ScienceWeek

SCIENCEWEEK

September 1, 2006

Vol. 10 - Number 35

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One thing I have learned in a long life: that all our science, measured against reality, is primitive and childlike -- and yet it is the most precious thing we have."

-- Albert Einstein (1879-1955)

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Contents (Full reports below):

1. Astronomy: On the Moons of Pluto.
Two small moons of Pluto, Nix and Hydra, were discovered in 2005 in images taken with the Hubble Space Telescope. Both travel on nearly circular orbits, in the same plane and direction as Pluto's much larger inner moon, Charon. Nix's orbital period is somewhat less than four times that of Charon, and Hydra orbits in slightly less time than it takes Charon to...

2. Ecology: Are Global Conservation Efforts Successful?
Human actions affect ecosystems worldwide, leading to irreversible losses in biodiversity. These changes were faster in the past 50 years than at any time in human history, and this acceleration is projected to continue, despite diverse efforts to prevent these losses. Do these efforts make any measurable difference in the global...

3. Cell biology: Centrosomes and Genome Stability.
The centrosome is one of the most intriguing organelles found in animal cells. It organizes the construction and maintenance of the microtubule cytoskeleton, a molecular "scaffold" that confers shape and polarity on the cell. The centrosome duplicates during the cell-division cycle, and the duplicated centrosomes then help to form the...

4. Solid-State Chemistry: On Solid Molecular Cages.
Small molecules can be trapped, like hostages in a prison cell, by networks of cages formed from another compound. The resulting solids -- known as clathrates, or inclusion compounds -- have been known about for more than 150 years, but have recently gained media attention for their potential to trap the greenhouse gas carbon dioxide. Many types of clathrate...

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Also Noted:

The Survival Imperative: Using Space to Protect Earth. William E. Burrows. Tom Doherty Assoc., New York, 2006. Hardback: 317 pp., $24.95. ISBN 0765311143. More information at: http://www.amazon.com/exec/obidos/ASIN/0765311143/scienceweek

Stargazer: The Life and Times of the Telescope. Fred Watson. Da Capo (Perseus), Cambridge, MA, 2006. Paperback: 352 pp., illus. $15.95, C$21.50. ISBN 0306814838. More information at: http://www.amazon.com/exec/obidos/ASIN/0306814838/scienceweek

The Storm: What Went Wrong and Why During Hurricane Katrina--the Inside Story from One Louisiana Scientist. Ivor van Heerden and Mike Bryan. Viking (Penguin), New York, 2006. Hardback: 320 pp., illus. $25.95, C$34. ISBN 0670037818. More information at: http://www.amazon.com/exec/obidos/ASIN/0670037818/scienceweek

Trust Is Not Enough: Bringing Human Rights to Medicine. David Rothman and Sheila Rothman. New York Review of Books, New York, 2006. Hardback: 229 pp. $24.95, C$32.95. ISBN 1590171403. More information at: http://www.amazon.com/exec/obidos/ASIN/1590171403/scienceweek

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1. ASTRONOMY: ON THE MOONS OF PLUTO.

The following points are made by Jack J. Lissauer (Science 2006 313:1054):

1) Two small moons of Pluto, Nix and Hydra, were discovered in 2005 in images taken with the Hubble Space Telescope (1). Both travel on nearly circular orbits, in the same plane and direction as Pluto's much larger inner moon, Charon. Nix's orbital period is somewhat less than four times that of Charon, and Hydra orbits in slightly less time than it takes Charon to complete six revolutions. The dynamical aspects of this system are analogous to the regular satellite systems of the giant planets Jupiter, Saturn, and Uranus. However, the giant planets contain considerable amounts of gas, and their moons, none of which exceeds 1/4000th the mass of its planet, grew within disks composed of gas and solid particles that orbited the planets in their youth (2). In contrast, Charon's mass is greater than 10% that of Pluto, and the pair is believed to have formed as the result of a partially elastic and disruptive giant impact (3). This giant impact model, which is also the preferred scenario for the origin of Earth's Moon, yields a closely bound pair, separated by only a few radii of the larger body. Tidal forces subsequently expanded the separation to its present distance of more than 16 Pluto radii. But analogous tides would barely move the recently found small moons of Pluto, and the impact model cannot account for the formation of moons on nearly circular orbits so far from the planet. New work (4) reports a mechanism by which Charon could have pushed Nix and Hydra outward from initial orbits much closer to Pluto, thereby providing a unified explanation for the origin and evolution of this intriguing four-body system.

