The shape of Mars before Global Surveyor: Results from reanalysis of the Viking control point network

Three-dimensional coordinates of 3739 globally distributed control points d erived from photogrammetric analysis of Viking Orbiter image data are studi ed with respect to the shape and large-scale morphology of Mars. Spheres, s pheroids, spherical functions, and a Digital Terrain Model (DTM) of 50 km g rid spacing are fitted to the data. Prominent topographic features in the t errain model include the Tharsis volcanoes, Olympus Mons, Alba Patera and p rominent mare and highland regions as well as the major impact basins. Whil e the global dichotomy is clearly visible in our data, our model does not r eveal any distinct signature associated with the presumed dichotomy boundar y at this global scale. Rather, it suggests a smooth topographic transition from southern to northern hemisphere. This model is unique among existing topographic data sets for Mars, as it combines global coverage, a spatial r esolution sufficient to resolve regional topography, and absolute elevation data more accurate than in previous control point network analyses. Formal errors and a comparison with the first released topographic profiles obtai ned by Mars Global Surveyor's Laser Altimeter (MOLA) suggest that 70% and 9 0% of the DTM grid elements represent the topography of the planet to bette r than 1000 m and 2000 m, respectively.