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ScienceWeek
SCIENCEWEEK
July 11, 2003
Vol. 7 - Number 28B
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CONTENTS:
1. Cell Biology: On Cell Division
2. Cell Biology: On Resolving Signal Propagation in Cells
3. Medical Biology: Blood Lead and Intellectual Impairment
4. Geology: On the Supercontinent Rodinia
5. Geology: Proposal: A Mission to Earth's Core
6. Condensed Matter Physics: On the Behavior of Hydrogen Ions
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1. CELL BIOLOGY: ON CELL DIVISION
The following points are made by J.M. Scholey et al (Nature 2003 422:746):
1) During the nineteenth century, the discovery that cells reproduce themselves by dividing into two illuminated the very origin of cells and became a cornerstone of the cell theory. Today, research on cell division flourishes because an improved understanding of its mechanism could lead to improvements in the treatment of diseases such as cancer and because we are fascinated by the cytoskeletal "nanomachinery" that is responsible for mitosis and cytokinesis.
2) The pathways by which the microtubule (MT)-based mitotic spindle and the actin-based contractile ring use cytoskeletal proteins to coordinate mitosis and cytokinesis are well understood. During mitosis, the spindle uses MTs and multiple mitotic motors to distribute identical copies of the replicated genome to the products of each division. Usually this process begins during prophase with the migration of duplicated centrosomes around the nuclear envelope. The envelope breaks down at the onset of prometaphase, allowing spindle MTs to capture chromosomes and move them to the equator (congression), so that by metaphase, pairs of sister chromatids lie on the spindle equator facing opposite spindle poles. Upon the onset of anaphase, cohesion between sister chromatids is lost, which allows sister chromatids to be moved to opposite spindle poles (anaphase A) as the spindle poles themselves move further apart (anaphase B). Also during anaphase, the spindle delivers a signal to the cortex that defines the position and orientation of the contractile ring, the machine that uses actin and myosin-II to drive cytokinesis. The contraction of this ring causes the furrow to ingress as the nuclear envelopes reassemble around sets of decondensing segregated sisters. Finally, the furrow "seals", completing the separation of the daughter cells.
3) Cells use a significant fraction of their proteins to divide -- functional proteomics indicates that the nematode worm Caenorhabditis elegans uses 6% of its open reading frames to encode proteins required for cell division and an important subset of these proteins comprise actin filaments, MTs, motor proteins and accessory proteins. MTs and actin filaments are linear, polar, multistranded polymers, built from 13 strands of -tubulin heterodimers and 2 strands of G-actin monomers, respectively. These polymers can generate pushing and pulling forces as they grow and shrink by addition and loss of subunits from their ends, and they also serve as tracks for motor proteins that use ATP hydrolysis to generate force and motility. At the single-molecule level, cytoskeletal proteins generate piconewton-scale forces and nanometre-scale movements, but during cell division they function as ensembles that are capable of generating forces in the range of nanonewtons and serve to accurately move intracellular components and rearrange areas of the cell surface over distances of tens of microns. How do these cytoskeletal force generators cooperate to drive the motility events underlying the mechanics and regulation of cell division?
4) In summary: In creating the mitotic spindle and the contractile ring, natural selection has engineered fascinating precision machines whose movements depend upon forces generated by ensembles of cytoskeletal proteins. These machines segregate chromosomes and divide the cell with high fidelity. Current research on the mechanisms and regulation of spindle morphogenesis, chromosome motility and cytokinesis emphasizes how ensembles of dynamic cytoskeletal polymers and multiple motors cooperate to generate the forces that guide the cell through mitosis and cytokinesis.
Related Material:
CELL BIOLOGY: ON ASYMMETRIC CELL DIVISION
Cell growth, which occurs throughout the cell cycle, typically causes a cell to double in size by the time it is ready to divide. Most cells then undergo "binary fission", which partitions the cell down the middle and generates two identical new cells. Certain types of cells, however, undergo an asymmetric division process. In the context of this report, the term "asymmetric cell division" refers to cell division producing two daughter cells that exhibit distinct fates. In all organisms in which development has been studied extensively, ranging from bacteria to mammals, asymmetric cell division is one of the apparent origins of cell diversity. The central question in this field is what are the molecular mechanisms responsible for the programmed asymmetry?
