ScienceWeek

SCIENCEWEEK

August 18, 2006

Vol. 10 - Number 33

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Back issues of ScienceWeek can be searched for subjects, names, terms, etc. at: http://scienceweek.com/swfr.htm

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All these fifty years of conscious brooding have brought me no nearer to the answer to the question, `what are light quanta?' Nowadays every Tom, Dick, and Harry thinks he knows it, but he is mistaken.

-- Albert Einstein (1879-1955)

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Contents:

1. Planetary Science: On Solving Laplace's Lunar Puzzle

Approximately two centuries ago, the eminent mathematician Pierre-Simon Laplace (1749-1827) noted something unusual about the Moon's gravity. With a mass ratio of about 80:1, Earth and its Moon are unique in the solar system and are sometimes referred to as a double planet. As the Moon revolves around Earth in about a month, this double planet itself revolves...

2. Climate: Aerosols, Cloudiness, and Climate

What is the net impact of anthropogenic aerosol emissions on Earth's climate? Is it similar in magnitude to that of greenhouse gases? Do aerosols mostly affect the amount of solar radiation reflected back into space, or do they also have a substantial effect on the hydrological cycle? Many recent studies have tried to answer these questions, but the picture gets ever more...

3. Evolutionary biology: On Building a Longer Beak

A classic illustration of nature's ability to generate morphological diversity comes from the finches that inhabit the Galapagos Islands. The beak shapes of these finches are remarkably diverse, and -- as described in new work --researchers have uncovered one of the mechanisms involved in achieving this. They have compared beak development of...

4. Chemical biology: On Small-Molecule Modulators of Proteins

One of the most exciting things about chemical biology is its potential to develop new tools for probing cellular processes. Small "drug-like" molecules that can diffuse into cells and quickly elicit a discernible response offer distinct advantages over genetics-based approaches for exploring the highly choreographed inner workings of a cell. In particular...

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Also Noted:

The Language of God: A Scientist Presents Evidence for Belief. Francis S. Collins. Free Press (Simon and Schuster), New York, 2006. Hardback: 303 pp. $26, C$32.95. ISBN 0743286391. More information at: http://www.amazon.com/exec/obidos/ASIN/0743286391/scienceweek

Many Worlds in One: The Search for Other Universes. Alex Vilenkin. Hill and Wang (Farrar, Straus and Giroux), New York, 2006. Hardback: 243 pp., illus. $24, C$32.50. ISBN 0809095238. More information at: http://www.amazon.com/exec/obidos/ASIN/0809095238/scienceweek

A Mind of Its Own: How Your Brain Distorts and Deceives. Cordelia Fine. Norton, New York, 2006. Hardback: 255 pp., illus. $24.95, C$34.50. ISBN 0393062139. More information at: http://www.amazon.com/exec/obidos/ASIN/0393062139/scienceweek

Science, Culture, and Modern State Formation. Patrick Carroll. University of California Press, Berkeley, 2006. Hardback: 289 pp., illus. $45, £29.95. ISBN 0520247531. More information at: http://www.amazon.com/exec/obidos/ASIN/0520247531/scienceweek

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1. PLANETARY SCIENCE: ON SOLVING LAPLACE'S LUNAR PUZZLE

The following points are made by Kimmo Innanen (Science 2006 313:622):

1) Approximately two centuries ago, the eminent mathematician Pierre-Simon Laplace (1749-1827) noted something unusual about the Moon's gravity (1). With a mass ratio of about 80:1, Earth and its Moon are unique in the solar system and are sometimes referred to as a double planet. As the Moon revolves around Earth in about a month, this double planet itself revolves around the Sun in a year. Both orbits are slightly elliptical, with the lunar orbit tilted a few degrees to Earth's solar orbit. The result is a three-dimensional example of what is called the gravitational three-body problem. Adding to the complexity is that both bodies are pear-shaped, with the Moon locked into a synchronous orbit with one face toward Earth. One can thus envision this choreography: a pair of slightly flexible (one covered with oceans), spinning, bulgy gyroscopes in mutual gravitational motion, with the pair in concurrent motion around the Sun. Laplace's problem was that he could not reconcile the observed orbital properties of the Moon with its shape and expected motion. Very simply, there is excess bulge material in the Moon's equatorial region. New work (2) presents an ingenious method to fill a gap in our knowledge of the earlier history of the Moon's orbit without using a full computer simulation of the entire complex system. The results now offer a credible solution to Laplace's problem.

