Meteorites, analyses

Zirconium is found in abundance in S-type stars, and has been identified in the sun and meteorites. Analysis of lunar rock samples obtained during the various Apollo missions to the moon show a surprisingly high zirconium oxide content, compared with terrestrial rocks. 

In the early days of meteorite analysis, it was difficult to detect N-heterocycles later, the Murchison meteorite was shown to contain xanthine, hypoxanthine, guanine, adenine and uracil (about 1.3 ppm in total). This meteorite seems to contain various classes of basic and neutral N-heterocycles, as well as isomeric alkyl derivatives. 

Meteorite analysis The chemistry and morphology of meteorites, specifically ALH84001 and the Murchison meteorite... 

Both pathways probably involved quite different conditions, the main difference being the absence of liquid water in the interstellar medium. Nevertheless the basic building blocks and chemical reactions should have been roughly similar, thus leading to important connections between these two routes. While a wide variety of amino acids are prebiotically relevant (as attested by either Urey-Miller experiments or meteorite analysis), we shall focus in this section on a-amino acids (as the most relevant to biochemistry) and closely related compounds. 

As for meteorite analysis, amino acids from such interstellar ice analogues are usually detected after (often acidic) hydrolysis of crude reaction products, and subject to the same interpretation restrictions. Pointing out the formation of peptides [85] (without further arguments) to explain the detection of amino acids in such acid-hydrolyzed irradiated ices is at least questionable since many other precursors can be involved. 

The astrophysical models of protoplanetary disks based on optical observations and laboratory experiments and meteoritic measurements provide the basis for theories of nebular evolution. The best and most precise relevant measurements are from meteoritic analysis. Meteorites from the Asteroid Belt of our Solar System are the best record of the evolution of the solar nebula from a gas-dust mixture to an organized planetary system. The addition of cometary and solar-wind sample analysis complement these data. Combination of fundamental laboratory-based experiments and modeling efforts has led to a highly resolved understanding of the chemical conditions and processes in the primordial solar nebula (see Chapter 6). In this chapter an overview of recent advances in our understanding of the chemical and isotopic evolution of the early Solar System and protoplanetary disks is presented. 

Percentage of meteorites seen to fall. Chondrites. Over 90% of meteorites that are observed to fall out of the sky are classified as chondrites, samples that are distinguished from terrestrial rocks in many ways (3). One of the most fundamental is age. Like most meteorites, chondrites have formation ages close to 4.55 Gyr. Elemental composition is also a property that distinguishes chondrites from all other terrestrial and extraterrestrial samples. Chondrites basically have undifferentiated elemental compositions for most nonvolatile elements and match solar abundances except for moderately volatile elements. The most compositionaHy primitive chondrites are members of the type 1 carbonaceous (Cl) class. The analyses of the small number of existing samples of this rare class most closely match estimates of solar compositions (5) and in fact are primary source solar or cosmic abundances data for the elements that cannot be accurately determined by analysis of lines in the solar spectmm (Table 2). Table 2. Solar System Abundances of the Elements ... 

Thermal neutron activation analysis has been used for archeological samples, such as amber, coins, ceramics, and glass biological samples and forensic samples (see Forensic chemistry) as weU as human tissues, including bile, blood, bone, teeth, and urine laboratory animals geological samples, such as meteorites and ores and a variety of industrial products (166). 

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... 

Whether this is true or not, meteorites give us one definite piece of information. Isotopic analysis shows that each element in a meteorite has the same isotopes in the same percentages that this same element has on earth. The accepted explanation for this fact is that meteorites and the earth share a common origin and that they became separated after the elements were created. 

Raptis K, Mayer K, Hendrickx F, De Bievre P (1998) Preparation and certification of new thorium isotopic reference materials. Fresenius J Anal Chem 361 400-403 Rehkamper M, Halliday AN (1999) The precise measurement of T1 isotopic compositions by MC-ICP-MS Application to the analysis of geological materials and meteorites. Geochim Cosmochim Acta 63 935-944... 

Fig. 3.9 Greatly simplified representation of the path taken by the material under study, beginning with nucleosynthesis and ending with laboratory analysis. Circumstellar dust (a component of the primeval presolar nebula) which was contained in asteroids or comets came to Earth in meteorites and was then available for exact study (Lugmair, 1999)...

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... 

We might think that the Murchison meteorite would have been studied thoroughly enough in the years since its arrival on Earth. But the results obtained always depend on the performance of the technical resources available in this case, the analytical methods and the apparatus. Thus, it is not really surprising that a new class of amino acid has been discovered in Murchison material diamino acids, such as DL-2,3-diaminopropionic acid, DL-2,4-diaminobutanoic acid etc. These were identified using a new enantioselective GLPC/MS method, which is also being used in analysis of material from the Rosetta mission. 

