Hard data indicate that biota survive over geological periods at subzero temperatures within the terrestrial cryosphere — ice sheets and permafrost. In such environments, dehydration leads to a considerable decrease of biochemical and metabolical activities. This allows the survival of ancient microbial communities that can physiologically and biochemically adapt much better in the cryosphere than in any other known habitat. The long-term subzero temperature regime of the cryosphere is not a limiting but a stabilizing factor. Organisms adapted to such balanced conditions represent a significant part of the biosphere, the cryobiosphere. Their ability to survive on a geological scale forces us to redefine the spatiotemporal limits of terrestrial and extraterrestrial biospheres.

Most planets of the Solar system, as well as their moons, asteroids, and comets, are of cryogenic nature, and the cryosphere is a common phenomenon in the cosmos. This is why the cells, their metabolic byproducts and bio-signatures (biominerals, bio-molecules and bio-gases) found in the Earth's cryosphere provide a range of analogues that could be used in the search for possible ecosystems and potential inhabitants on extraterrestrial cryogenic bodies. If life ever existed on other planets during the early stages of their development, then its traces may consist of primitive cell forms. Similar to life on Earth, they might have been preserved and could be found at depths within the ice or permafrost.

Most intriguing are the traces of past or existing life on Mars; these traces are of interest due to upcoming missions. Mars is the fourth and outermost Earth-like planet from the Sun, with an orbit between the Earth and the belt of asteroids. The orbits of both Earth and Mars are located in an intermediate position between Mercury and Venus, which are close to the Sun and therefore dehydrated, and the planets of the Jupiter group, mostly composed of volatile hydrogen, methane, and

David A. Gilichinsky

Soil Cryology Laboratory, Institute of Physicochemical & Biological Problems in Soil Sciences, Russian Academy of Sciences, 142290, Pushchino, Moscow Region, Russia e-mail: [email protected]

R. Margesin (ed.) Permafrost Soils, Soil Biology 16,

DOI: 10.1007/978-3-540-69371-0, © Springer-Verlag Berlin Heidelberg 2009

water. Due to such an astronomical location, Mars is known to be the only solid planet that, similar to the Earth, contains abundant water supplies that form the hydrosphere (Kasting 2003; Baker et al. 2005). The existence, quantity and phase of water on Mars during geological history and at present play a leading role in the life-searching theory based on comparison of terrestrial and Martian conditions. The exploration of Mars by spacecrafts started in 1962 and has included seven Soviet, one European, and 13 US missions. The information collected by these vehicles formed the general view of the Martian hydro- and cryospheres, and the "Mars Odyssey" observations of neutron fluxes that found water in the subsurface layer (Boynton et al. 2002) indicated Mars a "water reach" planet, where surface water mostly exists in the form of ice due to subzero temperatures. Similar to the Earth, these spheres unite into one at subzero temperatures, and the underground water represents only hydrosphere at depths below the zero isotherm.

Because of unfavorable factors, such as high irradiation intensity (~300-500 |Gy per day), absence of water etc., life is unlikely to exist on the surface, and no terrestrial habitats duplicate Martian conditions. Anderson et al. (1972) and Cameron and Morelli (1974) first advanced the idea of using terrestrial permafrost analogues, and this chapter considers these analogues as a bridge to possible Martian life forms and shallow subsurface habitats where the probability of finding life is highest. Since there is a place for water, the requisite condition for life, the analogous models are more or less realistic.

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