Water chondrites

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... 

New computer simulations of the accretion process of the protoearth indicate that only a few large bodies with a high water concentration collided with the Earth during the later bombardment. They apparently came from the same region of the asteroid belt as the carbonaceous chondrites. 

The authors chose pyruvic acid as their model compound this C3 molecule plays a central role in the metabolism of living cells. It was recently synthesized for the first time under hydrothermal conditions (Cody et al., 2000). Hazen and Deamer carried out their experiments at pressures and temperatures similar to those in hydrothermal systems (but not chosen to simulate such systems). The non-enzymatic reactions, which took place in relatively concentrated aqueous solutions, were intended to identify the subsequent self-selection and self-organisation potential of prebiotic molecular species. A considerable series of complex organic molecules was tentatively identified, such as methoxy- or methyl-substituted methyl benzoates or 2, 3, 4-trimethyl-2-cyclopenten-l-one, to name only a few. In particular, polymerisation products of pyruvic acid, and products of consecutive reactions such as decarboxylation and cycloaddition, were observed the expected tar fraction was not found, but water-soluble components were found as well as a chloroform-soluble fraction. The latter showed similarities to chloroform-soluble compounds from the Murchison carbonaceous chondrite (Hazen and Deamer, 2007). 

The question as to the potential availability of the requisite amphiphilic precursors in the prebiotic environment has been addressed experimentally by Deamer and coworkers, [143,145] who looked into the uncontaminated Murchison chondrite for the presence of such amphiphilic constituents. Samples of the meteorite were extracted with chloroform-methanol and the extracts were fractionated by thin-layer chromatography, with the finding that some of the fractions afforded components that formed monomolecular films at air-water interfaces, and that were also able to self-assemble into membranous vesicles able to encapsulate polar solutes. These observations dearly demonstrated that amphiphiles plausibly available on the primitive Earth by meteoritic infall have the ability to self-assemble into the membranous vesides of minimum protocells. ... 

Busfield A, Gilmour JD, Whitby JA, Turner G (2004) Iodine-xenon analysis of ordinary chondrite halide implications for early solar system water. Geochim Cosmochim Acta 68 195-202 Busso M, Gallino R, Wasserburg GJ (1999) Nucleosynthesis in asymptotic giant branch stars relevance for galactic enrichment and solar system formation. Annu Rev Astronom Astrophys 37 239-309 Cameron AGW (1969) Physical conditions in the primitive solar nebula. In Meteorite Research. Millman PM (ed) Reidel, Dordrecht, p 7-12... 

The analysis of fractionation law exponents quantifies the impression from the A -5 plots that aqueous Mg is related to primitive mantle and average crustal Mg by kinetic processes while carbonates precipitated from waters approach isotopic equilibrium with aqueous Mg. In any case, the positive A Mg values of carbonates relative to the primitive chondrite/mantle reservoir and crust is a robust feature of the data and requires a component of kinetic Mg isotope fractionation prior to carbonate formation, as illustrated schematically in Figure 3. 

D/H ratios in carbonaceons chondrites may hint on the origin of water on Earth. Robert (2001) suggested that since the contribntion of cometary water to terrestrial water should be less than 10%, most of the water on Earth should derive from a meteoritic source. 

EUer and Kitchen (2004) have re-evaluated the hydrogen isotope composition of water-rich carbonaceous chondrites by stepped-heating analysis of very small amounts of separated water-rich materials. Their special aim has been to deduce the origin of the water with which the meteorites have reacted. They observed a decrease in 5D with increasing extent of aqueous alteration from 0%c (least altered, most volatile rich) to —200%c (most altered, least volatile rich). 

Robert F, Epstein S (1982) The concentration and isotopic composition of hydrogen, carbon and nitrogen carbonaceous meteorites. Geochim Cosmochim Acta 46 81-95 Robert F, Merlivat L, Javoy M (1978) Water and deuterium content in ordinary chondrites. Mete-oritics 12 349-354... 

Oxygen isotopic compositions for bulk chondrites, after Clayton (2004). By convention, 170/160 and 180/160 ratios are plotted as deviations from the composition of standard mean ocean water (SMOW) in units of parts per thousand (permil). The 8 values are calculated as follows S170 = [((170/160)sampie/(170/160)SMOw)-1] x 1000, and similarly for S180. 

Many asteroids are dry, as evidenced by meteorites in which water is virtually absent. These samples include many classes of chondrites, as well as melted chunks of the crusts, mantles, and cores of differentiated objects. Anhydrous bodies were important building blocks of the rocky terrestrial planets, and their chemical compositions reveal details of processes that occurred within our own planet on a larger scale. The distributions of these asteroids within the solar system also provide insights into their formation and evolution. 

Bulk hydrogen and 36Ar contents versus mineral alteration index for CM carbonaceous chondrites. These trends reflect progressive incorporation of water and destruction of the noble gas carrier as alteration advances. After Browning et al. (1996). 

Brearley, A. J. (2006) The action of water. In Meteorites and the Early Solar System II, eds. Lauretta, D. S. and McSween, H. Y., Jr. Tucson University of Arizona Press, pp. 587-624. The best available review of aqueous alteration processes and materials in chondritic meteorites. 

Grimm, R. E. andMcSween, H. Y. (1989) Water and the thermal evolution of carbonaceous chondrite parent bodies. Icarus, 82, 244-280. 

Gravitational stirring of icy planetesimals by the giant planets could have sent many comets careening into the inner solar system, providing a mechanism for late addition of water to the terrestrial planets. Comets impacting the Earth and the other terrestrial planets would have delivered water as ice (Owen and Bar-Nun, 1995 Delsemme, 1999), whereas the accretion of already altered carbonaceous chondrite asteroids would have delivered water in the form of hydroxl-bearing minerals (Morbidelli el al., 2000 Dauphas et al., 2000). 

Noble gases may provide a constraint on the source of water and other volatiles. The abundance pattern of noble gases in planetary atmospheres resembles that of chondrites, perhaps arguing against comets. However, there are some differences, especially in the abundance of xenon. Relative to solar system abundances, krypton is more depleted than xenon in chondrites, but in the planets, krypton and xenon are present in essentially solar relative abundances (Fig. 10.11). This observation has been used to support comets as the preferred source of volatiles (even though measurements of xenon and krypton in comets are lacking). A counter-argument is that the Ar/H20 ratio in comets (if the few available measurements are accurate and representative) limits the cometary addition of volatiles to the Earth to only about 1%. 

The presence of organic molecules in samples of extraterrestrial matter has been known for more than a century. Some of the greatest chemists of the nineteenth century were involved in the analysis of samples of meteoritic material. They were able to show that carbonaceous chondrites (as they are now named) contain organic molecules. The first to detect carbon in a meteoritic sample was Thenard, in 1806, by analysis of a sample of the Alais meteorite. This result was confirmed in 1834 by Berzelius, who was also the first to detect the presence of water of crystallisation. Working on a sample of the Kaba meteorite, Wohler (1858) confirmed the presence of organic matter, and in a paper dated 1859 said, I am still convinced that besides free carbon this meteorite contains a low-melting point, carbon containing substance which seems to be similar to certain fossil hydrocarbon-like substances... . 

Amino acids - and more generally organic substances - are mainly detected in carbonaceous chondrites, a minor, carbon-rich class of meteorites. These are believed to have originated from parent bodies having underwent alteration by liquid water at some stages of their existence, as attested by geochemical studies [63]. 

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