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J.L. Palguta (UCLA), J.D. Anderson (JPL), G. Schubert, W.B. Moore (UCLA)
The discovery of mass anomalies on Ganymede was reported this summer (Anderson et al., Science 305, 989). We report here on a more detailed characterization of the source of the anomalies. Rather than two or three mass points as reported in Anderson et al., we find that five mass points provide an excellent fit to the data, including a newly revealed acceleration feature about 300 s before closest approach. Two of the five masses are near the previous masses from the two-point fit, and are in good agreement with their mass values. We conclude the two-point fit reveals two major anomalies on Ganymede, but misses three more revealed by the five-point fit. Further, three of the five masses could be indicating a single surface anomaly of horizontal extent under the outgoing flyby trajectory, and the other two masses could be indicating a single extended anomaly under the incoming trajectory. We also include results on placing the five masses at different depths from zero to 1450 km below the surface. A good fit is obtained at any depth from surface to rock-ice interface at about 800 km depth, but the fit deteriorates at greater depth. It is highly unlikely mass anomalies exist within Ganymede's ice shell. We prefer either the near surface or the rock-ice interface. The rock-ice interface is attractive based on rigidity arguments, and the suggestion of two major extended anomalies is even more striking at greater depth. In order to fit the acceleration data, the anomalies must be six or seven times more massive at the 800 km depth than at the surface.
The JPL contribution to this paper was performed at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. J.L.P, G.S. and W.B.M. acknowledge support by grants from NASA through the Planetary Geology and Geophysics program.
The author(s) of this abstract have provided an email address for comments about the abstract: jpalguta@ucla.edu
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.