Other Extraterrestrial Sources

There are some other far-fetched and perhaps not so far-fetched ideas about the source of earth chirality. In recent years, there have been numerous reports about meteorites found in the arctic region that have been attributed to coming from Mars, and some of these reports presented evidence that was interpreted to indicate the existence of remnants of living microbes. It is not too difficult to imagine that if there were events on Mars that ejected Martian stuff out into space that found its way to planet Earth, perhaps some chiral molecules were deposited on our planet. These molecules could then have started the process of life on earth. 

More to the liking of movie studios, perhaps, is the idea that we were visited in the distant past by extraterrestrial beings 

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In recent years. Znller and associates University of Maryland) have studied six active volcanoes iAugustine, Mount St. Helens. El Chiehdn. Arcnal. Poas. and Colima) and have found no evidence of lr enrichment. The new Kilauea evidence of volcanic action as an Ir source tends to conflict with that of other researchers who have generally attributed the Ir anomaly to an extraterrestrial source, such as resulting from a cataclysmic meteorite or asteroid impact, notably in connection with (he Cretaceous-Tertiary [K i t boundary layer. 

The question also arises as to where the chiral molecules came from. Were the L-amino acids or the D-sugars selected on the primeval Earth, or are extraterrestrial sources responsible for the homochirality This second possibility is dealt with by hypotheses on the effect of circularly polarised light, of extraterrestrial origin, on chiral molecules in the molecular clouds from which the solar system was formed. One such hypothesis was proposed by Rubenstein et al. (1983) and developed further by others, particularly A. W. Bonner (Bonner and Rubenstein, 1987) both scientists worked at Stanford University. The authors believe that the actual radiation source was synchrotron radiation from supernovae. The excess of one enantiomeric form generated by this irradiation process would have needed to be transported to Earth by comets and meteorites, probably during the bombardment phase around 4.2-3.8 billion years ago. 

Our knowledge of the state of the primitive solar nebula in its early stages of development comes from a number of sources. These include the observations and calculations made by astronomers, from the principles of elemental behavior in planetary systems and from the study of meteorites and other extraterrestrial materials. 

Fig. 17.40 The concentration ratios of elements (e.g., Na at Base Roi Baudouin/Na at Plateau Station) reflect the input from the ocean and from other terrestrial sources to the chemical composition of firn along the coast of East Antarctica. For example, firn at Base Roi Baudouin contains 8.8 times more sodium and 5.9 times more potassium than firn at Plateau Station. Nickel is derived from extraterrestrial sources by meteorites that disintegrate in the atmosphere (Data from Hanappe et al. 1968)...

Natural EMI sources - Sources that are associated with natural phenomena. They include atmospheric charge/discharge phenomena such as lightning and precipitation static and extraterrestrial sources including radiation from the sun and galactic sources such as radio stars, galaxies and other cosmic sources. 

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 ... 

Beginnings of cosmochemistry (and geochemistry) Philosophical foundations Meteorites and microscopy Spectroscopy and the compositions of stars Solar system element abundances Isotopes and nuclear physics Space exploration and samples from other worlds New sources of extraterrestrial materials Organic matter and extraterrestrial life ... 

An alternative to the terrestrial synthesis of the nucleobases is to invoke interstellar chemistry. Martins has shown, using an analysis of the isotopic abundance of 13C, that a sample of the 4.6 billion year old Murchison meteorite which fell in Australia in 1969 contains traces of uracil and a pyrimidine derivative, xanthine. Samples of soil that surrounded the meteor when it was retrieved were also analyzed. They gave completely different results for uracil, consistent with its expected terrestrial origin, and xanthine was undetectable [48], The isotopic distributions of carbon clearly ruled out terrestrial contamination as a source of the organic compounds present in the meteorite. At 0°C and neutral pH cytosine slowly decomposes to uracil and guanine decomposes to xanthine so both compounds could be the decomposition products of DNA or RNA nucleobases. They must have either travelled with the meteorite from its extraterrestrial origin or been formed from components present in the meteorite and others encountered on its journey to Earth. Either way, delivery of nucleobases to a prebiotic Earth could plausibly have been undertaken by meteors. The conditions that formed the bases need not have been those of an early Earth at all but of a far more hostile environment elsewhere in the Solar System. That environment may have been conducive to the production of individual bases but they may never have been able to form stable DNA or RNA polymers this development may have required the less extreme conditions prevalent on Earth. 

Blood, urine, cerebrospinal, and other physiological fluids contain a great number of posttranslationally modified amino acids (approximately 170 have been studied to date) and in a wider range of concentrations than protein hydrolysates [6], Additionally, plant sources produce about 500 nonprotein amino acids and, in geological samples, highly unusual amino acids may indicate extraterrestrial origin [7, 8],... 

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