Cosmochemistry

(redirected from cosmochemical)
Also found in: Dictionary, Wikipedia.

cosmochemistry

[¦käz·mō¦kem·ə·strē]
(astrophysics)
The science of the chemistry of the universe, particularly that beyond earth, concerned primarily with inferences on pre-solar-system events, solar nebular processes, and early planetary processes as deduced from minerals in meteorites and from chemical and isotopic compositions of meteorites and their parts.

Cosmochemistry

 

the science of the chemical composition of cosmic bodies, the laws of abundance and distribution of chemical elements throughout the universe, and the processes of the combination and migration of atoms during the formation of cosmic matter. Geochemistry is the best studied branch of cosmochemistry.

Cosmochemistry primarily investigates the “cold” processes on the atomic-molecular level of the interaction of substances, while physics primarily studies the “hot” nuclear processes in space, for example, the plasma state of matter and the formation of chemical elements within stars. Cosmochemistry, a new field of knowledge, has developed rapidly since the second half of the 20th century owing to the advancements in space exploration. Early investigations of chemical processes in space and of the composition of cosmic bodies were primarily conducted using spectral analysis of solar and stellar radiation and, on occasion, through the spectral analysis of the outer atmospheric layers of planets. This method led to the discovery of the element helium on the sun even before it was found to exist on the earth. The only direct method of studying cosmic bodies was the analysis of the chemical and phase compositions of various meteorites that had fallen onto the earth.

In this way, considerable data were accumulated having fundamental significance for further development of cosmochemistry. The advances in astronautics, the flights of unmanned space probes to the moon and the planets (Mars, Venus), and, finally, man’s landing on the moon have created new possibilities in Cosmochemistry. These include the direct study of lunar rock samples by astronauts or the gathering of soil samples with automatic (mobile or stationary) devices and their return to the earth for further study in chemistry laboratories. Furthermore, automatic landing vehicles have made it possible to study matter and the conditions determining its presence in the atmosphere and on the surface of other planets, Mars and Venus in particular.

One of the most important tasks of Cosmochemistry is to study the evolution of cosmic bodies on the basis of the composition and abundance of chemical elements and to explain the origin and history of these bodies from a chemical point of view. Attention is focused on problems involving the abundance and distribution of chemical elements. The abundance of chemical elements in space is determined by their formation in the interior of stars. The chemical composition of the sun, of the terrestrial planets of the solar system, and of meteorites appears to be almost identical. The formation of nuclei of chemical elements is connected with the various nuclear processes within the stars. Therefore, at various stages in their evolution different stars and star systems do not have identical chemical compositions. Stars with particularly strong spectral lines of Ba, Mg, Li, or other elements are known to exist.

The distribution of chemical elements in different phases in cosmic processes is highly varied. The following factors affect in various ways the state of aggregation and state of phase of a substance in space at various stages of its transformation: (1) the vast temperature range, from stellar to absolute zero; (2) the vast range of pressures, from millions of atmospheres under planetary and stellar conditions to a cosmic vacuum; (3) highly penetrating galactic and solar radiation of varying composition and intensity; (4) radiation accompanying the transformation of unstable atoms into stable ones; and (5) magnetic, gravitational, and other physical fields. It has been established that all these factors affect the composition of the material in the planets’ inner crust, of the planets’ gaseous envelopes, of the meteorite material, and of cosmic dust. In this case, the fractionation of matter in space affects both the atomic and isotopic compositions. The determination of isotopic equilibriums, which arise in the presence of radiation, makes it possible to investigate thoroughly the history of formation of planets, asteroids, and meteorites and to establish the age of the processes.

Owing to the extreme conditions in space, there exist processes and states of matter that are not encountered on the earth. These include (1) the plasma state of stellar material (for example, the sun); (2) the condensation of He, H2, CH4, NH3, and other highly volatile gases in the atmospheres of the planets at very low temperatures; (3) the formation of noncorrosive iron in a cosmic vacuum during explosions on the moon; (4) the chondritic structure of matter in stony meteorites; and (5) the formation of complex organic matter in meteorites and, probably, on planet surfaces (for example, Mars). Small concentrations of atoms and molecules of many elements, as well as minerals (quartz, silicates, graphite), are found to exist in interstellar space. Further-more, the synthesis of various complex organic compounds (arising from primary solar gases H, CO, NH3, O2, N2, S, and other simple compounds under equilibrium conditions in the presence of radiation) takes place. All these organic substances in meteorites and interstellar space are optically inactive.

