Conditions for the existence of sub-surface oceans on icy satellites and large KBO's

Abstract

One important result of the Galileo mission is the indirect evidence for oceans in the interiors of the icy Galilean satellites. This can be inferred from the magnetometer data, showing signals of induced fields, which are generated at shallow depth beneath the outermost ice layers of Europa, Ganymede, and Callisto, as a response to the time-varying Jovian magnetic field. So far, the only explanation for the induced fields are currents generated in a global liquid water layer containing ions (salts or ammonia). Subsurface oceans are consistent with thermodynamic models of the satellites, in which the heat production (due to radiogenic heating in the rocky component, and in case of Europa also due to tidal heating) and the rate of heat transfer are in equilibrium. If volatiles, like e.g. ammonia are taken into account, the possibility of liquid water layers is increasing because of the reduced melting temperature of water ice. In general, subsurface oceans are expected especially in large icy bodies, e.g. the icy Galilean satellites, Titan, and Triton, because of the slower cooling and the larger total amount of rocks and thus radiogenic heat sources, as compared to smaller bodies. However, depending on the ammount of volatiles incorporated in the icy component during accretion, oceans cannot be ruled out for the largest of the medium-sized satellites of Saturn and Uranus. Provided, that they are differentiated, the larger Kuiper-belt objects, like Pluto, Charon, Sedna, Quaoar, Ixion, and other 1000-km-class objects may also harbour internal oceans. On the basis of thermal modelling we investigate such a possibility for the mentioned objects. Because of the lack of knowledge on interior structure, a wide range of parameter variations, e.g. for the rock or ammonia content, is necessary. Triton, Pluto, Charon, and the satellites of Saturn and Uranus may serve as analogues for the bulk structure of the considered Kuiper-belt objects.