Tidal heating

Both tectonic cracks and thermally produced Chaotic terrain are caused by tides, and both involve linkage of the underlying ocean to the surface. The darkening associated with each of these formation processes probably involves substances from the ocean, which makes the features visible even at the low resolution of global images like Figure 5. The cracks are caused by tidal stress and the chaos is probably caused by melt-through of patches of crust by tidal heat. 

However, even a completely solid Europa would rotate non-synchronously as long as mass asymmetries are small enough. Non-synchronous rotation does not in itself imply the existence of an ocean. However, both the existence of an ocean and Non-synchronous rotation are made possible by the substantial tidal heating. 

It was a big surprise when on lo active volcanism was detected on its surface (see Eig. 4.1). On Earth, the heat source that produces volcanic activity comes from its interior where radioactive materials decay and release energy and also from heat left over from its formation, accretion heat. This however cannot explain the volcanism on lo. The satellite is too small and should have cooled out. The only mechanism to maintain active volcanism on lo is tidal heating (see also Moore, 2003 [236]). The semi major axis of the orbit of lo is 421800 km so it is close to Jupiter and exposed to strong tidal forces. One revolution around Jupiter takes only 1 d 18 h 27 min. The albedo of lo is 0.61 which is a relatively high value. Other parameters... 

Fig. 4.1 The Jupiter satellite lo due to tidal heating the body with the most active volcanism in the Solar System. Courtesy NASA/Galileo mission, JPL...

Fig. 4.2 The Jupiter sateUite Europa due to tidal heating beneath an ice curst an ocean of liquid water is highly probable. Courtesy NASA/GaUleo mission, JPL...

Sotin, Head and Tobie (2002, [313]) showed that the ice viscosity on Europa in connection with tidal heating will lead to heating of rising diapirs. This would result in a disruption of near-surface material and the formation of lenticulae and chaos. Also a relatively thick ice sheet of 20 km overlying an ocean might produce such effects. 

Tethys might have had a subsurface liquid ocean during its early history (Chen and Nimmo, 2008 [66]) when there was a 2 3 orbital resonance between Dione and Tethys. The resonance would have led to orbital eccentricity and tidal heating that may have warmed Tethys interior enough to form the ocean. Subsequent freezing of the ocean after the moons escaped from the resonance may have generated the extensional stresses that created Ithaca Chasma. 

Sotin, C., Head, J.W., Tobie, G. Europa Tidal heating of upwelling thermal plumes and the origin of lenticulae and chaos melting. Geophys. Res. Lett. 29(8), 080000 (2002)... 

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