title: Biosignatures on Mars: What, Where, and How? Implications for the Search for Martian Life.
abstract: The search for traces of life is one of the principal objectives of Mars exploration. Central to this objective is the concept of habitability, the set of conditions that allows the appearance of life and successful establishment of microorganisms in any one location. While environmental conditions may have been conducive to the appearance of life early in martian history, habitable conditions were always heterogeneous on a spatial scale and in a geological time frame. This "punctuated" scenario of habitability would have had important consequences for the evolution of martian life, as well as for the presence and preservation of traces of life at a specific landing site. We hypothesize that, given the lack of long-term, continuous habitability, if martian life developed, it was (and may still be) chemotrophic and anaerobic. Obtaining nutrition from the same kinds of sources as early terrestrial chemotrophic life and living in the same kinds of environments, the fossilized traces of the latter serve as useful proxies for understanding the potential distribution of martian chemotrophs and their fossilized traces. Thus, comparison with analog, anaerobic, volcanic terrestrial environments (Early Archean >3.5-3.33 Ga) shows that the fossil remains of chemotrophs in such environments were common, although sparsely distributed, except in the vicinity of hydrothermal activity where nutrients were readily available. Moreover, the traces of these kinds of microorganisms can be well preserved, provided that they are rapidly mineralized and that the sediments in which they occur are rapidly cemented. We evaluate the biogenicity of these signatures by comparing them to possible abiotic features. Finally, we discuss the implications of different scenarios for life on Mars for detection by in situ exploration, ranging from its non-appearance, through preserved traces of life, to the presence of living microorganisms.
  
  
  BACKGROUND
  Mars-Early Earth-Anaerobic chemotrophs-Biosignatures-Astrobiology missions to Mars.
authors.affiliation: 1 CNRS-OSUC-Centre de Biophysique Moléculaire , CS80054, Orléans, France .
authors.forename: Frances
authors.initials: F
authors.lastname: Westall
authors.affiliation: 1 CNRS-OSUC-Centre de Biophysique Moléculaire , CS80054, Orléans, France .
authors.forename: Frédéric
authors.initials: F
authors.lastname: Foucher
authors.affiliation: 2 CNRS-Conditions Extrêmes et Matériaux: Haute Température et Irradiation , CS90055, Orléans, France .
authors.forename: Nicolas
authors.initials: N
authors.lastname: Bost
authors.affiliation: 1 CNRS-OSUC-Centre de Biophysique Moléculaire , CS80054, Orléans, France .
authors.forename: Marylène
authors.initials: M
authors.lastname: Bertrand
authors.affiliation: 3 ESA-ESTEC , Noordwijk, The Netherlands .
authors.forename: Damien
authors.initials: D
authors.lastname: Loizeau
authors.affiliation: 3 ESA-ESTEC , Noordwijk, The Netherlands .
authors.forename: Jorge L
authors.initials: JL
authors.lastname: Vago
authors.affiliation: 3 ESA-ESTEC , Noordwijk, The Netherlands .
authors.forename: Gerhard
authors.initials: G
authors.lastname: Kminek
authors.affiliation: 1 CNRS-OSUC-Centre de Biophysique Moléculaire , CS80054, Orléans, France .
authors.forename: Frédéric
authors.initials: F
authors.lastname: Gaboyer
authors.affiliation: 4 School of Environment, The University of Auckland , Auckland, New Zealand .
authors.forename: Kathleen A
authors.initials: KA
authors.lastname: Campbell
authors.affiliation: 5 GéoHydrosytèmes Continentaux, Faculté des Sciences et Techniques, Université François-Rabelais de Tours , Tours, France .
authors.forename: Jean-Gabriel
authors.initials: JG
authors.lastname: Bréhéret
authors.affiliation: 6 CNRS-OSUC-Institut des Sciences de la Terre d'Orléans , Orléans, France .
authors.forename: Pascale
authors.initials: P
authors.lastname: Gautret
authors.affiliation: 7 School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK .
authors.forename: Charles S
authors.initials: CS
authors.lastname: Cockell
mesh_terms: D018559:Exobiology; D018535:Mars