2) It has been known for more than 40 years that a moon whose orbit is tidally evolving away from a planet can trap a more distant moon in an orbital resonance, pushing the exterior moon outward ahead of itself (5). As the inner orbit expands due to tidal forces, the satellites thereby maintain the commensurate mean motions and move outward together. The ratio of energy to angular momentum required to expand an orbit without changing its eccentricity is just the orbital frequency. So if energy is transferred outward to a moon with a longer orbital period, not enough angular momentum is available to maintain the circularity of the orbits, and the eccentricity of one or both moons increases. In the case of Jupiter's large inner moons Io, Europa, and Ganymede, which are locked in a 4:2:1 resonance identified by Pierre-Simon Laplace two centuries ago, tides raised by Jupiter on the satellites damp this eccentricity, producing persistent volcanic activity on Io and a liquid ocean below Europa's thin ice crust. Pluto itself was pushed hundreds of millions of kilometers away from the Sun by Neptune, as this giant planet migrated outward (as a back reaction from throwing solid bodies sunward, however, not solar tides). Pluto is still locked in a 3:2 resonance with Neptune, and its high eccentricity is evidence of this migration.

3) But what of Nix and Hydra, which travel on circular orbits and are too small and distant to have their orbital eccentricities tidally damped? The answer is that the resonances in which Pluto's tiny moons were trapped were both maintained by and acted to enhance (albeit not by much) the eccentricity of Charon. Charon, being much larger as well as closer to Pluto, has its eccentricity damped in a time that is much less than the age of the Solar System. Indeed, for this mechanism to work, Charon must have initially been on a highly eccentric orbit, which would be expected if it was captured nearly intact after an inelastic collision (3), rather than having accreted from a giant impact-produced disk (the preferred mechanism for the formation of Earth's Moon).

4) Ward and Canup (4) have offered a model that explains the origin of the orbital configuration of Pluto's three known satellites via tidal expansion from a compact system that was produced by a giant impact. Their model requires the impact origin of an intact Charon, which previous models (3) suggest is likely only if the Pluto impactor was a homogeneous mixture of ice and rock. Hence, the model of Ward and Canup (4) also predicts that Nix and Hydra are made of ice-rock mixtures. Pluto is by far the brightest and best known of the Trans-Neptunian Objects (TNOs). Once believed to be a planet-sized body, it is now viewed as one of the larger members of a populous class of distant Solar System bodies. Many other TNOs are known to have moons. Observations of the orbital characteristics of additional, smaller moons of such minor planets could indicate whether their observed large moons were formed by giant impacts or purely gravitational capture.

References (abridged):

1. H. A. Weaver et al., Nature 439, 943 (2006)

2. J. B. Pollack, Protostars and Planets II (Univ. Arizona Press, Tucson, 1985)

3. R. M. Canup, Science 307, [546] (2005)

4. W. R. Ward, R. M. Canup, Science 313, 1107 (2006)

5. P. Goldreich, Mon. Not. Roy. Astron. Soc. 130, 159 (1965)

Science http://www.sciencemag.org

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2. ECOLOGY: ARE GLOBAL CONSERVATION EFFORTS SUCCESSFUL?

The following points are made by Ana S. L. Rodrigues (Science 2006 313:1051):

1) Human actions affect ecosystems worldwide, leading to irreversible losses in biodiversity. These changes were faster in the past 50 years than at any time in human history, and this acceleration is projected to continue (1), despite diverse efforts to prevent these losses. Do these efforts make any measurable difference in the global state of biodiversity? The combined results of the 2006 World Conservation Union (IUCN) Red List of Threatened Species (2) and of a study by Butchart et al. (3) provide the first opportunity to assess the impact of global conservation investment on biodiversity.

2) Measuring global conservation impact is not simple. Biodiversity is not easily quantified (4), and resources for monitoring it fall woefully short (5). Moreover, innumerable conservation activities take place worldwide, with approaches as diverse as single-species management, ecosystem restoration, environmental education, and political lobbying. Their scales range from local to global, with various degrees of coordination, replication, and synergy among them. Overall conservation impact can thus not be measured as the summed impacts of individual actions.