The following points are made by N. Hawkins and G. Garriga (Genes & Development 1998 12:3625):
1) Asymmetric cell division can be achieved by either intrinsic or extrinsic mechanisms. Intrinsic mechanisms involve the preferential segregation of cell fate determinants to one of two daughter cells during mitosis. Asymmetrically segregated factors that bind cell fate determinants and orient the mitotic spindle may also be necessary to ensure the faithful segregation of determinants into only one daughter cell.
2) Extrinsic mechanisms involve cell-cell communication. In metazoans, the social context of a dividing cell provides positional information and opportunity for cell-cell interactions. Interactions between daughter cells or between a daughter cell and other nearby cells can specify daughter cell fate. Interaction between a progenitor cell and its environment can influence cell polarity by directing spindle orientation and an asymmetric distribution of developmental potential to daughter cells.
3) Recent studies have indicated that a combination of intrinsic and extrinsic mechanisms specify distinct daughter cell fates during asymmetric cell division. In the past few years, rapid progress has been made in elucidating the mechanisms underlying asymmetric cell division, but several outstanding questions remain to be addressed, in particular the mechanism of the establishment of neuroblast polarity.
Notes:
mitosis: (karyokinesis) Division of the cell nucleus in eukaryotic cells (i.e., cells with internal membrane-bound organelles).
mitotic spindle: A structure composed of *microtubules that separates the two sets of chromosomes during cell division in eukaryotes.
microtubules: Microtubules are part of the cytoskeleton of biological cells, the quasi-rigid matrix that among other things determines cell shape. The microtubules are 25 nanometers in diameter, and composed of the protein tubulin. They occur in regular arrays in cilia, flagella, the mitotic spindle, and in the cytoplasm in general, and they contribute not only to cell shape, but also to cell motility.
metazoans: In general, the term "metazoa" refers to all multicellular animals. Among important distinguishing characteristics of metazoa are cell differentiation and intercellular communication. For certain multicellular colonial entities such as sponges, some biologists prefer the term "parazoa".
cell polarity: In general, non-random organization of the interior of the cell.
neuroblast polarity: Neuroblasts are embryonic cells destined to be differentiated into the cells of nerve tissue.
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2. CELL BIOLOGY: ON RESOLVING SIGNAL PROPAGATION IN CELLS
The following points are made by Cornelis J. Weijer (Science 2003 300:96):
1) Advances in molecular genetics and biochemistry have led to the identification of many new signaling molecules and interactions between them, as documented in the elaborate signaling maps that are currently under development. These maps consist of boxes indicating molecules connected by arrows that delineate the possible flow of information (signals) between them to result in specific cellular actions such as gene expression, movement, cell division, etc. These maps, however, do not take into account the spatial and structural aspects of these signaling pathways, which in real cells are very important. Understanding these pathways and mechanisms of signal propagation in cells will require the measurement of many signaling reactions, with high spatial and temporal resolution. Most cells are small and the concentration of signaling molecules is generally low; therefore, these measurements require both considerable magnification and sensitivity. The most widely used detection methods are, therefore, based on fluorescent microscopic imaging techniques.
2) Microscopy and detection techniques have improved considerably in sensitivity over the last decades, and it is now possible to take fluorescence images in the microsecond to millisecond range. Through the use of confocal and deconvolution microscopy, it has also become possible to measure several fluorescence signals simultaneously in the same cell with high three-dimensional spatial and temporal resolution. Using total internal reflection microscopy it is now possible to image single fluorescent molecules in living cells. Data analysis requires the development of advanced visualization and analytical techniques. Furthermore, because many of the signaling reactions taking place in cells involve complex positive and negative nonlinear feedback as well as transport, their dynamics can give rise to a wide variety of nonintuitive behaviors. To interpret and understand these data it is becoming increasingly necessary to model and analyze them using qualitative and quantitative mathematical models.
3) In summary: Biological cells display a highly complex spatiotemporal organization required to exert a wide variety of different functions, for example, detection, processing, and propagation of nerve impulses by neurons; contraction and relaxation by muscle cells; movement by leukocytes; and adsorption and secretion of nutrients and metabolites by epithelial cells lining the gut. Successful execution of these complex processes requires highly dynamic information transfer between different regions and compartments within cells. Through the development of fluorescent sensors for intracellular signaling molecules coupled with improved microscopic imaging techniques, it has now become possible to investigate signal propagation in cells with high spatial and temporal resolution.