2) Following Newton's exact solution of the two-body problem (in the form of ellipses, with the center of gravity at one focus), the search for a complete analytical solution to the general three-body problem preoccupied and frustrated the best mathematical minds for more than three centuries. Newton himself is said to have suffered severe headaches in his own attempts to provide a more complete theory of the Moon's motion. Two of the most famous mathematicians of the 18th and early 19th centuries were Leonhard Euler (1707-1773) and Laplace. Euler made fundamental contributions (now known as Euler's equations) to the dynamics of solid and fluid bodies, introducing the concept of the three basic moments of inertia A, B, and C of symmetrical bodies with equatorial bulges. The moments of inertia capture the rotational properties of a body much the way its mass quantifies its inertia for motion in a straight line. Euler also applied his results in attempts to understand the Moon's motion. Laplace used Euler's work and his own mathematical skills to identify the most important secular perturbation terms and thereby to show that the lunar moments cannot be in equilibrium with its present orbit.

3) In the solar system, many of the mutual interactions (perturbations) between the planets, including their satellites, are of short term (say hundreds or thousands of years) and average to zero. Secular perturbations do not average out to zero, but accumulate significantly over much longer times, over, say, hundreds of thousands or millions of years. Something interesting must therefore have happened during the Moon's early history to "lock in" this disequilibrium, perhaps as it cooled and solidified, when it was much closer to Earth. The theoretical work on the Moon's orbit culminated in the late 19th and early 20th century with the analyses of Delaunay, Hill, and Brown (3). Their work used the perturbation method: Beginning with the elliptical orbit as a reference, they added perturbing terms to take account of the other physical influences affecting the system. The result was a list of more than 300 perturbing terms, each with its own periodic effect. (It is a testament to the care of Delaunay, Hill, and Brown that only a handful of modest errors have been discovered in this work by means of modern computer algebraic methods that have added many more terms.) Nevertheless, this huge list was enough to discourage later generations of mathematicians and astronomers from studying the problem.

4) Modern computer analyses of the general three-body problem (4, 5) have shown its incredible complexity, so that statistical approaches become viable. There are also additional important longer-period complexities in the problem: To conserve angular momentum (which is not affected by friction), the frictional energy losses due to Earth's ocean tides sloshing near the coastal shorelines cause the Earth's spin to slow down and the Earth-Moon distance to increase by 3.8 cm each year, to a present distance of some 60 Earth radii. (In comparison, geosynchronous communication satellites revolve around Earth at the same rate as it spins, at a distance of about 6 Earth radii.) This very slow lunar recession is known from two sources: the timing and location of ancient solar eclipses, and from accurate measurement of the Earth-Moon distance with lasers on Earth and reflectors left on the Moon's surface by the Apollo astronauts. It is therefore possible to run the Moon's position backward in time. In addition, the moments of inertia of both the Moon and Earth have been accurately determined from artificial satellite motions.

5) In the work of Garrick-Bethell et al (2), the central issue is the Moon's own nonspherical shape, which, together with its orbit, lead the authors to an interesting conclusion about its past history: Its orbit around Earth in the distant past must have been much closer and also more eccentric than it is now. In fact, their optimum solutions locate the young Moon at a time 100 to 200 million years after its formation, when it was at a distance of some 24 to 27 Earth radii. At this time it would have passed through the 3:2 spin-orbit resonance, reminiscent of the present-day behavior of the planet Mercury, which rotates three times about its own axis for every two revolutions about the Sun. The authors show that the distance and eccentricity at this time would have been optimal for the bulge to "freeze" into the solidifying Moon, a fossil bulge we observe to this day. These results appear to dovetail in a reasonable way with the most viable contemporary theory of the Moon's own origin through a giant impact of a Mars-like object with Earth, from which debris the primordial Moon formed at some 4 Earth radii.

References (abridged):

1. P.-S. Laplace, Traité de Mécanique Céleste (Duprat, Courrier, and Bachelier, Paris, 1798-1827), vol. 2, book 5, chap. 2.

