This Week in Science

Science  07 Mar 2003:
Vol. 299, Issue 5612, pp. 1481
  1. Impatient Earthquakes

    Some recent large earthquakes, such as the 1992 Landers and 1999 Izmit events, show evidence of supershear, in which the rupture velocities are actually higher than the shear wave velocity along a fault. Dunham et al. (p. 1557) have developed a numerical model of elastodynamic wave propagation in a three-dimensional medium around a rectangular fault plane. They find that stronger sections of the fault can drive the rupture forward at supershear velocities. Rather than stopping the rupture's propagation, strong crustal barriers may create stronger ground motions.

  2. Jupiter in Motion

    An analysis of the images from the Cassini mission to Jupiter by Porco et al. (p. 1541; see the cover and the Perspective by Esposito) have refined our understanding of the dynamics and complexity of the jovian system. In the atmosphere, zonal winds are stable and vortices are long-lived, and convective models accounted for observations of storms and lightning. The evolution and origin of the Great Red Spot and aurorae are illuminated in detail. Time-lapse images of the Galilean satellites being eclipsed by Jupiter were to record atmospheric emissions which suggest the presence of singly ionized oxygen on the volcanically active Io and neutral oxygen on icy Europa. Observations of smaller satellites, such as Himalia, Metis, and Adrastea, as well as images of the subtle jovian rings confirm that the rings are primarily evolved from impact debris ejected from the satellites.

  3. When Moms Move from Welfare to Work

    In the 1990's, dramatic changes were made in the way public welfare support was provided in the United States that moved hundreds of thousands of single mothers into the workforce. To study the effects of this social change, Chase-Lansdale et al. (p. 1548) interviewed more than 2000 low-income families in Boston, Chicago, and San Antonio in 1999 and again in 2001. They found no evidence that a working mother had any negative effect on the developmental progress of preschool-age children or young adolescents. Entrance of mothers into employment was associated with some improvement in adolescent mental health, cognitive development, and reductions in alcohol and drug use.

  4. Blasted to Bits

    Boron carbide (B4C) is a very hard material that has found use in armor plating. Although it can survive some impacts, if hit with a projectile with sufficient velocity, the material fails with much lower shear strength than expected. Chen et al.(p. 1563) demonstrate that the loss of strength is due to the formation of intergranular amorphous regions that occur along specific crystallographic directions. No evidence of melting is observed, indicating that these changes occur in the solid state.

  5. Melting Out the Stops?

    The disintegration of sea-ice shelves could facilitate the collapse of inland ice sheets that would contribute substantially to sea-level rise, but it has not been clear that ice shelves actually restrain the flow of ice from land to sea. De Angelis and Skvarca (p. 1560) report that the recent disintegration of Larsen Ice Shelf, located in the northeastern Antarctic Peninsula, has produced active surging in several former fast flowing ice tributaries. This finding contradicts recent theoretical results that have concluded that ice shelves should not affect the behavior of inland ice. However, the slow-moving ice piedmont and many smaller glaciers in the region have been unaffected.

  6. Cosmic Helium Balance

    How long a star will live can be estimated by dividing its helium content by its heavy-element content. Jimenez et al. (p. 1552) have determined the cosmic rate of helium production relative to heavy elements using improved spectroscopic data of K dwarf stars from Hipparcos and a refined evolutionary model. Their ratio can be used to estimate the ages of stars as well as the primordial abundance of helium in the early universe, which constrains the initial star formation rate and distribution.

  7. Stratospheric Sulfates

    It is believed that the high concentrations of stratospheric sulfate aerosols must be caused by the photolysis of H2SO4 to SO2, but estimates of the absorption cross section for this reaction indicate that is too small to make it a viable pathway. Vaida et al. (p. 1566) propose a photochemical mechanism for sulfuric acid decomposition by excitation of vibrational overtones of the molecule, based on quantum mechanical calculations of H2SO4 absorption cross sections. The cross sections they determine by this method are sufficiently large to explain the observed concentrations of SO2 and sulfate aerosols in the stratosphere.

  8. Tracking Squirrels

    Squirrels appear to be excellent indicators of environmental history because they are one of a very small number of mammalian families that is almost worldwide in distribution. Mercer and Roth (p. 1568) present a comprehensive genus-level molecular phylogeny of the squirrel family. They document striking chronological and geographic correspondence between events of divergence and diversification in the squirrel family (Sciuridae) and multiple tectonic, sea-level, and paleontological events documented in the geological record. Their findings may help in the understanding of the Cenozoic environmental and biogeographical history of such complex regions as Southeast Asia and the American tropics.

