Physics
Scientific paper
Dec 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p33f..03i&link_type=abstract
American Geophysical Union, Fall Meeting 2011, abstract #P33F-03
Physics
[1217] Geodesy And Gravity / Time Variable Gravity, [6281] Planetary Sciences: Solar System Objects / Titan
Scientific paper
The large eccentricity (e=0.03) of Titan's orbit causes significant variations in the tidal field from Saturn and induces periodic stresses in the satellite body at the orbital period (about 16 days). Peak-to-peak variations of the tidal field (from pericenter to apocenter) are about 18% (6e). If Titan hosts a liquid layer (such as an internal ocean), the gravity field would exhibit significant periodic variations. The response of the body to fast variations of the external, perturbing field is controlled by the Love numbers, defined for each spherical harmonic as the ratio between the perturbed and perturbing potential. For Titan the largest effect is by far on the quadrupole field, and the corresponding Love number is indicated by k2 (assumed to be identical for all degree 2 harmonics). Models of Titan's interior generally envisage a core made up of silicates, surrounded by a layer of high pressure ice, possibly a liquid water or water-ammonia ocean, and an ice-I outer shell, with variations associated with the dehydration state of the core or the presence of mixed rock-ice layers. Previous analysis of Titan's tidal response [1] shows that k2 depends crucially on the presence or absence of an internal ocean. k2 was found to vary from about 0.03 for a purely rocky interior to 0.48 for a rigid rocky core surrounded by an ocean and a thin (20 km) ice shell. A large k2 entails changes in the satellite's quadrupole coefficients by a few percent, enough to be detected by accurate range rate measurements of the Cassini spacecraft. So far, of the many Cassini's flybys of Titan, six were used for gravity measurements. During gravity flybys the spacecraft is tracked from the antennas of NASA's Deep Space Network using microwave links at X- and Ka-band frequencies. A state-of-the-art instrumentation enables range rate measurements accurate to 10-50 micron/s at integration times of 60 s. The first four flybys provided the static gravity field and the moment of inertia factor of the body[2]. In this previous analysis, tidal variations of the gravity field were neglected. Thanks to the availability of two additional flybys (on May 20, 2010 and Feb. 18, 2011) and the improvement of the data analysis tools, also the variable component of the gravity field could be estimated with good accuracy. In order to increase the confidence in the results, two independent analyses have been carried out, both resulting in very close and statistically indistinguishable values for k2. While the results are compatible (at the low end of k2) with interior models made up by a high viscosity core and a near-surface liquid water layer, the centroid of the k2 values requires additionally that some substantial fraction of the interior -ice mantle or core- be capable of significant deformations over time scales of the orbital period. References [1] Rappaport, N.J. et al., 2008. Can Cassini detect a subsurface ocean in Titan from gravity measurements?, Icarus, Vol. 194, No. 2. [2] Iess, L. et al., 2010. Gravity field, shape, and moment of inertia of Titan. Science 327, 1367-1369.
Armstrong John W.
Asmar Sami
Ducci M.
Iess Luciano
Jacobson Robert
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