Abstract.A differential rotation of the inner core with respect to the mantle has been recently detected from the variations in time of the inner core anisotropy symmetry axis. This detection relies on the assumption that the anisotropy axis is tilted with respect to the rotation axis. We show that such a tilt cannot be reliably detected with the presently available data. In addition, we observe an apparent correlation between the differential travel time residuals of the core phases used to infer inner core rotation, and the magnitude of the events. This correlation, which could result fi'om the analysis method of the data and from their varying frequency content, may partly bias the observations. Although these results do not rule out the possibility of an inner core differential rotation, they question whether it has actually been detected.
A new model of Mars gravity field in spherical harmonics up to eighteenth degree and order has been computed from all available Doppler tracking data of Mariner 9 and Viking 1 and 2 orbiters. It represents an achievement in the knowledge of Mars global gravitational potential, since no more data will be available for such a derivation before the next mission to the planet. The model has been extensively tested from the point of view of orbit representation over periods of time ranging from a few hours to 12 days, and it is currently used for geophysical studies of the Martian lithosphere.
Improved modeling of the earth's geopotential is one of the major tasks of satellite geodesy. The availability of a new set of precise LAGEOS satellite laser ranging (SLR) data as well as Seasat-acquired observations enabled us to compute an improved solution in the GRIM model series. The new geopotential GRIM3-L1 recombined the satellite perturbation normals of the former GRIM3 solution with new normal equation subsystems derived from an additional 16 months of LAGEOS SLR data, oceanic 1 ø x 1 ø mean free air anomalies recovered from Seasat altimetry, an updated set of terrestrial 1 ø x 1 ø mean free air anomalies, and several condition equations for zonal and resonant harmonics. The combined normal equations were solved for a complete set of spherical harmonics up to degree and order 36 and the coordinates of 109 tracking stations. Comparisons with altimetry-derived quantities, terrestrial gravity, anomalies from satellite-to-satellite tracking, longitude accelerations of 24-hour satellites, and satellite orbit fits show a substantial improvement over previous GRIM solutions and indicate that the LAGEOS data have contributed considerably to the reliable estimation of the long-wavelength portion of the gravity field.
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