Related Material:
ON FLUORESCENCE DETECTION OF DNA-JOINING REACTIONS
Various biochemical applications of fluorescence involve two fluorochromes, one of which absorbs light emitted by the other. The initially emitted light is absorbed and thus "quenched" by the second compound. Since such quenching can only occur if the two fluorochromes are in proximity, the process can be used as an indicator of such proximity. The term "quench" has other meanings in other contexts.
The following points are made by S. Sando and E.T. Kool (J. Am. Chem. Soc. 2002 124:2096):
1) A number of strategies for detection of nucleic acid reactions involve a change in fluorescence intensity or emission wavelength. Fluorescence-changing methods have the distinct advantage that unbound probe molecules can easily be distinguished from those bound to the desired target. Such approaches can be used either in solution or on solid supports, whereas static methods often cannot be used in solution, and typically require careful washing methods on solid supports. Approaches that rely on simple intensity variation produced by changes in quenching have the further advantage of freeing more spectral ranges so that multiple simultaneous probing can be achieved.
2) To date, the number of different quenching-approaches to nucleic acid sensing is limited. Perhaps the most well-developed approach is that of "molecular beacons", which consist of hairpin-forming DNAs labeled in the stem with fluorophore and quencher. Binding of the DNA molecule to a complementary sequence results in opening of the hairpin and moving of the quencher away from the emitting fluorophore. Beacons can be used in solution or in solid-support approaches, but their fluorescence change depends on solution conditions (e.g., temperature and ionic strength), and so one must monitor conditions carefully. Moreover, methods that rely on DNA hybridization alone are usually not as sequence selective as some recently developed DNA-sensing methods such as enzymatic approaches or some non-enzymatic autoligation methods.
Related Material:
GREEN FLUORESCENT PROTEIN AND BIOLUMINESCENCE
The following points are made by Marc Zimmer (Chem. Rev. 2002 102:759):
1) In the last 10 years, green fluorescent protein (GFP) has changed from a nearly unknown protein to a commonly used tool in molecular biology, medicine, and cell biology. GFP is used as a biological marker. It is particularly useful due to its stability and the fact that its chromophore is formed in an autocatalytic cyclization that does not require a cofactor. This has enabled researchers to use GFP in living systems, and it has led to GFP's widespread use in cell dynamics and development studies. Furthermore, it appears that fusion of GFP to a protein does not alter the function or location of the protein.
2) Pliny the elder (23-79 AD) described bioluminescence as early as the first century. Bioluminescence is the process by which visible light is emitted by an organism as a result of a chemical reaction. The reaction involves the oxidation of a substrate (called the luciferin) by an enzyme (the luciferase). Oxygen is usually the oxidant. Bioluminescent organisms are found in a variety of environments. Common examples are insects, fish, squid, sea cacti, sea pansies, clam, shrimp, and jellyfish. The bioluminescent systems in these organisms are not all evolutionarily conserved, and the genes coding for the proteins involved in bioluminescence are not homologous. The emitted light commonly has one of three functions: defense, offense, or communication.
3) Green fluorescent proteins are found in numerous organisms, but Aequorea aequorea (A. victoria; A. forskalea; a hydrozoan jellyfish) GFP was the first GFP for which the gene was cloned and expressed, and it is the GFP used in most tracer studies. It was first reported in 1955 that Aequorea fluoresced green when irradiated with ultraviolet light. Two proteins in Aequorea are involved in its bioluminescence, aequorin and green fluorescent protein. Aequorin (the luciferase) contains coelenterazine (the luciferin). Upon binding three calcium ions the aequorin oxidizes the coelenterazine with a protein-bound oxygen resulting in a Ca(sub3)-apo-aequorin-coelenteramide complex which in vitro emits blue light. However, Aequorea does not emit blue bioluminescence; instead, the aequorin complex undergoes radiationless energy transfer to GFP which gives off green fluorescence.(5) No binding between aequorin and GFP is observed in solution. In vitro energy transfer can be obtained by coadsorption of aequorin and GFP on DEAE cellulose membranes. The crystal structure of aequorin was recently solved.
