cosmochemistry

The science of the chemical composition of the universe.

The science of the chemistry of the universe, particularly that beyond the Earth. As currently practiced, cosmochemistry is 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. See also Isotope; Meteorite.

Minerals in meteorites

Meteorites provide a great deal of otherwise unobtainable information about the formation and early history of the solar system. The solar system formed from the solar nebula, a large cloud of gas and dust. The chemical composition of the solar system, which can be assumed to be the same as that of the solar nebula, is known in detail from spectroscopic analysis of the Sun, which contains 99.9% of its mass. From a combination of this knowledge, an estimate of temperature and pressure from hydrodynamic models, and thermodynamic data for all possible gas, liquid, and solid species, it can be determined which minerals would have become stable as the gas cloud underwent gravitational collapse, spun down into a disk, and cooled. Pressure in the solar nebula was probably so low that condensation occurred at temperatures below the melting points of the minerals, and they condensed as solids. Different minerals became stable at different temperatures, giving rise to a condensation sequence, which can be calculated. See also Astronomical spectroscopy; Elements, cosmic abundance of; Hydrodynamics; Protostar; Solar system; Sun.

Detailed chemical information, however, requires samples, and the Earth is too geologically active to provide samples dating to its formation. Meteorites are therefore studied because, in general, they have undergone much less modification than terrestrial rocks.

A certain type of meteorite, the carbonaceous chondrite, contains millimeter-sized rocklets, or refractory inclusions, that are composed of the very minerals (especially hibonite, perovskite, melilite, spinel, and fassaite) believed to have been among the first to condense from the solar nebula. This finding indicates that assumptions used to calculate the condensation sequence are reasonably accurate. See also Corundum; Melilite; Perovskite; Spinel.

Some meteorites also contain trace amounts of grains of carbon-rich minerals (silicon carbide, diamond, and graphite) whose condensation requires a gas with a carbon/oxygen ratio greater than 1, whereas that of the Sun is 0.6. This finding, along with very large isotopic anomalies for many elements, provides strong evidence that these grains are interstellar in origin, predating the Sun by as much as about 2.4 × 10⁹ years. These grains should have disappeared by reacting with the solar nebular gas, however; why they did not and how they survived long enough to be incorporated into meteorites are not yet understood. See also Diamond; Graphite; Interstellar matter; Silicon.

Chemical compositions of meteorites

The chemical composition of meteorites and independently formed objects within them have taught much about pre-solar-system events, solar nebular processes, and early planetary processes.

As mentioned above, several types of interstellar grains have been separated from primitive meteorites. Presolar grains identified include silicon carbide, graphite, diamond, titanium carbide, zirconium carbide, molybdenum carbide, aluminum oxide, titanium oxide, hibonite, and spinel. While chemistry alone has not unambiguously identified any of these grains as presolar, chemistry does help to understand the formation conditions of objects whose isotopic composition proves a presolar origin. A good example is silicon carbide, most of which is believed, on the basis of isotopic arguments, to come from the circumstellar envelopes of asymptotic giant branch stars. The silicon carbide with isotope anomalies that is separated from meteorites is enriched in a number of trace elements that are predicted to condense at high temperatures as carbides in reducing environments, such as zirconium, titanium, molybdenum, strontium, barium, and the rare-earth elements. These enrichments are entirely consistent with the proposed origin of silicon carbide by high-temperature condensation in the circumstellar envelopes of carbon-rich stars. See also Giant star.

Chemistry has been used to infer a number of important processes in the solar nebula. These include high-temperature condensation and vaporization, melting and crystallization of millimeter- to centimeter-sized objects in the solar nebula, separation of metal from silicate grains, and reaction of condensed grains with solar nebular gas. Investigation of these processes helps determine important properties of the solar nebula, such as its temperature, how long it lasted, and what the planets, which formed from the nebula, are made of.

A number of important planetary processes, such as separation and differentiation of a metallic core and differentiation of the silicate portion of a planet into mantle and crust, can be studied through the chemical compositions of meteorites.

Isotopic compositions of meteorites

Studies of the relative abundances of isotopes in meteorites provide a unique source of information about the age of the solar system, the time scales for formation of the first solid bodies in the solar system and the growth and evolution of small planets, the prehistory (nucleosynthesis) of the material out of which the solar system formed, and the interaction of solar and galactic cosmic rays with matter. See also Nucleosynthesis.

The oldest meteoritic samples are the refractory, calcium-aluminum-rich inclusions found in carbonaceous chondrites. The 4.566-billion-year age of these objects provides the best estimate for the condensation of the first solid material in the solar system. Chondrules, millimeter-size spherules of glass and crystals, began to form a few million years after refractory inclusions as the solar nebula cooled. The oldest basaltic meteorites crystallized from molten lavas on small asteroids within roughly 10 million years of this time (that is, ages of 4.562–4.552 billion years). Most chondritic meteorites have less precisely determined ages encompassing the range covered by chondrules and basaltic achondrites. The tight clustering of ages of the oldest meteorites reflects a period of rapid growth during the first 10 million years of solar system history, as millimeter-to-centimeter-size objects collided and coalesced to form meter-to-kilometer-size planetesimals. The younger ages of some meteorites indicate that disturbances due to shock and thermal metamorphism began as early as 4.5 billion years ago and continued for 100–200 million years. Collisions between planetesimals led to coagulation, the growth of larger bodies, and the eventual formation of the terrestrial planets, including the Earth-Moon system, 50–100 million years after most meteorites. See also Rock age determination.

