This Week in Science

Science  29 May 1998:
Vol. 280, Issue 5368, pp. 1321
  1. Cold dark matter heats up

    Density fluctuations present just after the big bang eventually resulted in the large-scale structure of the universe we see today. Gawiser and Silk (p. 1405) examined how 10 different cosmological models would predict the evolution of cosmic microwave background (CMB) fluctuations, less than one part in 105, into the large-scale structure of the galaxies. They found that only a universe composed of about 70 percent cold dark matter, 20 percent hot dark matter, and 10 percent ordinary matter can fit the data. In related commentaries, Kamionkowski (p. 1397) explains how the CMB, which represents remnants of particles scattered right after the big bang before any galaxies formed, provide a crucial glimpse of the geometry of the early universe, and Primack (p. 1398) relates how hot dark matter (fast moving particles consisting mostly of massive neutrinos that were swirling around the universe just after the big bang) may represent a significant mass contribution compared to the abundant cold dark matter (slower moving particles just after the big bang).

  2. Converting infrared light into soft x-rays

    Nonlinear optical materials such as lithium niobate are routinely used to convert laser light to higher frequencies; efficient conversion requires phase matching, that is, both the input and output colors must travel through the material at the same speed (have the same refractive index), or the beams will get out of phase and cease to couple. Few solid materials are transparent at wavelengths below 200 nanometers, so phase matching techniques have been limited at high frequencies. Rundquist et al. (p. 1412; see the news story by Kestenbaum, p. 1348) show that gases can be used as the nonlinear medium if they are surrounded by a wave guide; this confinement creates an additional geometrical parameter for matching the phase velocities of the input laser light and the emitted x-rays in the gas. This approach allowed infrared light at 800 nanometers to be converted to coherent soft x-rays in the 17- to 32-nanometer range.

  3. Conductivity two-step

    Models of the electrical conductivity of Earth's mantle are used to understand the propagation of the geomagnetic field to the surface and can also constrain mantle models derived from seismic and mineralogical data. Several models can fit the existing data and show either two or three discontinuities in the bulk electrical conductivity of the mantle in the transition zone (depths of 410 to 660 kilometers). Xu et al. (p. 1415) devised a molybdenum electrode assembly to measure directly the conductivity of mantle minerals at high pressures (up to 20 gigapascals) and temperatures (1400° Celsius). The conductivities of a magnesium (Mg)-rich wadsleyite and a Mg-rich ringwoodite (high-pressure polymorphs of olivine, the most abundant mineral phase in the upper mantle) were similar and much higher than the conductivity of an Mg-rich olivine. Modeling using these data is consistent with the two-step geophysical conductivity model. Lateral temperature variations of about 100° Celsius would not change the conductivity of these minerals by an amount that might be remotely detected in the bulk mantle.

  4. From dust we are formed

    Micrometer-sized graphite grains in a chondrule-free fragment from the unequilibrated ordinary chondrite, Khohar, show unusually large deuterium and nitrogen-15 isotopic excesses that are greater than those measured in other chondrites and interplanetary dust particles. Mostefaoui et al. (p. 1418) argue that these isotopic excesses represent a distinctive signature of an interstellar molecular cloud, possibly the cloud that later formed the solar system. Iron-nickel-rich grains in the fragment do not show the same isotopic characteristics but instead may represent unequilibrated metal particles formed in the solar nebula.

  5. An extra step in 2D melting

    The physics of materials in two dimensions differs from that of normal materials; for example, theory predicts that melting of a two-dimensional solid to an isotropic liquid passes first through an intermediate hexatic phase that has long-range order of molecular orientation and short-range positional order. However, experimental studies revealing the hexatic phase do not me all of the theoretical predictions; instead of a continuous Kosteritz-Thouless (KT) transition, strong thermal signatures are seen. Chou et al. (p. 1424) revisit this problem for freely suspended films of a smectic liquid crystal (whose smectic phase is the free liquid in this context) by performing electron diffraction, heat capacity, and optical scattering experiments. Their conclusion is that there are actually two phases between the solid and the isotropic liquid, a hexatic phase and a secondary smectic phase that has more positional order than the normal smectic phase. The large divergence in heat capacity is seen between these two smectic phases (a first-order transition), rather than between the hexatic phase and the smectics, which may be exhibiting KT behavior.