References (abridged):
1. Chalfie, M. Photochem. Photobiol. 1995, 62, 651-656
2. Cubitt, A. B.; Heim, R.; Adams, S. R.; Boyd, A. E.; Gross, L. A.; Tsien, R. Y. TIBS 1995, 20, 448-455
3. Gerdes, H.-H.; Kaether, C. FEBS Lett. 1996, 389, 4-47
4. Tsien, R. Annu. Rev. Biochem. 1998, 67, 510-544
5. Kendall, J. M.; Badminton, M. N. TIBTECH 1998, 16, 216-224
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3. MEDICAL BIOLOGY: BLOOD LEAD AND INTELLECTUAL IMPAIRMENT
The following points are made by R.L. Canfield et al (New Engl. J. Med. 2003 348:1517):
1) Lead is neurotoxic, and young children are at particular risk for exposure. Numerous studies indicate that blood lead concentrations above 10 micro-g per deciliter (0.483 micro-mol per liter) are associated with adverse outcomes on measures of intellectual functioning and social–behavioral conduct. Such studies led to the identification of a blood lead concentration of 10 micro-g per deciliter or higher as a "level of concern" by the Centers for Disease Control and Prevention (CDC) and the World Health Organization.
2) It remains unclear whether lead-associated cognitive deficits occur at concentrations below 10 micro-g per deciliter. The CDC and WHO recognized that no evidence of a threshold existed for lead-associated deficits but noted an absence of research on the possible effects of blood lead concentrations below 10 micro-g per deciliter. Although some studies in which the average blood lead concentration was below 10 micro-g per deciliter have reported associations between the blood lead concentration and cognitive deficits, the analyses did not focus specifically on children whose concentrations remained below 10 micro-g per deciliter throughout life. Other evidence suggesting lead-related deficits at concentrations below 10 micro-g per deciliter relied on linear extrapolation or on data unadjusted for important potential confounders such as maternal intelligence and the quality of caregiving.
3) The authors examined associations between low-level exposure to lead and children's performance on intelligence tests at the ages of three and five years in a population that included many children whose blood lead concentrations remained below 10 micro-g per deciliter.
4) The authors conclude: "Blood lead concentrations, even those below 10 micro-g per deciliter, are inversely associated with children's IQ scores at three and five years of age, and associated declines in IQ are greater at these concentrations than at higher concentrations. These findings suggest that more U.S. children may be adversely affected by environmental lead than previously estimated."
Related Material:
ON LEAD AS AN ENVIRONMENTAL POLLUTANT
The following points are made by Vincent T. Breslin (J. Chem. Educ. 2001 78:1647):
1) Despite the ban on lead-based paints and leaded gasoline in the US in the 1970s and 1980s, 4.4 percent of American children aged 1 to 5 years still have blood lead levels high enough to cause irreversible damage to the developing nervous system. In addition, almost 12 percent of children in older housing in large urban areas have elevated blood lead levels, and African-American children living in the major US inner cities are affected disproportionately (approximately 22 percent). Lead exposure in young children results primarily from ingestion or inhalation of soil particles, drinking water, paint, and dust particles in and around the home and play areas.
2) Lead was used extensively as a corrosion inhibitor and pigment in both interior and exterior oil-based paints prior to 1978, and some paints were manufactured with lead concentrations of 50 percent by weight. Therefore, weathering of lead-based exterior paint and deposition of paint chips and dust on soils remains a significant source of lead to soils surrounding homes. Soil lead concentration at or above 500 micrograms per gram will result in a 1 to 5 percent probability that a child will have a blood lead concentration that equals or exceeds 10 micrograms per deciliter.
3) Drinking water is another source of ingested lead. Household plumbing fixtures, including metal pipes, faucets, and soldered joints, are possible sources of lead in drinking water. The lower the pH of the water and the lower the concentration of dissolved salts in the water, the greater is the solubility of lead in the water. Leaching of lead from plastic pipes has also been documented and has been attributed to the use of lead stearate, a stabilizer used in the manufacture of polyvinyl plastics.
Related Material:
A DANGEROUS NEW SOURCE OF ENVIRONMENTAL LEAD
The following points are made by Howard W. Mielke (American Scientist 1999 87:62):
1) Since the 1920s, millions of US children have been quietly poisoned by lead, and thousands of deaths are attributed to this over the long term.
2) Although childhood lead exposure in the US has diminished during the past 2 decades, the problem has not been solved. Instead, the demographics has shifted.
