Cryoprotective properties of water in the Earth cryolithosphere and its role in exobiology.

In permanently frozen rocks, water occurs in all the three phases and plays a dual role from the biological point of view. About 93-98% of it is in the solid state. This, alongside with negative temperatures, contributes to cell cryoconservation. The remaining 2-7% is in the unfrozen state and represents thin films enveloping organic-mineral particles. These films play the role of cryoprotectors against cell damage by ice crystals during geologically significant time. Electron microscope examinations of prokaryotes revealed the well preserved outer cell structures, specifically strong envelopes and capsules. The cells are resistant to water phase transitions through 0 degrees C, i.e. to the freezing-thawing stress. The exobiological implication of this phenomenon is determined by the fact that the Earth permafrost at first approximation can he considered as a model of e.g. the Mars one. The latter protects the cells against radiation and simultaneously serves as a cryoconservant. However, most important is the possible presence of unfrozen (= liquid) water as prerequisite for the development of microbial life forms.