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It is now possible to view global maps depicting long-wavelength and short-wavelength features of the Earth's gravity potential field, both as geoid anomalies and as free-air anomalies. This progress has resulted from (1) an increased number of surface gravity measurements, (2) orbiting of artificial satellites, (3) instal-lation of radar altimeters in three spacecraft, and (4) improved computing and display capabilities. The ratio of gravity to geoid anomalies (g/N) has been utilized to suggest a decomposition of the Earth's gravity field. The g/N ratio for each individual spherical harmonic degree is independent of the harmonic coefficient values, being determined only by the degree and values for normal gravity and the Earth's radius. This g/W-ratio value has an associated point-mass depth that is more limiting of source depth than that provided by wavelength considerations. Mass anomalies that may exist in the Earth at great depth will be best represented in the coefficients of low-degree spherical harmonics, not only because of their longer wavelength anomalies at the surface but also to provide a better match of the resulting ratio of gravity to geoid at the center of the anomaly feature.

Shallow (less than 600-km-deep), broad masses that might be considered possible sources of the degree 2* and 3* Sri Lanka geoid low and the New Guinea geoid high are considered geologically unlikely, whereas 1 to 2 km of relief on the core-mantle boundary is plausible and can explain the combined degree 2 and 3 geoid and gravity anomalies. The core-mantle boundary may become warped as a result of stresses developed by hydrodynamic motions in the outer part of the core, possibly related to those that produce the Earth's magnetic field. Combined contributions from harmonic degrees 4 through 10 identify a narrow positive mass anomaly beneath convergent-plate zones. These positive anomalies are not due to the anomaly of a downgoing slab because they are much broader and have larger magnitude. The fact that association of this positive mass anomaly with convergent-plate zones is more sharply identified in the selected packet of harmonic coefficients than in the full field is a strong indication that the preliminary decomposition is proceeding in the correct direction.

Cumulative geoid degree contribution curves for the low harmonic terms of the gravity field of Venus have a different pattern than that for the Earth. Maps of the low harmonic coefficients for Venus indicate correspondence with surface topographic features in marked contrast to the Earth. Venus rotates very slowly and has no internal magnetic field. Perhaps coherent motion within the core is negligible, so that there is no magnetic field and no distortion of the core-mantle boundary. These relations support the conclusion that long-wavelength undulations at the Earth's core-mantle boundary may contribute significantly to the lowest degree harmonic coefficients of the Earth's potential field.

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