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ScienceWeek
COSMOCHEMISTRY: ISOTOPE COMPOSITION OF THE SOLAR SYSTEM
The following points are made by Alex N. Halliday (Nature 2003 424:137):
1) Long before astronomers presented us with spectacular images of the material swirling around other stars, Pierre-Simon Laplace (1749–1827) proposed that the planets of our Solar System formed from a circumstellar disk. The scale and degree of isotopic heterogeneity of the matter in our Solar System provide unique insights into the dynamics associated with the development of these disks.
2) Modern mass spectrometry techniques have revealed that the isotopic compositions of many of the more refractory elements in meteorites, including a primitive class of meteorite called chondrites, are, within error, identical to those found on Earth itself(2). It was unclear how the Solar System could have such a uniform composition without some kind of earlier mixing process, because the mechanism that produces these elements creates huge isotopic heterogeneity. The elements heavier than hydrogen and helium ("metals" to astronomers) are manufactured by fusion and irradiation in stars, in a process known as nucleosynthesis. Different kinds of mechanisms produce distinct isotopes, depending on the mass of the star, its stage of development and its metal composition. The most likely way to produce a uniform mix of atoms from the many stars that fed the molecular cloud that eventually formed our Solar System was for the building-blocks of the Earth and planets to have condensed from a hot, well-mixed gas. This idea led to a series of "hot nebula" models, but various lines of evidence have since shown that such models can only apply to localized portions of the Solar System.
3) The most important evidence has stemmed from the discovery of presolar grains(3). A variety of grains condense in stellar envelopes and so record the extreme isotopic composition generated in the star itself. These have been discovered in chondrites -- which are now viewed as having formed from relatively cold dust and debris in the circumstellar disk. The development of techniques for measuring the isotopic compositions of these submicrometer specks of stardust has produced data even for trace elements such as molybdenum and zirconium(4). These remarkable samples of other stars have isotopic compositions that are completely unlike that of our Solar System. But they do match the compositions predicted from theory for various kinds of stars(3,4).
4) Some types of presolar grain could not have survived the high temperatures and chemistry advocated in the hot nebula model. Therefore, this stardust must have been introduced to a cold disk. There is no obvious mechanism for large-scale mixing in the molecular cloud that preceded disk formation, so such heterogeneity was probably eliminated by mixing in the disk itself, otherwise we could not explain the isotopic uniformity of the Solar System.
5) Astronomers have found evidence that disks act like swirling conveyor belts, accreting gas and dust onto the central star(5). Most of the disk consists of hydrogen and helium, and its drag on the dust may result in lateral and radial mixing that eliminates variations. Precise isotopic measurements could potentially allow small variations to be resolved and provide a map of mixing in the disk at the start of our Solar System. Now, with improvements in mass spectrometric techniques, geochemists have indeed begun to report very small isotopic differences between bulk meteorites and planets in refractory elements that should have been predominantly inherited from the dust, such as chromium, zirconium and molybdenum.(1)
References (abridged):
1. Becker, H. & Walker, R. J. Nature 425, 152-155 (2003)
2. Suess, E. Rev. Astron. Astrophys. 3, 217-234 (1965)
3. Nittler, L. R. Earth Planet. Sci. Lett. 209, 259-273 (2003)
4. Nicolussi, G. K. et al. Astrophys. J. 504, 492-499 (1998)
5. Hartmann, L. Space Sci. Rev. 92, 55-68 (2000)
Nature http://www.nature.com/nature
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ON THE ORIGIN OF THE SOLAR SYSTEM
During the past two centuries, astronomers have considered two types of theories for the origin of our Solar System planets. Catastrophic theories proposed that these planets formed from some improbable cataclysm such as the collision of the sun and another star, while gradualist theories proposed that the planets formed naturally with the Sun. At the present time, as a result of evidence accumulated during the past five decades, the gradualist idea is the consensus idea, and nearly all astronomers now believe that planets form naturally as a by-product of star formation.
The following points are made by John A. Wood (Sky and Telescope 1999 January):
1) The current theory is that the Sun and the planets were born from a rotating disk of cosmic gas and dust (the "solar nebula"), and the flattened form of the disk constrained the planets that formed from it to have orbits lying in the same plane, or nearly so, the planets all moving in the same direction in which the disk had turned.
2) The idea of a solar nebula was first formulated in 1755 by *Immanuel Kant. Although his treatment of the problem was only qualitative, its precepts were remarkably similar to those considered fundamental today, and at the present time, Kant's original idea is considered to be correct: stars and their disks form in much the same way he pictured, the formation resulting from the gravitational collapse of huge volumes of thinly dispersed interstellar gas and dust onto appropriate nuclei.
3) The present view is that the solar nebula was hot near its center, tapering off to a cold region, then a very cold region at its outermost margins. Thus, the falloff of nebula temperature with heliocentric distance defined 3 radial zones. The innermost zone was too warm for water to condense as ice; objects forming in the innermost zone consisted entirely of *silicate minerals and other *refractory materials, and ultimately became the terrestrial planets (Mercury, Venus, Earth, and Mars). The next zone of the solar nebula was colder, water ice was stable, and a vast blizzard of snowflakes gave rise to the much larger Jovian planets (Jupiter, Saturn, Uranus, and Neptune). In the outermost and thus coldest zone of the solar nebula, condensed matter was also icy, but matter was too sparsely distributed to accrete into sizable planets; instead matter remained dispersed in small icy planetesimals -- comet nuclei -- in what is now called the *Kuiper belt. Evidence suggests the planets assembled themselves quickly: Although the process differed in detail from zone to zone, virtually everything was in place within 10 million years, by which time the solar nebula had largely dissipated.
4) Nearly four centuries of telescopic observation, combined with four decades of space exploration, have taught us this essential truth about the Solar System: While the Sun and its planetary system surely arose from one grand spiral of gas and dust in a flurry of collective activity, the results are hardly a homogeneous set of characterless orbiting entities. Instead this grand scheme of formation has yielded amazing diversity in the properties of the various objects in the Solar System.
Sky and Telescope http://www.skyandtelescope.com
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Notes:
Immanuel Kant (1724-1804): Kant is best known as a philosopher, but he first studied mathematics and physics, and the year he obtained his doctorate degree (1755) he published his physical view of the Universe in *General History of Nature and Theory of the Heavens). In this treatise, Kant described the solar nebula hypothesis of planet formation, suggested that our own galaxy is a lens-shaped collection of stars and that other such "island universes" exist, and suggested that *tidal friction slows the rotation of the Earth. All three propositions are the current view in astrophysics.
tidal friction: A force between the oceans of the Earth and the ocean floors caused by the gravitational attraction of the Moon.
silicate minerals: (silicates) The most important and abundant group of rock-forming minerals.
refractory materials: (refractory minerals) Minerals resistant to decomposition by heat, pressure, or chemical attack. The term is most commonly applied to heat resistance.
Kuiper belt: In 1951 the astronomer Gerard P. Kuiper (1905-1973) postulated the existence of a belt of objects beyond the orbit of Pluto. Both the existence and nature of the objects were matters of speculation for decades, and finally in 1992 Jewitt and Luu identified the first Kuiper object. The current estimate is that as many as 10^(8) objects larger than 10 kilometers in diameter may exist in what is called the "Kuiper belt", a disc that hugs the plane of the planetary system and lies between 35 and 1000 AU from the Sun. Observations to date have yielded some 55 trans-Neptune bodies with radii on the order of 100 km or larger, and Pluto is considered by some astronomers to be a member of this population.
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