TOPOGRAPHY OF THE MOON FROM THE CLEMENTINE LIDAR

Range measurements from the lidar instrument carried aboard the Clemen tine spacecraft have been used to produce an accurate global topograph ic model of the Moon. This paper discusses the function of the lidar; the acquisition, processing, and filtering of observations to produce a global topographic model; and the determination of parameters that d efine the fundamental shape of the Moon. Our topographic model; a 72nd degree and order spherical harmonic expansion of lunar radii, is desi gnated Goddard Lunar Topography Model 2 (GLTM 2). This topographic fie ld has an absolute vertical accuracy of approximately 100 m and a spat ial resolution of 2.5 degrees. The field shows that the Moon can be de scribed as a sphere with maximum positive and negative deviations of s imilar to 8 km, both occurring on the farside, in the areas of the Kor olev and South Pole-Aitken (S.P.-Aitken) basins. The amplitude spectru m of the topography shows more power at longer wavelengths as compared to previous models, owing to more complete sampling of the surface, p articularly the farside. A comparison of elevations derived from the C lementine lidar to control point elevations from the Apollo laser alti meters indicates that measured relative topographic heights generally agree to within similar to 200 m over the maria. While the major axis of the lunar gravity field is aligned in the Earth-Moon direction, the major axis of topography is displaced from this line by approximately 10 degrees to the east and intersects the farside 24 degrees north of the equator. The magnitude of impact basin topography is greater than the lunar flattening (similar to 2 km) and equatorial ellipticity (si milar to 800 m), which imposes a significant challenge to interpreting the lunar figure. The floors of mare basins are shown to lie close to an equipotential surface, while the floors of unflooded large basins, except for S.P.-Aitken, lie above this equipotential. The radii of ba sin floors are thus consistent with a hydrostatic mechanism for the ab sence of significant farside maria except for S.P.-Aitken, whose depth and lack of mare require significant internal compositional and/or th ermal heterogeneity. A macroscale surface roughness map shows that rou ghness at length scales of 10(1)-10(2) km correlates with elevation an d surface age.