With the development of astrophysics and a number of other sciences, the possibilities of obtaining information relating to cosmochemistry were greatly expanded. For example, radio astronomy is employed to search for molecules in the interstellar medium. More than 20 types of molecules had been discovered in interstellar space by the end of 1972, including several rather complex organic molecules containing up to seven atoms each. It was established that their observed concentrations were 10 to 100 million times smaller than the concentration of hydrogen. Radio astronomy also permits the investigation of the isotopic composition of interstellar gas by comparing the microwave lines of the isotopic varieties of one molecule (for example, H212CO and H213CO) and the verification of the validity of existing theories of the origin of chemical elements.

Of great importance in the study of the chemistry of space is the investigation of the complex multistage condensation of matter in the low-temperature plasma, for example, the transition of solar material into the solids of the planets, asteroids, and meteorites, accompanied by condensation growth, accretion (increase in mass, the “growth” of any substance by the external addition of particles, for example, from a gas-dust cloud), and the agglomeration of primary aggregates (phases) accompanied by loss of volatiles into the cosmic vacuum. In a cosmic vacuum, at relatively low temperatures (5000°-10,000°C), solid phases of varying chemical composition (depending on temperature), characterized by different bonding energies, oxidation potentials, and other factors, are successively precipitated from the cooling plasma. For example, silicate, metallic, sulfide, chromite, phosphide, carbide, and other phases are differentiated in chondrites; these phases agglomerate at some point in their history into a stony meteorite and, probably, in the same manner into the matter making up the terrestrial planets.

Furthermore, the planets undergo a differentiation of solid, cooled matter into shells—metallic core, silicate phases (mantle and crust), and atmosphere. This differentiation is the result of the secondary warming up of the planetary material by radiogenic heat emitted during the decay of radioactive isotopes of potassium, uranium, thorium, and possibly other elements. Such a process of melting and degassing of matter during volcanism is characteristic of the moon, the earth, Mars, and Venus. It is based on the universal principle of zone melting, which separates the low-melting material (for example, the crust and atmosphere) from the high-melting material of planetary mantles. For example, primary solar matter has the ratio Si/Mg ≈ 1, while for the planetary crust produced by melting of the mantle the ratio is Si/Mg ≈ 6.5. The preservation and nature of outer planetary shells depend primarily on the mass of the planets and the distances of the planets from the sun (for example, the lowdensity atmosphere of Mars and the high-density atmosphere of Venus). Owing to the proximity of Venus to the sun, a “greenhouse” effect has resulted from the CO2 in the Venusian atmosphere: at temperatures higher than 300°C, the process CaCO3 + SiO2 → CaSiO3 + CO2 reaches a state of equilibrium during which a 97 percent CO 2 content at a pressure of 90 atm is observed. Using the moon as an example, secondary (volcanic) gases cannot be retained by a celestial body of small mass.

Collisions in cosmic space (either among particles of meteoritic material or during the fall of meteorites and other particles onto planet surfaces) caused by enormous velocities can create a thermal explosion, which leaves traces in the structure of solid cosmic bodies and forms meteorite craters. Exchange of matter occurs between cosmic bodies. For example, according to a minimum estimate no less than 1 X 104 tons of cosmic dust whose composition is known is precipitated onto the earth annually. Among the stony meteorites that fall to the earth are the basalt achondrites, which are similar in composition to lunar rocks and the basalt on the earth’s surface (Si/Mg ≈ 6.5). In view of this, it has been hypothesized that the source of these achondrites is the moon (surface rocks of its crust).

These and other processes in space are accompanied by the irradiation of matter (high-energy galactic and solar radiation) during the numerous stages of its transformation, and this leads, in particular, to the transformation of one type of isotope into another and, in the general case, to a change in the isotopic or atomic composition. The more prolonged and varied the processes, the further is its chemical composition from the primary stellar (solar) composition. At the same time, the isotopic com-position of cosmic matter (for example, meteorites) makes it possible to determine the composition, intensity, and modulation of past galactic radiation.

The results of investigations in cosmochemistry are published in the journals Geochimica et Cosmochimica Acta (New York, since 1950) and Geokhimiia (since 1956).

REFERENCES

Vinogradov, A. P. “Vysokotemperaturnye protoplanetnye protsessy.” Geokhimiia, 1971, issue 11.
Aller, L. H. Rasprostranennost’ khimicheskikh elementov. Moscow, 1963. (Translated from English.)
Seaborg, G. T., and E. G. Valens. Elementy vselennoi, 2nd ed. Moscow, 1966. (Translated from English.)
Merrill, P. W. Space Chemistry. Ann Arbor, Mich., 1963.
Spitzer, L. Diffuse Matter in Space. New York, 1968.
Snyder, L. E., and D. Buhl. “Molecules in the Interstellar Medium.” Sky and Telescope, 1970, vol. 40. pp. 267, 345.

A. P. VINOGRADOV