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  • 1
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    Did Mars once have martians? (1993)
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    Publication Date: 2011-08-24
    Description: Pictures of Mars from the Viking and Mariner 9 missions provide evidence of liquid water in Mars' past, indicating a warmer climate. It is speculated that outflow channels and valley networks were formed 3.5 to 4 billions years ago, at the same time life was developing on Earth. Current theories suggest that chemical reactions in Mars' early atmosphere depleted the original amounts of carbon dioxide with no mechanism to recycle it back into the atmosphere. Ecosystems in the cold, dry Antarctic valleys provide models of Mars on Earth and indicate that if life evolved on Mars there may be organic or fossil evidence under the surface. Lake beds on Mars surface are considered to be the best place to hunt for fossils.
    Keywords: Exobiology
    Type: Astronomy (ISSN 0091-6358); Volume 21; 27-33
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  • 2
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    Origins of life: a comparison of theories and application to Mars (1996)
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    Publication Date: 2011-08-24
    Description: The field of study that deals with the origins of life does not have a consensus for a theory of life's origin. An analysis of the range of theories offered shows that they share some common features that may be reliable predictors when considering the possible origins of life on another planet. The fundamental datum dealing with the origins of life is that life appeared early in the history of the Earth, probably before 3.5 Ga and possibly before 3.8 Ga. What might be called the standard theory (the Oparin-Haldane theory) posits the production of organic molecules on the early Earth followed by chemical reactions that produced increased organic complexity leading eventually to organic life capable of reproduction, mutation, and selection using organic material as nutrients. A distinct class of other theories (panspermia theories) suggests that life was carried to Earth from elsewhere--these theories receive some support from recent work on planetary impact processes. Other alternatives to the standard model suggest that life arose as an inorganic (clay) form and/or that the initial energy source was not organic material but chemical energy or sunlight. We find that the entire range of current theories suggests that liquid water is the quintessential environmental criterion for both the origin and sustenance of life. It is therefore of interest that during the time that life appeared on Earth we have evidence for liquid water present on the surface of Mars.
    Keywords: Exobiology
    Type: Origins of life and evolution of the biosphere : the journal of the International Society for the Study of the Origin of Life (ISSN 0169-6149); Volume 26; 1; 61-73
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  • 3
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    Restoring Mars to habitable conditions: Can we? Should we? Will we? (1993)
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    Publication Date: 2011-08-24
    Description: Two scenarios are presented for terraforming Mars into a habitable state: one state suitable for plants and another with an atmosphere suitable for human habitation. Studies of Mars indicate enough necessary elements to provide raw materials. Transformation would take two stages: the first to warm the planet and release carbon dioxide, the second to convert carbon dioxide into oxygen and build up nitrogen in the atmosphere. Present knowledge indicates that it is possible to transform Mars into a habitable state. Environmental ethical concerns are reviewed from three axioms: human intervention is ultimately harmful to the environment, stewardship, and intrinsic worth.
    Keywords: Exobiology
    Type: Journal of the Irish Colleges of Physicians and Surgeons (ISSN 0374-8405); Volume 22; 1; 17-9
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  • 4
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    Thickness of tropical ice and photosynthesis on a snowball Earth (2000)
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    Publication Date: 2011-08-24
    Description: On a completely ice-covered "snowball" Earth the thickness of ice in the tropical regions would be limited by the sunlight penetrating into the ice cover and by the latent heat flux generated by freezing at the ice bottom--the freezing rate would balance the sublimation rate from the top of the ice cover. Heat transfer models of the perennially ice-covered Antarctic dry valley lakes applied to the snowball Earth indicate that the tropical ice cover would have a thickness of 10 m or less with a corresponding transmissivity of 〉 0.1%. This light level is adequate for photosynthesis and could explain the survival of the eukaryotic algae.
    Keywords: Exobiology
    Type: Geophysical research letters (ISSN 0094-8276); Volume 27; 14; 2153-6
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  • 5
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    Photochemical modeling of Titan's atmosphere (1995)
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    Publication Date: 2011-08-24
    Description: We have developed a new photochemical model of Titan's atmosphere which includes all the important compounds and reactions in spherical geometry from the surface to 1240 km. Compared to the previous model of Yung et al. (1984), the most significant recent change in the reactions used is the updated methane dissociation scheme (Mordaunt et al. 1993). Moreover, the transfer of the solar radiation in the atmosphere and the photolysis rates have been calculated by using a Monte Carlo code. Finally, the eddy diffusion coefficient profile is adjusted in order to fit the mean vertical distribution of HCN retrieved from millimeter groundbased observations of Tanguy et al. (1990); using new values for the boundary flux of atomic nitrogen (Strobel et al. 1992). We have run the model in both steady-state and diurnal modes, with 62 species involved in 249 reactions. There is little difference between diurnal and steady-state results. Overall our results are in a closer agreement with the abundances inferred from the Voyager infrared measurements at the equator than the Yung et al. results. We find that the catalytic scheme for H recombination invoked by Yung et al. only slightly improves the model results and we conclude that this scheme is not essential to fit observations.
