Carbonaceous chondrites

The carbonaceous chondrite meteorites are the most primitive remnants of the primeval nebula from which the Sun, Earth, and all other solar system bodies originated. The hydrocarbon molecules present in these meteorites indicate the types of carbon-bearing molecules that most likely were present on primitive Earth and may have been the building blocks of life.

Overview

Carbonaceous chondrites are a class of stony meteorites that are chemically and physically primitive. They are chemically primitive in that, except for the elements hydrogen, carbon, oxygen, and noble gases, the proportions of the elements in these meteorites are very similar to those observed in the Sun. They are physically primitive in that the carbonaceous chondrites escaped the thermal alteration (exposure to heat that causes changes in chemical composition and mineralogy) that affected almost all the meteorites in the other classes. Because of the primitive nature of the carbonaceous chondrites, they are thought to be the best samples of the solar nebula out of which the Sun, Earth, and other solar-system objects formed. Thus, the composition of carbonaceous chondrites is generally taken as the starting point for models of the formation and subsequent evolution of the Earth.

The carbonaceous chondrites are relatively scarce, constituting only about 5 percent of all meteorites recovered soon after their fall to Earth was observed. They are composed of millimeter-sized chondrules, individual grains whose mineralogy and texture indicate their crystallization from molten material, set in a matrix of finer-grained material. The carbonaceous chondrites are easily distinguished from all other meteorites by their dull black color, friability, generally low density, and almost total lack of nickel-iron grains, but not all carbonaceous chondrites are alike. Differences in composition, mineralogy, and texture allow the carbonaceous chondrites to be separated into several distinct types.

The most primitive type of carbonaceous chondrite, called the CI (or C1) type, is extremely rare, represented by only five meteorites. Of these, only Orgueil, a fall of about 127 kilograms, is large enough for extensive study. The others, Ivuna (0.7 kilogram), Alais (with little remaining of a 6-kilogram fall), Tonk (7.7 grams), and Revelstoke (only one gram), are all very small. The CI carbonaceous chondrites are different from all other chondrites, both carbonaceous and ordinary, in that they lack chondrules and consist almost entirely of low-temperature minerals, particularly clays.

Another type of carbonaceous chondrite, named CII (or C2 or CM), contains numerous organic compounds, including amino acids, that may have served as the basis for the development of life on Earth. At the very least, they provide clues as to the types of organic material likely to have been present on the early Earth to serve as building blocks for life.

The CB carbonaceous chondrites exhibit a high oxidation state with abundant volatile substances. The CO type is slightly less oxidized but contains metal and sulfides. The CV type closely resembles the CO type in mineral composition and oxidation state but contains large quantities of chondrules and whitish aggregates with a high calcium-aluminum content. The CV and CO types are sometimes called the CIII (or C3) type.

The carbonaceous chondrite types other than CI consist of a matrix, similar in chemical composition to bulk CI material, mixed with chondrules and aggregates of minerals that formed at high temperatures from a condensing gas and that exhibit a depletion in volatile elements from what is observed in the CI matrix. As the abundance of high-temperature material increases from about 1 percent in the CI type to about 60 percent in other types, the similarity of the bulk composition to that of the Sun decreases.

At one time, it was thought that the high-temperature material might be derived from the matrix by heating, eliminating the volatile material. However, some studies show significant differences between the chemical and isotopic compositions of the carbonaceous chondrites' high-low-temperature components, making it impossible to derive one from the other by any simple process.

Much modern research on carbonaceous chondrite meteorites has focused on understanding the process by which these objects formed from the solar nebula, the gas, and possibly dust that collapsed to form the Sun, Earth, and other solar-system objects. As the most primitive relics of the formation process for laboratory analysis, the carbonaceous chondrites have been used to determine the chemical composition of the solar nebula, to establish the sequence and duration of events in the formation process, and to determine the temperatures characteristic of the process.

