Chemistry: Space resources for teachers including suggestions for classroom activities and laboratory experiments

Curriculum supplement to assist general chemistry teachers in updating instruction materials with aerospace development

Viewpoint consistency in Z and LOTOS: A case study

Specification by viewpoints is advocated as a suitable method of specifying complex systems. Each viewpoint describes the envisaged system from a particular perspective, using concepts and specification languages best suited for that perspective. Inherent in any viewpoint approach is the need to check or manage the consistency of viewpoints and to show that the different viewpoints do not impose contradictory requirements. In previous work we have described a range of techniques for consistency checking, refinement, and translation between viewpoint specifications, in particular for the languages LOTOS and Z. These two languages are advocated in a particular viewpoint model, viz. that of the Open Distributed Processing (ODP) reference model. In this paper we present a case study which demonstrates how all these techniques can be combined in order to show consistency between a viewpoint specified in LOTOS and one specified in Z. Keywords: Viewpoints; Consistency; Z; LOTOS; ODP

Diagenetic responses to sea-level change: Integration of field, stable isotope, paleosol, paleokarst, fluid inclusion, and cement stratigraphy research to determine history and magnitude of sea-level fluctuation

Abstract Quantifying the history of changes in sea level is an important constraint for modeling sedimentary systems. Integration of diagenetic evidence for subaerial exposure with stratigraphic evidence of paleotopography is important for determining the history of relative changes in sea level. Surfaces of subaerial exposure can develop on marine strata from aggradation of sediment into the subaerial realm, from eustatic sea-level falls, or from uplift. Surfaces of subaerial exposure that result from aggradation alone can be distinguished from those that result from eustatic fall or uplift. If exposure surfaces directly overlie strata of subtidal origin or drape significant paleotopography, aggradation alone must be ruled out. The minimum relative fall in sea level can be quantified by tracing surfaces of subaerial exposure over reconstructed paleotopography or by determining the depth to which vadose-zone diagenesis occurred. Paleosols, paleokarst, trends in stable isotopes, calcite cement stratigraphy, calcite cement fabrics, and fluid inclusions provide diagenetic records that are useful in identifying ancient surfaces of subaerial exposure and determining the position of ancient vadose zones. Paleosols can be identified using the preserved records of desiccation and wetting, subaerial processes, and plant activity. Studies from the Pennsylvanian Holder Formation of New Mexico illustrate that paleosols can be laterally variable in nature and can be used to demonstrate a relative fall in sea level of at least 30 m (100 ft). Paleosol features can develop well below the subaerial surface, so caution must be used in applying this technique. Paleokarst is another useful record of subaerial exposure. Some karstification results in surface landforms, terra rossa paleosols, or vertical voids in which there is a clear relationship to an ancient surface of subaerial exposure. The depth of penetration of these karst features that developed in the vadose zone can be used as a minimum estimate of relative sea-level fall. For karst cavities that cannot be directly associated with a specific surface of subaerial exposure, the age of sediment fills or regional distribution of cavities can provide the most direct link to a particular surface of subaerial exposure. Whole-rock trends of relatively negative δ13C, relatively positive δ18O, and a baseline shift in δ18O can also reflect ancient surfaces of subaerial exposure. Variability of data from the Holder Formation shows that the trends predicted are not the result of stabilization in a homogeneous, relatively negative δ13C zone but the result of patchy cementation and replacement in a solution of heterogeneous carbon isotopic composition. Preservation of the most negative carbon signatures depends on sampling the highest volumes of soil-precipitated phases and those in closest proximity to organic structures in soils. Calcite cements with meniscus or pendant fabrics preserve a record of vadose diagenesis. Vertical pinchout of calcite-cement compositional zones may reflect surfaces of subaerial exposure. The lateral variability of such cements in the Pennsylvanian Holder Formation and the Lansing-Kansas City Groups of Kansas shows that such cements develop best in paleotopographically high settings. Fluid inclusions can provide a record of diagenesis in the vadose zone. Fluid inclusions trapped in the vadose zone are marked by variable ratios of vapor to liquid and all-liquid fluid inclusions. The distribution of such inclusions in Miocene rocks of Spain demonstrate a relative fall in sea level of at least 50-55 m (160-180 ft)

Diagenetic responses to sea-level change: Integration of field, stable isotope, paleosol, paleokarst, fluid inclusion, and cement stratigraphy research to determine history and magnitude of sea-level fluctuation

