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ScienceWeek
PLANETARY SCIENCE: ON THE MOONS OF THE OUTER PLANETS
The following points are made by Peter R. Sammonds (Science 2006 311:1250):
1) On Earth, ice in glaciers and ice sheets can flow superplastically (that is, this ice can deform much more than the normal range for a given stress). This is possible because of time-dependent creep that is sensitive to the grain size of the ice [1]. This recent realization, although controversial [2], has deeply influenced glaciological thinking and has led to models that could better explain modern and ancient ice sheet behavior [3]. Now new work[4] reports that grain size-sensitive creep occurs in a high-pressure water ice that is a major constituent of the moons of the outer Solar System. This realization could change our understanding of the dynamics and evolution of these planetary bodies.
2) The dynamics of glaciers and ice sheets on Earth are largely controlled by grain-scale deformation of ice [1]. Naturally occurring ice on Earth, Ice I, creeps along at low stresses by solid-state viscous flow. Microscopically, what is happening is that line defects in the crystal lattice, called dislocations, glide within the ice grains as carriers of deformation, enabling the ice bulk to creep along in time. This mechanism of deformation is called dislocation creep. Because the deformation is occurring within individual ice grains, dislocation creep does not depend on the size of the ice grains; it is grain size-insensitive. However, Goldsby and Kohlstedt [1] showed in laboratory experiments that for very fine-grained Ice I deforming at very low stresses, ice deformation does depend on grain size; it is grain size-sensitive. Under these conditions, grain boundary sliding accommodates deformation as ice grains slide past each other. Ice deforming under these low stresses is orders of magnitude less viscous than at high stresses. They described this as superplasticity in ice. At still lower stresses, ice can deform by defects diffusing through the ice lattice and along grain boundaries; this is called diffusional flow, which is also grain size-sensitive.
3) On Earth, Ice I, as Poirier [5] pointed out, is a unique rock-forming mineral in that it is the only one that is so close to the solid-liquid-vapor triple point at the temperatures and pressures of Earth's surface. Ice I has a crystal lattice structure of stacked puckered hexagonal rings of oxygen ions with a disordered hydrogen ion (proton) sublattice. On moons of the outer Solar System such as Jupiter's Europa, Ganymede, and Callisto and Neptune's Triton, water ice and its high-pressure polymorphs are also important rock-forming minerals and major constituents of these moons. Although the outer skins of the icy moons would be the low-pressure Ice I, in the interior the high-pressure polymorphs could be present. At high pressures and low temperatures, Ice I transforms to Ice II, which has a rhombohedral crystal lattice structure of hexagonal tubes of oxygen ions with an ordered proton sublattice. Higher pressure ice polymorphs Ices V, VI, and VII may also be present in the interiors of the larger moons.
4) What Kubo et al[4] have now shown is that a high-pressure polymorph of ice, Ice II, also deforms by grain size-sensitive creep at low stresses. They have done this by fabricating fine-grained Ice II by transformation from Ice I in a cryogenic high-pressure cell and then deforming the ice in the cell to measure the flow stress at which it creeps. By cycling an Ice II sample back to Ice I and then transforming it to Ice II again, they created still finer grained Ice II, measured its flow stress, and so on. They found that triply transformed Ice II flows at less than half the stress of a single-transformation sample. The grain sizes of their singly, doubly, and triply transformed samples were revealed by scanning electron microscope (SEM) analyses of the indium metal sleeves that jacket the ice samples, as well as by direct cryogenic SEM analyses of Ice II grains partially decorated with Ice I. They found a correlation between the number of transformation cycles and the flow stresses, demonstrating that the rheology of Ice II is grain size-sensitive at low stresses.
