The Moon's gravitational field is decidedly "lumpy," as shown in this map compiled from observations by orbiting satellites, with the nearside at left and farside at right. Red areas exert a stronger gravitational pull than blue and greeen areas. The red circular features correspond to large impact basins that are filled with volcanic rock. NASA's next lunar mission, GRAIL, will map the lumpiness in greater detail, revealing new details about the Moon's structure and formation. The mission also will probe the Moon's interior. [NASA]