This Week in Science

Science  03 Feb 2006:
Vol. 311, Issue 5761, pp. 573
  1. Slippery Melt Strands


    Some exhumed faults contain small pockets of melted rocks, presumably produced by frictional heat during an earthquake. The role of these melt strands-whether they inhibit faulting or lubricate it-and how they are produced in weak faults has been controversial. Di Toro et al. (p. 647) have now produced analogous features in the laboratory that they compare with actual field samples from an exposed fault. They used a rotary shear apparatus to slide rocks against each other at conditions that approximate natural earthquakes. Melt pockets were produced that lowered the friction and lubricated rather than sealed the fault.

  2. Assessing Nanomaterial Safety

    Scenarios of the dangers of nanotechnology that involve nanorobots running amok in our bodies or the world being taken over by “gray goo” are considered highly unlikely by many experts. However, a great deal remains unknown about the biological effects of human and environmental exposure to nanomaterials. Nel et al. (p. 622) review the important chemical and biological properties of nanomaterials and outline ways in which the safety and toxicity of these substances can be evaluated.

  3. Magnetic Maps

    Magnetic fields on stars like the Sun affect their interiors and their surrounding environment. In strongly convecting stars, turbulence is expected to break up aligned magnetic fields. Donati et al. (p. 633; see the Perspective by Basri) show that in a very-low-mass, fully-convective star, substantial fields remain, including a strong dipole component. The pattern of magnetic fields on the star's surface was recreated from observations of the fine Zeeman splitting of spectral lines caused by magnetic fields and other signatures in polarized light.

  4. Spectroscopy of Coupled Quantum Dots

    Single and coupled multiple quantum-dot structures have long been proposed as systems for storing and manipulating information in quantum information processing. However, finding routes to get the coupled dots to communicate are only now being explored. Stinaff et al. (p. 636, published online 12 January) present a spectroscopic study of pairs of neutral and charged quantum dots where coherent coupling between the dots can be induced by a combination of electric field resonances and optical excitation. The main spectroscopic features can be recovered with a relatively simple molecular model.

  5. Under Pressure to Separate

    When hydrogen is produced industrially, the gas stream is typically contaminated with H2S, CO2, steam, and other impurities that need to be removed. Ideally, the separation should occur at high pressure to avoid costly recompression, but current membrane materials do not work well at high pressure. Lin et al. (p. 639) have developed polymeric membrane materials that preferentially absorbed CO2 and other impurities and that showed greater efficiency as the pressure of the gas feed was increased. Unlike conventional membranes, the presence of impurities plasticizes the polymer membranes and improves their selectivity and permeability.

  6. Differences Without Diversity


    When adapting to varied environments, some plants and animals take on alternative phenotypes but retain the same genotype. The classic laboratory model organism, the tobacco hawk-moth Manduca sexta, is monophenic with a green larval phenotype. However, the sister species, the five-spotted hawkmoth M. quin-quemaculata, is polyphenic with a black phenotype at 20°C and green phenotype at 28°C. Suzuki and Nijhout (p. 650; see the news story by Pennisi) sensitized M. sexta to environmental temperature by using a black mutant line. Mutation of the black gene reduced juvenile hormone and increased melanization of the larval epidermis. Heat shock of the black mutant generated larva with colors ranging from black to green. Two lines were established with the desired phenotype (green or black) by selecting individuals from subsequent generations of black mutant populations. Polyphenism can thus evolve by genetic accommodation regulated by juvenile hormone.

  7. Hydrogenation with Less Guidance

    The selective addition of hydrogen across carbon-carbon double bonds to generate homochiral products is used to prepare a wide range of compounds, both in the lab and in industry. However, the scope of this reaction is often limited by the need for a specific group adjacent to the olefin, whether a phenyl or a coordinating oxygen or nitrogen substituent, to direct the catalyst. Bell et al. (p. 642, published online 8 December 2005; see the Perspective by Wills) show that a class of iridium compounds, coordinated by chiral ligands with both phosphinite and pyridine groups, can catalyze the asymmetric hydrogenation of olefins bearing only simple alkyl substituents. They reduce a vitamin E precursor at two noncontiguous C=C bonds in an alkyl chain with net selectivity exceeding 98%.