    Keywords: INORGANIC AND PHYSICAL CHEMISTRY
    Type: Icarus (ISSN 0019-1035); 113; 1; p. 2-26
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  • 6
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    The physics, biology, and environmental ethics of making mars habitable (2001)
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    Publication Date: 2011-08-24
    Description: The considerable evidence that Mars once had a wetter, more clement, environment motivates the search for past or present life on that planet. This evidence also suggests the possibility of restoring habitable conditions on Mars. While the total amounts of the key molecules--carbon dioxide, water, and nitrogen--needed for creating a biosphere on Mars are unknown, estimates suggest that there may be enough in the subsurface. Super greenhouse gases, in particular, perfluorocarbons, are currently the most effective and practical way to warm Mars and thicken its atmosphere so that liquid water is stable on the surface. This process could take approximately 100 years. If enough carbon dioxide is frozen in the South Polar Cap and absorbed in the regolith, the resulting thick and warm carbon dioxide atmosphere could support many types of microorganisms, plants, and invertebrates. If a planet-wide martian biosphere converted carbon dioxide into oxygen with an average efficiency equal to that for Earth's biosphere, it would take 〉 100,000 years to create Earth-like oxygen levels. Ethical issues associated with bringing life to Mars center on the possibility of indigenous martian life and the relative value of a planet with or without a global biosphere.
    Keywords: Exobiology
    Type: Astrobiology (ISSN 1531-1074); Volume 1; 1; 89-109
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  • 7
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    Organic synthesis in experimental impact shocks (1997)
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    Publication Date: 2011-08-24
    Description: Laboratory simulations of shocks created with a high-energy laser demonstrate that the efficacy of organic production depends on the molecular, not just the elemental composition of the shocked gas. In a methane-rich mixture that simulates a low-temperature equilibrium mixture of cometary material, hydrogen cyanide and acetylene were produced with yields of 5 x 10(17) molecules per joule. Repeated shocking of the methane-rich mixture produced amine groups, suggesting the possible synthesis of amino acids. No organic molecules were produced in a carbon dioxide-rich mixture, which is at odds with thermodynamic equilibrium approaches to shock chemistry and has implications for the modeling of shock-produced organic molecules on early Earth.
    Keywords: Exobiology
    Type: Science (ISSN 0036-8075); Volume 276; 5311; 390-2
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  • 8
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    Hydrogen consumption by methanogens on the early Earth (1998)
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    Publication Date: 2011-08-24
    Description: It is possible that the first autotroph used chemical energy rather than light. This could have been the main source of primary production after the initial inventory of abiotic organic material had been depleted. The electron acceptor most readily available for use by this first chemoautotroph would have been CO2. The most abundant electron donor may have been H2 that would have been outgassing from volcanoes at a rate estimated to be as large as 10(12) moles yr-1, as well as from photo-oxidation of Fe+2. We report here that certain methanogens will consume H2 down to partial pressures as low as 4 Pa (4 x 10(-5) atm) with CO2 as the sole carbon source at a rate of 0.7 ng H2 min-1 microgram-1 cell protein. The lower limit of pH2 for growth of methanogens can be understood on the basis that the pH2 needs to be high enough for one ATP to be synthesized per CO2 reduced. The pH2 values needed for growth measured here are consistent with those measured by Stevens and McKinley for growth of methanogens in deep basalt aquifers. H2-consuming autotrophs are likely to have had a profound effect on the chemistry of the early atmosphere and to have been a dominant sink for H2 on the early Earth after life began rather than escape from the Earth's atmosphere to space.
    Keywords: Exobiology
    Type: Origins of life and evolution of the biosphere : the journal of the International Society for the Study of the Origin of Life (ISSN 0169-6149); Volume 28; 3; 311-9
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  • 9
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    [Life on Mars] (1990)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: No abstract available
    Keywords: Exobiology
    Type: La Recherche (ISSN 0029-5671); Volume 21; 225; 1216-24, 35
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  • 10
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    Preliminary report on radiocarbon dating of cryptoendolithic microorganisms (1988)
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    Publication Date: 2011-08-24
    Description: The existence of microbial communities living inside desert rocks has been reported by FRIEDMANN et al. (1967, 1976), first in rocks collected from the hot and dry Negev desert and later in rocks in the frigid Ross Desert of Southern Victoria Land, Antarctica. The extremely inhospitable climatic conditions in both places has led to the suggestion that these organisms have very low rates of metabolism and may, in addition, be very old (FRIEDMANN 1982). Our preliminary measurements showed a 14C deficiency indicating a carbon age in the order of magnitude of 10(3) years.
    Keywords: Exobiology
    Type: Polarforschung (ISSN 0032-2490); Volume 58; 2-3; 199-200
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