Rotation variations and interior structure of icy satellites of the Solar System
(2018)
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- Abstract
- From measurements performed by the Galileo and Cassini-Huygens missions, there is now ample evidence of the presence of a subsurface ocean under the thin ice shell of large icy satellites such as Europa, Callisto, Ganymede and Titan. However, many physical properties (such as the thickness, the density and the rigidity) of the ocean and of the ice shell are still uncertain. In comparison with the other large icy satellites, more data have been collected about Titan thanks to the numerous flybys of this satellite by the Cassini probe and the landing of Huygens on the surface of Titan. Titan is the only satellite to possess a significant atmosphere. It also harbours methane lakes and seas on its surface. These methane lakes and seas are involved in a methane cycle similar to the water cycle on Earth. Recently, a deviation of the rotation rate of Titan from its mean synchronous value has been observed from a comparison between images taken by the Cassini probe at different times between 2004 and 2009 (Meriggiola et al. 2016). The goal of this doctoral Thesis is to study and develop methods to constrain the interior of the large icy satellites of the Solar System and Titan in particular. Due to the presence of the subsurface ocean, the interior and the shell of the large icy satellites can move differently from each other. Firstly, the oscillations of the interior and of the shell (« Slichter modes ») of the satellites are studied for a large range of internal structure models. Secondly, the rotation variations of Titan are modeled. Rotation variations include (1) variations of the speed of rotation due to the gravitational torque exerted by the central planet, or « librations in longitude »; (2) variations of the speed of rotation due to angular momentum exchanges with the atmosphere, or « length-of-day variations » and (3) variations of the orientation of the rotation axis with respect to the surface of the satellites, or « polar motion ». The diurnal librations of Titan can be largely increased by the deviation from hydrostaticity of Titan. For an ice shell thicker than 50 km, the polar motion has mainly an annual period with an amplitude that increases with decreasing ice shell thickness. For thinner shells, the period and amplitude can be different due to a resonance amplification. The length-of-day variations have mainly a semi-annual period but short-period variations can become dominant due to a resonance amplification. The observed non-synchronous rotation of Titan can be explained by the length-of-day variations in the 2004-2009 period.
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