STRONG TIDAL DISSIPATION IN SATURN AND CONSTRAINTS ON ENCELADUS' THERMAL STATE FROM ASTROMETRY

Abstract

Tidal interactions between Saturn and its satellites play a crucial role in\nboth the orbital migration of the satellites and the heating of their\ninteriors. Therefore constraining the tidal dissipation of Saturn (here the\nratio k2/Q) opens the door to the past evolution of the whole system. If\nSaturn's tidal ratio can be determined at different frequencies, it may also be\npossible to constrain the giant planet's interior structure, which is still\nuncertain. Here, we try to determine Saturn's tidal ratio through its current\neffect on the orbits of the main moons, using astrometric data spanning more\nthan a century. We find an intense tidal dissipation (k2/Q= (2.3 \\pm 0.7)\n\\times 10-4), which is about ten times higher than the usual value estimated\nfrom theoretical arguments. As a consequence, eccentricity equilibrium for\nEnceladus can now account for the huge heat emitted from Enceladus' south pole.\nMoreover, the measured k2/Q is found to be poorly sensitive to the tidal\nfrequency, on the short frequency interval considered. This suggests that\nSaturn's dissipation may not be controlled by turbulent friction in the fluid\nenvelope as commonly believed. If correct, the large tidal expansion of the\nmoon orbits due to this strong Saturnian dissipation would be inconsistent with\nthe moon formations 4.5 Byr ago above the synchronous orbit in the Saturnian\nsubnebulae. But it would be compatible with a new model of satellite formation\nin which the Saturnian satellites formed possibly over longer time scale at the\nouter edge of the main rings. In an attempt to take into account for possible\nsignificant torques exerted by the rings on Mimas, we fitted a constant rate\nda/dt on Mimas semi-major axis, also. We obtained an unexpected large\nacceleration related to a negative value of da/dt= -(15.7 \\pm 4.4) \\times 10-15\nau/day.\n