<P> A major feature of the Moon's gravitational field is the presence of mascons, which are large positive gravity anomalies associated with some of the giant impact basins . These anomalies significantly influence the orbit of spacecraft around the Moon, and an accurate gravitational model is necessary in the planning of both manned and unmanned missions . They were initially discovered by the analysis of Lunar Orbiter tracking data: navigation tests prior to the Apollo program showed positioning errors much larger than mission specifications . </P> <P> Mascons are in part due to the presence of dense mare basaltic lava flows that fill some of the impact basins . However, lava flows by themselves cannot fully explain the gravitational variations, and uplift of the crust - mantle interface is required as well . Based on Lunar Prospector gravitational models, it has been suggested that some mascons exist that do not show evidence for mare basaltic volcanism . The huge expanse of mare basaltic volcanism associated with Oceanus Procellarum does not cause a positive gravity anomaly . The center of gravity of the Moon does not coincide exactly with its geometric center, but is displaced toward the Earth by about 2 kilometers . </P>

What is the gravity like on the moon