  6. An icy Centaur

    Centaurs, objects with dynamically unstable orbits that cut across the orbital paths of the jovian planets, are thought to represent relatively primitive components of the outer solar. Twelve Centaurs have been detected, which indicates that these objects must be replenished from the boundaries of the solar system, either from the Kuiper Belt, the Oort Cloud, or beyond. Brown et al. (p. 1430) identified traces of water ice on the surface of Centaur 1997 CU26 from near-infrared spectroscopic observations with the Keck Telescope. Water ice has also been detected on Pholus, but has not been detected on Chiron, the only two other Centaurs whose composition has been measured. Centaurs may be compositionally diverse, complicating a determination of their source region, and may not be related to observed Kuiper Belt objects, which have exhibited surfaces that are hydrocarbon rich.

  7. Diamond origins

    Most diamonds are brought to Earth's surface by magmas from deep in the mantle. Diamonds can be separated into two main types (peridotitic and eclogitic) on the basis of their inclusions and chemistry. The large range of carbon isotope values for eclogitic diamonds suggests that the carbon producing these diamonds was introduced into the Earth's mantle by ancient subduction and recycling of shallow crust and organic-rich sediments. To test this hypothesis, Cartigny et al. (p. 1421) examined paired nitrogen and carbon isotopes in 40 diamonds. The nitrogen isotopic values are not consistent with derivation from ancient sediments, and the range of carbon isotope values can be produced by reactions in the mantle source regions.

  8. Superhelix formation

    The chirality of molecules can be used to direct their supramolecular assembly, which may in turn affect properties such as the optical responses of these larger structures. Nanosized supramolecular chiral structures such as helices have been shown to form from relatively small molecular building blocks. Cornelissen et al. (p. 1427) apply this concept to larger, block copolymer building blocks and show that the morphology of the supramolecular structures can be controlled through varying the polymer properties; among other structures, large superhelices formed from helical subunits were observed.

  9. TH1 role in immunity

    A new human genetic immunodeficiency is reported by Altare et al. (p. 1432) and de Jong et al. (p. 1435). Patients particularly susceptible to Salmonella and Mycobacteria infections were deficient in the interleukin-12 receptor, a protein that initiates a type 1 T helper cell (TH1) response, that is, cell-mediated immunity. The patients formed granulomas, which normally control the infections, but in these cases the infection still disseminated. The phenotype was milder than what might have been expected (childhood viral and bacterial infections were apparently handled normally), given what is known about mouse deficiencies in IL-12 and interferon-γ receptors. This result then raises questions about the real role of TH1 cells in immunology.

  10. Audio bars

    Although the stimulus-response properties of neurons early in the auditory processing pathway are known (such as frequency and intensity), the features characterizing mammalian auditory cortical neurons, analogous to the moving, oriented bar within a visual cortical neuron's receptive field, are not. deCharms et al. (p. 1439; see the commentary by Young, p. 1402) use a reverse correlation approach to search for “best stimulus” features for auditory cortical neurons in the owl monkey. They find that these neurons preferentially respond to stimuli with time-varying frequencies and “edges” in frequency and in time.

  11. Predicting RNA shape over time

    It is difficult to predict the three-dimensional structure of a protein from its primary structure (amino acid sequence). Fontana and Schuster (p. 1451) tackle the somewhat more computationally tractable problem of connecting RNA sequence and secondary structure with the wrinkle of examining this relation over time. Beginning with a population of sequences of fixed length but random shapes, they simulate the progression toward a transfer RNA-like target shape. They find that changes in the nucleotide sequence occur throughout but that changes in the secondary structure appear in a punctuated manner.

  12. Myosin and deafness

    A pair of reports by Probst et al. (p. 1444) and Wang et al. (p. 1447) discuss how mouse models and human family trees have been used to pinpoint a human gene, DFNB3, associated with a form of nonsyndromic recessive deafness (see the commentary by Steel and Brown, p. 1403). Synteny between mouse and human chromosomes had previously been used as the basis of suggesting that there might be homology between shaker-2 and DFNB3 mutations, respectively. Shaker-2 mice are deaf and exhibit abnormal circling behavior. DNA from the region of mouse chromosome 11 thought to be critical for the phenotype was inserted into bacterial artificial chromosomes (BACs) and injected into fertilized eggs of mutant mice. Transgenic mice that could hear and did not circle were found to contain a gene for an unconventional form of myosin designated MyO15. Affected individuals from three unrelated human families had mutations in the human form of this gene that were associated with deafness. MyO15 is expressed in a number of tissues, including those in the inner ear.

  13. Joint developments

    From the undifferentiated limb buds form the intricately constructed bones, joints, tendons, and ligaments of the mature vertebrate limb. Brunet et al. (p. 1455; see the news story by Dickman, p. 1350) show that the formation of the joints depends on Noggin, also known for its dorsalizing activity early in development. In mice lacking Noggin, the limbs and the joints are poorly specified. Correct specification of joint location and structure seems to depend on a delicate balance between BMP signaling factors and Noggin, acting antagonistically.

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