it may not be possible to retrieve provenance information by either petrographic or chemical means. Because of this, pseudomatrix-rich sandstones should be routinely included in chemical-petrological provenance analysis. Because of the many factors, both pre- and post-depositional, that operate to increase the compositional maturity of sandstones, petrologic studies must include a complete inventory of matrix proportions, grain size and sorting parameters, and an assessment of depositional setting.     

Numerical simulation of the final stages of terrestrial planet formation

Three representative numerical simulations of the growth of the terrestrial planets by accretion of large protoplanets are presented. The mass and relative-velocity distributions of the bodies in these simulations are free to evolve simultaneously in response to close gravitational encounters and occasional collisions between bodies. The collisions between bodies, therefore, arise in a natural way and the assumption of expressions for the relative velocity distribution and the gravitational collision cross section is unnecessary. These simulations indicate that the growth of bodies with final masses approaching those of Venus and the Earth is possible, at least for the case of a two-dimensional system. Simulations assuming an initial uniform distribution of orbital eccentricities on the interval from 0 to e_max are found to produce final states containing too many bodies with masses which are too small when e_max < 0.10, while simulations with e_max > 0.20 result in too many catastrophic collisions between bodies thus preventing rapid accretion of planetary-size bodies. The e_max = 0.15 simulation ends with a state surprisingly similar to that of the present terrestrial planets and, therefore, provides a rough estimate of the range of radial sampling to be expected for the terrestrial planets.     

Temperature and its control of isotope fractionation by a sulfate-reducing bacterium

A synthesis of previous results, which we dub the “standard model,” provides a prediction as to how isotope fractionation during sulfate reduction should respond to physiological variables such as specific rate of sulfate reduction and environmental variables such as substrate availability and temperature. The standard model suggests that isotope fractionation should decrease with increasing specific rates of sulfate reduction (rate per cell). Furthermore, the standard model predicts that low fractionations should be found at both high and low temperatures whereas the highest fractionations should be found in the intermediate temperature range. These fractionation trends are controlled, as a function of temperature, by the balance between the transfer rates of sulfate into and out of the cell and the exchange between the sulfur pools internal to the organism. We test this standard model by conducting experiments on the growth physiology and isotope fractionation, as a function of temperature, by the sulfate-reducing bacterium Desulfovibrio desulfuricans (DSMZ 642). Our results contrast with the “standard model” by showing a positive correlation between specific rates of sulfate reduction and fractionation. Also by contrast with the standard model, we found the highest fractionations at low and high temperatures and the lowest fractionations in the intermediate temperature range. We develop a fractionation model which can be used to explain both our results as well as the results of the “standard model.” Differences in fractionation with temperature relate to differences in the specific temperature response of internal enzyme kinetics as well as the exchange rates of sulfate in and out of the cell. It is expected that the kinetics of these processes will show strain-specific differences.     

Quantitative charge state analysis of manganese biominerals in aqueous suspension using scanning transmission X-ray microscopy (STXM)

We have applied Scanning Transmission Soft X-ray Microscopy (STXM) to investigate the charge state distribution of Mn in two kinds of Mn-biominerals, Mn nodules collected from Lake Michigan sediments and Mn precipitates formed by spores of a marine bacillus SG-1 under transport limited reaction conditions. A data analysis technique was developed, which allows for extraction of spatially resolved 2-d charge state maps of manganese on a submicron level. We find that the charge state of Mn dominates the spectral shape of L-edge spectra of environmentally important single oxidation state Mn minerals and that spectra of mixed oxidation state oxides can be modelled by a combination of appropriate single oxidation state reference spectra. Two-dimensional maps of charge state distributions clearly reveal domains of different oxidation states within single particles of Mn-micronodules. Spots of preferred accumulation of Mn(II) were found, which indicates biogenic precipitation of Mn(II)-species as a first step of nodule formation. The presence of Mn(III) in the studied sediment samples suggests the involvement of one-electron oxidation processes and reaction conditions which inhibit or slow down the disproportionation of Mn(III)-products. Under transport limited conditions, Mn oxidation products formed by spores of the marine bacillus SG-1 can vary from cell to cell. The presence of significant amounts of Mn(III) containing species points to the involvement of one-electron oxidation reactions as in the case of the micro-nodules. Our technique and the results obtained form a new basis for the mechanistic understanding of the formation of Mn biominerals in the environment.     

