The mystery of Titan’s rotation
1er mai 2008
Usually, the tidal effects applied by the parent planet on their natural satellites are strong enough to slow down their spin velocities and lock them in a synchronous rotation. As a consequence, the satellite always shows the same side to its planet, as it is the case for the Moon. Cassini spacecraft has recently observed a super-synchronous rotation for Titan, and several hypotheses are introduced to explain this observation. Some astronomers of IMCCE (Paris Observatory) are part of a team who suggests that the spin rate measurements might be altered.
Due to tidal effects, the rotation of the natural satellites is expected to be synchronous with their revolution, i.e. they perform a whole rotation during a complete revolution around their parent planet. It corresponds to a dynamical equilibrium state, that involves that the satellite always shows the same side to its planet.
- Figure 1:Titan vu par Cassini. La période de Titan autour de Saturne est d’environ 16 jours (image NASA/ESA) Cliquer sur l’image pour l’agrandir
Cassini spacecraft has recently observed a slight super-synchronous rotation of Titan (+0.004%) and the authors interpret it as a signature of an internal ocean, that would dissociate the rotation of Titan’s crust from this of the core, justified by the fact that the motion of the crust would be sensitive to the energy exchanges with the atmosphere.
Another explanation is being proposed by researchers of the University of Namur (Belgium), the University of Lille 1 and Paris Observatory (IMCCE). Following Benoît Noyelles, Anne Lemaître and Alain Vienne, there could be a dynamical forcing of the angle (the wobble) between Titan’s rotation and pole axes, that could false the measurement of Titan’s spin rate if these axes were considered as equal.
Cassini space mission has been extended for at least two years, so more accurate measurements would allow to confirm or refine these explanations.
References
Lorenz R., Stiles B., Kirk R. et al., 2008, "Titan’s Rotation Reveals an Internal Ocean and Changing Zonal Winds", Science, 319, 1649
Noyelles B., Lemaitre A. & Vienne A., 2008, "Titan’s rotation : A 3-dimensional theory", Astron. Astrophys., 478, 959-970
Noyelles B., 2008, "Titan’s rotational state : The effects of a forced "free" resonant wobble", Cel. Mech. Dyn. Astr., in press, arXiv:0709.2293
Contact
Benoît Noyelles (Observatoire de Paris, IMCCE)
Alain Vienne (Observatoire de Paris, IMCCE, et Université de Lille)
Dernière modification le 4 mars 2013