3) Over 50 percent (and perhaps even 70 percent) of children living in the inner city of New Orleans and Philadelphia have blood lead levels above the current guideline of 10 micrograms per deciliter [*Note #2]. In contrast, in the concrete "jungle" of Manhattan, where very little of the soil is exposed and almost all apartments and housing contain lead-based paints, only between 5 and 7 percent of children under the age of 6 have been reported to have blood-lead levels of 10 micrograms per deciliter or higher. It is of significance that in Brooklyn, across the river from Manhattan, where yards containing soil are common, the percentage of affected children is several times higher than in Manhattan.
4) The serious of the problem has been recognized by the US Centers for Disease Control and Prevention since the early 1990s, which has called pediatric lead poisoning "entirely preventable".
5) The author suggests that effective prevention assumes an accurate identification of the environmental reservoirs of lead, and that current policies to reduce lead exposure are based on the assumption that the greatest lead hazard comes from lead-based paints [*Note #3]. Most lead-based have now been removed from the market, and parents have been instructed to guard their children from eating paint flakes. However, for children, paint is now neither the most abundant nor the most accessible source of lead. The common problem is lead dust in the environment, with the soil a giant reservoir of tiny particles of lead. The greatest risk for exposure of inner city children is in the yards around houses and to a lesser extent in public playgrounds.
6) The author suggests that an accurate and complete appreciation of the distribution of lead in the environment can help shape policies that more effectively protect the health of children. The author concludes: "It took nearly 10 decades for lead to accumulate to its current levels in urban areas. With judicious planning, the problem can be resolved in much less time."
Notes:
Note #1: There is much data concerning certain syndromes, e.g., fetal alcohol syndrome, lead poisoning, etc. One research problem is that effects of low levels of environmental toxins on the developing nervous system can be subtle and not detected unless specific rather than general behavioral measures are applied.
Note #2: There is hardly a consensus concerning acceptable levels of lead in the whole blood of children. Some clinicians consider the danger point to be in the region of 50 micrograms per deciliter whole blood; other clinicians consider anything above 10 micrograms per deciliter as a cause for alarm. In terms of low-level effects on the developing central nervous system, general concentration cut-off points are perhaps arbitrary, since there is considerable individual variation in toxic susceptibility.
Note #3: In the US, lead was used in residential paint between 1884 and 1978, and leaded paint remains on the walls of many old buildings.
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4. GEOLOGY: ON THE SUPERCONTINENT RODINIA
The following points are made by Trond H. Torsvik (Science 2003 300:1379):
1) Earth's surface is divided into a dozen tectonic plates that either drift apart, creating new oceanic crust, or collide, generating mountain belts such as the Himalayas. In the past, continents have coalesced into single supercontinents, which had dramatic effects on both surface and deep Earth processes. But while much is known about Pangaea (the most recent supercontinent on Earth), the earlier Rodinia supercontinent remains shrouded in mystery.
2) Pangaea started to form approximately 330 million years ago and reached its maximum extent in the Late Permian (250 million years ago). Not all continents coalesced simultaneously; some were added along Pangaea's margins just as others rifted off. The supercontinent changed the distribution of land and sea areas and brought about unusual climatic and biological conditions. Increased mantle temperatures and continental bulging in the interior of Pangaea may also have occurred as a result of long-term shielding of large parts of the underlying mantle. The ultimate breakup of Pangaea approximately 175 million years ago was preceded by and associated with widespread magmatic activity.
3) There is some evidence that supercontinents have formed periodically during Earth's history. The existence of a supercontinent in the Precambrian (before 544 million years ago) was proposed in the 1970s, when many geologists noted a large number of mountain belts with similar ages (1300 to 1000 million years old) that are today located on different continents. In the early 1990s, the name Rodinia was adopted for this supercontinent.
4) Most Rodinia models have sought to match the 1300- to 1000-million-year-old mountain belts. In these models, Laurentia forms the core of the supercontinent, with Australia-East Antarctica situated along its present-day western margin and Baltica-Amazonia along the eastern margin.
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5. GEOLOGY: PROPOSAL: A MISSION TO EARTH'S CORE
The following points are made by David J. Stevenson (Nature 2003 423:239)):
1) Planetary missions have enhanced our understanding of the Solar System and how planets work, but no comparable exploratory effort has been directed towards the Earth's interior, where equally fascinating scientific issues are waiting to be investigated. The author proposes a scheme for a mission to the Earth's core, a scheme in which a small communication probe would be conveyed in a huge volume of liquid-iron alloy migrating down to the core along a crack that is propagating under the action of gravity. The grapefruit-sized probe would transmit its findings back to the surface using high-frequency seismic waves sensed by a ground-coupled wave detector. The probe should take about a week to reach the core, and the minimum mass of molten iron required would be 10^(8) to 10^(10) kg -- or roughly between an hour and a week of Earth's total iron-foundry production.
