HR: 09:30h
AN: P21D-07 INVITED    [Abstracts]
TI: Titan's Spin State from Cassini SAR Data: Evidence for an Internal Ocean
AU: * Stiles, B W
EM: Bryan.W.Stiles@jpl.nasa.gov
AF: Jet Propulsion Laboratory, California Inst. Of Technology, 4800 Oak Grove Dr, Pasadena, CA 91214, United States
AU: Lorenz, R D
EM: Ralph.Lorenz@jhuapl.edu
AF: Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Rd., Laurel, MD 20723, United States
AU: Kirk, R L
EM: rkirk@usgs.gov
AF: United States Geological Survey, 225 N Gemini Dr., Flagstaff, AZ 86001, United States
AU: Hensley, S
EM: Scott.Hensley@jpl.nasa.gov
AF: Jet Propulsion Laboratory, California Inst. Of Technology, 4800 Oak Grove Dr, Pasadena, CA 91214, United States
AU: Lee, E M
EM: elee@usgs.gov
AF: United States Geological Survey, 225 N Gemini Dr., Flagstaff, AZ 86001, United States
AU: Allison, M D
EM: Michael.D.Allison@nasa.gov
AF: NASA Goddard Institute for Space Sciences, 2880 Broadway, New York, NY 10025, United States
AU: Perci del Marmo, P
EM: p_perci@yahoo.it
AF: University of Rome, via Eudossiana 18, Rome, 00184, Italy
AU: Lunine, J I
EM: jlunine@lpl.arizona.edu
AF: University of Arizona, Lunar and Planetary Lab, 1629 E University Blvd, Tucson, AZ 85721, United States
AU: Ostro, S J
EM: Steven.J.Ostro@jpl.nasa.gov
AF: Jet Propulsion Laboratory, California Inst. Of Technology, 4800 Oak Grove Dr, Pasadena, CA 91214, United States
AU: Gim, Y
EM: Yonggyu.Gim@jpl.nasa.gov
AF: Jet Propulsion Laboratory, California Inst. Of Technology, 4800 Oak Grove Dr, Pasadena, CA 91214, United States
AU: Hamilton, G A
EM: Gary.A.Hamilton@jpl.nasa.gov
AF: Jet Propulsion Laboratory, California Inst. Of Technology, 4800 Oak Grove Dr, Pasadena, CA 91214, United States
AU: Johnson, W T
EM: Williamt.K.Johnson@jpl.nasa.gov
AF: Jet Propulsion Laboratory, California Inst. Of Technology, 4800 Oak Grove Dr, Pasadena, CA 91214, United States
AU: West, R D
EM: Richard.D.West@jpl.nasa.gov
AF: Jet Propulsion Laboratory, California Inst. Of Technology, 4800 Oak Grove Dr, Pasadena, CA 91214, United States
AB: Nineteen areas on Titan's surface have been imaged with Cassini SAR on two separate flybys with intervals from 2 months to 2 years. We have used the apparent misregistration of features between separate flybys (which is 10-30 km) to construct a refined model of Titan's spin state, estimating six parameters: pole right ascension and declination, spin rate, and these quantities' first time derivatives. Because we have only observed Titan for 2-3 years, our dataset is unlikely to be sensitive to higher order derivatives. We have studied the uncertainty and degree of correlation of the model parameters, and have also searched the parameter space to eliminate the possibility of more than one solution. Our model spin state differs significantly from both the zero-inclination synchronous model and from any other plausible Cassini state. The previously estimated pole location and spin rate used by the IAU and the Cassini mission definitely cannot account for the observed misregistration. Because our imaging resolution is between 300 m and 1 km, we are very sensitive to the pole location and spin rate. Our estimated corrections to the pole and spin rate exceed their corresponding standard errors by factors of 40 and 4, respectively. We examined 150 different features in 19 different twice-observed regions. Applying our pole correction reduces the feature misregistration from tens of km to 3-4 km. Applying the spin rate and derivative corrections further reduces the misregistration to 1-2 km. We propose that our result reflects coupling between atmospheric angular momentum changes and an internal water ocean, for two reasons. First, astrodynamical theory predicts that if Titan is in a dynamically relaxed Cassini state there is a relationship between the moment of inertia factor C/MR² and the obliquity of a few tenths of a degree. Our results (from two independent analyses of the overlaps) show an appreciable deviation from the expected range of states: either Titan suffered a recent dynamical excitation, or the theory does not hold because the surface is decoupled from the deep interior. We cannot identify an evident source of a recent excitation, so we favor the latter. Second, much as the Earth's length-of-day changes by ~1 ms over a year, seasonal changes in Titan's atmospheric angular momentum (Tokano and Neubauer, 2005) will manifest themselves in a change in surface rotation rate. The change in rate is ~10x higher, amounting to some hundreds of seconds, when the surface is decoupled from the interior by a water-ammonia ocean. Our preliminary rotation solutions indicate a present- day spin rate offset of several tenths of a degree per year that may be accelerating. The spin rate and its rate of change suggest that significant atmospheric changes are occurring and that Titan has an internal ocean. The research described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
DE: 5450 Orbital and rotational dynamics (1221)
SC: Planetary Sciences [P]
MN: 2007 Fall Meeting