Element production in massive objects
- Also called:
- element
- Related Topics:
- rare-earth element
- isotope
- transition metal
- periodic table
- hydrogen
Although there is no direct evidence for the existence of stars more than about 50 times as massive as the Sun, there is no obvious reason why much more massive objects should not exist. If they were sufficiently massive, they would not behave as ordinary stars because their gravitational attraction would be so strong that not even the energy released by conversion of hydrogen into helium would prevent such supermassive stars from continuing to collapse rapidly. According to present theoretical ideas, if such a collapse is spherically symmetrical, nothing can prevent the supermassive object from collapsing to an extremely high or infinite density; but, if it is asymmetrical—because it is, for example, rapidly rotating—there is some possibility that the catastrophic collapse, called an implosion, might be followed by explosion. At the high-density, high-temperature phase of such an object, some nucleosynthesis (manufacture of nuclei from smaller nuclei) would occur, primarily of helium but with a small amount of heavier elements according to the arguments given early in this article. Such objects have been suggested as a possible important source of helium.
There is some observational evidence that explosions on a very much greater scale than single supernovae are occurring in galaxies. In some peculiar galaxies that are strong emitters of radio waves, there is evidence that explosions have thrown a large quantity of gas hundreds of thousands of light-years into intergalactic space. Such galactic explosions may not be related to the theoretical supermassive objects mentioned above, but it is difficult to believe that some nucleosynthesis does not take place during the phases of extreme conditions that must occur in such objects. The suggestion that heavy-element abundances may be higher near the centre of the Galaxy could be related to a past explosion there.
Radioactive chronologies
Radioactive elements in the Earth, the Moon, and in meteorites can provide useful information about the ages of these objects and about the dates of formation of the heavy elements themselves. The elements uranium and thorium gradually decay into lead, different isotopes of lead arising from the various isotopes of uranium and thorium; some isotopes of lead are, however, not produced by any radioactive decay process. When the rocks of the Moon or the Earth’s crust or the meteorites solidified, further chemical separation of the radioactive elements and their decay products was prevented. By studying the relative amounts of the radioactive isotopes and their decay products, it is possible to obtain an estimate of when the rocks solidified. Estimates can also be made using radioactive isotopes other than uranium and thorium.
The results of these discussions indicate that the meteorites, or at least the parent body of the meteorites, solidified between 4.5 × 109 and 4.6 × 109 years ago. It is possible to speak with such confidence of this age because two isotopes of uranium and one of thorium have very different decay times that bracket that value. There is no unique age for the rocks of the Earth’s crust because there has been considerable volcanic activity during the Earth’s history and rocks have solidified at all stages. All indications are that the oldest rocks have ages of the same order as the ages of the parent bodies of the meteorites. Only a very small region of the Moon’s surface has been studied so far, but it has been found to have very old rocks of age up to about 4.5 × 109 years. No conclusions can be drawn about the date of solidification of the Moon from these few observations, as nothing is known about its past geological history, but they are certainly not inconsistent with the view that the Earth, the Moon, and meteorites have a similar age and origin.
It has also been found possible to obtain information about the time of formation of the radioactive elements. Assuming that both radioactive nuclei and their stable neighbours are produced by the neutron-capture process discussed earlier, theory predicts a relative production rate for all of the nuclei. The radioactive nuclei can be divided into three groups: short-lived, medium-lived, and long-lived, where short-lived means considerably less than the believed age of the universe and long-lived means comparable with that age. If radioactive nuclei are produced and decay steadily, then at some point in time the total amount of a short-lived isotope reaches a steady value. In meteorites, one can study the decay products of such short-lived nuclei and can discover their abundance when the meteorites were formed. This amount is lower than the expected value, suggesting that nucleosynthesis ceased in the solar system material about 2 × 108 years before the meteorites and planets solidified.
Study of the decay products of nuclei with medium decay rates indicates that their abundance is higher than if nucleosynthesis has occurred at a constant rate throughout galactic history. This suggests that the solar system material was significantly enriched in heavy elements shortly before the cessation of nucleosynthesis—that is, before the Sun and planets were formed. Finally, the very long-lived isotopes give information about the total time scale of nucleosynthesis that is not inconsistent with the galactic age estimated by other methods.
Although there is not unanimous agreement concerning these results, it appears that it is, in principle, possible to obtain a considerable amount of information about the past rate of nucleosynthesis and possibly about the types of objects in which it has occurred. In particular, it may eventually be possible to decide whether most element production has occurred in a large number of supernovae or in a much smaller number of massive objects.
Roger John TaylerGeochemical distribution of the elements
Knowledge of the geochemical distribution of elements involves elucidation of the relative and absolute abundances of the chemical elements in the Earth and in its various parts—the crust, interior, atmosphere, and hydrosphere. This comprises a major part of the science of geochemistry, which is the study of the distribution of the chemical elements in space and time and the laws governing this distribution. Basic knowledge in this area was largely accumulated during the 19th century. As noted above, the concept of a limited number of chemical elements had been established by 1800, and the appearance of the periodic table, in 1869, provided a new insight into the limitations on the number of elements. Concurrent with these advances in chemical understanding, from about 1850 onward there was a steadily increasing output of analytical data on the Earth’s rocks, minerals, and waters, mainly from laboratories in Europe and North America. The output from North America was materially increased following the establishment of the United States Geological Survey in 1879 and the appointment of Frank W. Clarke as chief chemist in 1884.
