This Week in Science

Science  12 Mar 1999:
Vol. 283, Issue 5408, pp. 1601
  1. Hotter Than Expected

    How hot is Earth's interior? Heat flow from Earth can be measured accurately, but only rough estimates of its internal temperatures have been made; for example, some estimates of temperatures at the base of the mantle differ by 500 to 1000 kelvin. Internal temperatures depend both on the heat flow and the thermal conductivity, which varies greatly with mineralogy, pressure, and temperature. Because there are only a few experimental determinations of the thermal conductivity at the relevant conditions, theoretical estimates must fill the gap. Hofmeister (p. 1699; see the Perspective by Anderson) develops a new theory for determining thermal conductivity at high pressures and temperatures based on consideration of the lattice phonon and radiative contributions. The theoretical measurements fit available infrared data of relevant minerals and imply that Earth's interior is hotter by about 500 kelvin than most current models.

  2. Electron Interferometer

    The thickness of the cavity in an optical interferometer is on the same scale as that of the wavelength of the incident light. When an integer number of wavelengths fit into the cavity, constructive interference occurs and a peak in the transmission results. The ability to grow perfect silver thin films up to 100 monolayers in thickness, which is the same length scale as the electron wave function, has allowed Paggel et al. (p. 1709; see the Perspective by Himpsel) to create an interferometer for electrons. Photoemission spectra from the metal “cavity” can be analyzed in much the same way as in an optical interferometer and yield a wealth of information about the fundamental electronic properties of the metallic film.

  3. CO2 Complexities

    Generally, atmospheric carbon dioxide (CO2) concentrations were low during glacial times and high during interglacial periods, but whether increases in CO2, and thus the greenhouse effect, and the retreat of ice sheets have been directly related has been difficult to evaluate. Fischer et al. (p. 1712) present a detailed look at how atmospheric CO2 changed during the last three glacial cycles using new analyses of gases trapped in Antarctic ice. The resulting relation is complex: During deglaciations, increases in CO2 seem to follow the initiation of warming, and during some glaciations, CO2 remained high. The interaction is likely controlled by the biosphere and its response to the duration of interglacials.

  4. Coherent Kicks

    Extracting the atoms from a Bose-Einstein condensate in a controlled fashion is the first step in achieving a coherent atomic laser. Previous methods have had a gravity boost, essentially allowing the atoms to drop from the condensate, but this approach severely limits the beam direction. Hagley et al. (p. 1706; see the news story by Voss) now report the use of stimulated Raman emission to kick out a fraction of the condensate with a well-defined momentum and direction.

  5. All Quiet on the Basin and Range

    The Basin and Range Province of the western United States has nearly doubled its area over the last 15 million years and continues to extend today. Thatcher et al. (p. 1714) combined Global Positioning System (GPS) data from surveys in 1992, 1996, and 1998 along an east-west transect across the northern Basin and Range and found almost no current deformation in the middle of the section. All of the deformation was concentrated along the edges of the Basin and Range: adjacent to the Sierra Nevada block in the west and adjacent to the Colorado Plateau in the east. Most of the deformation is oriented in the direction of the motion of the Sierra Nevada block to the northwest. These observations indicate that the internal deformation of the Basin and Range is determined by the traction of the adjacent blocks.

  6. Weighing Brown Dwarfs

    Many brown dwarfs, dead stars without enough mass to burn hydrogen, have been detected, but the actual mass of one of these objects has not been measured. Martin et al. (p. 1718) found an isolated, nearby brown dwarf binary system during the deep near-infrared survey and determined the separation between these brown dwarfs with the near-infrared camera and multiobject spectrometer (NICMOS) on the Hubble Space Telescope. By following the orbital motion of these brown dwarfs for a short time, they will be able to determine the masses of the brown dwarfs.

  7. Leaving an Imprint on C60

    If materials are put under severe pressure, the arrangement of chemical bonds may change and can be preserved in the material after release of pressure. If the force used to establish the pressure is directional, then a greater compression can result in some directions than in others. Marques et al. (p. 1720) show that if polycrystalline C60 is put under such directional pressure, a large pressure memory signature is retained in the sample even at atmospheric pressure. The high symmetry of C60, which has 30 isotropically distributed equivalent bonding directions, allows such a pressure memory effect to be retained in grains oriented randomly with respect to the applied pressure. The resulting three-dimensional polymerized C60 lattice joins the previously synthesized one- and two-dimensional polymerized C60 structures.

  8. Pulling and Tumbling Polymers

    The motion of a polymer chain that enters a region of strong shearing flow is difficult to predict—it may get pulled and stretched, but it can also be tumbled and compacted. This variability makes it difficult to understand the dynamics with methods that average over many molecules, such as light scattering. Smith et al. (p. 1724) used video fluorescence microscopy to image individual λ-phage DNA molecules undergoing shear flow and directly observe large fluctuations in extension corresponding to molecules moving in and out of tumbling flow.

