Sub-surface oceans on mid-sized icy satellites and large TNO's

Kurzfassung

The detection of induced magnetic fields in the vicinity of the Jovian satellites Europa, Ganymede, and Callisto was one of the most surprising findings of the Galileo mission to Jupiter. The observed magnetic signature cannot be generated in solid ice or in silicate rock. It rather suggests the existence of electrically conducting reservoirs of liquid water beneath the satellites' outermost icy shells that may contain even more water than all terrestrial oceans combined. The maintenance of liquid water layers at a depth of several tens of kilometers is closely related to the internal structure, chemical composition, and thermal state of the corresponding satellite interior. Controlling parameters for sub-surface ocean formation are the radiogenic heating rate of the silicate component, additional contributions due to, e.g., the dissipation of tidal energy in case of Europa, and the effectiveness of the heat transfer to the surface. Furthermore, the melting temperature of ice will be significantly reduced by small amounts of salts and/or incorporated volatiles such as methane and ammonia that are highly abundant in the outer solar system. In general, large icy bodies such as, e.g. the icy Galilean satellites, Titan and Triton, are more likely to harbour sub-surface oceans because of the slower cooling rate and the more intense radiogenic heating caused by their larger rock mass fractions, as compared to smaller icy bodies. However, depending on the amount of volatiles incorporated in the icy component during accretion, internal oceans cannot be ruled out for the largest of the medium-sized satellites of Saturn and Uranus and the biggest Transneptunian objects (TNO's), provided they are differentiated into a rock core and a water ice/liquid shell . Based on this assumption and using an equilibrium condition between the heat production rate in the rocky cores and the heat flow through the ice shell, we find that oceans are possible on Rhea, Titania, Oberon, Triton, and Pluto and on the largest TNO's Sedna and 2004 DW. The presence of oceans requires that small amounts of ammonia are available. The liquid sub-surface reservoirs are situated deeply underneath an ice-I shell of more than 100 km thickness. However, they may be indirectly detectable by their interaction with the surrounding magnetic fields and charged particles and by the magnitude of the satellite's response to tides exerted by the primary. The latter is strongly dependent on the occurrence of a sub-surface ocean which provides greater flexibility to the satellite's rigid outer ice shell. Time-dependent models show that even small satellites may have harboured oceans in the past due to the then higher radiogenic heat production rate. Apart from the above mentioned objects, past oceans are obtained for Dione, Iapetus, Ariel, Umbriel, Charon, the Kuiperbelt objects Quaoar and Ixion, and other 1000-km-class objects. We do not find indications for past oceans in case of Tethys, Enceladus, Miranda, and Mimas as the smallest object considered in this study.