STRATIGRAPHIC MAPPING IN PLANETARY EXPLORATION

Two centuries ago, William Smith’s remarkable geologic map of an entire country was pieced together from his own local maps. Conversely, the modern maps made by planetary explorers start at global scales and progress to regional and local scales as spatial resolution allows. Nonetheless, the stratigraphic principles espoused by Smith are directly applicable to mapping planets and planetesimals, but with a twist necessitated by the origin of strata on other worlds. Using telescopic observations, Eugene Shoemaker (1962) first defined time-rock units on the Moon as the blankets of material ejected from large impact basins. These superposed ejecta layers are distributed over large areas and can be ordered into a global stratigraphic column, with correlations made using relative ages from crater-density measurements. Volcanic units, recognizable by their geomorphic characteristics, can be integrated into the Moon’s impact stratigraphy. The same mapping approach, using spacecraft imagery, has been applied successfully to Mars, Mercury, moons of the giant planets, and a few asteroids. In the case of Mars, lacustrine and aeolian units can also be distinguished. The geochemical and mineralogical compositions of units determined from orbital remote sensing, as well as radiometric ages from samples where available, greatly enhance the geologic interpretations of these maps. Using radar, which provides information on topography and reflectivity, maps of even the cloud-obscured surfaces of Venus and Titan can be constructed. And surface exploration by robotic rovers allows mapping at finer scales with which field geologists can readily identify. Traverse and outcrop maps of the regions around Mars rover landing sites still utilize stratigraphic principles, and subsurface strata can be observed in the walls and central uplifts of craters. The tried-and-true, smithian methods of geologic mapping (sans correlation by fossils) work on other planets too.