  8. Neurons Navigate Downstream

    Neurons born near the brain's ventricles travel out to the olfactory bulb to function in olfaction. A steady stream of migrating neurons makes the journey not only in early development, but also during adulthood. Sawamoto et al. (p. 629, published online 12 January) now provide insight into how these neurons find their way in mice. The ventricles of the brain are lined with cells bearing cilia on their surface. The coordinated beating of these cilia develops a stream of fluid coursing through the ventricles carrying signaling factors that guide the traveling neurons. Mutations that disrupt the cilia also disrupt establishment of the signaling gradient and the migration of the neurons to the olfactory bulb.

  9. Flexible RNA

    Conformational flexibility of RNA molecules arises from a complex set of local motions, collective domain motions, and overall rotational diffusion. Zhang et al. (p. 653) describe a domain-elongation strategy that allows them to resolve picosecond local motions and nanosecond domain motions by nuclear magnetic resonance (NMR) spectroscopy. By comparing the structural dynamics to the conformational differences evident in eight HIV-1 transactivation response element structures, they show that a hierarchical network of local and collective internal motional modes underlies RNA's ability to change conformation adaptively.

  10. Keeping Tabs on Schools of Fish


    A technology for continuously monitoring fish populations over areas on the scale of continental shelves has been developed by Makris et al. (p. 660) that uses the ocean as an acoustic waveguide. Its areal survey rate is several orders of magnitude greater than that of current survey methods. The technology has been used to provide instantaneous images of enormous fish shoals in their entirety, as well as to reveal rapid temporal and spatial changes in these shoals.

  11. HIV Decline in Zimbabwe

    The human immunodeficiency virus epidemic in Zimbabwe is slowing down because of a large-scale change in sexual behavior, particularly among young and educated people. Gregson et al. (p. 664; see the Perspective by Hayes and Weiss) present an analysis that disentangles decline from the mortality of high-risk subpopulations and a lower infection rate of young people. These trends may be taking place across much of sub-Saharan Africa and seem to result from a combination of national program activities, condom use, and increased fear of death from AIDS.

  12. Smelling in Stereo

    Stereo sound localization uses both intensity and phase differences between the ears to determine source direction. Rajan et al. (p. 666) report that olfaction can use similar cues. Trained rats can locate an odor source to the left or right using concentration differences or time-of-arrival differences. Rats can perform this task within a single sniff. Olfactory bulb neuronal responses recorded in response to directional odor stimuli were highly selective for the direction of odor stimulation.

  13. Processing Nothing But Faces

    Are there areas in the brain that are solely dedicated to the processing of faces? Tsao et al. (p. 670; see the Perspective by Kanwisher) used functional magnetic resonance imaging on monkeys in order to identify areas responding to faces, and then implanted electrodes in the principal area in order to identify its properties at the single-cell level. In this region, virtually all of the cells only responded to faces. This finding supports the idea that the cortex has a modular architecture.

  14. Mantle Minerals as Waveguides

    Seismic waves travel a bit faster horizontally through the deepest part of Earth's mantle than they do radially. This difference may be caused by a preferred orientation in the minerals induced by flow or deformation. The dominant mineral phase is MgSiO3 postperovskite (produced by a transformation with pressure from perovskite), but determining many of its properties can be problematic because of the high pressures and temperatures required for its stability. Merkel et al. (p. 644) examined an analog phase, MgGeO3, which forms the postperovskite structure at much lower pressures. Compression of MgGeO3 caused slip on some lattice planes such that the one crystallographic direction becomes preferentially aligned. This alignment, when extended over a large area, could account for observed difference in seismic waves.

  15. Closing In on Receptor Binding

    Binding of the urokinase plasminogen activator (uPA) to its cellular receptor (uPAR) mediates biological activities that play a role in tumor progression and metastasis. Huai et al. (p. 656) have determined the crystal structure of uPAR complexed with the amino-terminal receptor-binding fragment of uPA and an antibody to the receptor at 1.9 angstrom resolution. The receptor displays some conformational flexibility that may allow it to interact with a variety of ligands. The structure provides a basis for the design of uPA-uPAR antagonists.