Meteorite carbonate

One other mechanism has been suggested71 which might involve recycling of carbon already in the samples or in meteorites. Carbon species vapourised and contributed to the tenuous atmosphere as a result of impact, are expected to become ionized by interaction with solar wind ions or ultraviolet radiation, (then accelerated by the electric fields present), and subsequently implanted like solar wind ions, but at lower energy. At present, however, there is no proof that this mechanism does operate. 

Opportunities for such secondary reactions certainly existed in the history of meteorites. Temperatures in the nebula (360-400 K, Table 1) may alone have been high enough for secondary reactions in the time available, 10 -10 yr. Kinetic studies of a similar reaction (formation of benzene from alcohols, amines, or fatty acids on Fe Oj or iron-rich peat catalysts Galwey, 1972) indicate a benzene formation rate of 5 x 10 molecules g yr at 360 K. At this rate it would take only 5000 years to transform all the meteoritic carbon to benzene. Further opportunities were provided by brief thermal pulses during chondrule formation, impact, or transient shocks. Of couree, any high-temperature episodes must have happened early or on a local scale, to permit survival of other, more temperature-sensitive compounds. 

Clayton R. N. (1963) Carbon isotopes in meteoritic carbonates. Science 140, 192-193. 

Grady M. M., Wright 1. P., Swart P. K., and PiUinger C. T. (1988) The carbon and oxygen isotopic composition of meteoritic carbonates. Geochim. Cosmochim. Acta 52, 2855-2866. 

The planetary CO2 inventory has been estimated from C/N ratios of possible accreted volatile sources (chondrites or comets). Carbon dioxide estimates, expressed as equivalent global thicknesses of carbonate, vary from 3 m to 20 m (Bogard et al., 2001). Carbonates have not been detected spectroscopically on Mars as of early 2000s, although they are present in small quantities as martian weathering products in SNC meteorites. Carbon in soils at the Mars Pathfinder site is below detection limit, and the visible inventory of carbon on Mars is only 10 that of Earth or Venus (Bogard et al., 2001). 

There now exist numerous observations of mass-independent isotopic compositions in nature. Most of these have recently been reviewed and will not be repeated here. When the first laboratory measurements of the mass-independent isotope effect were reported by Thiemens and Heidenreich (1983), their occurrence in nature was not expected, except possibly for the early solar system to produce the observed meteoritic CAI data. It is significant to note that, at present, all oxygen-bearing molecules in the atmosphere (except water) possess mass-independent isotopic compositions. These molecules include O2, O3, CO2, CO, N2O, H2O2, and aerosol nitrate and sulfate. Mass-independent sulfur isotopic compositions are also observed in aerosol (solid) sulfates and nitrates and sulfide and sulfate minerals from the Precambrian, Miocene volcanic sulfates, Antarctica dry valley sulfates, Namibian Gypretes, and Chilean nitrates. In addition, martian (SNC meteorites) carbonates and sulfates possess both mass-independent sulfur and oxygen isotopic compositions. These studies have been reviewed recently (Thiemens et al., 2001 Thiemens, 1999). 

Latin carbo, charcoal) Carbon, an element of prehistoric discovery, is very widely distributed in nature. It is found in abundance in the sun, stars, comets, and atmospheres of most planets. Carbon in the form of microscopic diamonds is found in some meteorites. 

In the late 1960s El Goresy and Donnay [17] discovered a new form of carbon which they called white carbon or Chaoite in a carbon-rich gneiss in the Ries meteorite crater in Bavaria. Chaoite has an hexagonal crystal stmcture and it... 

Despite many publications on carbynes, their existence has not been universally accepted and the literature has been characterised by conflicting claims and counter claims [e.g., 27-29]. This is particularly tme of meteoritic carbynes. An interesting account of die nature of elemental carbon in interstellar dust (including diamond, graphite and carbynes) was given by Pillinger [30]. Reitmeijer [31] has re-interpreted carbyne diffraction data and has concluded that carbynes could be stratified or mixed layer carbons with variable heteroelement content (H,0,N) rather than a pure carbon allotrope. 

Analysis of carbon compounds—even amino acids—from extraterrestrial sources might provide deeper insights into this mystery. John Cronin and Sandra Pizzarello have examined the enantiomeric distribution of unusual amino acids obtained from the Murchison meteorite, which struck the earth on September 28, 1969, near Murchison, Australia. (By selecting unusual amino... 

It is estimated that the earth s age is in the neighborhood of 4 to 7 billion years. These estimates are basically derived from carbon-14, potassium-40, uranium-235, and uranium-238 dating of earth rocks and meteorites. The meteorites give important data as to the age of our solar system. Geologic time is felt to be represented by the presence of rock intervals in the geologic column (layers of rock formations in vertical depth) or by the absence of equivalent rocks in correlative columns in adjacent locations [25,26]. The two basic factors that are used to determine geologic time are ... 

