This Week in Science

Science  21 Apr 2006:
Vol. 312, Issue 5772, pp. 333
  1. Wet and Dry Martian Processing


    The main spectrometer on Mars Express, called OMEGA, has now returned a planet-wide data set, and Bibring et al. (p. 400) have used these results in combination with related observations by other Mars orbiters and the two rovers to reconstruct the history of water alteration on Mars. Hydrous minerals are abundant only in the oldest rocks; sulfur-rich minerals are present in some younger rocks, but more recent alteration is anhydrous. This record implies that there was likely surface water only early in Mars history, which gave way to more ephemeral acidic alteration. Water-rock interactions are not apparent after about 3.5 billion years ago.

  2. Laser Acceleration Hits the Spot

    One application of ultra-intense laser pulses is particle acceleration, but protons and ions accelerated from surfaces tend to have a large spread in energy and spatial extent. Toncian et al. (p. 410, published online 16 February; see the Perspective by Dunne) placed a hollow cylinder in the path of the accelerated protons and hit the cylinder with a well-timed, high-intensity laser pulse. The transit time of the protons is energy dependent, so varying the timing between the ion or proton generation pulse and the cylinder pulse allowed for energy selection and collimation of the protons exiting the cylinder.

  3. Better to Be Left Hanging

    The electronic properties of single-walled carbon nanotubes (SWNTs) have been understood in terms of both band and excitonic carrier models. An argument made in favor of the band model is that the photoexcitation spectra of the SWNTs matches their absorption spectra. Itkis et al. (p. 413) found that by suspending SWNT films, they could increase the photoconductivity response by at least five orders of magnitude, a value large enough to consider these materials as infrared detectors. Because the photoconductivity is bolometric (that is, has a thermal origin), these effects cannot be directly related to photoexcitations and conductivity models.

  4. Faster Than Femtoseconds

    The time resolution of chemical dynamics studies has generally been limited by the duration of laser pulses used as probes. Pulse durations now approach 1 femtosecond (fs), but some molecular events occur on even more rapid time scales. Baker et al. (p. 424, published online 2 March; see the Perspective by Bucksbaum) show that an 8-fs laser pulse can be used to observe nuclear dynamics of H2 and methane after ionization with 0.1-fs (10−16 s) resolution. The technique relies on the electrons being ejected from the molecule by the laser pulse with a spread of velocities, which in turn leads to a spread, or chirp, in frequency of the photons released upon electron-ion recombination. The emitted photon frequency acts as a clock that is more precise than the excitation pulse.

  5. Of Gold, Silver, and Diamonds


    Nanoparticles can be assembled into a variety of crystalline lattices that are close-packed in nature, but more open structures reminiscent of the diamond lattice are harder to form. Kalsin et al. (p. 420, published online 23 February; see the Perspective by Velev) exploit electrostatic effects to assemble gold and silver nanoparticles, of the same size but coated with oppositely charged monolayers, into the diamond-like sphalerite lattice. Unlike the formation of elemental salt crystals, the screening interactions are on the same scale as the nanoparticles, and so only short-range forces direct the assembly. The presence of smaller charged nanoparticles that act as counterions improved crystalline quality.

  6. Dating the Drake Passage

    The opening of the Drake Passage, between the southernmost tip of South America and the Antarctic Peninsula, was an essential step in the development of the Antarctic Circumpolar Current. However, estimates of the age of the passage range from as early as 49 million years to as late as 17 million years ago, so it has been difficult to assess what role the opening played in climate change. Scher and Martin (p. 428; see the news story by Kerr) present a marine sedimentary record of ocean circulation derived from Nd isotopes in fish teeth found downstream from the Drake Passage for the interval between 46 and 33 million years ago. They find that the passage must have begun to open 41 million years ago, in the middle Eocene. This event long preceded the opening of the last remaining corridor, the Tasmanian Gateway, around 35 million years ago, and major ice sheet growth in Antarctica, which began around 34 million years ago.

  7. Bird Flu H5 Structure Defined

    The H5N1 “bird flu” virus is highly contagious and deadly in poultry. To date, infection of humans seems limited to direct bird-to-human transmission, but mortality in humans is high, and the question of whether the virus may adapt into a pandemic human strain is pressing. Stevens et al. (p. 404, published online 16 March) determined the structure of H5N1 hemagglutinin (HA) at 2.9 angstrom resolution and examined the receptor-binding preference of this HA and specific mutants using a glycan microarray system. Mutations that convert avian H2 and H3 HAs to human receptor specificity did not cause a similar specificity switch in the H5N1 HA, but did permit binding to a natural human α2-6 glycan.

