This Week in Science

Science  05 Mar 1999:
Vol. 283, Issue 5407, pp. 1413
  1. Incidentally Porous

    Amorphous solid water (ASW), a metastable phase of water that forms at low temperatures, is believed to be abundant in astrophysical environments and has also attracted attention as a model system for supercooled liquids. Its formation and properties depend strongly on deposition and annealing conditions. Stevenson et al. (p. 1505) report a new twist: Decreasing the incident angle at which water molecules arrive at the surface during growth to more glancing angles creates a highly porous material. Discrepancies in density and surface area measurements in previous studies may be resolved by these measurements.

  2. From Wobbles to Wallop?

    The Yarkovsky effect is a subtle thermal radiation force that perturbs the orbit of a spinning object because of its nonuniform surface temperature. Although it has been assumed that this effect is negligible for the orbits of asteroids and planets, orbital dynamic simulations by Farinella and Vokrouhlicky (p. 1507) performed on objects with radii between 1 to 10 kilometers show that the Yarkovsky effect can alter the semimajor axes. Such small perturbations may push asteroids into orbits where other forces initiate larger perturbations that eventually transport asteroids into Earth-crossing orbits.

  3. Carbon-Based Quartz

    Quartz contains tetrahedra of oxygen atoms covalently bonded to silicon. Iota et al. (p. 1510) have synthesized a quartz-like polymorph of carbon dioxide (CO2). Liquid CO2 was laser heated to 1800 kelvin and pressurized in a diamond anvil cell to 40 gigapascals. The recognition of a transition of a simple molecular species, CO2, to an extended, covalently bonded phase is unexpected and may fundamentally alter ideas about simple molecular oxide chemistry. The quartzlike CO2 may also have useful applications because of its high thermal conductivity and high energy density.

  4. Now You See It ...

    Whether the Antarctic and Arctic ice sheets are actually increasing or decreasing in size remains a subject of debate; it is also of crucial importance to global climate change and future sea-level changes. Altimetry surveys measure the elevation across the ice sheet and thus capture the entire volume of the ice sheets at different times, but such data are limited. For example, satellite techniques are useful only for determining elevations above about 1700 meters. In 1998, Krabill et al. (p. 1522) performed an altimetry survey from aircraft of the Southern Greenland Ice Sheet, which resurveyed the same flight lines flown in a 1993 survey. Comparison of the two data sets indicates that thinning and thickening of the ice sheet depends on altitude and location, but that the overall mass balance is negative—the ice sheet is diminishing.

  5. Shake Your Nanotube

    One measure of the mechanical strength and response of an object is its resonant frequencies (as exemplified in the resonant failure of the Tacoma Narrows Bridge). Poncharal et al. (p. 1513) have now imaged resonances of carbon nanotubes. The nanotubes were mounted in a transmission electron microscope (TEM) so that they could be charged, and a time-dependent electric field was used to apply an oscillatory force. The deflections, including first and second harmonic resonances, could be imaged with the TEM. The elastic bending moduli decreased an order of magnitude as the tube widths increased from 8 to 40 nanometers. Nanoparticles could be attached to the end of the nanotube and their mass determined from the change in resonant frequency.

  6. Long-Term Growth

    The formation of the continents and the oxidation of Earth's atmosphere both affected the recycling of certain elements back into the mantle. Hence, the history of continent formation and atmospheric evolution might be inferred from the accumulation or depletion of these elements in the mantle over time. Collerson and Kamber (p. 1519) examined thorium, uranium, and niobium in volcanic rocks believed to be derived from the mantle since 3.8 billion years ago. The data imply that continents grew variably with time and that Earth's atmosphere became significantly oxidized about 2.0 billion years ago.

  7. Fault Finding

    Earthquakes on blind thrust faults, so named because they do not extend to the surface, have been responsible for much recent damage in the Los Angeles area, but their hazard potential has been difficult to assess. Shaw and Shearer (p. 1516) used a new seismic velocity model for the Los Angeles Basin and data from drill cores in the region to map the extent of a major blind thrust fault in the basin. The fault they found was responsible for the Whittier Narrows Earthquake in 1986 (magnitude 6.0). The extent of the fault implies that it could be associated with a larger earthquake.

  8. A Reducing Plan for Life

    Making and breaking bonds to carbon is central to the chemistry of life. A critical step in the current scenarios of the evolution of biomolecules is the replacement of the ribose 2′-hydroxyl with a hydrogen, which converts a ribonucleotide into a deoxyribonucleotide. Logan et al. (p. 1499) describe the structure of a class III ribonucleotide reductase (RNR), an enzyme that uses a glycine-derived free radical to accomplish this difficult chemical step. On the basis of structural homologies to other RNR classes and to other radical reactions involving simple metabolites, they propose that this enzyme may be the closest extant ancestor of the original reductase.

