Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p11d..03b&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P11D-03
Other
5417 Gravitational Fields (1221), 5430 Interiors (8147), 5450 Orbital And Rotational Dynamics (1221)
Scientific paper
The rotational state of Titan remains somewhat puzzling, despite recent observational progress by the Cassini mission. It has long been known that Titan's orbit pole precesses about Saturn's spin pole, with a period of ~700 years. Tidal dissipation is expected to damp Titan's spin pole to a Cassini state, in which it would precess about the orbit pole while remaining coplanar with the orbit pole and Saturn's spin pole, much like the Moon does with the Earth. If Titan were known to exactly occupy such a state, then measurements of its obliquity and degree two gravity field would provide constraints on the polar moment of inertia. Analysis of radar returns from the surface of Titan yields an obliquity of 0.3 degree [1]. Analysis of Doppler data from two flybys of Titan yield gravity field coefficients J2 = (27.22 + 0.18) 10-6, and C2,2 = (11.16 + 0.04) 10-6 [2]. The polar moment of inertia value implied by these parameter values, assuming occupation of a Cassini state, is C = 0.55 M R2 [3], which is greater than the homogeneous value. This argument provides independent evidence that Titan has a subsurface fluid layer which mechanically decouples the surface from the deeper interior, as has been separately argued [4] on the basis of a small departure from synchronous rotation. The Titan spin pole apparently lies slightly out of the plane defined by the orbit pole and Saturn's spin pole [1], which suggests that it is not in a pure Cassini state. For bodies with non-uniform orbital precession rates, like the Galilean satellites of Jupiter [5], the coplanarity constraint only applies on a mode-by-mode basis. However, in the case of Titan, the other satellites of Saturn make only small perturbations to the orbit plane. The presumed cause of the observed departure from synchronous rotation is a torque applied by zonal winds in the atmosphere of Titan [6]. This wind is driven by solar heating and is thus modulated over Saturn's orbital period, with a substantial semi-annual (14.8 year period) variation. Because of Titan's non-zero obliquity, the periodic atmospheric torque will also perturb the spin pole orientation, and could be the cause of the angular departure from a conventional Cassini state. However, the semi-annual forcing is quite far from the spin pole precession period (of 700 years) and will thus yield a relatively small amplitude response. An alternative suggestion is that the wobble period of the spin pole is in a 2:1 resonance with the orbit pole precession period [7]. If that resonance were occupied, then the spin pole would be forced to precess about the Cassini state configuration, with an amplitude governed by the rate of tidal dissipation. However, that requires a polar moment of inertia value of 0.355 M R2, which though plausible for Titan as a whole, is quite far from the value suggested by the observed obliquity. References [1] B.W. Stiles, et al. Astron. J., 135, 1669-1680, 2008; [2] L. Iess et al. NASA/CP-2007-214158, 2007; [3] B.G. Bills, F.Nimmo, Icarus, 196, 293-297, 2008;[4] R.D. Lorenz et al. Science, 319, 1649-1651, 2008; [5] B.G. Bills, Icarus, 175, 233-247, 2005; [6] T. Tokano, N. Neubauer, GRL, 32, L24203, 2005; [7] B. Noyelles, Celest. Mech. Dyn. Astr. 101, 13-30, 2008.
class="ab'>
Bills Bruce G.
Nimmo Francis
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