  8. Network Interactions in Ecological Communities

    Most studies on coevolution and mutualism between plants and animals have focused on interactions between pairs of species and ignore the wider network of interactions at the level of the ecological community. To fill this gap, Bascompte et al. (p. 431; see the Perspective by Thompson) analyzed a large set of coevolved networks, drawing on data from the tropics to the poles, to assess their structure and the implications for their stability and coevolution. Mutualistic networks are dominated by weak, asymmetric interactions, in which one partner in each mutualism depends strongly on the other while the other is only weakly dependent. This network structure confers stability to the wider ecological community.

  9. MicroRNA and Innate Immune Responses in Plants


    Plants mount an innate immune response when they detect pathogen-associated molecular markers such as bacterial flagellin. Navarro et al. (p. 436) now show that in Arabidopsis, bacterial flagellin induces the expression of the microRNA miR393, which in turn reduces the expression of three auxin receptors and eventually leads to the down-regulation of auxin signaling pathways that are implicated in disease susceptibility. This down-regulation then increases the plant's resistance to infection. This miRNA expression seems to act in parallel with independent transcriptional repression of the auxin receptors to ensure that an immune response is generated.

  10. Nuclear Pore Production Line

    The nucleus of eukaryotic cells is surrounded by a double membrane structure, the nuclear envelope, that is punctuated by nuclear pore complexes. During interphase, nuclear pores represent the exclusive sites of transport between the nucleus and the cytoplasm. Are these nuclear pore complexes generated by splitting of existing pores, or are they produced de novo? D'Angelo et al. (p. 440) present real-time imaging of nuclear pore complex assembly in living cells and suggest that nuclear pore complexes form de novo and are assembled from both sides of the nuclear envelope.

  11. From in Silico to in Vitro Drug Discovery

    Many currently available therapeutic drugs act by modulating signaling through G protein (heterotrimeric GTP-binding protein)-coupled receptors. The G-protein βγ subunit transmits signals from G protein-coupled receptors to their targets, and many crystal structures of such complexes have been solved. Bonacci et al. (p. 443; see the Perspective by Tesmer) used a computer program to predict which chemical compounds would bind to the interaction site on the βγ subunits and obtained potent small molecule inhibitors of protein-protein interactions. Furthermore, these molecules showed specificity for disrupting signaling-specific downstream targets, which suggests that such reagents might be both effective and relatively free of side effects.

  12. Predicting Flu Dynamics

    Taking influenza mortality data collected in the United States from 1972 to 2002 as a measure for seasonal influenza virus circulation and disease, Viboud et al. (p. 447, published online 30 March) investigated the synchrony of influenza epidemics across the United States. They found that severe epidemics were more synchronous than mild ones, and that work-related movement of people correlated with spread of infection better than long-distance travel or geographical distance between states. Adults were the primary transmitters of seasonal influenza, rather than children, as has been previously assumed. These findings have implications for the design of pandemic control strategies.

  13. The Hardening-Fast Rules

    Aluminum alloys are often used in applications where lighter is better. One commonly used alloy is AlMgSi, which can be rapidly strengthened with a heat-treating step after being pressed into a given shape. However, this quick-bake hardening can be hindered if the alloy is stored for a long time before processing. Chen et al. (p. 416) studied the hardening process and identified double columns of silicon that form the skeleton of the evolving nanoparticle precipitates. They were able to monitor how the reinforcing precipitates change in composition with annealing.

  14. Measuring Metapopulation Dynamics

    Natural populations of organisms are not uniformly distributed, but rather are clustered into smaller subpopulations connected by dispersal and migration. Dey and Joshi (p. 434) tested theoretical predictions regarding the effects of dispersal rate on the size, dynamics, and stability of metapopulations in a laboratory system of replicated metapopulations of Drosophila. Low levels of dispersal induced asynchrony among subpopulations and led to more stable metapopulation dynamics, whereas higher rates led to subpopulation synchrony and more unstable dynamics at the metapopulation level. Most of the experimental observations could be captured by simulations that used a simple, non-species-specific model of population growth.

  15. RNA Interference with Viruses

    RNA interference (RNAi) plays a role in the defense against viral infection in plants, but does it play a similar role in animals? The evidence from tissue culture cells and Caenorhabditis elegans-based systems has suggested that this is so, but definitive evidence in a system that uses endogenous viruses and whole animals has been lacking. Wang et al. (p. 452, published online 23 March) now show that both Drosophila embryos and adult flies mount a substantial innate immune response to insect viruses that requires the RNAi machinery, and that this innate defense is separable from the innate bacterial defense provided by the Toll and immune-deficiency pathways.