2. I. Garrick-Bethell, J. Wisdom, M. T. Zuber, Science 313, 652 (2006)

3. C. D. Murray, S. F. Dermott, Solar System Dynamics (Cambridge Univ. Press, Cambridge, UK, 1999)

4. M. Valtonen, H. Karttunen, The Three-Body Problem, (Cambridge Univ. Press, Cambridge, UK, 2006)

5. C. D. Murray, in Chaotic Motion in the Solar System in Encyclopedia of the Solar System, T. Johnson, P. Weissman, L. McFadden, Eds. (Academic Press, Orlando, FL, 1998)

Science http://www.sciencemag.org

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2. CLIMATE: AEROSOLS, CLOUDINESS, AND CLIMATE

The following points are made by François-Marie Bréon (Science 2006 313:623):

1) What is the net impact of anthropogenic aerosol emissions on Earth's climate? Is it similar in magnitude to that of greenhouse gases? Do aerosols mostly affect the amount of solar radiation reflected back into space, or do they also have a substantial effect on the hydrological cycle? Many recent studies have tried to answer these questions, but the picture gets ever more complex. New work (1) brings some clarity to the issue by quantifying the positive impact of aerosol load on the cloud cover and demonstrating the opposite effect in the presence of absorbing aerosols.

2) Aerosols were long thought to affect climate mainly by reflecting incoming solar radiation back to space, thus cooling Earth's surface. But in the late 1990s, it was shown that some aerosols can absorb substantial amounts of solar energy, thereby increasing solar heating, particularly when aerosol layers are located above cloud decks. Even worse news for attempts to quantify the effect of aerosols on climate came with the identification of the so-called indirect effects, through which aerosols change the optical properties and the life cycle of clouds.

3) In the first indirect effect, the presence of aerosols leads to the formation of more numerous and smaller cloud droplets (2, 3), resulting in brighter clouds that reflect more solar energy back into space. This reduction in cloud droplet size tends to reduce precipitation and, together with other aerosol-cloud processes, changes the geographical extent of cloudiness (the second indirect effect) (4). Both processes lead to an increase in the solar energy reflected back to space, and thus a net cooling of climate.

4) However, the opposite effect has been observed above the Indian Ocean (5) and over the Amazon Basin: Aerosols that can absorb substantial amounts of solar energy -- particularly black carbon, which is produced by incomplete combustion -- tend to warm the atmosphere and inhibit cloud formation. Aerosols may also alter convective cloud dynamics. The net impact of aerosols on cloud cover may thus depend not only on local atmospheric conditions, but also on the magnitude of the aerosol absorption.

5) To address these issues, Kaufman and Koren (1) performed a statistical analysis of ground-based remote sensing data from around the globe. The analysis confirms a positive and robust correlation between aerosol load and cloud cover, with a mean slope that gets smaller as aerosol absorption increases. On the basis of these results, the authors estimate that anthropogenic aerosols increase the global cloud cover by 5%. Assuming a typical cloud albedo of 0.5, this corresponds to an increase of the reflected solar flux by 5 W m^(-2) -- a forcing on climate that is larger than, and of opposite sign to, that of greenhouse gases. These estimates are based on global average cloud reflectance, aerosol load, and absorption. Furthermore, the statistical analysis is limited to specific cloud cover conditions. Many regions have overcast skies or a meteorology that is not suitable for the formation of clouds, even in the presence of large aerosol loads. The figures above are therefore probably overestimates, and considerable uncertainties remain.

References (abridged):

1. Y. J. Kaufman, I. Koren, Science 313, 655 (2006)

2. F.-M. Bréon, D. Tanré, S. Generoso, Science 295, [834] (2002)

3. J. E. Penner, X. Q. Dong, Y. Chen, Nature 427, 231 (2004)

4. D. Rosenfeld, Science 287, [1793] (2000)

5. A. S. Ackerman et al., Science 288, [1042] (2000)

Science http://www.sciencemag.org

ScienceWeek http://scienceweek.com

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3. EVOLUTIONARY BIOLOGY: ON BUILDING A LONGER BEAK

The following points are made by Nipam H. Patel (Nature 2006 442:515):

1) A classic illustration of nature's ability to generate morphological diversity comes from the finches that inhabit the Galapagos Islands. The beak shapes of these finches are remarkably diverse, and -- as described in new work (1) --researchers have uncovered one of the mechanisms involved in achieving this. They have compared beak development of several finch species at the molecular level. In combination with experimental analyses in chickens, they show that changes in calcium-dependent molecular signalling during development are involved in the evolution of beak shape.

2) When Darwin visited the Galapagos Islands, he collected several birds that are now known as Darwin's finches. There are roughly a dozen species of them, and we recognize today that they are closely related to one another, despite their remarkable differences in beak shape and size, and in eating habits. Indeed, they are so varied that Darwin did not immediately realize that they were all finches, and did not initially use them to support his emerging theories of evolution.

