A 70TH DEGREE LUNAR GRAVITY MODEL (GLGM-2) FROM CLEMENTINE AND OTHER TRACKING DATA

A spherical harmonic model of the lunar gravity field complete to degr ee and order 70 has been developed from S band Doppler tracking data f rom the Clementine mission, as well as historical tracking data from L unar Orbiters 1-5 and the Apollo 15 and 16 subsatellites. The model co mbines 361,000 Doppler observations from Clementine with 347,000 histo rical observations. The historical data consist of mostly 60-s Doppler with a noise of 0.25 to several mm/s. The Clementine data consist of mostly 10-s Doppler data, with a data noise of 0.25 mm/s for the obser vations from the Deep Space Network, and 2.5 mm/s for the data from a naval tracking station at Pomonkey, Maryland. Observations provided Cl ementine, provide the strongest satellite constraint on the Moon's low -degree field. In contrast the historical data, collected by spacecraf t that had lower periapsis altitudes, provide distributed regions of h igh-resolution coverage within +/-29 degrees of the nearside lunar equ ator. To obtain the solution for a high-degree field in the absence of a uniform distribution of observations, we applied an a priori power law constraint of the form 15 x 10(-5)/l(2) which had the effect of li miting the gravitational power and noise at short wavelengths. Coeffic ients through degree and order 18 are not significantly affected by th e constraint, and so the model permits geophysical analysis of effects of the major basins at degrees 10-12. The GLGM-2 model confirms major features of the lunar gravity field shown in previous gravitational f ield models but also reveals significantly more detail, particularly a t intermediate wavelengths (10(3) km). Free-air gravity anomaly maps d erived from the new model show the nearside and farside highlands to b e gravitationally smooth, reflecting a state of isostatic compensation . Mascon basins (including Imbrium, Serenitatis, Crisium, Smythii, and Humorum) are denoted by gravity highs first recognized from Lunar Orb iter tracking. All of the major mascons are bounded by annuli of negat ive anomalies representing significant subsurface mass deficiencies. M are Orientale appears as a minor mascon surrounded by a horseshoe-shap ed gravity low centered on the Inner and Outer Rook rings that is evid ence of significant subsurface structural heterogeneity. Although dire ct tracking is not available over a significant part of the lunar fars ide, GLGM-2 resolves negative anomalies that correlate with many farsi de basins, including South Pole-Aitken, Hertzsprung, Korolev, Moscovie nse, Tsiolkovsky, and Freundlich-Sharonov.