2) We live on the Earth's surface, which divides what is above from what is below. The part above us (the rest of the Universe) is mostly empty, mostly unknown and about 10^(57) times larger by volume. The part below is crammed with interesting stuff and is also mostly unknown, despite its much greater proximity to us. Space probes have so far reached a distance of about 40 astronomical units (6 x 10^(9) km), but subterranean probes (drill holes) have descended only some 10 km into the Earth.
3) Travel downwards is impeded by the dense intervening matter, and the energy required to penetrate it by melting is about 10^(9) times (per unit distance travelled) the energy needed for space travel -- a fact that partly explains the large difference in distances attained. Travel downwards has also been impeded by the much more limited allocation of financial and material resources relative to those provided for space travel -- there is no underground equivalent of NASA.
4) One possible means of reaching the core appeals to the "China syndrome" idea and requires melting of the rock, but the trip times in these scenarios are thousands of years or more --geologically short but too long on a human timescale for any government to contemplate funding. However, a liquid-iron-filled crack initiated in the Earth would propagate downwards (despite very high pressures), closing up behind as it travels, and a neutrally buoyant, insoluble probe could be carried along for the ride. The author states: "This proposal is modest compared with the space programme, and may seem unrealistic only because little effort has been devoted to it. The time has come for action."
Related Material
CONDITIONS AT EARTH'S CORE
The following points are made by A. Jephcoat and K. Refson (Nature 2001 413:27):
1) Although in the beginning, shortly after the Earth was formed, there was apparently no inner core, this has since grown to a spherical region of dense and nearly pure solid iron approximately 2240 kilometers in diameter. The size and some of the characteristics of the internal structure of the core are known because of their effect on seismic waves produced by earthquakes, which pass through the core on their way to detector stations at the surface.
2) A longstanding puzzle is that the speed of a seismic compressional wave (an ultra-low frequency sound wave) depends on its direction across the core. Such waves travel faster along the axis of Earth's rotation (north-south) than in the equatorial plane (east-west), and this difference (called "seismic anisotropy") appears to increase with depth within the inner core and is patchy on a variety of length scales. Other vital evidence for seismic anisotropy comes from free oscillations, studies of the natural vibrational frequencies of the Earth.
3) Steinle-Neumann et al (2001) have presented calculations of the behavior of iron at high temperatures and pressures, and predicted that the crystal structure of iron will distort unexpectedly when subjected to the conditions at the center of the Earth. Together with a model of the orientation of iron crystals in the inner core by Buffet and Wenk (2001), these results apparently explain the observed seismic anisotropy, and it seems the fundamentals for understanding the phenomenon are now in place.
Related Background:
ON EARTH'S CORE AND THE GEODYNAMO
Seismic wave propagations are the propagated shock waves produced by earthquakes, and quantitative analysis of these waves can tell us much about the structure of the Earth and its interior physical discontinuities. Seismic studies indicate the interior of the Earth consists of three parts: a metallic core, a dense rocky mantle, and a thin low-density crust. The central part of the core is solid, but the outer part of the core is evidently liquid. The mantle, the layer of dense rock and metal oxides between the molten part of the core and the surface, has plastic properties (i.e., it is a solid capable of flow under pressure). Apparently, the Earth's magnetic field is a direct result of the combination of its rapid rotation and its molten core, and the theoretical account of this is called the "dynamo effect" (the source of the effect is called Earth's "geodynamo"). The essential idea is that the liquid metallic core is stirred by convection, the rotation of the Earth couples this motion into a circulation that generates electric currents, and the electric currents in turn generate a magnetic field according to classical electromagnetic theory.
The following points are made by Bruce A. Buffett (Science 2000 288:2007):
1) Earth apparently evolved into a layered body early in its history. Molten metal (mainly iron) descended to form the present-day core, while silicates and oxides were confined to the thick shell of the mantle. The innermost part of the core is now a solid, whereas the outer portion of the core is liquid. The apparent viscosity of the liquid outer core is comparable to that of water, which permits vigorous convection as the core cools. Fluid velocities of the order of 10 kilometers per year are evidently sufficiently rapid to sustain Earth's magnetic field via the geodynamo.
