HR: 1340h
AN: P33C-0263    [Abstracts]
TI: How Jupiter helped Titan retain a finite orbital eccentricity
AU: * Bills, B G
EM: bbills@ucsd.edu
AF: NASA GSFC, Planetary Geodynamcis Laboratory, Greenbelt, MD 20771 United States
AU: * Bills, B G
EM: bbills@ucsd.edu
AF: Scripps Institution of Ocenaography, IGPP, La Jolla, CA 92093 United States
AU: Nimmo, F
EM: fnimmo@es.ucsd.edu
AF: University of California, Dept. Earth Sciences, Santa Cruz, CA 95064 United States
AB: The present eccentricity (2.88%) of Titan's orbit is surprisingly high, and, in contrast to the situation for the Galilean satellites, this eccentricity is not excited by resonant interactions with other satellites. If the eccentricity is primordial, then its present value places strong constraints on tidal dissipation within Titan over the past 4.5 Gyr. This constraint, in turn, means that only certain models of internal structure and surface fluid bodies are acceptable. We show that this eccentricity, rather than being primordial, may have been generated more recently, as Titan's orbit evolved through a secular orbit-orbit resonance. This result opens up the parameter space of acceptable models of Titan's interior. Furthermore, passage through this resonance is likely to have strong (and potentially detectable) effects on Titan's surface geology and tectonics. Since tidal dissipation removes energy from Titan's orbit, it is expected that the orbit will have shrunk considerably over the age of the system, and the past apsidal precession period will have been rather longer than the current ~700 yr. If the dissipation rate is sufficient, then the apsidal period will have passed through a 3:2 resonance with the Great Inequality (an ~800 yr period of circulation of Jupiter-Saturn inferior conjunction locations), and that resonance passage will have influenced the eccentricity of Titan's orbit. The condition for resonance is that the apsidal period is ~1200 yr. The apsidal rate depends sensitively on distance from the oblate figure of Saturn, and the resonance occurs at a semimajor axis value 16% larger than present. The eccentricity at that distance, without the resonant effect, would be 0.378. The rate of dissipation, and associated orbital evolution, is proportional to the ratio of the tidal Love number k and the tidal quality factor Q. Assuming a fluid value of k = 3/2, the resonance would occur at time 870 (Q/100) Ma. Conversely, if the age of resurfacing were known, we could infer the value of k/Q. The magnitude of the eccentricity change incurred upon resonance passage depends on two parameters: the duration of the transit through resonance, and the phase of the apsidal line at the moment of exact resonance. The duration depends on the rate of dissipation, and the phase at resonance is currently unknown. If the phase at resonance was favorable to an increase in eccentricity, the resulting maximum eccentricity during passage through resonance would slightly exceed the value of 0.378, but only briefly. The surface heat flow, assuming equilibration with heat generation, would be 280 (100/Q) mW m^-2. Heat fluxes above 100 mW m^-2 would almost certainly imply significant resurfacing. The eccentricities of all the satellites of Saturn are gravitationally coupled, and resonant pumping of the eccentricity of Titan will influence the other orbits. The strongest coupling between Titan and its neighbors is with Iapetus, and the substantial eccentricity of Iapetus (2.84%) may also be a result of the Titan resonance passage. In summary, we have identified a resonance which may have excited Titan's orbital eccentricity in the relatively recent past. A recent excitation invalidates arguments based on Titan's eccentricity being a primordial remnant, and may also have had detectable geological effects.
DE: 5418 Heat flow
DE: 5430 Interiors (8147)
DE: 5450 Orbital and rotational dynamics (1221)
SC: Planetary Sciences [P]
MN: Fall Meeting 2005