3) Subsequently, however, the finches have figured prominently in our understanding of the mechanisms of evolution. In particular, over the past 30 years or more, Peter and Rosemary Grant have shown that variation in the finches is driven by ecological forces, and that the birds' adaptive radiation -- their rapid speciation from a common ancestor to fill many ecological niches -- has occurred in just the past few million years. The precise dimensions (length, depth and width) of each species' beak are crucial to their lifestyle and survival (2), and fluctuations in the environment lead to selection that changes the relative success of birds with various beak shapes. Indeed, these evolutionary processes are evident in real time on the Galapagos Islands (3).

4) An initial insight into the molecular and genetic underpinning of the different beak morphologies came from Abzhanov et al two years ago (4). They focused on genes that regulate the skeletal and cartilaginous development of the face in laboratory species such as mice and chickens. Using this "candidate gene approach", they showed that a gene encoding a cell-cell signalling protein, known as bone morphogenetic protein 4 (BMP4), was more broadly expressed during the embryonic development of the deep and wide beaks of ground finches than during the development of finches with narrower beaks. To go beyond correlation, they then showed that broadening the expression of the gene encoding BMP4 in chicken embryos resulted in chickens with wider beaks.{5}

References:

1. Abzhanov, A. et al. Nature 442, 563-567 (2006)

2. Grant, P. R. The Ecology and Evolution of Darwin's Finches (Princeton Univ. Press, 1999)

3. Grant, P. R. & Grant, B. R. Science 313, 224-226 (2006)

4. Abzhanov, A. et al. Science 305, 1462-1465 (2004)

5. Doebley, J. , Stec, A. & Hubbard, L. Nature 386, 443-445 (1997)

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

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4. CHEMICAL BIOLOGY: ON SMALL-MOLECULE MODULATORS OF PROTEINS

The following points are made by Tom W. Muir (Nature 2006 442:517):

1) One of the most exciting things about chemical biology is its potential to develop new tools for probing cellular processes. Small "drug-like" molecules that can diffuse into cells and quickly elicit a discernible response offer distinct advantages over genetics-based approaches for exploring the highly choreographed inner workings of a cell (1). In particular, small molecules can allow cellular processes to be rapidly perturbed in a reversible and often tunable fashion, allowing the dynamic features to be teased out. But finding a small-molecule modulator that is specific for one protein is a formidable challenge; finding one molecule that could somehow modulate the function of any desired protein seems impossible. But new work (2) describes the results of a study, based on the pharmacological regulation of protein splicing, that suggest this might not be a complete pipedream.

2) Protein splicing is one of the most dramatic protein modifications known (3). It is a self-catalyzed process in which an internal protein domain, known as an intein, removes itself from a host protein with concomitant linking together of the flanking polypeptides, the exteins. Some inteins are remarkably promiscuous with respect to the extein sequences within which they sit. Indeed, inteins have found widespread applications in protein engineering as a way to introduce biochemical and biophysical probes into proteins (4).

3) Protein splicing results in a new polypeptide sequence being produced at the site of intein excision. Because a protein's function is intimately linked to its sequence, splicing has the potential to regulate the activity of the host protein. With this in mind, conditional inteins have been reported whose splicing activity is triggered by changes in temperature (5) or by the application of small molecules. The appeal of these systems is that inducible inteins can be dropped into target proteins with standard molecular-biology techniques.

4) The work of Yuen et al (2) builds on a previous study from the same group, in which directed protein evolution was used to develop a controllable protein-splicing element that could be activated by the addition of a ligand -- a molecule that binds to the protein. The controllable intein was based on a hybrid formed between the ligand-binding domain of the estrogen receptor (ER-LBD) and the Mycobacterium tuberculosis RecA intein. Addition of the high-affinity ER-LBD ligand 4-hydroxytamoxifen (4HT) led to dose-dependent, rapid splicing of the engineered intein out of a series of model proteins. This work was performed in yeast, so it was unclear whether the technology would work in a mammalian cell, or whether it could be used to control an actual biological pro-cess. These questions are addressed in the new study (2).

References (abridged):

1. Shogren-Knaak, M. A. , Alaimo, P. J. & Shokat, K. M. Annu. Rev. Cell Dev. Biol. 17, 405-433 (2001)

2. Yuen, C. M. , Rodda, S. J. , Vokes, S. A. , McMahon, A. P. & Liu, D. R. J. Am. Chem. Soc. 128, 8939-8946 (2006)

3. Paulus, H. Annu. Rev. Biochem. 69, 447-496 (2000)

4. Muralidharan, V. & Muir, T. W. Nature Meth. 3, 429-438 (2006)

5. Zeidler, M. P. et al. Nature Biotechnol. 22, 871-876 (2004)

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

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