2) Planetary rotation promotes the types of flows that are needed to generate the magnetic field, but the resulting magnetic field exerts a strong feedback on
convection, and this complicates quantitative predictions of the field generation. An important advance in recent years is the development of numerical simulations that produce self-sustaining dynamo action. Computational limitations prevent these simulations from reaching known Earth-like conditions, but the models obtained so far have external magnetic fields that are similar to Earth's magnetic field.
3) The operation of the geodynamo depends on the internal evolution of the planet, since convection in the core is linked to the rate of cooling. (Cooling of the core causes growth of the inner core by solidification; the current rate of growth is approximately 1 millimeter per year.) The transport of heat through the mantle is crucial for powering the geodynamo, and even the existence of *plate tectonics at the surface is an important factor. Interactions between the core and the mantle are expected from theory, but it is not clear how these interactions are expressed in the magnetic field. The apparent persistence of the magnetic field over most of the history of Earth implies continual cooling and convection in the core. In contrast, the absence of magnetic fields in our nearest planetary neighbors indicates that other planetary thermal histories are possible. As we gain a better understanding of the geodynamo and the dynamics of the core, new perspectives about the processes that drive the internal evolution of Earth are expected to emerge.
Notes:
plate tectonics: The term "lithosphere" refers to the outer layer of the Earth, comprising the crust and upper mantle, and extending to a depth of 50 to 70 kilometers. The traditional view of tectonics (changes in the structure of the Earth's crust) is that the lithosphere consists of a strong brittle layer overlying a weak ductile layer. "Plate tectonics" is the current consensus theory that the Earth's lithosphere is broken into fairly rigid plates, seven or eight major plates and many smaller plates, and that convection within the underlying less rigid "asthenosphere" causes the plates (and the associated continents and crust) to move relative to each other.
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6. CONDENSED MATTER PHYSICS: ON THE BEHAVIOR OF HYDROGEN IONS
The following points are made by R.M. Martin and G. Galli (Nature 2003 423:595):
1) The role of hydrogen in controlling electrochemical activity is essential to both living systsms and computers. The former involvement is well known to any student of chemistry, as the pH of aqueous solutions is defined by the concentration of H+ ions. The crossover from a dominance of H+ to OH- defines the transition from acidic to alkaline solutions, which has profound consequences for chemical and biological processes. Perhaps less well known is that hydrogen is also essential for the operation and lifetime of every modern solid-state electronic device. The ability of hydrogen to take on either of two charge states, H+ or H-, means that it can "passivate" electrically active defects of either sign in the device material. Introducing hydrogen into the processing makes it possible to achieve the perfection required to reliably produce millions of transistors on a chip. Recently, Van de Walle and Neugebauer (Nature 2003 423:626) have proposed that these two phenomena are closely related: in fact, these authors claim to have found a universal alignment of the electrochemical level for the change of charge state of hydrogen complexes across a wide range of materials.
2) Why should there be any relation between the H+ and OH- ions solvated in liquid water and the hydrogen bound to defects in strongly bonded solids? In the case of water, the role of the solvation shell around the ion has been the subject of studies for many years; the details are still obscure, but interesting suggestions are emerging from simulations that start from first principles. In the case of semiconductors, the mechanism of hydrogen passivation has been elucidated, and the structures of the passivated defects vary greatly for different materials. How could such variety be captured in a "universal" relation?
3) The proposal made by Van de Walle and Neugebauer (2003) is based on calculations done using state-of-the-art methods including "density functional theory"). The authors are able to predict a large range of formation energies of the individual defect configurations involving H+ and H- ions and neutral hydrogen, H(sup0). Under all circumstances, they find that states involving either H+ or H- are more stable than those involving H(sup0). Whether the H+ or H- charge state is more stable depends on the chemical potential of the reservoir of electrons in the material -- called the "Fermi energy" -- which determines how easily the conversion between an H+ and an H- ion can be made (by adding or removing two electrons). A high Fermi energy means that the system lowers its energy by accepting electrons to become H-; as the Fermi energy is lowered, a transition point is reached at which both states have the same energy; beyond this, the H+ state becomes energetically preferred. Remarkably, Van de Walle and Neugebauer have found that the energy at the transition has almost the same value for a wide variety of materials.
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