  9. Making the Most of the Light

    Plants optimize their photosynthetic processes in response to changing light quality through the state transition, which is mediated by alterations in the phosphorylation state of the light-harvesting complex. Dep├Ęge et al. (p. 1572; see the Perspective by Allen) have now identified the kinase responsible for this regulatory phosphorylation step in Clamydomomas. The nuclear gene encodes a protein that resides in the chloroplast thylakoids, and homologs were found in other plant genomes.

  10. When Innate Immunity Is Critical

    When viruses infect higher animals, they are confronted first with the innate immune response and later by the more highly tuned adaptive response. Karst et al. (p. 1575) provide evidence that a class of virus with an important but poorly understood impact on public health (the Norwalk-like caliciviruses associated with epidemic gastroenteritis) may be particularly sensitive to the actions of the innate immune system. They found that a murine virus belonging to the Norovirus genus was most lethal in mice lacking innate immune response genes for type-1 interferon receptors and the signal transducer and activator of transcription 1 (STAT 1). The debut of a mouse Norovirus model and the unexpectedly rigid requirement for the innate, rather than adaptive, immune response to deal with infection may lead to more appropriate means of studying and treating this class of virus in humans.

  11. Modifier Gene in Sight

    Primary congenital glaucoma (PCG) is one of the most devastating forms of glaucoma because it causes blindness in young children. Most cases of PCG are caused by mutations in the gene encoding a cytochrome p450 protein called CYP1B1, but not all individuals with such mutations develop glaucoma. Studying mice deficient in CYP1B1, which display many of the ocular abnormalities seen in human congenital glaucoma, Libby et al. (p. 1578; see the Perspective by Alward) show that the presence of tyrosinase gene (Tyr) reduced the severity of the disease phenotype in two different mouse models of glaucoma, as did administration of the tyrosinase product L-dopa to mice doubly deficient in CYP1B1 and Tyr.

  12. Traveling on Our Stomachs

    Stomach ulcers and cancer in human beings are commonly caused by the bacterium Helicobacter pylori, which has a long evolutionary history with human beings. By taking a population genetic (rather than a phylogenetic) approach, Falush et al. (p. 1582; see the Perspective by Spratt) have identified specific bacterial populations and assigned particular isolates to each bacterial population. The populations of H. pylori have characteristic geographical distributions. A reconstruction of the ancestral state of these populations allows the current distribution of the bacterial isolates to be mapped onto historical migrations of human beings around the world.

  13. Guided by Experience

    Synapses, the contacts between neurons, work more efficiently after being stimulated simultaneously by two inputs, a phenomenon thought to underlie certain types of learning. In tissue culture slices, this enhanced efficiency results from increased trafficking of one type of glutamate receptor, the AMPA receptors (AMPA-Rs), into synapses. To test if this same mechanism operates in animals receiving environmental inputs, Takahashi et al. (p. 1585) injected recombinant AMPA-Rs to neurons in the rat barrel cortex with a Sindbus virus vector. They compared normal synapses in the barrel cortex at postnatal day 15, just after a period of rapid experience-dependent growth with synapses corresponding to an area on the animal's face where the whiskers had been. The synapses subject to sensory experience through intact whiskers showed increased trafficking of AMPA-Rs, while synapses deprived of whisker input did not.

  14. Dynamically Different

    The traditional way to understanding chemical reactions is to find a pathway, in terms of changing bond distances and angles, that hugs the minimum in the potential energy of the system. Recent work has suggested, however, that at realistic temperatures, vibrational and kinetic energy can play a large enough role that this intrinsic reaction coordinate is not actually followed. Ammal et al. (p. 1555) performed molecular dynamics simulations of an internal rearrangement of protonated pinacolyl alcohol, which eliminates water. The trajectories did not follow the local minimum pathway that produces the tertiary cation directly (a concerted reaction), but instead created the secondary cation before rearranging to produce the tertiary product. The propensity for methyl group migration changes introduced the dynamical effects; an analogous reaction in which a bulkier oxymethyl group would have to migrate followed the expected concerted pathway.

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