Major advances in the study of carbonaceous chondrites began in 1969. In February of that year, the Allende carbonaceous chondrite fell in northern Mexico, and about 2,000 kilograms of material were recovered for analysis. Later that same year, an even more primitive carbonaceous chondrite, the Murchison, fell in Australia. In addition, the return of lunar samples in 1969 spurred the development of research laboratories for the study of extraterrestrial materials. With the end of the Apollo lunar landing program, many of these laboratories shifted their emphasis to meteorite research and began to use highly sophisticated instruments perfected for lunar sample analysis to study meteorites.

Organic matter (chemical compounds of carbon, nitrogen, and oxygen) has been detected by spectroscopic methods in comets, on some asteroid surfaces, and on some planetary satellites. A study of the properties of this extraterrestrial organic matter would provide indications of the organic material likely to have been present on the Earth at the time life developed on this planet. The meteorites, particularly the carbonaceous chondrites, have been subjected to intensive examinations to determine whether they contain samples of this organic matter.

The search for organic matter in the carbonaceous chondrites was hampered for decades by terrestrial organic contamination of these meteorites from the time of their recovery until their analysis. By the time of the fall of the Murchison meteorite in 1969, researchers were aware of the contamination problem, and efforts were made to preserve samples properly for organic analysis. In addition, several laboratories had recently developed procedures and instrumentation to search for organic material in returned lunar samples and to distinguish terrestrial contaminations from extraterrestrial organic material. The first analyses of the Murchison meteorite provided evidence for the presence of amino acids, which are the building blocks of proteins. Subsequent analysis of carbon in the organic matter indicated that the isotopic composition, the ratio of carbon 13 to carbon 12, was inconsistent with terrestrial contamination. It has been demonstrated that several carbonaceous chondrites contain a varied suite of organic compounds. These same organic compounds have been duplicated in laboratory experiments by purely chemical processes and consequently are not evidence for life in space, but they are taken to indicate the types and variety of organic material likely to have been present on the Earth to serve as building blocks for life.

Methods of Study

Scientists have employed various techniques and instruments to uncover the secrets locked in the carbonaceous chondrite meteorites. The chemical compositions, molecular abundances, mineralogies, isotopic ratios for individual elements, and present radioactivity have all been studied. Because of the small amount of carbonaceous chondrite material available for scientific study, especially of the rare CI type, many techniques employed to examine these meteorites have benefited greatly from the sophisticated instrumentation developed in support of the lunar sample analysis program.

The observation that the carbonaceous chondrites are primitive—that is, relatively unaffected by thermal processes—was established by detailed chemical analyses of individual mineral grains. The effect of prolonged heating is to cause the compositions of minerals of the same type to equilibrate, meaning that all grains of the same mineral from a single meteorite would have approximately the same composition if the meteorite were heated above the equilibration temperature. Such an effect is seen in most ordinary chondrites but not in carbonaceous chondrites.

The compositions of small mineral grains are usually determined using an electron microprobe, an instrument that bombards the sample with an intense beam of electrons and detects the X-rays emitted by the sample. When struck by an electron, each element emits X-rays of specific energies. For example, the number of X-rays emitted at the energy characteristic of the element iron gives the iron abundance in the sample. When the mineral grains in carbonaceous chondrite meteorites are examined by this technique, grains of olivine, the most easily altered of the major minerals in the matrix, exhibit a relatively wide range of compositions. In the Allende meteorite, for example, magnesium-rich olivine chondrules are found in direct contact with iron-rich matrix olivine. Such contacts eliminate the possibility that significant thermal events have occurred since the time at which the Allende meteorite was formed.

Observation of radioactive effects can also indicate the thermal history of these meteorites. The elements uranium and plutonium decay by nuclear fission, a process by which the nucleus splits into two fragments, each about one-half the mass of the original nucleus. These fragments fly apart with high energy, traveling a few thousandths of a centimeter before coming to rest. The host material can be damaged along the path of each fragment. This damage, called a fission track, is more easily attacked by reactive chemicals than is the surrounding mineral. After chemical etching, the fission tracks can be observed through a microscope. This damage, however, can be healed by heating. Thus, if fission tracks are revealed by chemical attack, the mineral has not been heated above the healing or annealing temperature after the fission event. The presence of tracks from uranium and plutonium fission in minerals from the carbonaceous chondrites indicates that they have not been heated above a few hundred degrees Celsius since their formation.