Water and carbon play critical roles in many of the Earth s chemical and physical cycles and yet their origin on the Earth is somewhat mysterious. Carbon and water could easily form solid compounds in the outer regions of the solar nebula, and accordingly the outer planets and many of their satellites contain abundant water and carbon. The type I carbonaceous chondrites, meteorites that presumably formed in the asteroid belt between the terrestrial and outer planets, contain up to 5% (m/m) carbon and up to 20% (m/m) water of hydration. Comets may contain up to 50% water ice and 25% carbon. The terrestrial planets are comparatively depleted in carbon and water by orders of magnitude. The concentration of water for the whole Earth is less that 0.1 wt% and carbon is less than 500 ppm. Actually, it is remarkable that the Earth contains any of these compounds at all. As an example of how depleted in carbon and water the Earth could have been, consider the moon, where indigenous carbon and water are undetectable. Looking at Fig. 2-4 it can be seen that no water- or carbon-bearing solids should have condensed by equilibrium processes at the temperatures and pressures that probably were typical in the zone of fhe solar... 

The photo below, taken by the Ttl/ng spacecraft, shows that the surface of Mars has been eroded, apparently by liquid water. More recent photos transmitted by Spirit and Opportunity convince scientists that this was the case. Apparently, Mars was once much warmer than it is today. Planetary scientists speculate that at one time the atmosphere of Mars may have contained large amounts of carbon dioxide, setting up a greenhouse effect that made the surface of that planet warmer and wetter. Might there, then, have been life on Mars at some earlier time Molecular stmctures found in meteorites thought to come from Mars have been interpreted to show that there was once life there, but these results are controversial. 

Thinking Critically Based on your answer to question 3, explain why radiochemical dating using carbon is an inappropriate technique for dating meteorites. 

Alexander von Humboldt (1769-1859) recognised meteorites as being a source of extraterrestrial material. Several well-known chemists carried out analyses of material from meteorites, starting at the beginning of the nineteenth century. Thus Louis-Jacques Thenard (1777-1857) found carbon in Alais meteorites these results were confirmed in 1834 by Jons Jacob Berzelius, who by dint of very careful work was also able to detect water of crystallisation in meteoritic material. 

Carbonaceous chondrites (C-chondrites) account for only 2-3% of the meteorites so far found, but the amount of research carried out on them is considerable. C-chondrites contain carbon both in elemental form and as compounds. They are without doubt the oldest relicts of primeval solar matter, which has been changed only slightly or not at all by metamorphosis. C-chondrites contain all the components of the primeval solar nebula, apart from those which are volatile they are often referred to as primitive meteorites . 

The analysis of extraterrestrial matter is concentrated on the detection of nucleic acid and protein building blocks, i.e., N-heterocycles and amino acids. The search for such compounds began immediately after the fall of the Murchison meteorite. Twenty-two amino acids were detected in it as early as 1974 eight of them pro-teinogenic, ten which hardly ever occurred in biological material, and four which were unknown in the biosphere. Up to now, about 70 amino acids have been identified (Cronin, 1998), the most common being glycine and a-aminoisobutyric acid. The latter is a branched-chain amino acid with the smallest possible number of carbon atoms. The most frequently found amino acids occur in concentrations of... 

Now and then, projectiles from outer space cause excitement and surprises, as in January 2000, when a meteorite impacted the frozen surface of Lake Targish in Canada. It was a new type of C-chondrite with a carbon concentration of 4-5%, and probably came from a D-type asteroid (Hiroi et al., 2001). More exact analysis of the Targish meteorite showed the presence of a series of mono- and dicarboxylic acids as well as aliphatic and aromatic hydrocarbons (Pizzarello et al., 2001). Aromatic compounds and fullerenes were detected in the insoluble fraction from the extraction this contained planetary helium and argon, i.e., the 3He/36Ar ratio was... 

The earlier assumption of a reducing atmosphere has been modified in favour of a neutral one. It is hypothetically possible that iron vapour and reduced forms of carbon from meteorite impacts on the ocean could have led to limited regions with reducing properties. 

The Murchison meteorite shown in Figure 6.7, like all meteorites, is named after the place from which it was recovered and in this case it is the town of Murchison, Victoria in Australia about 100 km north of Melbourne. The fall occurred in 1969 and was followed by an analysis of the chemical composition in some considerable detail. The Murchison meteorite is a carbonaceous chondrite containing about 2 per cent carbon, some as inorganic carbonates, and some as soluble compounds such as amino acids but the bulk as a macromolecular heterogeneous material referred to as kerogen. 

Figure 6.13 The /xL2MS spectrum of ALH84001 (a) from the carbonate granules (b) from the bulk meteorite. (Reproduced from Becher 1999 with permission from Elsevier)...

Amino acid measurements in ALH84001 are almost certainly the result of Antarctic ice contamination. Amino acids are readily soluble in water but PAHs are practically insoluble. Isotopic measurements of 14C show that terrestrial carbon is incorporated into the meteorite during extended stays in the Antarctic ice fields. In addition, microbial activity on the exposed surfaces provides an additional source of biogenic organic material that may be incorporated over time. 

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