Other
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p23c..02i&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P23C-02
Other
[6280] Planetary Sciences: Solar System Objects / Saturnian Satellites
Scientific paper
On Apr. 28, 2010, the Cassini spacecraft flew above Enceladus’s south pole at an altitude of 100 km. The flyby was especially designed to allow a good sensitivity to gravity anomalies that may occur in the south polar regions of the satellite. Continuous tracking of the spacecraft in X- and Ka-band across closest approach was provided by NASA’ Deep Space Network antennas located in Spain and California. The favourable solar elongation angle (141 degrees) contributed to reduce solar plasma noise in range rate data to about 0.03 mm/s at 60 s integration times. Other eight flybys occurred earlier in the mission, but of those the only one devoted to gravity investigations (Feb. 17, 2005) had such a large closest approach distance (1511 km) that only the mass could be determined. In all other flybys the spacecraft was not tracked at closest approach. Although a single polar flyby of such a small satellite (200 km radius, GM = 7.2 km3/s2) does not allow a good determination of the quadrupole field, exploiting data from other flybys partially removes the degeneracy in the orbital solutions and leads to a considerable reduction of the uncertainties. Two combined solutions have been produced, one obtained only from the two existing gravity flybys, the other from a global fit that includes all Cassini flybys as well as optical observations from the spacecraft and the Earth. A pure quadrupole field proves fully adequate for a consistent orbital fit. In addition to the quadrupole coefficients, our solutions include a correction to the orbit of the spacecraft and Enceladus and the estimation of the satellite’s mass. The range rate residuals do not show any signature neither at closest approach nor in the tracking passes preceding and following the flyby, and are consistent with white noise. The estimated gravity field is consistent with the one expected for a tidally-locked body in a relaxed shape and for this interpretation, the estimated moment of inertia is about 0.34 to 0.36 in units of mass times radius squared. However, the current uncertainties do not rule out significant non-hydrostatic components, at a level of 5-10% of the estimated values so inferring an imperfectly differentiated Enceladus may be premature. The relatively large uncertainties in the harmonic coefficients will be strongly reduced after the third gravity flyby to occur on Nov. 30, 2010, when Cassini will fly at an altitude of about 50 km at a latitude of 50N degrees. Numerical simulations show that the combination of all three gravity flyby allows not only a good estimation of the quadrupole field, but also the detection of relatively small North-South asymmetries in the gravity field. A density anomaly with a density contrast (of either sign) of 8%, occupying a spherical cap of 20 km radius at the south pole, would be detected at a 3-sigma level.
Armstrong John W.
Asmar Sami W.
Ducci M.
Iess Luciano
Jacobson Abram R.
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