Astrobiology through the Ages of Mars: The Study of Terrestrial Analogues to Understand the Habitability of Mars Alberto G. Faire ´ n, 1,2 Alfonso F. Davila, 1 Darlene Lim, 1,2 Nathan Bramall, 1 Rosalba Bonaccorsi, 1,2 Jhony Zavaleta, 2 Esther R. Uceda, 2 Carol Stoker, 2 Jacek Wierzchos, 3 James M. Dohm, 4 Ricardo Amils, 5 Dale Andersen, 1 and Christopher P. McKay 2 Abstract Mars has undergone three main climatic stages throughout its geological history, beginning with a water-rich epoch, followed by a cold and semi-arid era, and transitioning into present-day arid and very cold desert conditions. These global climatic eras also represent three different stages of planetary habitability: an early, potentially habitable stage when the basic requisites for life as we know it were present (liquid water and energy); an intermediate extreme stage, when liquid solutions became scarce or very challenging for life; and the most recent stage during which conditions on the surface have been largely uninhabitable, except perhaps in some isolated niches. Our understanding of the evolution of Mars is now sufficient to assign specific terrestrial environments to each of these periods. Through the study of Mars terrestrial analogues, we have assessed and constrained the habitability conditions for each of these stages, the geochemistry of the surface, and the likeli- hood for the preservation of organic and inorganic biosignatures. The study of these analog environments provides important information to better understand past and current mission results as well as to support the design and selection of instruments and the planning for future exploratory missions to Mars. Key Words: Mars—Astrobiology—Mars analogues—Climatic evolution of Mars. Astrobiology 10, 821–843. 1. Introduction: The Evolution of Knowledge T he search for evidence of extant or extinct life on Mars began with the Viking mission (Klein, 1999). The am- biguous results of the experiments, interpreted by most to be due to chemical reactions (Klein, 1978, 1979; see a counter argument in Levin and Straat, 1981), largely re-shaped our approach to searching for life on Mars. Subsequent missions have focused on understanding the climatic and aqueous evolution of the planet in an attempt to derive present and past habitability models, which can be used later to target specific surface and near-subsurface localities with the highest potential to contain evidence of extant or fossilized life. The first step in assessing the habitability of the surface began with the search for past or present existence of water. Evidence of liquid water flowing and ponding on the surface of the planet is found over most of the martian landscape, highlighted collectively by orbiter, lander, and rover missions. The scale and diversity of the martian hy- drological regime is evidenced by the variety of aqueous environments that have been identified, which include ocean-related landforms (Parker et al., 1993; Clifford and Parker, 2001; Faire ´n et al., 2003; Faire ´n, 2010a), a large plain surrounding the north pole that resembles a sediment-filled ocean basin with shoreline features (Faire ´n et al., 2003; Perron et al., 2007; Faire ´n, 2010a), anastomosing and meandering rivers and deltas (Malin and Edgett, 2003; Di Achille and Hynek, 2010), massive layered outcrops interpreted as water-deposited sediments (Malin and Edgett, 2000), cross- stratification in sedimentary outcrops (Squyres et al., 2004), water-related mineralogies that extend over regional scales (Hynek, 2004; Squyres et al., 2004; Arvidson et al., 2005; Poulet et al., 2005; Mustard et al., 2008; Wray et al., 2009; Carter et al., 2010), and elemental soil compositions in re- gions suspected to have been occupied by oceans when compared to other regions of Mars (Dohm et al., 2009). 1 SETI Institute, Mountain View, California, USA. 2 Space Science and Astrobiology Division, NASA Ames Research Center, Moffett Field, California, USA. 3 Instituto de Recursos Naturales, CSIC, Madrid, Spain. 4 Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona, USA. 5 Centro de Astrobiologı ´a (INTA-CSIC), Madrid, Spain. ASTROBIOLOGY Volume 10, Number 8, 2010 ª Mary Ann Liebert, Inc. DOI: 10.1089/ast.2009.0440 821