One unusual group, the SNC meteorites, whose appearance and composition is similar to that of terrestrial basalts, is the exception to the rule with much younger crystallization ages of approximately 1.3 billion years. The young ages require an origin on a planetary body larger than the Moon, on which radioactive heating could sustain igneous activity for at least 3 billion years. It is generally believed that this group of meteorites originated on Mars, an argument strengthened by recent evidence that the isotopic compositions of hydrogen, nitrogen, and noble gases in the SNC meteorites and the Martian atmosphere are strikingly similar and distinct from those in the Earth and other meteorites.

Nine short-lived species with half-lives ranging from 0.10 to 103 million years are known to have existed at the birth of the solar system. The evidence of short-lived radionuclides is distributed across all classes of meteorites and provides a compelling argument not only that meteorites are the oldest objects in the solar system but that they formed rather quickly over a very short interval. Although these nuclides can no longer be observed directly, evidence that they were present during the earliest epoch of solar system history is preserved as variations in the relative abundances of the daughter isotopes of the extinct radionuclides. For example, large excesses of ²⁶Mg, produced by the decay of ²⁶Al (half-life of 0.72 million years), are found in aluminum-rich, magnesium-poor minerals in refractory inclusions in carbonaceous chondrites.

The short-lived nuclides are not restricted to primitive chondritic meteorites but are also prominent in meteorites believed to have been produced by planetary-scale melting. The widespread presence of short-lived nuclides indicates that the time interval between nucleosynthesis and the formation of the solar system must be short. Refractory inclusions containing ⁴¹Ca, an isotope with a half-life of only 100,000 years, must have formed within a few mean lives, that is, a few hundred thousand years of ⁴¹Ca production. This time scale is so short that it suggests the formation of the solar system may have been triggered by a shock wave from a nearby exploding star containing freshly synthesized nuclear material.

Except for the variations in isotopic abundances related to decay of radionuclides discussed above, for the most part there is a close similarity between the isotopic compositions of meteorites, the Moon, and the Earth. For many years this apparent homogeneity was taken as evidence that the solar nebula was chemically and physically well mixed before the formation of the first solid bodies. The development of new analytical techniques permitting the determination of isotopic compositions in small (<0.01 mm) constituents of meteorites has, however, led to the discovery of large differences in isotopic abundances for many elements which cannot be explained in terms of radioactive decay or cosmic-ray interactions. These isotope anomalies provide unequivocal evidence that dust grains that condensed around a variety of different stars were incorporated into primitive meteorites and offer a window through which nucleosynthesis and stellar evolution are revealed with unprecedented clarity.

The isotope anomalies found in refractory inclusions represent a mixture of exotic stardust from other stars diluted with normal solar system material; the inclusions themselves clearly are not presolar material. A major new discovery in meteorite research is the identification in primitive chondrites of whole interstellar grains, formed outside the solar system in stellar atmospheres and incorporated into primitive meteorites essentially intact. These grains survived the formation of the solar system and preserve the isotope compositions of the stellar nuclear sources for both major and trace elements. Presolar grains show isotope anomalies for all elements, and the size of the anomalies is at least a factor of 100 times larger than that of anomalies found in refractory inclusions.

Studies of isotope anomalies and presolar grains in primitive meteorites have yielded important new clues to nucleosynthesis and the types of stars contributing material to the nascent solar nebula. The very large range of isotope abundances measured in presolar grains cannot be generated by a single stellar source, and is ample proof that the solar system is the product of contributions from disparate stars. Hydrogen and the bulk of the helium in the solar system are relicts of big bang nucleosynthesis, while the elements carbon through iron were produced dominantly in charged particle reactions in massive stars. The radionuclides ¹²⁹I, ²³²Th, ²³⁵U, ²³⁸U, and ²⁴⁴Pu are all produced by neutron-rich nucleosynthesis in supernovae. The abundance of short-lived radionuclides requires the late addition of fresh nucleosynthetic material, possibly from a massive, red-giant star, to the solar nebula less than a few million years prior to the formation of the first meteorites. See also Big bang theory; Supernova.

Meteorites may be studied as part of cosmochemistry.

Cosmochemistry is concerned with the origin and development of the elements and their isotopes, primarily within the Solar System. The term was coined by Harold Urey.^[1] Closely related fields are astrochemistry, a branch of astronomy concerned with measuring chemical elements in other parts of our Galaxy and in other galaxies; astrophysics, which includes the study of physical processes in stars, supernovas, and our own solar system that may result in chemical and isotopic changes; and planetary science, of which cosmochemistry is in large part a subdiscipline.

Cosmochemistry frequently involves direct measurement of physical samples in laboratories on the Earth. Samples are most often meteorites and micrometeorites, which include material that originated on the Moon, Mars, many different asteroids, and quite possibly comets, as well as with samples returned from the Moon by manned and robotic missions. Other materials available for direct study by cosmochemists are tiny particles from Comet Wild 2 returned to Earth in 2006 by NASA's Stardust mission, and samples of solar wind, captured by the Genesis mission and returned to Earth in 2004. The most exotic available samples are tiny presolar grains that are embedded in some primitive chondritic meteorites, which originated elsewhere in the galaxy prior to the formation of the solar system. Similar interstellar dust particles may also have been collected by the Stardust mission, although this has not yet been confirmed.

NASA has funded a research program in cosmochemistry since the 1970s.^[2]

See also

• Geochemistry
• Nucleocosmochronology
• Extraterrestrial materials

References

1. ^ "DMG Cosmochemistry project group". University of Koln. http://www.cosmochemistry.org/. Retrieved on 2006-11-07. 
2. ^ "NASA Cosmochemistry program". http://www.higp.hawaii.edu/ccp/index.html. 

External links

• Planetary Science Research Discoveries Educational journal with articles about cosmochemistry, meteorites, and planetary science.

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