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Europa: Chaos Terrains Formation and Its Exobiological Potential

Introduction: Processes and conditions of Europa's crust support assumptions about habitability of this icy satellite. A combined ice/liquid water shell ~80-170 km thick is indicated by Galileo space craft gravity measurements [1], this ice shell has undergone through geological resurfacing in a time period of ~10 million of years [2]. Different models predict tidal heating strong enough to maintain a global subsurface ocean of liquid water [3] and geothermal activity sufficient to cause regional melting of icy lithosphere [4, 5]. Features like chaotic terrains might be formed by this kind of processes [6], which will involve mixing of subsurface liquid water with ice material from surface. With cover of 40% of Europa's surface, obvious mixing of water and ice and past presence of thermal activity, the chaotic terrains present area where we should look for habitable zones of Europa. Geophysical background: Thermal activity on the sea floor can be the cause of the origin of the chaotic terrains. Previous models predicted global periodic thermal activities of rocky interior can occur within a period of ~10 8 years [9]. We have estimated that for a minimum increasing of water temperature of 1 0 C, a time period of 7x10 4 years is necessary [6]. This rate of changes in the temperature will cause significant melting of the ice shell. Such a global changes are probably followed by opening of regional vents on the purported ocean floor. Formation of chaotic terrains is more likely the result of internal regional processes driven by thermal perturbations. According to the thermodynamical model we used, ~3.5 km thick icy shell can be melt in 10 6-10 8 years [6]. This means that during this period emissions from the sea floor would be active, and create a stable and warm environment on the ocean floor. This warm environment can be extended to the circulating water column above. Convective currents may produce mixing of water from upper layer enriched in H 2 O 2 , H 2 SO 4 , CO 2 [7] with deep ocean water. Chaos-forming thermal processes leads to formation of lateral cracking and migration of rafts [4]. Due to the very low temperature (~110 K) on the surface, in areas of cracking and mixing of ice and water, is possible that the liquid water is not directly exposed to the surface, and forming slush. This could shield biocommunities in water from outer radiation influence.