This Week in Science

Science  13 Mar 1998:
Vol. 279, Issue 5357, pp. 1605
  1. Mars Global Surveyor

    The areally limited but intriguing measurements made during the aerobraking and assessment orbits of Mars Global Surveyor between September 1997 and February 1998 are the subject of an overview (Albee et al., p. 1671) and five reports. The accelerometer (Keating et al., p. 1672) measured the density, temperature, and pressure in the martian thermosphere (the upper atmosphere, between 110 to 170 kilometers). These measurements are essential for planning the aerobraking maneuvers to obtain a circular orbit and also suggest that an observed regional dust storm caused a global thermospheric response and that topographically forced planetary waves may produce the measured thermospheric density anomalies seen at nearly opposite ends of Mars. The magnetometer (Acu-a et al., p. 1676) did not find a global-scale magnetic field. However, small-scale magnetic anomalies were seen that appear to be concentrated in the oldest (most heavily cratered) surface deposits of the crust and suggest that Mars may have had a magnetic field early in its evolution. The thermal emission spectrometer (Christensen et al., p. 1692) returned spectra of surface materials, atmospheric dust, clouds, CO2 gas, and water vapor. Pyroxene is abundant with some plagioclase in the regions where spectra were obtained, whereas carbonates, olivine, clay minerals, and quartz are minor components limited to low albedo surfaces. The laser altimeter (Smith et al., p. 1686) measured the topography along 18 tracks in the northern hemisphere. The Ares Vallis channel is much deeper than previously estimated, implying a much greater water discharge rate and possibly more flowing water. The narrow-angle, high-resolution camera and two wide-angle cameras (Malin et al., p. 1681; see the cover) provided detailed and global views of the planet. Windswept terrains of dunes, sand sheets, and drifts can be seen on the scale of a few meters, whereas the canyons of Valles Marineris reveal layered deposits extending from the top to the bottom of the canyon walls. The complex and in some cases intersecting eolian features provide clues to wind direction, wind intensities, and the possible ages of these landforms that may be related to seasonal changes on Mars.

  2. Earlier settlements

    Several large central archaeological sites thought to mark the origin of organized agricultural societies in southwestern United States and northern Mexico have been recognized and dated to about 1200 to 600 years ago. These villages, known as cerros de trincheras, typically extend over several hundred meters and are marked by extensive terraces on hills and house rings or other residential structures. Hard and Roney (p. 1661; see the commentary by Smith, p. 1651) describe a large cerros de trincheras in Chihuahua, Mexico, that is dated much earlier, to about 3000 years ago. This age is coeval with the introduction and first use of maize in the region. Thus, large centralized agricultural villages developed rapidly in this region, not gradually over many centuries as had been previously thought.

  3. More deuterium in Hale-Bopp

    Deuterium-to-hydrogen (D/H) ratios can be used to estimate the likely origin of such materials as stardust in the laboratory and comets in space by tracing their content of modified versus unmodified (primitive) material. Previous measurements of the D/H ratio in water in comet Hale-Bopp of about 3 × 10−4 were consistent with the only other cometary D/H ratios measured in comets Halley and Hyakutake. Meier et al. (p. 1707) have now measured hydrogen cyanide (HCN and DCN) in Hale-Bopp and found a D/H ratio of 2.3 × 10−3. The different D/H ratios imply that Hale-Bopp has preserved unmodified interstellar material because if the HCN and H2O species had time to exchange D with the abundant H2 gas (with an estimated maximum D/H of 4 × 10−5) in the solar nebula, then both ratios would have been lowered and in closer agreement with each other. These measurements indicate that cometary ices are primitive materials formed in interstellar clouds, probably near the Uranus-Neptune region of the early solar nebula.

  4. Mantle boundaries

    The abrupt increase in seismic velocities at the boundary between the upper and lower mantle at 660 ± 30 kilometers has been partially attributed to a change in mineral structure of the dominant upper mantle mineral phase, spinel, which is transformed into perovskite and MgO-periclase. Irifune et al. (p. 1698), using synchrotron radiation and a high pressure multianvil apparatus, estimated the pressure and temperature at which this mineral phase transformation occurs for the pure magnesium silicate end members. The phase boundary occurs at about 21 gigapascals and 1600°C, which corresponds to a depth of only about 600 kilometers. This depth discrepancy is significant and remains unresolved, although the authors postulate some possible causes. The structure at and above the core-mantle boundary (CMB) is determined from small-amplitude compressional sound waves, which travel through the mantle and the outer core faster and with slightly diffracted paths from the larger amplitude primary mantle-outer core sound wave phase, or PKP. Wen and Helmberger (p. 1701) used complimentary short- and long-period precursors to PKP that sampled the CMB beneath the Western Pacific Ocean. The long-period precursors suggest ultralow velocity zones (ULVZ; a decrease in the velocity of about 7 percent) that have Gaussian shapes with heights of 60 to 80 kilometers from the CMB and widths of 100 to 300 kilometers. The short-period precursors indicate that these ULVZ contain partial melts created by small-scale convection or by a thermal instability at the CMB.

  5. Viscous bubbles

    We normally think of foams and bubbles forming in liquid films stabilized by surfactants, but highly viscous fluids can also support bubble formation, at least for awhile. Debrégeas et al. (p. 1704) studied drainage and bursting of air bubbles formed in melts of a silicone rubber and of a borosilicate glass, liquids that are approximately a million times more viscous than water. Although the systems displays very different relaxation behaviors, they can be described with a relatively simple hydrodynamic model.

