Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.p21d..07s&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #P21D-07
Other
5450 Orbital And Rotational Dynamics (1221)
Scientific paper
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.
Allison Michael David
Gim Yonggyu
Hamilton Gary A.
Hensley Scott
Johnson Timothy W.
No associations
LandOfFree
Titan's Spin State from Cassini SAR Data: Evidence for an Internal Ocean does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Titan's Spin State from Cassini SAR Data: Evidence for an Internal Ocean, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Titan's Spin State from Cassini SAR Data: Evidence for an Internal Ocean will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1405416