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An Angular Momentum Approach to the study of the Cassini States of large triaxial icy satellites in synchronous rotation
https://publi2-as.oma.be/record/6239
Coyette, AlexisFri, 27 Jan 2023 11:41:53 GMThttps://publi2-as.oma.be/record/62392022The rotation of Ganymede and Callisto
https://publi2-as.oma.be/record/5614
The rotation rates of Ganymede and Callisto, the two largest satellites of Jupiter, are on average equal to their orbital mean motion but cannot be constant as a result of the varying gravitational torque exerted by Jupiter on the satellites. For a Keplerian orbit, the period of the torque and of the rotation variations is equal to the orbital period. Gravitational interaction with the other Galilean satellites and the Sun induces deviations from a purely Keplerian orbital motion, leading to changes in the gravitational torque of Jupiter on the satellites with respect to the mean Keplerian orbital motion and therefore to additional rotation variations. Here we discuss small variations from the average rotation on different time scales and assess the potential of using rotation as a probe of the interior structure. The ESA JUICE (JUpiter ICy moons Explorer) mission will measure the rotation and tides of Ganymede and Callisto in the early 30s, and will in particular very accurately determine those quantities for Ganymede during the orbital phase of the spacecraft around that satellite starting in 2032. We report on different theoretical aspects of the rotation for realistic models of the interior of the satellites, include tidal deformations and take into account the low-degree gravity field and topography of Ganymede and Callisto. We assess the advantages of a joint use of rotation and tides to constrain the satellite's interior structure, in particular its ice shell and ocean.Van Hoolst, Tim Tue, 25 Jan 2022 11:09:47 GMThttps://publi2-as.oma.be/record/56142021The rotation and interior of Ganymede
https://publi2-as.oma.be/record/5170
The rotation rate of Ganymede, the largest satellite of Jupiter, is on average equal to its orbital mean motion but cannot be constant on orbital time scale as a result of the gravitational torque exerted by Jupiter on the moon. Here we discuss small deviations from the average rotation rate, evaluate polar motion, and discuss Ganymede's obliquity. We examine different time scales, from diurnal to long-period, and assess the potential of using rotation as probes of the interior structure. The ESA JUICE (JUpiter ICy moons Explorer) mission will accurately measure the rotation of Ganymede during its orbital phase around the satellite starting in 2032. We report on different theoretical aspects of the rotation for realistic models of the interior of Ganymede, include tidal deformations and take into account the low-degree gravity field and topography of Ganymede. We assess the advantages of a joint use of rotation and tides to constrain the satellite's interior structure, in particular its ice shell and ocean.Van Hoolst, TimMon, 25 Jan 2021 12:59:02 GMThttps://publi2-as.oma.be/record/51702020Cassini state of Galilean Moons: Influence of a subsurface ocean
https://publi2-as.oma.be/record/5169
Large moons such as the Galilean satellites are thought to be in an equilibrium rotation state, called a Cassini state (Peale, 1969). This state is characterized by a synchronous rotation and a precession rate of the rotation axis that is equal to the precession rate of the normal to its orbit. It also implies that the spin axis, the normal to the orbit and the normal to the Laplace plane are coplanar with a (nearly) constant obliquity. For rigid bodies, up to 4 possible Cassini states exist, but not all of them are stable. It is generally assumed that the Galilean satellites are in Cassini State I for which the obliquity is close to zero (see e.g. Baland et al. 2012). However, it is also theoretically possible that these satellites occupy or occupied another Cassini state. We here investigate how the interior structure, and in particular the presence of a subsurface ocean, influences the existence and stability of the different possible Cassini states.Coyette, AlexisMon, 25 Jan 2021 12:55:08 GMThttps://publi2-as.oma.be/record/51692020Possible observations of Length-of-day Variations of Titan from Cassinidata between 2004 and 2009
https://publi2-as.oma.be/record/4597
The determination of rotation variations of Titan is based on measurements of the shift in orientation ofCassini RADAR images taken different flybys. Between 2004 and 2009, these images have shown that Titanwas rotating slower than expected for a satellite in synchronous rotation (non-synchronous rotation (NSR) of-0.024±0.018◦/year, Meriggiola et al., 2016).We here model the rotation of Titan from the angular momentum equations (or Liouville equations) of thedifferent interior layers of Titan. Our model includes the effect of tidal deformations of the different layers, of thedeviation from the hydrostatic equilibrium, of the dense atmosphere of Titan and of the flow in the subsurfaceocean on the rotation of Titan.Our results (published in Coyette et al., 2018) show that length-of-day variations with a period of a fewyears and due to angular momentum exchanges between Titan and its atmosphere can be large enough to explainthe observed deviation from synchronous rotation. This observed NSR could therefore be interpreted as anobservation of the length-of-day variations of Titan in that period of time. If our interpretation is correct, weexpect a rotation faster than the synchronous rotation rate between 2009 and 2014, period of time that has notbeen studied yet.Coyette, AlexisFri, 24 Jan 2020 10:28:24 GMThttps://publi2-as.oma.be/record/45972019Influence of non-hydrostatic equilibrium and subsurface ocean flow on the polar motion of Titan
https://publi2-as.oma.be/record/4594
We investigate the influence of a deviation from hydrostaticity and of a flow inside the subsurface ocean on the polar motion of Titan.Coyette, AlexisFri, 24 Jan 2020 09:41:34 GMThttps://publi2-as.oma.be/record/45942019