References (abridged):
1. D. L. Goldsby, D. L. Kohlstedt, J. Geophys. Res. 106, 11017 (2001)
2. P. Duval, M. Montagnat, J. Geophys. Res. 107, 10.1029/2001JB000946 (2002)
3. W. R. Peltier, D. L. Goldsby, D. L. Kohlstedt, L. Tarasov, Ann. Glaciol. 30, 163 (2000)
4. T. Kubo, W. B. Durham, L. A. Stern, S. H. Kirby, Science 311, 1267 (2006)
5. J. P. Poirier, Nature 299, 683 (1982). [ADS]
Science http://www.sciencemag.org
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JUPITER'S MOONS
The following points are made by Douglas P. Hamilton (Nature 2003 423:235):
1) The preeminence of Jupiter as the planet with the largest number of natural satellites (moons) has been dramatically and decisively re-established by new observations. Fending off strong challenges from Saturn and Uranus, Jupiter, the Solar System's largest planet, now has nearly as many known moons as all of its competitors combined. Nearly two dozen new jovian moons have recently been discovered.
2) The search for planetary satellites has a long history, dating back to 1610 and the discovery by Galileo Galilei (1564-1642) of four star-like objects orbiting Jupiter -- Io, Europa, Ganymede and Callisto. Saturn's splendid ring system and its largest moon, haze-enshrouded Titan, were first seen by Christiaan Huygens (1629-1695) about 50 years later, and, in 1684, the discoveries of icy Dione and Tethys established Saturn as the planet with the most moons, a title it held for 230 years. Jupiter's Sinope, spotted in 1914, evened the score at nine known moons apiece, and two additional findings in 1938 allowed the giant planet to surge into the lead. Saturn staged a surprise comeback in 1980, when seven new satellites were spotted by the Voyager spacecraft and ground-based observers. Then came the great upset of 1999: dark horse Uranus revealed three additional outer satellites and vaulted to the forefront. But the title has since been reclaimed, first by Saturn, with a dozen new discoveries reported in 2000, and now by Jupiter, with the 23 new findings.
3) Currently, the number of known planetary moons stands at 128. More than half of this total has been added since 1997, when B. Gladman and colleagues found the first two distant, or "irregular", satellites of Uranus. The large number of satellite discoveries over the past six years, at an ever quickening pace, is reminiscent of the situation following Jewitt and Luu's 1992 discovery of the first transneptunian (or Kuiper belt) objects. Both population explosions have been fuelled by major improvements in digital-camera technology.
4) Nearly two-thirds of the known moons (including all of the recent discoveries) are irregular satellites, orbiting far from their planets along highly tilted, elliptical paths. These objects are believed to have been captured by their planets from independent orbits around the Sun early in the history of the Solar System. Regular satellites, by contrast, have much smaller, untilted, circular orbits, and were probably formed out of the disks of gas and dust that surrounded the giant planets in their youth. Energy dissipation in these early accretion disks also acted to facilitate the capture of the irregular satellites.
Nature http://www.nature.com/nature
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THE MOONS OF SATURN
The following points are made by B. Gladman et al (Nature 2001 412:163):
1) The giant planets in the Solar System each have two groups of satellites. The "regular satellites" move along nearly circular orbits in the planet's orbital plane, revolving about the planet in the same sense as the spin of the planet. In contrast, the so-called "irregular satellites" are generally smaller in size and are characterized by large orbits with significant eccentricity, inclination, or both. The differences in the characteristics of the two groups of satellites suggest that the regular and irregular satellites formed by different mechanisms: the regular satellites are believed to have formed in an accretion disk around the planet, like a miniature Solar System, whereas the irregular satellites are generally thought to be captured planetesimals.
2) The authors report the discovery of 12 irregular satellites of Saturn, along with the determinations of the orbits of these satellites. These orbits, along with the orbits of irregular satellites of Jupiter and Uranus, fall into groups on the basis of their orbital inclinations. The authors interpret this result as indicating that most of the irregular moons are collisional remnants of larger satellites that were fragmented after capture, rather than being captured independently.
Nature http://www.nature.com/nature
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