Uncertainty in adaptive capacity

The capacity to adapt is a critical element of the process of adaptation: it is the vector of resources that represent the asset base from which adaptation actions can be made. Adaptive capacity can in theory be identified and measured at various scales, from the individual to the nation. The assessment of uncertainty within such measures comes from the contested knowledge domain and theories surrounding the nature of the determinants of adaptive capacity and the human action of adaptation. While generic adaptive capacity at the national level, for example, is often postulated as being dependent on health, governance and political rights, and literacy, and economic well-being, the determinants of these variables at national levels are not widely understood. We outline the nature of this uncertainty for the major elements of adaptive capacity and illustrate these issues with the example of a social vulnerability index for countries in Africa. To cite this article: W.N. Adger, K. Vincent, C. R. Geoscience 337 (2005).     

Micro- and nano-textural evidence of Ti(–Ca–Fe) mobility during fluid–rock interactions in carbonaceous lawsonite-bearing rocks from New Zealand

Understanding the mobility of chemical elements during fluid–rock interactions is critical to assess the geochemical evolution of a rock undergoing burial and metamorphism and, more generally, to constrain the geochemical budget of the subduction factory. In particular, determining the behavior and mobility of Ti in aqueous fluids constitutes a great challenge that is still under scrutiny. Here, we study plant fossils preserved in blueschist metasedimentary rocks from the Marybank Formation (New Zealand). Using scanning and transmission electron microscopies (SEM and TEM), we show that the carbonaceous material (CM) composing the fossils contains abundant nano-inclusions of Ti- and Fe-oxides. These nanocrystals are mainly anatase, rutile, and Fe–Ti oxides. The mineral composition observed within the fossils is significantly different from that detected in the surrounding rock matrix. We propose that Ti and Fe might have been mobilized by the alteration of a detrital Ti–Fe-rich protolith during an early diagenetic event under acidic and reducing conditions. Aqueous fluids rich in organic ligands released by the degradation of organic matter may have been involved. Moreover, using mass balance and petrological observations, we show that the contrasted mineralogy between the rock matrix and the fossil CM might be the consequence of the chemical isolation of fossil CM during the prograde path of the rock. Such an isolation results from the early formation of quartz and Fe-rich phyllosilicate layers enclosing the fossil as characterized by SEM and TEM investigations. Overall, this study shows that investigating minerals associated with CM down to the nanometer scale in metamorphic rocks can provide a precious record of early prograde geochemical conditions.     

A modeling of the structure and favorable H-docking sites and defects for the high-pressure silica polymorph stishovite

Employing first-principles methods, the docking sites for H were determined and H, Al, and vacancy defects were modeled with an infinite periodic array of super unit cells each consisting of 27 contiguous symmetry nonequivalent unit cells of the crystal structure of stishovite. A geometry optimization of the super-cell structure reproduces the observed bulk structure within the experimental error when P1 translational symmetry was assumed and an array of infinite extent was generated. A mapping of the valence electrons for the structure displays mushroom-shaped isosurfaces on the O atom, one on each side of the plane of the OSi₃ triangle in the nonbonded region. An H atom, placed in a cell near the center of the super cell, was found to dock upon geometry optimization at a distance of 1.69 Å from the O atom with the OH vector oriented nearly perpendicular to the plane of the triangle such that the OH vector makes a angle of 91° with respect to [001]. However, an optimization of a super cell with an Al atom replacing Si and an H atom placed nearby in a centrally located cell resulted in an OH distance of 1.02 Å with the OH vector oriented perpendicular to [001] as observed in infrared studies. The geometry-optimized position of the H atom was found to be in close agreement with that (0.44, 0.12, 0.0) determined in an earlier study of the theoretical electron density distribution. The docking of the H atom at this site was found to be 330 kJ mol^–1 more stable than a docking of the atom just off the shared OO edge of the octahedra as determined for rutile. A geometry optimization of a super cell with a missing Si generated a vacant octahedra that is 20% larger than that of the SiO₆ octahedra. The valence electron density distribution displayed by the two-coordinate O atoms that coordinate the vacant octahedral site is very similar to those displayed by the bent SiOSi angles in coesite. The internal distortions induced by the defect were found to diminish rather rapidly with distance, with the structure annealing to that observed in the bulk crystal to within about three coordination spheres.