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3 Detection of Evidence of Earlier Life
Pages 21-32

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From page 21...
... The failure of the Viking gas chromatography-mass spectrometry experiment (detection limits of 1 part in 106 to 109) to detect organic material in martian surface and subsurface materials at two landing sites dealt a severe blow to the prospect of life on Mars.1 Because the infall of carbonaceous chondrite meteorites, if nothing else, should build up a finite level of organic compounds in the martian soil, it seems likely that such compounds are systematically destroyed in the martian environment, perhaps by ultraviolet radiation and/or an oxidizing agent such as hydrogen peroxide or hydroxyl radicals in the atmosphere,2 although they may survive inside sufficiently large, impermeable rock fragments.
From page 22...
... · Examples of organismal properties include cellularity and cell division states (reproduction) seen in optical microscopic studies of Precambrian and Phanerozoic fossils, and the heritable morphologic variability routinely established by optical microscopic study of populations of Precambrian and Phanerozoic microscopic fossils of a single taxon.
From page 23...
... Additionally, life leaves biomarkers (from "biological marker compounds") in the rocks, distinctive geochemically altered biomolecules such as various hydrocarbons, fatty acids, amino acids, and porphyrins that by their presence provide evidence of the existence of particular biosynthetic pathways and the biological groups in which they occur.
From page 24...
... These chromatograms show the distribution of hydrocarbons, alcohols, organic acids, and other compounds generated from formic acid in hydrothermal synthesis experiments. The peaks are labeled with the number of carbon atoms in each molecule.
From page 25...
... ~~ _ 24 Lo 28 i:( .. ~ _ ~ ~ ~ ~ :~ 20 30 40 50 Time' Minutes 25 FIGURE 3.2 Gas chromatograms of compounds formed by biological processes; shown are total hydrocarbon extracts from surface marine sediments in the Guaymas Basin (above)
From page 26...
... It can be extended or deepened in proportion to the detail available and has the potential to use all of the information provided by the analytical data. And it can be made multidimensional, with patterns of order being searched for in isotopic distributions, in relative abundances of structurally related compounds, and in distributions of structural types.
From page 27...
... , which nearly doubles the known period in which these eukarya have existed.20 In another recent breakthrough driven by biomarker analysis, Brocks and co-workers report evidence of steranes,21 which are indicative of eukaryotic organisms, in 2.7 billion-year-old shales from Australia. Not only does this study demonstrate that biomarkers can survive for great lengths of time under geologically gentle conditions, but this evidence also strongly suggests that eukaryotic organisms were present 500 million to 1 billion years earlier than the fossil record indicates.
From page 28...
... Dissimilarity between the isotopic compositions of martian and terrestrial organic carbon would demonstrate that the martian carbon analyzed is indigenous to that planet, possibly the product of an extraterrestrial life process. Similarity between the isotopic compositions of organic carbon from the two planets could mean that the martian carbon analyzed is actually terrestrial contamination, or it could mean that fractionation processes operate in the same way on both planets.
From page 29...
... 25 in The Proterozoic Biosphere, a Multidisciplinary Study, J.W. Schopf and C
From page 30...
... 30 THE QUARANTINE AND CERTIFICATION OF MARTIAN SAMPLES FIGURE 3.4 A comparison of living cyanobacteria with their fossil counterparts. A, C, E, and G show living bacteria from stromatolitic microbial mats in Baja California, Mexico.
From page 31...
... and also such structures as tracks, trails, burrows, and carbonate microborings that evidence the presence of living systems but are themselves not organismal remnants;34~36 see Figure 1.2. Fossil evidence on this scale is unlikely to be found in the martian samples, although the possibility cannot be ruled out that a fragment of an ancient stromatolite will be found.
From page 32...
... CLOSING OBSERVATIONS in summary, there are significant limits on the ability to detect life, even in samples that are in-hand, on Earth. Methods based on molecular biology and PCR provide some of the best tools for detecting terrestrial contamination of samples, or Mars life that had an ancient common origin with terrestrial life.


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