3) One useful approach is to compare the observed change in global biodiversity with the predicted change in the absence of such efforts. In the best-case scenario, conservation action would maintain biodiversity at a stable level, because global biodiversity does not increase on a time-scale relevant to human enterprise. More realistically, conservation action might be considered successful if it slows down the human-induced rate of global biodiversity decline.

4) This approach can now be applied to birds, the best-studied vertebrate group. The 2006 IUCN Red List reports 135 bird species that have become extinct since 1500 (2). The numbers of extinctions per century increased steadily to 49 in the 19th century, but then appear to decline to 43 in the period from 1901 to 2006. This could be mistaken as evidence that the human impact on birds has weakened. However, recent extinctions are underestimated because of the time lag between the disappearance of a species from the wild and the confirmation of its extinction.

5) Butchart and colleagues (3) add the missing piece required to evaluate conservation impact. Using data on population sizes, population trends, threatening processes, and conservation actions, they identify at least 26 bird species surviving in the wild that would have very probably gone extinct if conservation programs for them had not been undertaken. Four additional species are classified as "Extinct in the Wild" and one as "Critically Endangered (Possibly Extinct in the Wild)," only surviving (or possibly only surviving) in captive breeding programs (2). These 31 species represent the gain in extant bird species attributable to conservation action, providing a measure of the success of global conservation in preventing bird extinctions (see the second figure). In the absence of conservation, the rates of bird extinctions would thus have increased dramatically into the present.

References (abridged):

1. Millennium Ecosystem Assessment, Ecosystems and Human Well-Being: Biodiversity Synthesis (World Resources Institute, Washington, DC, 2005). [publisher's information]

2. IUCN, 2006 IUCN Red List of Threatened Species (IUCN, Gland, Switzerland, 2006); available at www.iucnredlist.org.

3. S. H. M. Butchart, A. J. Stattersfield, N. J. Collar, Oryx 40, 266 (2006).

4. K. J. Gaston, Ed., Biodiversity--A Biology of Numbers and Difference (Blackwell Science, Oxford, 1996).

5. A. Balmford et al., Science 307, [212] (2005).

Science http://www.sciencemag.org

ScienceWeek http://scienceweek.com

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3. CELL BIOLOGY: CENTROSOMES AND GENOME STABILITY

The following points are made by Erich A. Nigg (Nature 2006 442:874):

1) The centrosome is one of the most intriguing organelles found in animal cells. It organizes the construction and maintenance of the microtubule cytoskeleton, a molecular "scaffold" that confers shape and polarity on the cell (1). The centrosome duplicates during the cell-division cycle (2), and the duplicated centrosomes then help to form the spindle apparatus that segregates the duplicated chromosomes into the daughter cells. Because aberrant centrosome numbers can cause chromosome mis-segregation, such abnormalities have been proposed to contribute to the development of cancer (3,4) -- cancer cells often have abnormal chromosome numbers. The way in which centrosome numbers are controlled has been shrouded in mystery, but new work (5) provides evidence supporting a simple control mechanism.

2) Every centrosome comprises two centrioles -- barrel-shaped structures made of nine microtubule triplets -- embedded in a complex protein matrix (1). So, from a mechanistic perspective, the problem of centrosome duplication essentially comes down to the question of how centrioles are duplicated. At the morphological level, the centrosome/centriole duplication cycle is well understood (2). One notable aspect of the centrosome cycle is that parent and progeny centrioles show a tight right-angled (orthogonal) association. This "engagement" (5) is established during duplication and persists through the subsequent phases of cell-cycle progression until "disengagement" late in M phase and/or early G1 phase of the cycle separates the progeny centriole from its parent. So, during G1 phase, the two (future parent) centrioles are clearly separated, albeit loosely tethered to one another, and centriole disengagement has long been thought to be a prerequisite for duplication. However, the full significance of this peculiar process in the cell cycle is just beginning to emerge.

3) Three years ago, researchers provided evidence that there is a block intrinsic to centrosomes that prevents the centrosome from duplicating again during late S and G2 phases -- thus, it can duplicate only once per cell-division cycle. This implies that passage through M and/or G1 phases might be required to create permissive conditions -- or "issue a license" -- for a new round of centriole duplication in the next S phase. Tsou and Stearns (5) now use an in vitro assay based on frog (Xenopus) egg extracts and purified centrosomes to monitor simultaneously centriole disengagement and the growth of new centrioles under different experimental conditions. Their results strongly suggest that centriole disengagement requires the activity of separase, a protein-digesting enzyme previously shown to have a leading role in triggering the separation of duplicated chromosomes. Furthermore, their data show that centriole disengagement is a prerequisite for the subsequent growth of new centrioles. This work provides an appealing model for how centriole duplication is normally limited to occurring only once in every cell cycle. In essence, centriole engagement (established during centriole duplication in S phase) prevents further centriole duplication until the activation of separase during M phase triggers centriole disengagement. Disengagement licenses the centrioles to undergo a new round of duplication. This mechanism for limiting centrosome duplication to once per cycle is reminiscent of the controls that prevent multiple rounds of DNA replication.

