Soil Cover

Water ice within the top metres of the high-latitude regolith, as well as visual similarities on the terrestrial and Martian surfaces (polygons formed by frost cracking), lead to the consideration of frost-affected, seasonally thawed soil cover with a mean annual temperature below 0°C underlain by permafrost as an extraterrestrial analogue. The leading factor in differentiation of these soils, named cryosol, is temperature crossing through 0°C, resulting in freezing-thawing processes and ice-water phase exchange. Temperature oscillations crossing through the freezing point are also observed on the Martian surface. With respect to Mars it is important to note that cryosol microbial communities, formed under the impact of multi-time freezing-thawing stress, did not change under such stress. Their maximal number and biodiversity correlate with the horizon A, decrease with depth from the surface beneath the seasonal thaw layer, and have an accumulative sharp peak on the permafrost table. In spite of the tundra, the day surface is under the influence of solar radiation, the snow and vegetation covers decrease and minimize this impact, as well as temperature oscillations. Thus, Arctic cryosol has distant similarities with the Martian surface.

The surface conditions in the Antarctic desert (the intensive level of solar radiation, the absence of snow and vegetation covers, and the ultra-low subzero temperatures, which can be as low as -60°C) and on Mars are closer. At elevations above 1,500 m, there are no summer air temperatures above freezing. However, the surface temperatures of soil or rock can be 15°C warmer than the air temperature due to solar heating, may exceed 0°C for several hours (McKay et al. 1998), and for short periods even reach 10°C (Campbell and Claridge 1987). In addition to sharp temperature oscillations and high insolation, the main similarity between Antarctic Dry Valleys and Mars is the vertical structure of their "active layers". In the Dry Valleys, the upper 10-25 cm-thick sandy layer does not form a stable soil cover on the ice-cemented permafrost table. It is dry (water content ~2%) and lacks ice-cement due to sublimation. This frosty ground throughout the upper 100 cm (including the active layer) covers 61% of Dry Valley's area (Bockheim et al. 2007). The overcooled ground, with no water and thus no ice, is often mobilized by storm winds similar to the instability of Martian dunes. Such double-layering structure and distribution of water ice within the first surface metre on Mars (dry top layer and ice-rich bottom layer) is proposed according to HEND/Odyssey and MOLA/MGS data (Mitrofanov et al. 2007), and consistent with present knowledge of environments on Mars. This is why Dry Valley's active layer which overlies permafrost could be considered as an analogue of the dry regolith layer on Mars.

The upper ~2 cm layer of the Dry Valley surface often contains a low number of viable cells compared with the underlying horizons (Horowitz et al. 1972). In some cases, these microorganisms cannot be isolated on agar plates, and correlate with a poor diversity of bacterial phylotypes, a low number of mycelia fungi strains, and a minimum of chlorophyll content. The occurrence and biodiversity of microorganisms is higher at depth than in the top of the active layer, and suggests that a search for life on Mars should not sample the surface but the bottom of the "active layer". In particular because the upper horizons contain low cell counts, Antarctic frosty soils are useful for testing equipment for searching for life on Mars (Gilichinsky et al. 2007a).

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