Title:

Evidences for the presence of water oceans inside large icy moons: Implications for thermal evolution models

Authors:

Christophe Sotin

Abstract:

The Galileo mission detected the presence of an induced magnetic field around Europa, Callisto, and Ganymede. It implies the presence of a conductive layer at depth which can be a global conductive ocean. Four Cassini/Huygens measurements are performed for investigating the presence of a deep ocean within Titan: the induced magnetic field, the values of the degree two periodic gravity coefficients, the non- synchronous rotation, and the Schumann resonance. These different measurements are discussed: to date, the presence of a Schumann resonance is the most compelling measurement to suggest the presence of an ocean at about 45 km depth.

The presence of a deep ocean has implications for the thermal structure, the thermal evolution and the degree of differentiation of a satellite. Most models agree on the fact that a liquid layer formed in between the high-pressure ice layer (or silicate core for Europa) and the ice I crust during accretion. Maintaining a deep ocean over geological time-scales requires large internal energy sources or/and low freezing temperatures. After several decades of modeling, the following models seem to emerge. On Europa, the large amount of tidal heating would prevent a complete freezing of the liquid layer. Ganymede is very much differentiated with a liquid iron core suggesting that the large amount of accreting energy is still being released. Callisto and Titan would have ammoniac in their ocean, which decreases so much the freezing temperature that an ocean is still present after 4.5 Gyr of evolution. One issue that will be discussed is whether the presence of an ocean is consistent with a partly differentiated interior structure as suggested by the interpretation of the degree two gravity coefficients of Titan and Callisto.