References (abridged):

1. Bornens, M. Curr. Opin. Cell Biol. 14, 25-34 (2002)

2. Sluder, G. in Centrosomes in Development and Disease (ed. Nigg, E. A.) 167-189 (Wiley-VCH, Weinheim, 2004)

3. Boveri, T. Zur Frage der Entstehung maligner Tumoren (1914) (Engl. transl. The Origin of Malignant Tumors; Williams & Wilkins, Baltimore, Maryland, 1929)

4. Nigg, E. A. Nature Rev. Cancer 2, 815-825 (2002)

5. Tsou, M. -F. B. & Stearns, T. Nature 442, 947-951 (2006)

Nature http://www.nature.com/nature

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4. SOLID-STATE CHEMISTRY: ON SOLID MOLECULAR CAGES

The following points are made by Michel Pouchard (Nature 2006 442:878):

1) Small molecules can be trapped, like hostages in a prison cell, by networks of cages formed from another compound. The resulting solids -- known as clathrates, or inclusion compounds -- have been known about for more than 150 years, but have recently gained media attention for their potential to trap the greenhouse gas carbon dioxide. Many types of clathrate have been identified, and it seemed that little remained to be discovered. But new work (1) describes the synthesis of a clathrate with a unique cage structure.

2) Clathrates are formed when a host compound encloses guest molecules without using strong ionic interactions or covalent bonds. This definition was proposed by the X-ray crystallographer Herbert Powell (2) in the 1940s, after he analysed the structure of the archetypal clathrate, beta-hydroquinone. Powell found that solvent molecules are embedded in a hydrogen-bonded network of hydroquinone during crystallization. The name "clathrate" was later applied to inclusion compounds with lattices constructed from covalent bonds, as exemplified by a series of silica (silicon dioxide) structures that are now known as clathrasils (3). Nanoporous frameworks of silicon or germanium semiconductors (4) and even some networks of inorganic covalent salt complexes (5) have also been described as clathrates.

3) In true clathrates, a tetrahedral framework of hydrogen bonds creates voids in which guest species are trapped. A good example of this is methane hydrate, discovered deep in the oceans, in which methane molecules are embedded in a lattice of water. The resulting material looks like dirty ice, and could be a source of fossil fuel. Only extremely weak interactions are required for clathrates to hold guest molecules -- for example, the gases xenon and krypton are renowned for being generally unreactive, but both can act as guests in inclusion compounds. The voids are often in the shape of symmetric polyhedra, such as a pentagonal dodecahedron.

4) Pioneering work (3) in the 1960s on melanophlogite, a rare mineral form of silica with a clathrasil structure, led directly to the discovery of microporous materials in the 1990s. These materials contain a framework of pores that either has no electric charge, as represented by the formula APO4 (where A can be boron, aluminium, gallium or indium, and P is phosphorus), or that is negatively charged, such as the well-characterized aluminosilicates (AlSiO4-). The chargeless frameworks enclose neutral molecules, whereas the negatively charged systems host large, positively charged guests, such as the cations of the alkali metals rubidium and caesium. Microporous materials can be made with different pore sizes, but as the pore size increases, the cavities connect to form channels. Such channel-containing materials are known as zeolites.

References (abridged):

1. Karau, F. & Schnick, W. Angew. Chem. Int. Edn 45, 4505-4508 (2006)

2. Palin, D. E. & Powell, H. M. J. Chem. Soc. 61, 815-821 (1948)

3. Kamb, B. Science 148, 232-234 (1965)

4. Kasper, J. , Hagenmuller, P. , Pouchard, M. & Cros, C. Science 150, 1713-1714 (1965)

5. Hofmann, K. A. & Küspert, Z. Anorg. Allg. Chem. 15, 204-224 (1897)

Nature http://www.nature.com/nature

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