  6. Selective ceramic membranes

    Membranes of amorphous silica supported on more porous alumina hold the potential for separating small molecules of industrial importance, as their pore diameters (less than 2 nanometers) can lead to large permeation ratios. Such materials can be impractical, however, in terms of their reproducibility and can contain defects that lower performance. De Vos and Verweij (p. 1710) show that when these membranes, which are made by dip-coating, are produced under clean-room conditions, high reproducibility is achieved in the separation of hydrogen from methane.

  7. Interfacial phosphor

    Combinatorial methods that search for new inorganic materials often make use of “inert” substrates for deposition of the elements of interest. Wang et al. (p. 1712), in searching for new photoluminescent materials, identified an efficient blue material prepared on a silicon substrate as a gadolinium-gallium oxide. When this material was prepared on LaAlO3, however, no emission was observed. They show that interfacial effects with the silicon substrate contribute to its luminescence.

  8. Recruiting tRNAs

    Transfer RNA (tRNA) molecules with more than one type of anticodon region that recognizes messenger RNA can have the same identity, that is, accept the same amino acid, and form an isoacceptor group. The classical model of tRNA evolution suggests that all of the tRNAs with the same identity have a common ancestor. An alternative model based on recent experimental evidence suggests that the anticodon interacts sufficiently with the aminoacyl tRNA synthetases so that a point mutation that changes the isoaccepting group could also change identity. Saks et al. (p. 1665) inactivated an essential threonine tRNA with a UGU anticodon in an Escherichia coli strain and show that an arginine tRNA, with a UCU anticodon, underwent a point mutation to UGU and changed its specificity to threonine.

  9. Reducing oxidation

    Oxidation can be damaging to cells. In the presence of oxidizing agents, Escherichia coli activates antioxidant genes by the action of transcription factors such as OxyR. Zheng et al. (p. 1718; see the commentary by Demple, p. 1655) show that OxyR is specifically activated by hydrogen peroxide through the formation of an intramolecular disulfide bond, and the disulfide bond is reduced by glutaredoxin 1. An autoregulatory loop in the defense mechanism occurs through the regulation of glutaredoxin 1 by OxyR.

  10. Connecting brain to brawn

    Early in development, neuromuscular connections are formed in excess and then pruned as development proceeds. Nguyen et al. (p. 1725) show that overexpression of a neurotrophic factor, GDNF, in the target muscle cells slows that pruning process. The transgenic mice show a greater than normal amount of innervation to their muscles in the few weeks after birth and display an unusual sort of tremor. Both features fade as the pruning process continues and the mice mature. The results suggest that GDNF may be the physiologically relevant trophic factor for formation of neuromuscular junctions.

  11. ORC complexes and cell replication

    The origin recognition complex (ORC) is a complex of six protein subunits that control initiation of DNA replication. Dillin and Rine (p. 1733) present evidence for a role of an ORC subunit in the mitotic phase of the cell division cycle as well as in S phase (when DNA replication actually occurs). They found that yeast cells with mutations in the ORC subunit Orc5p arrest either in early M phase or at the G1-/S phase boundary. After binding of inactive Orc5 during M phase, arrest of cells at the G1-/S phase border could not be overcome by introduction of wild-type Orc5 protein. Thus, the ORC complex appears to be irreversibly formed in the M phase before replication occurs. Inactive ORC complexes apparently produce an inhibitory signal within the nucleus that can prevent initiation of replication even at origins at which wild-type ORC is bound.

  12. Role of gamma delta T cells

    Recently it has been argued that the limited repertoire of γ δ T cells in various anatomical sites is of little significance and is not due to selection. Mallick-Wood et al. (p. 1729) have now shown that this may not be the case. They used mice that lacked the Vγ5 gene and an antibody that recognizes the idiotype (“shape”) of the Vγ5-Vδ 1 T cell receptor (TCR). Although no T cells in the skin had the Vγ5-V δ 1 receptor, the T cells used other TCRs that made the same shape and were recognized by the antibody. This result strongly suggests that these cells are being selected by some self antigen and serve a physiologic purpose. Human intestinal T cells are largely γδ T cells that express Vδ1 as part of their T cell receptor. It has been proposed that the restricted repertoire of these T cells makes them ideal for acting as sentinels for self proteins that are expressed in response to damage or infection of the intestinal epithelium. Their localization is matched by the location of the nonclassical class I major histocompatibility complex molecules MICA and MICB, which contain heat shock promoters and are expressed in response to stress. Groh et al. (p. 1737) have isolated intestinal γδ T cells and show that they recognize MICA and MICB, independent of antigen processing. Thus, they may be a primitive T cell “first line of defense” for the gut.

  13. Sensory representation

    Portions of the brain that encode incoming sensory information are known to be highly plastic; that is, reducing or increasing a particular aspect of the input leads to diminished or expanded representation. Kilgard and Merzenich (p. 1714; see the commentary by Juliano, p. 1653) find that electrical stimulation of the nucleus basalis in conjunction with auditory input remodels the auditory cortex in the rat, leading to increased representation of those areas responding to the specific frequencies used and contraction of other areas. They suggest that output from the cholinergic nucleus basalis mediates this stimulus-induced plasticity and that remodeling apparently can occur in the absence of any behavioral content of the input.

  14. Calcium sensor

    The metabotropic family of glutamate receptors bind to the neurotransmitter glutamate in the brain and influence synaptic transmission and plasticity. Kubo et al. (p. 1722) now show that the receptor also responds to the levels of extracellular calcium ions, which may provide a further way to modulate synaptic plasticity.

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