Signs of Life A Report Based on the April 2000 Workshop on Life Detection Techniques (2002) / Chapter Skim
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Workshop Papers Session 1: Introduction to the Detection of Life
Workshop Papers Session 1: Introduction to the Detection of Life
Pages 53-78
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Session 1: Introduction to the Detection of Life .¢ 53
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The device called the "Wolf Trap" was developed and field-tested on Earth, but was not included onboard the Viking missions that landed the first life detection experiments on Mars in 1976. Another life detection instrument funded by NASA utilized a radioactive tracer, 14C, to measure the respiratory products of a growing culture of microorganisms.
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Deposition by meteoric infall. The three life detection experiments on the Viking missions were to detect metabolism, growth, or organic synthesis.
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Since they require unusual types of technology, most of those efforts have been supported through nongovernment funds. Acknowledgment NASA has initiated and developed the program of extraterrestrial life detection over the past 40 years.
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H.E. Newell, Beyond the Atmosphere: Early Years of Space Science, Scientific and Technical Information Branch, National Aeronautics and Space Administration, NASA SP-4211, Washington, D.C., 1980.
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Speculation on the nature of life elsewhere generally has paid little heed to constraints imposed by the nature of biochemistry, however. A century of fanciful science fiction writings has resulted not only in social enthusiasm for the quest for extraterrestrial life, but also in fanciful notions of the chemical and physical forms that life can take, what the nature of life can be.
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The simple organic molecules then are used as building blocks for large molecules. Amino acids, for instance, are used to construct the long chains of proteins; simple sugars combine with purine and pyrimidine bases and phosphate to construct the nucleic acids.
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Even in the absence of other biochemical information, genetic sequences could provide criteria to identify the evolutionary source of DNA-containing organisms and to distinguish terrestrial organisms from one another or from extraterrestrial. The gene sequence-based overview of terrestrial biological diversity is embodied in universal phylogenetic trees such as that shown in Figure 1.3 The construction of such a relatedness diagram is conceptually simple.
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\3 ~ FIGURE 1. Universal phylogenetic tree based on small-subunit ribosomal RNA sequences.
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Biochemistry and Molecular Biology in Life Detection Techniques in biochemistry and molecular biology are applicable to the detection and evaluation of life, regardless of its evolutionary origin. Thus, knowledge of the chemical requirements for metabolism can point to chemical settings where life may be found.
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2. A single biopolymer life form, presumably characteristic of all new life, will recruit cofactors with structures suited for binding to it.
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A decade ago, we suggested that the transition from a single biopolymer life form to a two-biopolymer system might have been slow on Earth, and discussed evidence for this hypothesis that could be found in contemporary metabolisms This USA. thnt ~in~le.-hinnolvm~.r life.
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From page 66... ...
The "palimpsest" of modern metabolism can be read exactly as such.~4 Reconstruction of the metabolism of the putative RNA world on early Earth suggests that cofactors having structural features analogous to the structure of the catalytic biopolymer might be one way to solve these contradictions. Should non-terrean samples become available and a putative genetic biopolymer identified based on its repeating charge, a rational chemical approach to confirm the suspicion of non-terrean life would be to identify small organic molecules that are plausible building blocks of the biopolymer carrying appended reactive groups not found in the biopolymer itself.
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Organic Molecules on the Surface of Mars Ultimately, the search for organic vestiges of life must involve experiments on non-terrean material. In designing these experiments, we must recognize that organic matter of nonbiogenic origin is abundant and may generate a large signal beneath which a smaller signal of biogenic organic material must be sought.
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From page 68... ...
In contemporary cellular life, two polymers are central to this process: Nucleic acids store and express genetic information, and proteins have structural and catalytic functions. In order to have the capacity for evolution, the two polymers are necessarily linked through a genetic code and translation process that couples mutational changes to catalytic function.
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Modern lipids are highly evolved products of several billion years of evolution and typically contain two hydrocarbon chains 16 to 18 carbons in length. However, much simpler amphiphilic molecules, including fatty acids, can also form reasonably stable vesicles composed of bilayer membranes.2~ 22 Furthermore, permeability is strongly dependent on chain length, so that shortening the chains of a given membrane lipid dramatically increases permeation rates of ionic solutes.23 Such liposomes can encapsulate enzymes, nucleic acids, and other macromolecules, yet they are sufficiently permeable to allow influx of smaller solutes that are potential substrates.
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Again, the ionic substrate ADP is able to penetrate the bilayer membranes fast enough to supply the enzyme with substrate for RNA synthesis. The encapsulated RNA polymerase provides a partial answer to the question of substrate permeation in a primitive cell.
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Relevance to Life Detection Given that terrestrial life has a system of replicating polymers at its core, it is reasonable to assume that life elsewhere might also incorporate linear polymers as a structural and functional framework. It follows that life detection, for instance, in the europan ocean, could involve a search for linear ionic polymers that have been released into the environment by extant life, just as plasmids and nucleic acid fragments are released by many terrestrial microorganisms.
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A pore in a lipid bilayer membrane separates two compartments containing a salt solution such as 1.0 M KC1. A voltage imposed across the bilayer causes an ionic current to flow through the pore of the channel.
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A nanopore is able to detect single molecules of linear ionic polymers such as nucleic acids, and to identify them according to unique signatures related to electrical signals that are generated when the molecule is driven through the pore. Nanopores can also be designed to detect particles ranging in size from nanoscopic to microscopic, and a nanopore instrument would therefore be useful for extant life detection missions to Europa and Mars.
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The absence of liquid water at the surface would inhibit oxygen loss by weathering, whereas the lack of volcanism would eliminate oxygen loss by reaction with reduced volcanic gases. Oxygen left behind by H2O photodissociation followed by hydrogen escape could therefore accumulate indefinitely in such a planet's atmosphere.
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This presumably represents the time at which net O2 production from photosynthesis followed by organic carbon burial exceeded the rate of supply of reduced volcanic gases.55 If so, the atmosphere prior to that time should have been almost completely anoxic. Some O2 should have been present at high altitudes as a consequence of CO2 photolysis, but the ground-level concentration should have been vanishingly small.56 The column depth of O3 would have been much less than is required to produce a strong 9.6-,um band.
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The abiotic source would then be within a factor of 25 of the present biological source (Figure 24. But the intensity of midocean ridge volcanism might also have been much higher in the past as a consequence of higher geothermal heat flow.
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From page 77... ...
Powell, "The Missing Organic Molecules on Mars," Proc.
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From page 78... ...
J.E. Lovelock, "A Physical Basis for Life Detection Experiments,"Nature 207:568-570, 1965.
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