Abstract Quantifying the history of changes in sea level is an important constraint for modeling sedimentary systems. Integration of diagenetic evidence for subaerial exposure with stratigraphic evidence of paleotopography is important for determining the history of relative changes in sea level. Surfaces of subaerial exposure can develop on marine strata from aggradation of sediment into the subaerial realm, from eustatic sea-level falls, or from uplift. Surfaces of subaerial exposure that result from aggradation alone can be distinguished from those that result from eustatic fall or uplift. If exposure surfaces directly overlie strata of subtidal origin or drape significant paleotopography, aggradation alone must be ruled out. The minimum relative fall in sea level can be quantified by tracing surfaces of subaerial exposure over reconstructed paleotopography or by determining the depth to which vadose-zone diagenesis occurred. Paleosols, paleokarst, trends in stable isotopes, calcite cement stratigraphy, calcite cement fabrics, and fluid inclusions provide diagenetic records that are useful in identifying ancient surfaces of subaerial exposure and determining the position of ancient vadose zones. Paleosols can be identified using the preserved records of desiccation and wetting, subaerial processes, and plant activity. Studies from the Pennsylvanian Holder Formation of New Mexico illustrate that paleosols can be laterally variable in nature and can be used to demonstrate a relative fall in sea level of at least 30 m (100 ft). Paleosol features can develop well below the subaerial surface, so caution must be used in applying this technique. Paleokarst is another useful record of subaerial exposure. Some karstification results in surface landforms, terra rossa paleosols, or vertical voids in which there is a clear relationship to an ancient surface of subaerial exposure. The depth of penetration of these karst features that developed in the vadose zone can be used as a minimum estimate of relative sea-level fall. For karst cavities that cannot be directly associated with a specific surface of subaerial exposure, the age of sediment fills or regional distribution of cavities can provide the most direct link to a particular surface of subaerial exposure. Whole-rock trends of relatively negative δ13C, relatively positive δ18O, and a baseline shift in δ18O can also reflect ancient surfaces of subaerial exposure. Variability of data from the Holder Formation shows that the trends predicted are not the result of stabilization in a homogeneous, relatively negative δ13C zone but the result of patchy cementation and replacement in a solution of heterogeneous carbon isotopic composition. Preservation of the most negative carbon signatures depends on sampling the highest volumes of soil-precipitated phases and those in closest proximity to organic structures in soils. Calcite cements with meniscus or pendant fabrics preserve a record of vadose diagenesis. Vertical pinchout of calcite-cement compositional zones may reflect surfaces of subaerial exposure. The lateral variability of such cements in the Pennsylvanian Holder Formation and the Lansing-Kansas City Groups of Kansas shows that such cements develop best in paleotopographically high settings. Fluid inclusions can provide a record of diagenesis in the vadose zone. Fluid inclusions trapped in the vadose zone are marked by variable ratios of vapor to liquid and all-liquid fluid inclusions. The distribution of such inclusions in Miocene rocks of Spain demonstrate a relative fall in sea level of at least 50-55 m (160-180 ft)

Nonlinear Scattering of a Bose-Einstein Condensate on a Rectangular Barrier

We consider the nonlinear scattering and transmission of an atom laser, or Bose-Einstein condensate (BEC) on a finite rectangular potential barrier. The nonlinearity inherent in this problem leads to several new physical features beyond the well-known picture from single-particle quantum mechanics. We find numerical evidence for a denumerably infinite string of bifurcations in the transmission resonances as a function of nonlinearity and chemical potential, when the potential barrier is wide compared to the wavelength of oscillations in the condensate. Near the bifurcations, we observe extended regions of near-perfect resonance, in which the barrier is effectively invisible to the BEC. Unlike in the linear case, it is mainly the barrier width, not the height, that controls the transmission behavior. We show that the potential barrier can be used to create and localize a dark soliton or dark soliton train from a phonon-like standing wave.Comment: 15 pages, 15 figures, new version includes clarification of definition of transmission coefficient in general nonlinear vs. linear cas

Electron Shock Waves with a Large Current Behind the Shock Front

The propagation of breakdown waves in a gas, which is primarily driven by electron gas pressure, is described by a one-dimensional, steady-state, three-component (electrons, ions, and neutral particles) fluid model. We consider the electron gas partial pressure to be much larger than that of the other species and the waves to have a shock front. Our set of equations consists of the equations of conservation of the flux of mass, momentum, and energy coupled with Poisson’s equation. This set of equations is referred to as the electron fluid dynamical equations. In this study we are considering breakdown waves propagating in the opposite direction of the electric field force on electrons (return stroke in lightning) and moving into a neutral medium. For Breakdown waves with a significant current behind the shock front, the set of electron fluid dynamical equations and also the boundary condition on electron temperature need to be modified. For a range of experimentally observed current values and also some larger current values which few experimentalists have been able to observe, we have been able to solve the set of electron fluid dynamical equations through the dynamical transition region of the wave. Some experimentalists have reported the existence of a relationship between return stroke lightning wave speed and current behind the shock front; however, some others are skeptical of the existence of such a relationship. Our solutions to the set of electron fluid dynamical equations within the dynamical transition region of the wave confirm the existence of such a relationship. We will present the method of solution of the set of electron fluid dynamical equations through the dynamical transition region of the wave and also the wave profile for electric field, electron velocity, electron temperature and electron number density, within the dynamical transition region of the wave

DICER-LIKE2 plays a primary role in transitive silencing of transgenes in Arabidopsis.

