Physics – Geophysics
Scientific paper
Nov 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001dps....33.3509a&link_type=abstract
American Astronomical Society, DPS Meeting #33, #35.09; Bulletin of the American Astronomical Society, Vol. 33, p.1101
Physics
Geophysics
4
Scientific paper
Radio Doppler data from two Ganymede encounters (G1 and G2) on the first two orbits in the Galileo Mission have been analyzed previously for gravity information (Anderson et al. Nature 384, 541-543, 1996). Here additional Ganymede encounters are included in the analysis. Only two (G7 and G29) provide additional gravity information by means of reasonably close encounters and also a tracking mode coherent with ground-based atomic frequency standards. We find that a gravity field complete through degree and order four is required in order to fit the data to the noise level, and we interpret the resulting field in terms of a gravity anomaly in the vicinity of the G2 closest approach point on Ganymede (latitude 79.29o, west longitude 123.68o). Analysis of images of Ganymede's limb yield a physical mean radius of 2631.2 +/- 1.7 km. The imaging data reveal no significant deviations from sphericity. The new analysis yields GM = 9887.83 +/- 0.03 km3/s2. The uncertainty in the gravitational constant G has increased in recent years as a result of inconsistent laboratory measurements, but because of the improved value of mean radius, Ganymede's mean density is a factor of 4.6 more accurate than reported in 1996. The new mean density based on G = 6.673 +/- 0.010 x 10-11 m3/s2/kg is 1942.0 +/- 4.8 kg/m3. Consistent with the 1996 results, the new gravity and shape determination implies a strongly differentiated satellite with a large concentration of mass towards its center. This work was sponsored by the Galileo Project and was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. G.S., W.B.M., and P.C.T. acknowledge support by grants from NASA through the Galileo Project at JPL and the Planetary Geology and Geophysics program.
Anderson John D.
Galileo Gravity Science Team
Jacobson Abram R.
Lau Eunice L.
Moore William B.
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