  9. Forcing Bonds Apart

    Direct measurements of force needed to break a single chemical bond have been made by Grandbois et al. (p. 1727) using an atomic force microscope (AFM). Polysaccharide chains adsorbed on silica surfaces were covalently attached to the AFM tip, and the rupture events as the chains were pulled off the surface were analyzed to identify single bond-breaking events. The force measured for rupturing a silicon-carbon bond, 2 nanonewtons, is in good agreement with estimates made from density functional calculations.

  10. Maintaining Integrity

    During cell division, microtubules are arranged into a mitotic spindle along which chromosomes are transported. The ends of the spindle are formed by the centrosomes, which include a ring of γ-tubulin thought to be important in nucleating spindle microtubules. Avides and Glover (p. 1733) examined the role of a Drosophila protein known as Asp and found that it was required to maintain the association between microtubules and the centrosomal γ-tubulin ring.

  11. Getting a Leg Up

    Much is known about limb patterning, but the genetic factors that specify hindlimbs versus forelimbs—whether they are arms in humans or wings in birds—are poorly understood. Logan and Tabin (p. 1736; see the news story by Vogel) now show that when the Pitx1 gene is misexpressed in the chick embryonic forelimb (wing bud), the result is a wing that is transformed into a leg. Characteristics that indicate distinct leg specification are seen in the skeletal and muscle development and are evidenced in the expression of leg-specific markers.

  12. Signaling and Segregation in Drosophila

    Two reports focus on identifying molecular species that play key roles in Drosophila development and genetics. The Wnt/Wingless (Wg) pathway functions in many signaling processes during development by stabilizing the protein β-catenin, which in turn regulates expression of Wnt/Wg responsive genes. In mammals, Axin interacts with β-catenin, the adenomatous polyposis coli protein, and glycogen synthase kinase-3β. Hamada et al. (p. 1739) have now isolated the Drosophila homolog of Axin, D-Axin, which displays many of the same biochemical interactions with other proteins as seen with mammalian Axin. Furthermore, genetic analyses show that D-Axin negatively regulates Wg signaling by down-regulating Armadillo, the Drosophila homolog of β-catenin. Segregation Distorter (SD) in Drosophila is an example of meiotic drive, where one allele or chromosome is preferentially transmitted to offspring through meiosis. Mechanistic insights have been restricted because the identity of the key component of Sd was unknown. Merrill et al. (p. 1742; see the Perspective by Crow) show that Sd encodes a truncated Drosophila homolog of RanGAP. Because RanGAP is a guanosine triphosphatase-activating protein that is involved in nuclear import in yeast and mammalian systems, the solution to the mystery of Sd may include defective nuclear transport.

  13. Diversity in Defense

    An early theory to explain the enormous diversity of class I and II genes of the major histocompatability complex was that individuals carrying different alleles (heterozygous) at HLA loci could present a greater variety of antigenic peptides, and therefore would mount a better immune response against infecting organisms. By looking at heterozygosity at HLA in three cohorts of HIV-infected individuals, Carrington et al. (p. 1748) found that heterozygotes progressed to AIDS and death much more slowly than homozygotes and that two alleles correlated with accelerated AIDS pathogenesis.

  14. Not Just Following Orders

    Traditional industrial organizations localize decisions to managers and production to workers. Recent thinking has opened the door to incorporating workers in some aspects of the decision process. Carpenter et al. (p. 1752; see the news story by Wickelgren) explore the possibility of a distributed network in the brain for encoding serial order in a stimulus-response task. The primary motor cortex constitutes the first station in the descending pathway leading to the muscles and movement. Nevertheless, some motor cortical neurons appear to encode the presentation of serial stimuli at a time prior to the point at which a response is selected and initiated, which suggests the potential for cognitive processing in this area of the brain.

  15. How We Think

    The convergence of new techniques (functional brain imaging) and old disciplines (neuropsychology) has invigorated cognitive neuroscience, which aims to explicate the neural basis of human cognition. Smith and Jonides (p. 1657) review what the current state of agreement and discordance is concerning the temporary storage of information in “working memory” as a function of location and of the type of information. They outline what directions research will take in teasing apart the executive processes of working memory that operate on this information for the purposes of focusing attention, encoding into long-term memory, or formulating a course of action.

  16. Monolayer Mechanics

    Single layers of molecules at an air-water interface are usually described thermodynamically in terms of surface area and pressure, but they can also be described mechanically, in that the pressure at which collapse occurs varies with temperature and compression rate. Kampf et al. (p. 1730) studied large branched polymer (dendrimer) amphiphile layers and were able to describe the collapse behavior for different temperatures with a single equation in terms of surface strain and stress. The response is more reminiscent of metallic systems than that of linear polymers; the dendrimers tend to behave mechanically as large, non-interpenetrating “atoms.”

  17. Return Engagements

    Without memory T cells, the immune response to specific pathogens would be much less effective. The rapid “recall” of the immune system usually prevents infectious agents from reproducing faster than the immune system could keep them in check. Opferman et al. (p. 1745) now show in a murine model system that memory T cells for a cytotoxic T cell response are generated directly from the killer cells themselves, and not from a separate lineage, as had been previously thought. If generally true, it will have implications for vaccine development in those cases where cell-mediated immunity is paramount—if the immunogen does not generate cytotoxic cells, the recall response cannot be expected to be effective.

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