Similar radioactive decay processes can be used as clocks, providing a way to determine the ages of these meteorites. One such clock depends on the radioactive decay of rubidium 87, an isotope of the element rubidium, into strontium 87, one of the four stable isotopes of strontium. One-half of any initial sample of rubidium 87 decays to strontium 87 in 47 billion years. In any given sample, if the abundance of rubidium 87 and the amount of strontium 87 produced by radioactive decay could be measured, the elapsed time, or age, required for that amount of decay could be determined. In practice, the application of this radioactive clock is complicated by a number of factors, including the migration of strontium 87 from its decay site because of heating and the fact that not all strontium 87 in the sample is from rubidium 87 decay. When appropriate corrections are made for these effects, however, the rubidium-strontium clock, as well as similar decay clocks using other pairs of elements, gives a consistent picture that the carbonaceous chondrites formed 4.55 billion years ago. Results for the oldest rocks on the Moon give essentially the same age. The extensive thermal activity in the early history of the Earth has apparently destroyed most or all evidence of the earliest rocks to form on this planet, but the observation that both the meteorites and the oldest lunar rocks have a common age suggests that the entire solar system, including the Earth, formed at that time.

Radioactive elements also provide clues to the duration of solar system formation. The decay of aluminum 26, reduced to one-half of its starting abundance in only 720,000 years, produces magnesium 26. Magnesium has three stable isotopes, which are usually found in fixed ratios to one another, but the ratio of magnesium 26 to the other two isotopes of magnesium will increase when aluminum 26 decays. In some of the high-temperature aggregates from the Allende meteorite, significant enrichments in magnesium 26 were found by mass spectrometry. Detailed examination of the minerals containing these enrichments showed that the size of the magnesium 26 enrichment increased in proportion to the aluminum concentration in that mineral. This suggested that radioactive aluminum 26 was incorporated into the mineral and subsequently decayed to magnesium 26. For this to be true, however, the high-temperature aggregates would have to have formed within a few million years of the isolation of the solar nebula, or most of the aluminum 26 would already have decayed. Thus, the high-temperature aggregates in Allende and some other carbonaceous chondrites provide evidence that mineral grains condensed very early in the solar-system formation process.

Context

As a group, the carbonaceous chondrites have provided a wealth of information on many different aspects of planetary formation and the development of biological processes that may have occurred in the early solar system. They exhibit a variety of conditions of formation that range from a high-temperature, low-pressure, volatile-poor environment to one that was at a lower temperature and volatile-rich. They appear to have experienced only minor alteration since their formation from the collapsing gaseous nebula that became our solar system, and thus, they preserve a record of that early era of solar-system history. The chemical composition of the least altered of these meteorites is almost identical to the composition of the Sun, except for a few gaseous elements. Thus, the carbonaceous chondrite composition is taken to indicate the bulk composition of the Earth, which cannot be measured directly since the Earth’s interior is inaccessible.

Radioactive clocks in the carbonaceous chondrites indicate that they formed 4.55 billion years ago. The consistency of this age with the age of the oldest rocks brought back from the Moon by the Apollo lunar landings is taken to indicate that the entire solar system, including the Earth, formed at that time. Isotopic relics of other radioactive elements, now extinct, demonstrate that some minerals in the carbonaceous chondrites formed within as little as a few million years of the isolation of the solar nebula from the addition of new galactic radioactive isotopes.

The carbonaceous chondrites also contain organic molecules, including amino acids, which are the building blocks of proteins. Although there is no evidence of biological activity on the parent body of the carbonaceous chondrites, these or similar organic molecules are likely to have been available on Earth to serve as building blocks for the development of life.

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