Dicer-like (DCL) enzymes play a pivotal role in RNA silencing in plants, processing the long double-stranded RNA (dsRNA) that triggers silencing into the primary short interfering RNAs (siRNAs) that mediate it. The siRNA population can be augmented and silencing amplified via transitivity, an RNA-dependent RNA polymerase (RDR)-dependent pathway that uses the target RNA as substrate to generate secondary siRNAs. Here we report that Arabidopsis DCL2-but not DCL4-is required for transitivity in cell-autonomous, post-transcriptional silencing of transgenes. An insertion mutation in DCL2 blocked sense transgene-induced silencing and eliminated accumulation of the associated RDR-dependent siRNAs. In hairpin transgene-induced silencing, the dcl2 mutation likewise eliminated accumulation of secondary siRNAs and blocked transitive silencing, but did not block silencing mediated by primary siRNAs. Strikingly, in all cases, the dcl2 mutation eliminated accumulation of all secondary siRNAs, including those generated by other DCL enzymes. In contrast, mutations in DCL4 promoted a dramatic shift to transitive silencing in the case of the hairpin transgene and enhanced silencing induced by the sense transgene. Suppression of hairpin and sense transgene silencing by the P1/HC-Pro and P38 viral suppressors was associated with elimination of secondary siRNA accumulation, but the suppressors did not block processing of the stem of the hairpin transcript into primary siRNAs. Thus, these viral suppressors resemble the dcl2 mutation in their effects on siRNA biogenesis. We conclude that DCL2 plays an essential, as opposed to redundant, role in transitive silencing of transgenes and may play a more important role in silencing of viruses than currently thought

Casimir-Polder force density between an atom and a conducting wall

In this paper we calculate the Casimir-Polder force density (force per unit area acting on the elements of the surface) on a metallic plate placed in front of a neutral atom. To obtain the force density we use the quantum operator associated to the electromagnetic stress tensor. We explicitly show that the integral of this force density over the plate reproduces the total force acting on the plate. This result shows that, although the force is obtained as a sum of surface element-atom contributions, the stress-tensor method includes also nonadditive components of Casimir-Polder forces in the evaluation of the force acting on a macroscopic object.Comment: 5 page

Computational science and re-discovery: open-source implementations of ellipsoidal harmonics for problems in potential theory

We present two open-source (BSD) implementations of ellipsoidal harmonic expansions for solving problems of potential theory using separation of variables. Ellipsoidal harmonics are used surprisingly infrequently, considering their substantial value for problems ranging in scale from molecules to the entire solar system. In this article, we suggest two possible reasons for the paucity relative to spherical harmonics. The first is essentially historical---ellipsoidal harmonics developed during the late 19th century and early 20th, when it was found that only the lowest-order harmonics are expressible in closed form. Each higher-order term requires the solution of an eigenvalue problem, and tedious manual computation seems to have discouraged applications and theoretical studies. The second explanation is practical: even with modern computers and accurate eigenvalue algorithms, expansions in ellipsoidal harmonics are significantly more challenging to compute than those in Cartesian or spherical coordinates. The present implementations reduce the "barrier to entry" by providing an easy and free way for the community to begin using ellipsoidal harmonics in actual research. We demonstrate our implementation using the specific and physiologically crucial problem of how charged proteins interact with their environment, and ask: what other analytical tools await re-discovery in an era of inexpensive computation?Comment: 25 pages, 3 figure

LiSc(BH_4)_4 as a Hydrogen Storage Material: Multinuclear High-Resolution Solid-State NMR and First-Principles Density Functional Theory Studies

A lithium salt of anionic scandium tetraborohydride complex, LiSc(BH_4)_4, was studied both experimentally and theoretically as a potential hydrogen storage medium. Ball milling mixtures of LiBH_4 and ScCl_3 produced LiCl and a unique crystalline hydride, which has been unequivocally identified via multinuclear solid-state nuclear magnetic resonance (NMR) to be LiSc(BH_4)_4. Under the present reaction conditions, there was no evidence for the formation of binary Sc(BH_4)_3. These observations are in agreement with our first-principles calculations of the relative stabilities of these phases. A tetragonal structure in space group I (#82) is predicted to be the lowest energy state for LiSc(BH_4)_4, which does not correspond to structures obtained to date on the crystalline ternary borohydride phases made by ball milling. Perhaps reaction conditions are resulting in formation of other polymorphs, which should be investigated in future studies via neutron scattering on deuterides. Hydrogen desorption while heating these Li−Sc−B−H materials up to 400 °C yielded only amorphous phases (besides the virtually unchanged LiCl) that were determined by NMR to be primarily ScB_2 and [B_(12)H_(12)]^(−2) anion containing (e.g., Li_2B_(12)H_(12)) along with residual LiBH_4. Reaction of a desorbed LiSc(BH_4)_4 + 4LiCl mixture (from 4LiBH_4/ScCl_3 sample) with hydrogen gas at 70 bar resulted only in an increase in the contents of Li_2B_(12)H_(12) and LiBH_4. Full reversibility to reform the LiSc(BH_4)_4 was not found. Overall, the Li−Sc−B−H system is not a favorable candidate for hydrogen storage applications