The number of scientific articles published on meteorites has increased dramatically in the last few years few of these, however, concern themselves with small meteorites, the size of which lies between that of the normal meteorites (from centimetres to metres in size) and that of interplanetary dust particles. In the course of an Antarctic expedition, scientists (mainly from French institutions) collected micrometeorites from 100 tons of Antarctic blue ice (Maurette et al 1991). These micrometeorites were only 100 400 pm in size five samples, each consisting of 30-35 particles, were studied to determine the amount of the extraterrestrial amino acids a-aminoisobutyric acid (AIBS) and isovaline—both of which are extremely rare on Earth—which they contained. The analysis was carried out using a well-tested and extremely sensitive HPLC system at the Scripps Institute, La Jolla. Although the micrometeorites came from an extremely clean environment, the samples must have been contaminated, as they all showed traces of L-amino acids. Only one sample showed a significantly higher concentration of AIBS (about 280 ppm). The AIBS/isovaline ratio in the samples also lay considerably above that previously found in CM-chondrites. 

Analysis of the Murchison meteorite led to a completely different type of phosphorus compound the only phosphorus-containing compounds found were alkanephos-phonic acids. Spurred on by these results, de Graaf et al. (1995) irradiated mixtures of o-phosphorous acid in the presence of formaldehyde, primary alcohols or acetone with UV light (low pressure Hg lamp, 254 nm with a 185-nm component) and obtained phosphonic acids, including hydroxymethyl and hydroxyethyl phosphonic acids, which had been found in the Murchison meteorite. Alkanephosphonic acids can be derived from phosphorous acid, with a P-H bond being replaced by a P-C bond. 

How could such a complex information transfer system have evolved No witnesses from the archaic period, three to four billion years ago, have survived even the analysis of meteoritic rock does not help here. 

Stony principally silicates or rocky meteorites. It is harder to determine the extraterrestrial origin of these meteorites and it usually requires careful laboratory analysis. 

Chemical analysis of meteorites proceeds along classical analytical chemistry lines but with added precautions to prevent terrestrial contamination. Careful laboratory procedures have been developed, together with blank trials to enable the contamination in the laboratory to be eliminated. However, there is always the possibility of terrestrial contamination associated with the period of time on the ground before the find , in particular ice-melt water in the case of ALH84001. This meteorite... 

The desorption laser can be tracked across the surface of the meteorite so the plume of molecules can be associated directly with internal structures and morphology on the meteorite surface. The internal morphology of a meteorite is accessed by cleaving the sample in an ultraclean environment to prevent terrestrial contamination, followed by the laser desorption analysis. Nearly all meteorites have been studied in this way producing a mature field of research, however, the details of just two meteorites will be discussed in some detail. The Murchison and ALH84001 meteorites have provoked considerable interest, particularly the Antarctic meteorite ALH84001, which was the subject of a NASA announcement regarding life on Mars. 

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 6.10 Meteorite ALH84001 - 1.93 kg of the most studied rock of all time (a) as it was found with the outer edge showing a fusion crust (b) after the sample was cut for analysis of the interior. (Reproduced from photos by courtesy of NASA)...

The structures seen in the ALH84001 meteorite could easily have formed from molten material solidifying rapidly or even by precipitation of minerals from saturated solutions. Neither explanation is as romantic as Martian nanobacteria. Similar sized features have been seen following electron microscopy analysis of basalt rock structures found in riverbeds, such as from the Columbia river (Figure 6.15). 

Essentially the same amino acids, and nearly equal quantities of D and L enantiomers, were detected in the Murray meteorite, another type II carbonaceous chondrite [6]. Recent expeditions to Antarctica have returned with a large number of meteorites, many of which are carbonaceous chondrites. These may have been protected from terrestrial contamination by the pristine Antarctic ice. Careful analysis of two of these, the Yamato (74662) and the Allan Hills (77306), both type II carbonaceous chondrites, by ion exchange chromatography, gas chromatography, and GC/MS, have detected a wide variety of both protein and non-protein amino acids in approximately equal D and L abundances [9,10]. Fifteen amino acids were detected in the Yamato meteorite and twenty in the Allan Hills, the most abundant being glycine and alanine. The amino acid content of the Yamato meteorite is comparable with that of the Murchison and Murray, but the Allan Hills contains 1/5 to 1/10 that quantity. Unlike earlier meteorites from other locations, the quantities of amino acids in the exterior and interior portions of the Yamato and Allan Hills meteorites are almost identical [9,10]. Thus, these samples may have been preserved without contamination since their fall in the blue ice of Antarctica, which js 250,000 years old in the region of collection. 

In Gubbio, Italy, a 1 cm layer of clay between extensive limestone formations marks the boundary between the Cretaceous and Tertiary Periods. This clay layer was known to have been deposited about 65 million years ago when many life forms became extinct, but the length of time associated with the deposition was not known. In an attempt to measure this time with normally deposited meteoritic material as a clock, extensive measurements of iridium abundances (and those of many other elements) were made on the Gubbio rocks. Neutron activation analysis was the principal tool used in these studies. About 50 elements were searched for in materials like the earth s crust, about 40 were detected and about 30 were measured with useful precision [26-28]2. 

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