  9. Mimicking Oxygen Evolution

    Plants and cyanobacteria generate O2 through photosystem II at the oxygen-evolving complex, which contains a cluster of four manganese (Mn) atoms bridged by oxygen atoms. Limburg et al. (p. 1524) have now synthesized a small complex, in which two Mn atoms are bridged by two oxygen atoms that mimic O2evolution. Although NaOCl must be used as the oxidant to drive this reaction, labeling studies show that water is the source of oxygen atoms in the O2 produced.

  10. ''Junk'' Vehicles Still Running

    Most of the sequences composing the human genome have been termed “junk” DNA because these sequences do not encode functional genes. Instead, they contain elements such as introns, repeats, and transposons. Moran et al. (p. 1530; see the Perspective by Eickbush) have examined the most abundant retrotransposons, the long interspersed nuclear elements (L1s). Here, they report that L1s can retrotranspose into transcribed genes, they can mobilize 3′ flanking DNA into new genomic locations, and they can generate new genes by exon shuffling. Hence, L1s represent a vehicle for genome evolution.

  11. Sticking by Mimicking

    The yeast Candida albicans opportunistically severely infects some immunocompromised patients, and filamentous forms can embed themselves in host tissue. Staab et al. (p. 1535) show that a surface protein of the yeast's hyphae, Hwp1, can serve as a substrate for mammalian transglutinases, which cross-link epithelial cell proteins to create the keratinized barrier layer. Yeast lacking Hwp1 could not attach stably to human buccal cells and were less able to cause systemic candidiasis in mice.

  12. Nonlinear Fish Dynamics

    The relation between egg production, larval supply, and recruitment is important but currently opaque to fisheries assessors and population ecologists alike. Now Dixon et al. (p. 1528), using biological and physical oceanographic time series, demonstrate a distinct nonlinear signature in the larval record of an Australian reef fish and identify wind turbulence as a critical nonlinear factor affecting larval survival. This case study illustrates how the use of nonlinear techniques and a top-down systems approach can shed light on a natural system.

  13. Revealing Rote's Roots

    One popular method for learning new facts is to use repetition to increase familiarity; another is to use an aide-mémoire, where something familiar is associated with the new item, like a string tied around the finger. The expected neural correlates of these methods are that brain-processing effort would diminish as the fact became familiar and that brain processing in two regions would become more tightly connected as the association was learned. Büchel et al. (p. 1538) used functional brain imaging to find both of these correlates and to observe that the rate of learning was reflected in the increase of connection strength.

  14. Making Plants Grow

    Cell proliferation is one of the responses to the plant hormone cytokinin. Riou-Khamlichi et al. (p. 1541) found that cyclin D3 is part of the early molecular response to cytokinin. Thus, development and senescence, phases that are coordinately regulated by hormonal control of cellular proliferation, may both depend on cyclin D3.

  15. Eschewing the Fat

    Can animals eat a high-fat diet and still resist weight gain and loss of sensitivity to insulin? Elchebly et al. (p. 1544; see the news story by Ferber) report that mice in which the gene encoding protein tyrosine phosphatase-1B (PTP-1B) has been disrupted can indeed do so. PTP-1B inhibits insulin signaling, possibly by dephosphorylating the insulin receptor. Mice lacking the phosphatase had enhanced insulin sensitivity in muscle and liver but not in adipose tissue. When fed a diet high in fat, mice heterozygous for the disrupted allele and those completely lacking PTP-1B were both resistant to weight gain and protected from loss of insulin sensitivity. PTP-1B, thus, may represent an effective target in the development of therapies for obesity and type 2 diabetes.

  16. Polarized Stellar Light

    J. Bailey et al. (Reports, 31 July, p. 672) stated that “circular polarization at shorter wavelengths might have been important in inducing chiral asymmetry in interstellar organic molecules that could [have been] subsequently delivered to the early Earth by comets, interplanetary dust particles, or meteors.”

    E. Rubenstein et al. comment that circular polarization can also occur in high-frequency radiation that is “emitted along the axis parallel to the magnetic field.” They state that “optical synchrotron radiation from the Crab Nebula has been observed … [and] circularly polarized ultraviolet synchrotron radiation must also be present.”

    In response, Bailey states that the situation described in the comment is correct “only for low electron energies … In the highly relativistic case (synchrotron radiation), relativistic beaming means that radiation is not emitted parallel to the magnetic field.”

    The full text of these comments can be seen at www.sciencemag.org/cgi/content/full/283/5407/1415a

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