Clarke’s name will always be linked with the study of the geochemical distribution of the elements—indeed, the term clarke was proposed as the unit for the average percentage of an element in the Earth’s crust by Soviet scientists and has been generally adopted. In 1889 Clarke wrote the first of his many publications on the geochemical distribution of the elements. He assembled many chemical analyses of rocks from different continents, calculated average values, and showed that the overall chemical compositions of continental areas are remarkably similar. By combining these averages he obtained values for the abundances of the commoner elements in the continental crust of the Earth, values that have not been materially changed in spite of the vast increase of available data since that time. He also estimated abundances for many of the less common elements; these estimates were based in many instances on very limited and imprecise data and subsequently have been improved.
A further development of great significance was the assemblage of comprehensive data on the abundances of individual elements in terrestrial materials and in the Cosmos (based on solar and meteorite abundances) by the Norwegian geochemist Victor Moritz Goldschmidt during the 1930s. Goldschmidt’s tables provided the basis for modern research on the geochemical distribution of the elements, and his compilation of data on cosmic abundances was the key to later theories on element synthesis in stars and supernovae.
Goldschmidt also contributed to the understanding of elemental distribution within the Earth through his geochemical classification of the elements into lithophile, siderophile, chalcophile, and atmophile. Lithophile elements are those with a strong affinity for oxygen; they are concentrated in the crust or lithosphere as silicate and oxide minerals. Siderophile elements are principally metals that alloy readily with iron; Goldschmidt explained their scarcity in the Earth’s crust by their concentration in the nickel–iron core (the siderosphere). Chalcophile elements are those with strong affinity for sulfur; they occur mainly as sulfides. And atmophile elements are gases, such as nitrogen, argon, and other rare gases, which are unreactive and hence accumulate in the atmosphere. (Goldschmidt also proposed a group of biophile elements, for those that concentrate in living matter—essentially carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus.)
Terrestrial distribution
The study of earthquake waves passing through the body of the Earth has shown that the interior is not uniform; it consists of distinct shells separated by concentric discontinuities at which the velocities of the passing waves change. The two major discontinuities that are universally recognized are the Mohorovičić Discontinuity, which divides the Earth’s crust from its underlying mantle, and the Wiechert–Gutenberg Discontinuity, which separates the mantle from the core. The latter discontinuity exists at a depth of 2,900 kilometres (1,800 miles); it is marked by a sudden increase in density, from about 5.7 at the base of the mantle to 9.7 at the top of the core. The only reasonable interpretation of this discontinuity is that the mantle consists of silicates and oxides of the common elements (largely magnesium and iron), and the core consists of metallic iron alloyed with minor amounts of other elements (analogous to the nickel-iron in meteorites). The Mohorovičić Discontinuity varies in depth from place to place; it averages about 33 kilometres (20 miles) below the continents and about 8 kilometres (5 miles) below the bottom of the deep oceans. It too is marked by a density increase from crust to mantle—a comparatively small one, from about 3 to 3.3.
To the three spherical divisions—crust, mantle, and core—two more should be added: the hydrosphere, which is the discontinuous shell of fresh and salt water, on and within the crust; and the atmosphere, the ocean of air that surrounds the Earth, gradually thinning into the vacuum of outer space.
The Earth’s core
The evidence for the composition of the core is all indirect because no means have yet been devised for directly sampling the deep interior of the Earth. The moment of inertia of the Earth indicates that there is a concentration of mass around the centre, and seismic data have shown that below the Wiechert–Gutenberg Discontinuity the density of the material is high, ranging upwards from 9.7. The only heavy element with high cosmic abundance is iron, and because an iron–nickel alloy is an important meteorite component, it is reasonable to conclude that the Earth’s core consists largely of metallic iron with a minor admixture of other elements. This conclusion is supported by geophysical evidence that indicates that the mean atomic number of the material of the core is about 22. The atomic number of iron is 26, so this implies that the core also contains elements of lower atomic number. Sulfur, with atomic number 16, and carbon, 6, are relatively abundant in meteoritic matter, and the presence of minor amounts of these elements in the core would effectively reduce the mean atomic number. Some authorities have advocated silicon (atomic number 14) as the major alloying component in the core, but this seems less likely; if silicon were the sole alloying element, then the core would have to contain more than 30 percent silicon in order to reduce its mean atomic number to 22. In addition, free silicon requires extremely reducing conditions (lack of oxygen), and the presence of ferrous iron in the mantle is inconsistent with this requirement.
It is not possible to give definite figures for the abundances of the elements in the Earth’s core. It is certainly made up largely of metallic iron, however, probably with some nickel, a little cobalt, and appreciable amounts of such lighter elements as carbon and sulfur.
