This Week in Science

Science  05 Dec 1997:
Vol. 278, Issue 5344, pp. 1685
  1. A view of Mars, then and now

    Mars Pathfinder safely bounced onto the surface of Mars on 4 July 1997, unfolded the solar panel petals of the lander, and deployed its rover, Sojourner, to sample the rocks and soil surrounding the lander (Golombek et al., p. 1743). Radio tracking from the lander allowed Folkner et al. (p. 1749) to improve the estimate of Mars rotation rate and the wobble of the polar axis; the results suggest that Mars has a dense, iron-rich core. Pressure, temperature, and density measurements of the atmosphere (Schofield et al., p. 1752) show a slightly warmer daytime temperature than Viking 1 and evidence for dust devils. The imager on the lander provided stunning panoramic images (see foldout) of a variety of rock shapes and textures on a dusty surface of ridges and troughs that may be related to water, wind, and impact erosion of the landing site (Smith et al., p. 1758; see the cover). The Rover Team (p. 1765) found evidence of sedimentary processes, including a possible conglomerate, in their analyses of soil and rock characteristics from the rover, while a series of magnets on the lander allowed Hviid et al. (p. 1768) to conclude that water may have been involved in the oxidation of magnetic minerals. Finally, the -proton x-ray spectrometer (APXS) on the rover conducted about a dozen chemical analyses and determined that the sampled rocks are high in silica, unlike the martian meteorites, and probably represent a differentiated crust (Rieder et al., p. 1771). The sampled soils have a different composition from the rocks, and these soils may have formed by the addition of magnesium and iron from mafic rocks, like the martian meteorites, to the locally eroded rocks during weathering. All of the observations and analyses suggest that Mars was indeed a warmer and wetter place a long time ago.

  2. On the quantum dots

    Quantum dots, nanometer-scale semiconductor structures that can be charged with electrons, can exhibit excited-state transitions that mimic those of atoms (see the Perspective by McEuen, p. 1729). Kouwenhoven et al. (p. 1788) show that in circular, defect-free quantum dots, quantum numbers and transitions corresponding to Hund's rule in atoms can be identified. Schedelbeck et al. (p. 1792) studied the formation of artificial molecules from such artificial atoms. The overlap between the wave functions from two quantum dots led to the formation of bonding and antibonding levels whose coupling strength depended on the distance between the dots. Stewart et al. (p. 1784) studied irregular quantum dots to investigate electronic transitions in systems lacking symmetry. Strong correlations in excitation spectra were observed for quantum dots with different numbers of electrons, despite the difference in charging energies, and one electron, rather than two electrons with paired spins, were sufficient to fill a level.

  3. Isotopes and N2O

    Nitrous oxide is both a greenhouse gas, and thus important in the troposphere, and is also involved in ozone destruction in the stratosphere. It has been difficult to obtain a global budget for N2O and unidentified sources or sinks. A key tracer is provided by the many isotopes of N2O, as shown in three reports. Rahn and Wahlen (p. 1776) show that heavy isotopes of N and O are enriched in stratospheric N2O, and Cliff and Thiemens (p. 1774) show that tropospheric N2O is enriched in 17O through a mass-independent process. Yung and Miller (p. 1778) propose that the stratospheric enrichment may reflect selective photolytic destruction of the isotopically lighter species.

  4. Hydrogen in the core

    Earth's core consists mostly of iron, but seismic observations infer that there must be a lighter element or elements in the core to account for the lower observed density of the core compared to iron metal at core pressures. Okuchi (p. 1781; see the Perspective by Wood, p. 1727) kept hydrogen bubbles from segregating from iron metal at high pressure (7.5 gigapascals) and temperature (1200 to 1500C). By determining the volume of hydrogen in the quenched samples he infers that about 60 percent of the core's density deficit can be explained by hydrogen.

