Biota of the Greenland ice sheet (120,000 years old) and Antarctic ice sheet (~400,000 years old) have been widely studied to depths of more than 3 km (Abyzov 1993; Kapitsa et al. 1996; Priscu et al. 1998; Karl et al 1999; Petit et al. 1999; Skidmore et al. 2000; Deming 2002; Miteva et al. 2004; Miteva and Brenchley 2005). The age of the oldest glacial ice, as well as immured bacteria, is still under discussion: >500,000 years old at Guliya ice cap on the Tibetan Plateau (Thompson et al. 1997; Christner et al. 2003), ~2 million years at the bottom of the Vostok ice core (Salamatin et al. 2004) or even ~8.1 million years (Sugden et al. 1995; Bidle et al. 2007) in Beacon Valley, Antarctica. The data from Vostok cores showed that the upper young (<12,000 years old) layers are the most abundant ones, in spite of extremely low temperatures of -50°C (Abyzov 1993).
The number of mostly air-born microorganisms isolated from snow and seasonal ice covers are not high (102 cells ml-1) and are of the same order as viable cells within the cores of ancient ice sheets. This fact could be interpreted as the absence of reduction of the microbial population once immured in ice during thousands of years, and could be explained by the near-zero background radiation in the ice (~2-4mGy per year, 0.23 mGy h-1).
Ice sheets are considered to be the Earths most representative analogues of icy habitats like Jupiter's ice-covered moon Europa, the icy moon in Saturn's system Enceladus, and firstly, ice caps on Martian poles. Correspondingly, microorganisms isolated from the ice cores of both hemispheres, and traces of life, such as genomic DNA well-preserved in ice cores (Willerslev et al. 1999; Christner et al. 2001), have been interpreted to be most representative analogues of inhabitants, and their fingerprints exist within these extraterrestrial icy habitats. The age of permanent Martian water-ice polar caps could be established on the basis of the amount of impact craters. On the north cap, large craters were not found, indicating the young geological age of the cap surface (not more than 100,000 years). On the south polar cap, 15 craters with a diameter >800 m were found, indicating the geological age of the cap surface to be about 7-17 million years (Hvidberg 2005). Because ice thaws under geostatic pressure, even at subzero temperatures, the existence of very old microorganisms is unlikely on the above-mentioned moons and caps. However, they and probably immured microorganisms are of the same order of age as on the Earth's ice sheets.
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