  5. The grass is greener

    Why do large grazing animals prefer particular patches of grasslands? Observations made by McNaughton et al. (p. 1798) in the Serengeti National Park support a positive feedback loop in which grazing accelerates nutrient availability. The rate of nitrogen mineralization was enhanced, and levels of sodium in the soil increased in grazed areas. The increased sodium availability more likely benefits the grazers than the plants.

  6. Handy virus

    PBCV-1 is a virus that infects a green algae that normally lives inside a paramecium. DeAngelis et al. (p. 1800) show that this virus can synthesize the carbohydrate hyaluronan, rather than following the normal route of relying on host enzymes or host carbohydrates. One speculation is that production of this polysaccharide may make it hard for the algae to be taken up by paramecium and thus leave it vulnerable to infection.

  7. Producing the pituitary

    Mammalian pituitary glands develop from a complex interplay between the floor of the neural tube and the roof of the oral cavity. Differentiation then generates a variety of cell types, each with specific hormonal and physiological function. Sheng et al. (p. 1809) have further elucidated the transcription factors that control this process. The Lhx3. and Lhx4. genes redundantly control formation of Rathke's pouch, the precursor from the oral ectoderm. Subsequently, Lhx3. independently controls fate choices of some of the pituitary precursor cells, and Lhx4. seems to have more of an effect on the proliferation of cells.

  8. The protein's graveyard

    Misfolded and misassembled secretory and membrane proteins in the endoplasmic reticulum (ER) are sent to the cytosolic side of the ER membrane for degradation by ubiquitin-dependent processes, including defective proteins that had been translocated into the ER membrane. Biederer et al. (p. 1806; see the Perspective by Riezman, p. 1728) now reveal that one of the key enzymes in ubiquitin-dependent degradation, Ubc7p, localizes at the cytoplasmic surface of the ER by association with a newly identified protein, Cue1p. The interaction of Ubc7p with Cue1p is required not only for the degradation of ER proteins, but also for bona fide cytosolic proteins. The cell apparently uses the ER surface as a scaffold to concentrate the components of the cytosolic protein degradation material, which include the Ubcs and the proteasome.

  9. Hybrid mesostructures

    Hybrid inorganic-organic materials may form locally phase-separated structures with large periodicities (mesostructures) that are of interest in applications ranging from catalysis to optics. Templin et al. show that by choosing an appropriate block copolymer, silica precursor combination, a range of microstructures with length scales determined by the chain length of the polymer can be obtained. The microstructures are determined by the phase diagram of the block copolymer. The approach can be potentially extended to a range of component materials and to length scales of up to 100 nanometers.

  10. Regulator for Stat3

    Binding of many hormones or cytokines to receptors on the cell surface causes changes in gene expression by altering the activity of transcription factors known as Stat (signal transducer and activator of transcription). The receptors for interleukin-6, epidermal growth factor, and the hormone leptin, for example, are all linked to the activation of Stat3. Chung et al. have characterized a molecule that interacts with and regulates Stat3. The molecule, which they call PIAS3 (protein inhibitor of activated STAT), associates with the activated form of Stat3. Association with PIAS3 inhibits DNA binding and the resultant activation of transcription by Stat3. These effects are specific for Stat3 because PIAS3 did not bind to or alter the function of Stat1.

  11. Mixed signals

    Recent evidence indicates that oncogenic, activated mutants of the small guanine nucleotide-binding protein Ras send multiple signals to the host cell that can promote distinct-even contradictory-signals. In order to cause unregulated growth of cells, oncogenes may not only stimulate cell division but also suppress cell death. Mayo et al. explored the mechanisms by which activated Ras allows transformed cells to escape cell death. Ras caused activation of the transcription factor NF-B, an event that inhibits apoptosis. Removal of this effect of Ras, by inhibition of NF-B activity, tipped the balance in favor of pro-apoptotic signals from Ras and enhanced cell death in Rat-1 cells expressing activated Ras. Understanding the balance between the various signals from oncogenes that lead to tumorigenesis could reveal strategies to inhibit growth or promote apoptosis of cancerous cells.